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doi:10.2204/iodp.proc.342.101.2014

Principal results: Newfoundland sediment drifts

Paleogene drift sedimentation in the northwest Atlantic Ocean

Expedition 342 drill sites are in plastered drifts that exist in two places: (1) the southern flank of J-Anomaly Ridge and (2) the slopes of seamounts on Southeast Newfoundland Ridge (Fig. F15). Both accretionary drifts and plastered drifts have formed adjacent to passages between bathymetric highs, most commonly on the northeastern and southwestern sides of seamounts. Some of these drifts are highly localized features that form lenticular bodies over a depth gradient of >1000 m in some cases, which gives them their typical “slug” appearance on seismic reflection profiles (Fig. F16). Other drifts are clearly accretionary features that have built toward moats adjacent to seamounts. A particularly elongate sediment ridge exists next to Titanic Canyon between J-Anomaly Ridge and Southeast Newfoundland Ridge (Fig. F11). Seismic records show that the Titanic Canyon drift is made entirely of sediment and was built as a series of spur ridges lapping against the northwestern flank of J-Anomaly Ridge. These ridges appear to be constructed largely of mudwaves based upon their seismic character and enclose ponds of young, likely Pleistocene, sediment. The accretionary drifts, in particular, offer an attractive drilling target for future expeditions because they offer the opportunity to core very high deposition rate sequences with offset shallow penetration sites.

Expedition 342 cored four of these drift sequences, each covered by a thin veneer of Pliocene and Pleistocene, or sometimes Miocene to uppermost Oligocene, sediment. The sites in the deepest water were drilled on J-Anomaly Ridge, where drilling encountered a carbonate-rich Cretaceous to Paleocene sequence overlain by a clay-rich Eocene to lower Miocene sequence (Sites U1403–U1406) (Fig. F16). Three additional drifts were drilled on the crest of Southeast Newfoundland Ridge near the wreck of the Titanic. Each of these drifts appears to preserve a middle Eocene to Lower Cretaceous sequence overlying an Albian or older reef complex. Coring at Sites U1407 and U1408 recovered much of this sequence in one of the drifts. Coring in another drift (Sites U1409 and U1410) focused on the younger, middle Eocene to Paleocene portion of the record. Finally, Site U1411 targeted still another drift that proved to have a highly expanded record of the upper Eocene and Oligocene overlying an uncored section that is likely of middle Eocene age based on seismic ties to Site U1410.

Results from Expedition 342 indicate that the onset of relatively clay-rich, high accumulation rate (>3 cm/k.y.) sedimentation, which we interpret as the onset of drift deposition, is early middle Eocene age (~47 Ma) at both J-Anomaly Ridge and Southeast Newfoundland Ridge (Fig. F17). This result closely matches new findings of the timing of drift onset in the Greenland-Scotland Ridge basins of the northeast Atlantic (Hohbein et al., 2012). Onset of drift deposition to the north (Labrador Sea and Northeast Atlantic) and to the south (Blake, Hatteras, and Chesapeake Drifts) is thought to occur no older than middle to late Eocene (~40 Ma) (Mountain and Tucholke 1985; Arthur et al., 1989). However, erosion associated with a significant regional unconformity that developed during the EOT (Reflector A_u) is interpreted to have removed sediment possibly associated with early to middle Eocene drifts along the eastern United States margin (Mountain and Tucholke, 1985). The discovery and recovery of an expanded middle Eocene section at multiple sites preserved in the Newfoundland drift complex is a significant achievement of Expedition 342.

Drift accumulation through the EOT was recovered at two sites on J-Anomaly Ridge (Sites U1404 and U1406) and one site on Southeast Newfoundland Ridge (Site U1411). The drifts with the highest sedimentation rates of the expedition (>10 cm/k.y.) span the latest Oligocene and Oligocene/Miocene boundary (Fig. F17). These higher maximum sedimentation rates (three times higher than the Eocene drifts) across the Oligocene–Miocene transition and during the Miocene isotope Event 1 (Mi1) glaciation are suggestive of an intensification of boundary current flow strength, sediment transport, and drift development. Why the drift deposits on J-Anomaly Ridge are of late Oligocene and early Miocene age and not of early Oligocene age remains uncertain. It is unclear at this time whether this pattern of lower accumulation rate in the older drifts is due to diminished supply of terrigenous sediment (e.g., as a result of climatic change), weaker currents (resulting in less sediment transport), or perhaps stronger currents (i.e., less material being deposited).

Insignificant thicknesses of late Miocene to Pliocene age drift deposits were recovered during Expedition 342. It is important to reiterate that the drilling strategy of Expedition 342 was to avoid thick accumulations of Neogene sediments to access and recover Paleogene sediments with the APC. Thus, the record of drift sedimentation depicted in Figures F17 and F18 shows the late Miocene to Pliocene dominated by nondeposition and inherently emphasizes the Paleogene history. However, as discussed above, seismic reflection data indicate that many areas on Southeast Newfoundland Ridge have Neogene drift accumulations several hundreds of meters to nearly 1 km in thickness.

Acoustic units of the Newfoundland ridges

Expedition 342 provided strong constraints on the ages of the major acoustic units on the Newfoundland ridges. Correlation of a prominent regional reflector interpreted as volcanic basement is straightforward and shows that the overlying sedimentary package generally drapes this inherited topography. However, some modern bathymetric features are primarily composed of drift sediments—these include the majority of the crest of Southeast Newfoundland Ridge and the linear ridge within Titanic Canyon. The deepest sedimentary reflector package consists of strong, usually highly continuous reflections that intersect mounded, internally structureless reflector units interpreted as buried reefs and flanking shallow-marine sediments. The reef sediments have been sampled at both the crest of J-Anomaly Ridge (Site 384) (Tucholke and Vogt, 1979) and on a seamount crest on Southeast Newfoundland Ridge (Site U1407). Overlapping the seismic expression of the reefs is a partially transparent reflector package that passes upward into a zone of discontinuous reflections. In some cases the discontinuous reflector unit grades laterally into well-developed, parallel, strong reflections. Site U1407 showed that this entire package consists of Albian–Campanian chalk and black shale. The interval from the K/Pg boundary through the middle early Eocene is represented by mostly strong, often continuous reflections associated with a zone of chert interbedded with chalk and calcareous ooze. Above this, the sedimentary pile is represented by a thick sequence of mostly acoustically transparent units that pass upward into more distinct wavy reflections suggestive of large-wavelength mudwaves. The entire transparent sequence is well dated by Expedition 342 coring and ranges in age from the middle early Eocene to the middle Miocene. The uppermost parts of the mudwave package have not been cored, but the absence of strong reflectors that could be unconformities suggest that the mudwave unit probably preserves an extensive Miocene and Pliocene record. Pleistocene (and possibly Pliocene) sediments, represented by strong parallel reflections, cap the entire sedimentary pile.

Subdivisions of the general reflector stratigraphy are possible. Most of the time during Expedition 342 was spent coring the upper acoustically transparent unit of Eocene to Miocene age. This unit can be divided into three subunits based upon reflection character. The lowest of these is a zone of generally fuzzy, acoustically dense reflections that typically do not have well-developed layers. Coring at Sites U1403–U1406 on J-Anomaly Ridge shows that this acoustically dense reflector unit is of early Eocene to middle Eocene age and tends to contain more calcium carbonate and opal than overlying sediments. Above the lower acoustically fuzzy subunit is an interval of dispersed reflections that are often discontinuous or faint. Coring at Site U1411 showed that the discontinuous reflections correspond to the zone of high sedimentation rates and changes in calcium carbonate abundance around the Eocene/Oligocene boundary. This boundary is not visible everywhere because there is very little seismic expression in the Eocene/Oligocene boundary sections on J-Anomaly Ridge. The upper part of the acoustically transparent unit typically is completely transparent but may, in places, show patches of wavy reflections similar to but far less continuous than the overlying mudwave-dominated sequence. Sites U1404–U1406 and U1411 encountered a lower Miocene to middle Oligocene section of clay and nannofossil clay. Other sites (U1403 and U1407–U1410) encountered condensed sequences, often of clay with low carbonate content and multiple sharp surfaces indicative of nondeposition or erosion.

Reef and shallow-marine carbonates form a pair of strong reflectors just above volcanic basement over much of Southeast Newfoundland Ridge. Drilling at Site U1407 showed that the prominent reflectors at the very base of the sedimentary sequence on Southeast Newfoundland Ridge include shallow-marine carbonate grainstone and packstone of probable late Albian and older age. The late Albian reflector overlies acoustic features that indicate probable reef buildups. Morphological reefs without internal reflectors are also visible in seismic lines from much of the expedition drilling area. Similar shallow-marine carbonates were extensively drilled on the crest of J-Anomaly Ridge at Site 384, where they were dated to the early Albian, early Aptian, and late Barremian (Tucholke and Vogt, 1979).

Lithostratigraphic summary

The sedimentary sequence on J-Anomaly Ridge and Southeast Newfoundland Ridge comprises six general expedition lithostratigraphic units (Table T1).

Unit A

Unit A is composed of clay, foraminifer sand, and nannofossil ooze containing dropstones, ice-rafted sand, and manganese nodules of Pleistocene to Pliocene age. Lithic fragments interpreted as IRD include subrounded pebbles and cobbles of basalt, granitic rocks, amphibolite, dolomite, and fossiliferous packstone and grainstone. Sediment is commonly shades of brown, tan, red, and green, reflecting variations in clay content, ice-transported sediment, and siliceous and calcareous microfossils and is extensively bioturbated, but centimeter to decimeter bedding is common. Pleistocene cover sediments tend to exhibit high magnetic susceptibility (Fig. F19) and strong contrasts in other physical properties, reflecting the large lithologic differences between glacial and interglacial sediments (Figs. F20, F21, F22). These sequences were cored only in thin, surficial cover during Expedition 342, but seismic reflection data suggest this unit reaches thicknesses of at least 200–500 m on the crest of Southeast Newfoundland Ridge and its northern slope.

Unit B

Unit B is composed of clay (often noncalcareous) and nannofossil ooze of Pliocene to early Oligocene age (Figs. F23, F24). Clay deposits were recovered at most of the Expedition 342 sites as condensed sequences. Sediment is commonly yellow, tan, brown, or pale green. Sand- and silt-sized lithic grains are common, probably reflecting ice rafting (Fig. F25). Typically, sediment in Unit B has low magnetic susceptibility and shows an abrupt shift in many physical properties compared to the overlying Pleistocene sediment (Figs. F19, F20, F21, F22). Multiple unconformities are common in this sequence, sometimes marked by burrowed firmgrounds or glauconitic horizons (Fig. F26B). Burrowing is often intense, creating smooth-textured clay-rich sediment without much physical stratification. Lithologic cycles are usually faint, either because of very low sedimentation rates or very high deposition rates in fairly uniform sediment. This general lithostratigraphic unit is likely roughly time-correlative to seismic sequences of mudwaves elsewhere on J-Anomaly Ridge and Southeast Newfoundland Ridge that are 1000–2000 m thick. We did not core thick sequences of mudwaves, so their lithologic expression is unknown in detail.

Unit C

Unit C is composed of nannofossil clay, nannofossil ooze, and biosiliceous ooze of early Miocene to middle Eocene age. In smear slides, the carbonate and biosilica are typically well preserved (Figs. F23, F24). Such sediment was cored at all Expedition 342 sites and consists of green, gray, and white deposits with often well-developed cycles in color, magnetic susceptibility, and bulk density (Figs. F20). Sedimentation rates are often higher than in other lithologic units, with rates as high as 3–10 cm/k.y. largely because of high clay fluxes. Probable ice-rafted sand and silt occurs in the Miocene, Oligocene, and uppermost Eocene part of this unit, often as subrounded grains of fine carbonate-cemented sandstone that degrades to blebs of silt- to sand-sized angular quartz (Figs. F26D, F25). The paler sediment in Unit C is typically carbonate rich (40–80 wt%) and displays cycles in carbonate abundance on decimeter to tens of meters wavelengths. Calcareous microfossils are often well preserved, particularly in clay-rich facies. Siliceous sediment, sometimes with well-preserved diatoms and radiolarians, is common in the Miocene part of this sequence (Figs. F23, F24). A prominent horizon interpreted to represent ejecta from the Chesapeake Bay impact event is notable at Site U1403, whereas a rare occurrence of a native copper-filled vein occurs in sediment of middle to late Eocene age at Site U1406 on J-Anomaly Ridge (Fig. F26A, F26C). Lithostratigraphic Unit C is represented by acoustically transparent drift packages in the deeper parts of the Southeast Newfoundland Ridge sedimentary record as well as in plastered drifts around seamounts.

Unit D

Unit D is composed of nannofossil ooze, chert, and biosiliceous ooze of early Eocene to Campanian age (Figs. F23, F24). These sediments are commonly white, brown, and pink ooze and chalk with mottles and interbedded tan, pale green, and dark brown chert. Sedimentation rates are low (~1 cm/k.y. or less), reflecting mostly pelagic biogenic sediment deposition and low terrestrial clay fluxes. Burrows are common, and bedding is mostly confined to decimeter-scale cycles of varying carbonate concentration. In sites on Southeast Newfoundland Ridge, carbonate content can be ~80–90 wt% and microfossil preservation is commonly moderate to poor with overgrowths and carbonate infilling. This lithostratigraphic unit typically is rich in radiolarians, particularly in the Paleocene and early Eocene, although the chalk immediately above and below chert layers commonly has rare, poorly preserved radiolarians. Magnetic susceptibility and bulk density are commonly high in this unit, and natural gamma radiation (NGR) characteristically is low, reflecting the low–terrigenous sediment fraction and relatively extensive diagenetic cementation (Figs. F19, F20, F21, F22). The contact with the middle Eocene gray-green nannofossil clay is often unconformable, but at least one of the sites (U1410) apparently has a biostratigraphically complete contact. Prominent volcanic ash layers are notable in sediment of Paleocene age and are associated with downhole spikes in magnetic susceptibility that can be readily correlated among sites (Fig. F27). Lithostratigraphic Unit D exists as pelagic sediments below the main drift sequences over much of Southeast Newfoundland Ridge and was encountered in the deeper parts of Expedition 342 drill sites.

Unit E

Unit E is composed of nannofossil chalk and organic-rich claystone of Santonian to Albian age (Figs. F23, F24). We cored this sequence at Site U1407, where we found it to be brown, tan, and pale green nannofossil chalk. Burrowing is extensive, and bedding is confined to decimeter-scale cycles of darker and lighter sediment. Carbonate content is typically high (80–90 wt%), and chert is present but rare. Bulk density and magnetic susceptibility are typically high and variable (Figs. F19, F20, F21, F22). Black shale and green laminated nannofossil clay is present in intervals near the Cenomanian/Turonian boundary. Total organic carbon reaches 11 wt% in the black shale, from background levels of ~0.5 wt% in the surrounding nannofossil chalk. A gradational contact exists between Unit E and both the overlying and underlying units.

Unit F

Unit F is composed of shallow-marine carbonate sandstone, pelletal packstone, skeletal grainstone, and oolitic limestone of Albian or older age cored at Site U1407 (and at Site 384) (Figs. F23, F24). This lithostratigraphic unit is interpreted to have been deposited in a variety of shallow-marine carbonate-dominated environments, ranging from reefs to lagoons and shelf-carbonate sandstones. We did not recover sediment from this unit with much fidelity, but the small rollers we did obtain are typically pale tan to brown moldic skeletal grainstone, pelletal grainstone, and pelletal packstone. A variety of shallow-marine fossil groups are represented, including belemnites, gastropods, bivalves, echinoderms, and corals. The seismic expression of this unit is of morphologic reefs as thick as 400 m and strong parallel reflectors that represent hardgrounds composed of carbonate grainstone and lagoon sediments.

Interstitial water geochemistry

Alkalinity and ammonium concentrations are expected to increase with depth in interstitial water as sedimentary organic matter undergoes degradation. At the J-Anomaly Ridge sites, alkalinity and ammonium concentrations are <10 mM and 380 µM, respectively, which are typical in an organically lean sediment (Fig. F28). The overall downhole trends in alkalinity and ammonium concentrations increase with depth to between 100 and 250 mbsf, suggesting that this depth interval marks the locus of peak organic matter consumption. The depth profile of dissolved manganese concentration closely follows those of alkalinity and ammonium, indicating that the organic matter decomposition process drives the reduction of manganese (IV) oxide as expressed by

A comparison of alkalinity and ammonium concentrations from the J-Anomaly Ridge sites indicates low values at Sites U1403 and U1406, especially in ammonium. This suggests that the sedimentary organic matter at Site U1406 exists at lower concentration and/or lesser reactivity than at other J-Anomaly Ridge sites (Fig. F28). The prediction of higher sedimentary organic matter content at Sites U1404 and U1405 is borne out by measurements of nitrogen at Site U1404 (Fig. F29), which has the highest total nitrogen content of all J-Anomaly Ridge sites.

Dissolved iron concentrations are low at Sites U1403, U1404, and U1406 (<15 µM) (Fig. F28). Iron concentrations are similarly low at Site U1405 from 0 to 75 mbsf but then increase downhole to values as high as 35 µM between 75 and 250 mbsf, to 100 µM at 265 mbsf, and then to <5 µM at 300 mbsf. Dissolved manganese concentrations at Site U1405 reach a maximum at 220 mbsf and decrease with greater depth, suggesting that manganese (IV) reduction changes to iron (III) reduction with depth, as expressed in the following equation:

These data indicate that sedimentary organic matter at Site U1405 was present in higher concentrations during the deposition of the sediment than at the other J-Anomaly Ridge sites.

Concentrations of calcium and strontium generally increase monotonically with depth at Sites U1403–U1405 (Fig. F30). The profiles for calcium and strontium at Site U1406 reach maxima at 180 and 210 mbsf, respectively, and then decease slightly, suggesting that the effect of carbonate recrystallization deep within the sequence (~200–250 mbsf) in Eocene-aged sediments is greater at Site U1406 than at other J-Anomaly Ridge sites. Magnesium concentration profiles show overall decreases downcore, with some minor fluctuations. Mg/Ca ratios decrease smoothly and eventually flatten with depth and, in the case of Site U1406, undergo a slight reversal below ~200 mbsf (Fig. F30).

Concentration gradients of calcium, strontium, and magnesium appear to reflect the effects of exchange reactions within basaltic basement with some contribution from carbonate recrystallization mainly from below the drilled interval at Site U1405 and from the base of the drilled interval at Site U1406. The gradient of calcium concentrations at Site U1403 is the steepest, whereas that of Site U1406 is the weakest among all J-Anomaly Ridge sites. At Site U1406, the decrease in calcium concentrations below 200 mbsf corresponds to a decrease in alkalinity and an interval of well-cemented chalks, indicating precipitation of carbonate (Fig. F30). The downhole increase in magnesium below 210 mbsf suggests “fresh” seawater flux to the deeper layers at this site, presumably a function of their outcropping close to the “nose” of the sediment drift.

Interstitial water profiles at Southeast Newfoundland Ridge Sites U1407–U1410 display evidence of compartmentalization with prominent abrupt downhole shifts in magnesium, manganese, and potassium at 100–120, 180–190, 125–130, and 220–230 mbsf, respectively (Figs. F31, F32), suggesting that the unrecovered sequences of Eocene chert act as aquicludes, a feature also characteristic of the roughly correlative sections drilled on Blake Nose (Leg171B; Rudnicki et al., 2001).

Despite smaller scale fluctuations (see discussion below), the overall trends of alkalinity, ammonium, manganese, and iron in interstitial water at Southeast Newfoundland Ridge sites increase with depth, reaching maximum values of 6 mM and 160, 28, and 80 µM, respectively. These concentrations are much lower than those observed in cores at the J-Anomaly Ridge sites, suggesting lower TOC contents or that less reactive organic matter survives into the interstitial water at the Southeast Newfoundland Ridge sites. Depth profiles of alkalinity and manganese concentrations are well correlated with each other. This co-variation of alkalinity and manganese concentrations indicates that organic matter degradation associated with manganese reduction controls the observed profiles, analogous to what was observed at the J-Anomaly Ridge sites (e.g., Fig. F30). However, in contrast to J-Anomaly Ridge sites, Southeast Newfoundland Ridge depth profiles of alkalinity, manganese, and iron show fluctuations with depth, displaying multiple local maxima through the recovered sequences. This implies that the levels of organic matter varied considerably over the sampled depth interval, with the subsequent degradation of organic matter resulting in the complex observed patterns, with some modification by subsequent diffusional processes. Sulfate concentrations steadily decrease with depth, suggesting its distribution is driven by diffusion of seawater sulfate as well as organic matter variations. Where cherts occur, sulfate concentrations drop rapidly, especially at ~150 mbsf at Site U1409 (Fig. F31).

Depth profiles of calcium increase to ~150 mbsf, whereas magnesium and Mg/Ca ratios generally decrease over this interval, suggesting exchange reactions of pore fluid with volcanic basement as seen at J-Anomaly Ridge sites. At greater depths, most profiles tend to flatten (Site U1410 is an exception) and in some cases slightly increase. These inflections can be attributed to chert layers acting as aquicludes. At Site U1407, however, calcium concentrations tend to decrease slightly below the chert layer, whereas magnesium increases; again, these observations imply a source of fresh seawater below the bottom of the hole (Fig. F32).

Paleoproductivity indicators

Three of the four sites drilled on the J-Anomaly Ridge depth transect (Sites U1404, U1405, and U1406) captured a record of biostratigraphically datable upper Oligocene to lower Miocene sediments. Downhole records of paleoproductivity indicators suggest elevated export production during the Oligocene–early Miocene. The abundant occurrence of infaunal calcareous benthic foraminifers, including abundant Globobulimina pacifica at Site U1404 (Fig. F33), suggests the presence of low-oxygen bottom water conditions associated with high organic matter supply to the seafloor. The high abundance and excellent preservation of the diatom assemblage, including the dominance of high-productivity indicator Chaetoceros resting spores, further suggest high productivity during this period.

These data suggest a first-order pattern of high export production to the seafloor at J-Anomaly Ridge (Sites U1404–U1406) from the late Oligocene to early Miocene. At the deepest of these three sites, Site U1404 (4746 mbsl), there is benthic foraminifer evidence (from G. pacifica) of persistent low-oxygen bottom water conditions in the lower Miocene. At Site U1405 (4286 mbsl; 460 m shallower water depth), intermittent low-oxygen bottom water conditions are also indicated during the lower Miocene, whereas at Site U1406 (3814 mbsl), ~930 m shallower than Site U1404, oxic seafloor conditions are maintained throughout the Oligocene to Miocene.

Oligocene to lower Miocene sediment was drilled on Southeast Newfoundland Ridge at all five sites, but apart from Site U1411, the sediment is restricted to only short intervals of time (Fig. F34). Downhole records of benthic foraminifers are remarkably similar to the J-Anomaly Ridge sites and suggest elevated export production through the Oligocene–early Miocene interval. In agreement with the upslope trend toward more oxygenated deep water across the J-Anomaly Ridge depth transect, benthic foraminiferal assemblages at the shallower Southeast Newfoundland Ridge sites (3000–3500 mbsl) are dominated by infaunal taxa (Fig. F34) but lack clear indicators of suboxic bottom water (e.g., G. pacifica). Although diatoms and radiolarians are abundant in lower Miocene and upper Oligocene sediment at Sites U1404 and U1405, they are very rare or absent at the Southeast Newfoundland Ridge sites (compare Figs. F23, F24).

Given the high accumulation rates of drift sediment identified in the late Oligocene to early Miocene at J-Anomaly Ridge sites and in the Oligocene at Southeast Newfoundland Ridge sites, this sediment will allow for high-resolution studies of paleoproductivity proxies like benthic foraminiferal assemblages, siliceous microfossils (diatoms and radiolarians), and nitrogen isotopes. Especially high sedimentation rates across the Oligocene–Miocene transition hold promise for future studies related to the Oligocene–Miocene transition glaciation and suborbital-scale variability in paleoproductivity. Another valuable aspect of the recovered Expedition 342 sequences is the opportunity to study changes in oxygenation and paleoproductivity across the drilled depth transect at J-Anomaly Ridge, covering 1200 m of water depth (3813–4946 mbsl). This record can be combined with Site U1411 (3300 mbsl) at Southeast Newfoundland Ridge to expand the transect even further.

All Southeast Newfoundland Ridge sites show sporadic occurrences of infaunal-dominated benthic foraminifer assemblages in the middle Eocene (Fig. F34). This dominance of infaunal taxa suggests short-term increases in export production to the seafloor. For example, the upper interval of nannofossil Zone NP16 and lower interval of Zone NP17, spanning the MECO, are characterized by increased abundances of infaunal taxa, suggesting a response to warming associated with the MECO event.

Paleogene and Cretaceous organic matter

Organic matter preserved in marine sediments provides important information on paleoceanographic and paleoclimatologic changes. In the past few decades, organic biomarker studies have significantly advanced our understanding in this field (e.g., Brassell, 1984; Simoneit, 1986; Volkman et al., 1988; Hayes, 1993; Schouten et al., 2003; Pagani et al., 2005). During this expedition, we performed >1600 organic analyses using the Flash EA to derive TOC and total nitrogen contents and also analyzed a small set of sediment samples for organic matter characterization using the Source Rock Analyzer. Organic-rich sediment from OAE 2 was recovered at Site U1407 and characterized in some detail on board (see “Ocean Anoxic Event 2”).

TOC and total nitrogen profiles from J-Anomaly Ridge sites indicate that organic matter preservation peaked in the early Miocene (Fig. F29), which, together with the abundance of diatoms and infaunal benthic foraminifers (see “Paleoproductivity indicators”), suggests high productivity during the early Miocene. Sites U1403–U1406 comprise a depth transect (4946–3813 mbsl) along the southwestern flank of J-Anomaly Ridge. TOC values reach ~2 wt% at the two middle sites on J-Anomaly Ridge (Sites U1404 and U1405) and are somewhat lower (1.5 wt%) at the sites on either end of the depth transect (Sites U1403 and U1406). Total nitrogen contents at all four sites vary between 0.1 and 0.2 wt% in the early Miocene section. In early Eocene–Paleocene sediment recovered from Site U1403, TOC content is highly variable, but total nitrogen values remain low (mostly <0.05 wt%) through the sedimentary sequence (Fig. F29).

TOC and total nitrogen profiles from Southeast Newfoundland Ridge sites are remarkably similar in that organic matter concentration increases substantially across the early/middle Eocene boundary (Fig. F35), corresponding to changes in carbonate content and lithostratigraphy. TOC content changes from ~0.5 wt% or lower in the early Eocene to 1.0–1.5 wt% in the middle Eocene, whereas total nitrogen content ranges from almost 0 to 0.05–0.15 wt%.

Results suggest that sediment recovered during Expedition 342 provides great opportunities for organic geochemistry work, such as sea-surface temperature reconstruction based on the TEX₈₆ and alkenone proxies, alkenone-based pCO₂ reconstruction, biomarker reconstruction of terrestrial and marine ecosystems, and other specific biomarkers that indicate past oceanic and climatic conditions (e.g., Schouten et al., 2003; Pagani et al., 2005; Liu et al., 2009). In general, organic matter content is higher at J-Anomaly Ridge sites than at Southeast Newfoundland Ridge sites. Given the high sedimentation rates of drift sediment identified in the late Oligocene to early Miocene at J-Anomaly Ridge sites (as high as 10 cm/k.y.) and in the middle Eocene at Southeast Newfoundland Ridge sites (~3 cm/k.y.), this sediment will allow high-resolution studies on organic compounds. Variance in geochemical parameters such as TOC and total nitrogen through events such as OAE 2 (see “Ocean Anoxic Event 2”), the K/Pg boundary, and the PETM (Figs. F29, F35) also hold promise for future organic geochemical studies.

Exceptional preservation in drift clay-rich sediment

One of the primary objectives of Expedition 342 was the recovery of high accumulation–rate, clay-rich sediment containing well-preserved microfossils suitable for trace element geochemistry, isotope geochemistry, and faunal studies. At almost all Expedition 342 sites where drift sediment was recovered, we observed calcareous microfossil preservation that was good and moderate to good and significantly better than the quality of preservation that is typical of deep-sea deposits. Furthermore, significant intervals of these drift successions contained exceptionally well preserved calcareous microfossils, including glassy foraminifers, and diverse, minute, and fragile calcareous nannofossils. Such high-quality preservation is usually only found in clay-rich shelf and slope sections (Pearson et al., 2001; Bown et al., 2008), so the recovery of stratigraphically continuous and expanded middle Eocene through lower Miocene successions with exceptional microfossil preservation is a significant outcome of the expedition.

The preservation of calcareous microfossils is strongly affected by the carbonate saturation state of bottom water and, therefore, proximity to the CCD and lysocline (e.g., Berger, 1970). Preservation is also affected by the composition of the sediment, in particular clay and carbonate contents (Norris and Wilson, 1998; Pearson et al., 2001; Wilson and Norris, 2001; Sexton et al., 2006b; Bown et al., 2008). Carbonate-rich sediment (e.g., nannofossil ooze) tends to promote the overgrowth and recrystallization of calcareous microfossils, whereas clay-rich sediment (e.g., nannofossil clay) typically contains well-preserved microfossils (Pearson et al., 2001; Sexton et al., 2006b; Bown et al., 2008). The spectrum of observed preservation states is dependent on the clay-carbonate balance. Exceptional preservation of planktonic and calcareous benthic foraminifers is characterized by the observation of glassy, translucent, unfilled tests with surface ornamentation (Fig. F36) (Sexton et al., 2006b; Sexton and Wilson, 2009). In calcareous nannofossils, unusually excellent preservation is identified by high diversity and the presence of abundant minute coccoliths (<3 µm) and larger preservation-sensitive taxa such as holococcoliths and Blackites spp. (Fig. F37) (Bown et al., 2008).

Siliceous plankton preservation is also affected by sediment composition. The best preservation occurs in clay-rich sediment, although good preservation is often found in carbonate-rich sediment. The oceans are everywhere undersaturated with respect to biogenic opal (Archer et al., 1993; Racki and Cordey, 2000), so to a first approximation preservation of siliceous microfossils is controlled by interstitial concentrations of dissolved silica. Dissolved silica is largely determined by rates of supply of opal and the other sedimentary components, such as clay and carbonates, to the seafloor. Hence, preservation of radiolarians and diatoms tends to be maximized at sites of high-export production, such as zones of surface ocean divergence and upwelling.

Sediment recovered from J-Anomaly Ridge and Southeast Newfoundland Ridge spans a wide range of lithologies, from siliceous and calcareous biogenic ooze to clay with minor biogenic components. This range of lithologies is the result of several factors, including original biotic production and export rates, clastic particle flux, and seafloor dissolution. The amount of dissolution is time dependent as a function of both the paleodepth (subsidence) history of each site and basin-scale fluctuations in the CCD in response to long-term variability in global biogeochemical cycles. This dissolution signal is most clearly seen at the deepest sites on the J-Anomaly Ridge depth transect (Sites U1403 and U1404), where the preserved sedimentary record reflects varying degrees of seafloor carbonate dissolution close to or beneath the CCD or lysocline (Figs. F38, F39). The occurrence of high carbonate contents in the Paleocene and Maastrichtian at Site U1403, the deepest site on the Newfoundland drifts sediment transect, suggests that the CCD was deeper at this time compared to previous estimates based on Site 384 (Fig. F2). Our records also indicate that the CCD in the North Atlantic was substantially deeper (by ~1.5 km) during the early Eocene than it was in the equatorial Pacific (see “Paleogene and Cretaceous carbonate compensation depth in the northwest Atlantic”) and then shoaled substantially by the middle Eocene. At Site U1404, short-lived intervals of good calcareous microfossil preservation within middle Eocene strata that are otherwise largely noncalcareous suggest transient (million year scale) shoaling and deepening events during this interval akin to those documented in the equatorial Pacific (Lyle et al., 2005; Pälike et al., 2012). The sites at Southeast Newfoundland Ridge are currently at 3500 mbsl or less and typically contain abundant and moderate to exceptionally well preserved calcareous microfossils (Fig. F39) that are less affected by CCD-related dissolution. However, intervals of low or no carbonate and accompanying poor calcareous microfossil preservation occur in the condensed Pliocene, Miocene, and upper Oligocene sequences and may be related to relatively extended exposure at the seafloor.

A compilation of shipboard qualitative estimates of microfossil preservation and abundance made during biostratigraphic analyses reveals a strong link between the preserved fossil record and the interlinked factors of site paleodepth/subsidence history and ocean CCD but also highlights the strikingly different sensitivities to dissolution across the fossil groups (Figs. F38, F39). Planktonic foraminifers have the most dissolution-sensitive record of the calcareous microfossil groups and are absent for long intervals at the deepest Sites U1403 and U1404. In strata of Eocene age at these deep sites, it appears that carbonate values of ~50 wt% are required to ensure the conservation of planktonic foraminifers, but foraminifers are present at far lower carbonate values in the clay-rich Oligocene and Miocene (<10 wt%). Planktonic foraminifers are consistently present at the shallower Sites U1406–U1411, where a range of preservation is associated with fine-scale clay-carbonate variability. Preservation is typically consistently good and very good in the drift sediment at Southeast Newfoundland Ridge Sites U1407–U1411. Here, the glassy unfilled tests indicative of exceptionally high quality foraminifer preservation are common (Fig. F36) and are likely the result of the clay-rich nature of the sediment entombing carbonate microfossils and reducing their interaction with surrounding interstitial water (Norris and Wilson, 1998; Pearson et al., 2001; Wilson and Norris, 2001; Sexton et al., 2006b; Bown et al., 2008; Sexton and Wilson, 2009). Where high carbonate contents coincide with low clay values, for example in the lower Eocene and, where present, the Paleocene (Sites U1406–U1409), preservation declines, with foraminifers exhibiting a frosty appearance (sensu Sexton et al., 2006b), reflecting both dissolution and secondary calcite overgrowth.

Calcareous nannofossils and benthic foraminifers are typically less susceptible to dissolution than planktonic foraminifers, particularly in the Eocene section, and their states of preservation closely track the carbonate content at the deeper sites (U1403–U1404) (Fig. F38). This is not surprising given that coccoliths are, for the most part, the major carbonate contributors to this sediment. Nevertheless, both nannofossil and benthic foraminiferal preservation co-vary strongly with carbonate content, even when carbonate values are low and planktonic foraminifers are absent. Abundant and moderate to well-preserved nannofossils and good to very well preserved benthic foraminifers are consistently present through most of the succession at Sites U1405–U1411 (Figs. F38, F39).

Our data sets do not differentiate a “very good” preservation category for nannofossils, but the drift deposits do contain assemblages that can be described as excellent to exceptional (Fig. F37). High diversity and the presence of holococcoliths, minute coccoliths, and fragile forms indicate high-quality preservation. These forms are not usually found in typical deep-sea nannofossil ooze lithology because of both dissolution and secondary calcite overgrowth (Bown et al., 2008; Dunkley Jones et al., 2009). The occurrence of these high-quality nannofossil preservation indicators is typically coincident with observations of glassy planktonic foraminifers. It is notable that the benthic foraminifer records show consistently excellent preservation below the preservation quality threshold that marks the step-up to glassy preservation in planktonic foraminifers and holococcolith and minute coccolith preservation in nannofossils (Figs. F38, F39).

Wherever siliceous microfossils have been found in abundance during Expedition 342, they are generally very well preserved. At J-Anomaly Ridge Sites U1404–U1406, abundant and well-preserved radiolarians occur in the lower Miocene to upper Oligocene drift sediment, typically accompanied by abundant diatoms. By contrast, at the Southeast Newfoundland Ridge sites radiolarians were absent from the Eocene to Oligocene drift sediment. At all sites, radiolarians are typically abundant and well preserved in the subdrift sediment in the lower middle and lower Eocene (Sites U1403 and U1406–U1410) and the lower Paleocene (Sites U1403, U1407, and U1409).

The pattern of radiolarian occurrence may be explained by two factors: productivity and temperature. Radiolarians are abundant and well preserved in intervals of varying carbonate content in the upper Oligocene–lower Miocene drift sediment at Sites U1404–U1406. Here, the abundance of diatoms and benthic foraminifer high-productivity indicators suggest high organic productivity and export production of biosilica and therefore enhanced preservation potential for siliceous microfossils, in general. In the Paleocene and lower Eocene, radiolarian occurrence is more likely related to the northward expansion of subtropical–tropical waters, as the assemblages are very similar to the diverse and rich assemblages previously reported from Blake Nose (Sanfillipo and Blome, 2001) and lower latitudes (Kamikuri et al., 2012). The absence of radiolarians in the middle Eocene through lower Oligocene may reflect the contraction of subtropical–tropical waters in response to cooling. Radiolarian preservation tends to worsen in the proximity of chert horizons, especially in the early Eocene and through the Paleocene–Eocene transition, reflecting the diagenetic transformation of biogenic silica into the cryptocrystalline quartz that constitutes the chert.

Biochronology

Coring at nine sites and 25 holes during Expedition 342 recovered sequences ranging from upper Pleistocene to upper Albian, representing 100 m.y. of geological history (Figs. F40, F41). The youngest 15 m.y. portion of this record (middle Miocene to recent) is typically represented by thin Pleistocene foraminifer-rich sandy clay and thin stratigraphically short sections of Pliocene and upper Miocene clay, often including manganese nodules. These Neogene sections are frequently barren of all microfossils, therefore preventing us from unequivocally identifying hiatuses within these condensed sections (see “Unit B”). Below the middle Miocene, the stratigraphic histories of the sites can be divided into two distinct groups, corresponding to the J-Anomaly Ridge and Southeast Newfoundland Ridge locations.

J-Anomaly Ridge sites typically comprise lower Miocene to upper Oligocene sequences with high sedimentation rates and occasional minor hiatuses, whereas middle Oligocene to Paleocene sequences show lower sedimentation rates (Fig. F40). The lower Eocene to upper Paleocene sequence is condensed (Site U1406) and/or contains one or two minor hiatuses (Sites U1403 and U1406). Stratigraphic highlights include apparently complete Oligocene/Miocene boundary sections at Sites U1404–U1406 and complete but comparatively condensed Eocene/Oligocene boundary sections at Sites U1404 and U1406. For the most part, these intervals contain well-preserved and continuous calcareous microfossil records (see “Exceptional preservation in drift clay-rich sediment”). Radiolarians are abundant and well preserved through the Oligocene–Miocene transition but are consistently absent through the EOT. Site U1403 is the deepest site drilled during the expedition and has a somewhat different stratigraphic history compared with the other J-Anomaly Ridge sites, comprising a lower Eocene section with relatively high sedimentation rates over a Paleocene through Upper Cretaceous sequence. Coring at this site recovered several key events, including the Eocene Thermal Maximum 2 (ETM2), PETM, and the K/Pg boundary mass extinction event (see “Cretaceous/Paleogene boundary”).

Southeast Newfoundland Ridge sites comprise short Pleistocene and Neogene sequences overlying Oligocene through Paleocene sections with higher sedimentation rates (Fig. F41). The highest sedimentation rates are found in clay-rich drift sediments of middle Eocene age at Sites U1408–U1410 and late Eocene through Oligocene age at Site U1411. The drift lithologies contain exceptionally well preserved calcareous microfossils, including glassy planktonic and benthic foraminifers and a diverse range of fragile and small nannoplankton. These Paleogene drift sequences, together with the slightly younger Oligocene–Miocene drift successions at J-Anomaly Ridge sites, will provide middle Eocene through lower Miocene paleoceanographic and paleobiologic records at unprecedented orbital to suborbital temporal resolution. The carbonate-rich lower Eocene lithologies contain less well preserved calcareous microfossils but, compared to previously drilled sequences, provide a relatively expanded and extremely valuable stratigraphic record through this interval of peak greenhouse climatic warmth (Fig. F42). PETM excursion microfossils have been identified at Site U1409, but the stratigraphy is condensed and/or includes minor hiatuses and, in the core of the event, sediment is indurated and silicified and includes chert beds. Coring at Site U1407 recovered an extensive Cretaceous sedimentary record from the upper Maastrichtian to upper Albian, although a relatively large hiatus cuts out the lower Maastrichtian and upper Campanian. Sedimentation rates are low but there appears to be a relatively continuous record from the upper Albian through lower Campanian. Stratigraphic highlights include a striking Cenomanian/Turonian boundary black shale sequence representing OAE 2 and an Albian–Cenomanian section overlying shallow-water carbonate facies with photic zone paleontological indicators (e.g., larger benthic foraminifers, coralline algae, and ooids).

The middle Miocene through mid-Cretaceous sections recovered during Expedition 342 have great potential for postcruise studies that will

• Improve age calibrations of biostratigraphic datums through integration of paleomagnetic and cyclostratigraphic data;

• Provide records of biotic response in unprecedented detail through periods of rapid environmental change during past greenhouse climates and during times of widespread glaciation;

• Provide the opportunity to discover new, short-lived species of phyto- and zooplankton as well as studies of evolutionary rates by exploiting the high sedimentation rates of the Miocene and Paleogene drifts; and

• Allow a detailed assessment of possible temporal (10⁴ to 10⁶ y timescale) diachroneity of microfossil datums during the Paleogene and early Neogene, an achievement previously restricted to expanded sequences of late Neogene age (e.g., Raffi et al., 2006; Sexton and Norris, 2008; Wade et al., 2011).

Magnetochronology and paleolatitudes

To establish magnetostratigraphic age models for Expedition 342 sites, APC- and extended core barrel (XCB)-recovered archive section halves were demagnetized using a peak alternating field (AF) of 20 mT and measured using a pass-through superconducting rock magnetometer (SRM). Discrete samples were also collected and measured from working section halves to verify the data from archive section halves. Bulk magnetic susceptibility and the anisotropy of magnetic susceptibility were also measured on some of these discrete samples.

Overall, paleomagnetic inclination from archive section half data is biased toward positive values, indicating a substantial drilling overprint even after 20 mT AF demagnetization. However, APC-recovered cores oriented with the FlexIT orientation tool often show declination values that cluster at ~0° and ~180°. We interpret intervals with declination values of ~0° (~180°) to indicate normal (reversed) magnetozones. In addition, discrete samples often give more shallow inclinations than their counterpart values in the archive section halves, particularly in intervals with ~180° declinations (i.e., probable reversed magnetzones). With the aid of the discrete sample measurement data, we established magnetostratigraphic age models for most APC-recovered intervals from all Expedition 342 sites. A few intervals at each site, as well as much of Site U1404, are characterized by especially low magnetization intensity (e.g., 10^–5 A/m) or ambiguous magnetic polarity; shipboard magnetostratigraphies were not established for these intervals. At some sites, shipboard paleomagnetic data from XCB-recovered intervals are especially good, enabling us to construct a shipboard magnetochronostratigraphic age model for these more deeply recovered intervals. We emphasize that the success of the magnetostratigraphic age models for Expedition 342 is due in large part to the routine use of nonmagnetic core barrels and the FlexIT core orientation tool in APC-recovered intervals.

Downhole paleomagnetic inclination and magnetization intensity variations for Sites U1403–U1406 (J-Anomaly Ridge) are shown in Figures F43 and F44. At Site U1403, several early to late Eocene chrons are recognized: lower Chron C16n.2n (~36.7 Ma) to lower Chron C22n (~49.3 Ma). Paleomagnetic results from Site U1404 are correlated to early Oligocene to middle Eocene chrons of lower Chron C12r (~33.2 Ma) through lower Chron C19r (~42.3 Ma). For Site U1405, chrons from upper Chron C5Cn.1n (~16.0 Ma) to middle Chron C6Cr (~23.7 Ma) are identified. For Site U1406, the magnetostratigraphy is correlated to the Miocene–Oligocene upper Chron C5Dr.1r (~17.5 Ma) through upper Chron C15n (~35.0 Ma).

The similar highly expanded late Oligocene–early Miocene magnetostratigraphy observed in many of the J-Anomaly Ridge sites facilitates not only precise dating of key oceanographic, biologic, and climatic events at each site but also precise age correlation among these sites to examine depth- and time-dependent changes in ocean chemistry, faunal assemblages, and drift sedimentation dynamics during the Oligocene–Miocene transition and Mi1. For example, shipboard magnetostratigraphic age models revealed several short (<1.5 m.y.) and lithostratigraphically subtle hiatuses during the late early Miocene at several of the J-Anomaly Ridge sites. Except at Site U1403, magnetization intensity for J-Anomaly Ridge sediment is generally weak (~10^–5 A/m) throughout the entire recovered interval (Fig. F44). Although the magnitude of magnetic intensity at J-Anomaly Ridge sites is low, the downhole record at many sites shows low-amplitude, high-frequency oscillations that are promising for stratigraphic correlation, cyclostratigraphy, and long-term environmental magnetic studies.

For Sites U1407–U1411 (Southeast Newfoundland Ridge), downhole paleomagnetic inclination and magnetization intensity variations are illustrated in Figures F45 and F46. The magnetostratigraphy at Site U1407 is correlated to lower Chron C20r (~43.4 Ma) to upper Chron C22r (~49.4 Ma). Sediment at Site U1408 records Chrons C17n.3n through 21n (~38.3–45.7 Ma). The magnetostratigraphy at Site U1409 primarily consists of two time intervals, the first from lower Chron C6Cr (~23.9 Ma) through upper Chron C13r (~33.7 Ma); the second is from lower Chron C19r (~42.3 Ma) through upper Chron C22r (~49.4 Ma). Sediment at Site U1410 also represents two distinct but different time intervals, the first from Chron C1n (Brunhes; modern) through upper Chron C2An.1n (Gauss; ~2.6 Ma) and the second from upper Chron C18n.1n (~39.6 Ma) through upper Chron C21r (~47.4 Ma). Two distinct time intervals are also recognized at Site U1411, the first spanning Chron C1n (Brunhes; modern) to upper Chron C1r.3r (~1.2 Ma) and the second from lower Chron C8n.2n (~25.9 Ma) through upper Chron C15n (~35.0 Ma).

The similar highly expanded middle Eocene magnetostratigraphy observed in many of the Southeast Newfoundland Ridge sites facilitates precise dating of key oceanographic climatic events, including the EOT and MECO. It also enables precise age correlation among these sites to examine depth- and time-dependent changes in ocean chemistry, faunal assemblages, and drift sedimentation dynamics. Exceptionally detailed records of two successive Eocene geomagnetic field transitions (Chrons C18n.1n to C18n.1r to C18n.2n) are recorded over ~7 m of sediment at Sites U1408 and U1410. The paleomagnetic records of these transitions from two widely separated sites are remarkably coherent, suggesting that Paleogene drift sediment has recorded Eocene geomagnetic field behavior in unprecedented detail. Downhole magnetic intensity trends at the Southeast Newfoundland Ridge sites are characterized by distinct zones of low and high magnetic intensity, with low-amplitude, high-frequency intensity oscillations superposed on these first-order trends (Fig. F46). The magnetic intensity records at Southeast Newfoundland Ridge show promise for stratigraphic correlation and cyclostratigraphic and environmental magnetic studies. Moreover, intervals with strong and stable downhole magnetization intensity are promising for long-term middle Eocene relative paleointensity records.

Coring during Expedition 342 recovered sediment deposited during the break-up and subsequent seafloor spreading between North America, Greenland, and Eurasia. These tectonic events established new physiographic boundary conditions in the North Atlantic region that set the stage for the regional and global Cenozoic oceanographic circulation changes recorded in the sediment collected at J-Anomaly and Southeast Newfoundland Ridges. A series of paleogeographic reconstructions for the North Atlantic from 70 Ma to present are shown in Figure F47. This kinematic reconstruction is described in detail by van Hinsbergen et al. (2011) and shows relative plate motions. Time intervals highlight the opening of the northeast Atlantic in the early Paleogene, restriction of the Tethys during the late Paleogene, and development of deeper connections between the North Atlantic and Arctic Oceans during the Neogene. The excellent magnetics recorded in the expanded Paleocene–Eocene sediment drifts recovered at J-Anomaly and Southeast Newfoundland Ridges are promising for developing a continuous and robust paleolatitude record for the North Atlantic. When coupled with terrestrial paleomagnetic poles, this paleolatitude record will not only anchor the reconstructions shown in Figure F47 in latitudinal space but also may reveal gradual and minor paleolatitude changes that could be significant to threshold climate events in the early Paleogene.

Astrochronology and calibration of the Cenozoic timescale

A major objective of the expedition was to obtain records of the Cenozoic, particularly the Eocene, that can be used to link the astronomic timescale developed for the last ~40 m.y. to the “floating” timescale of the early Paleogene developed over a series of IODP and earlier drilling expeditions. From this perspective, acquisition of sedimentary records suited to generating an astronomically tuned record of the late Eocene and the early middle Eocene is an extremely important expedition result that should make it possible to span existing gaps in our tuning efforts. Coring at Sites U1404–U1406 and U1411 captured records of the late Eocene through early Miocene with high-quality biostratigraphic and magnetostratigraphic control (Magnetochrons C17 through C6 and nannofossil Zones NP17 through NN2), including spectacular records through the EOT and Oligocene–Miocene transition. Coring at Sites U1403 and U1408–U1410 recovered records of the middle Eocene through early Eocene, including the Paleocene/Eocene boundary. Many of these sites display striking color banding that has orbital frequencies, with particularly notable examples at Sites U1408–U1410. For the time intervals that have already been astronomically tuned elsewhere, the expedition will allow a comparison between low-latitude sites from the Pacific and Atlantic, offering the chance to decipher the processes controlling the amplification of, for example, ~41 k.y. obliquity versus ~100 and 405 k.y. eccentricity cycles and to test the hypothesis of different dominant astronomical forcing between Earth’s “warm” and “cool” periods (Boulila et al., 2011).

Orbital cycles in early Miocene data series

The lower Miocene sequence at Site U1405 is highly expanded (10 cm/k.y. sedimentation rates) and is composed of clayey sediment with varying contributions of biogenic components, diatoms, radiolarians, and calcareous nannofossils. Color reflectance (L*) data show cyclic variations throughout site-specific lithostratigraphic Subunit IIb (Fig. F48A, F48B). Power spectrum and filtering of these data reveal two prominent wavelength bands of 16.67 m and 3.89–4.74 m exceeding the 99% confidence level (Fig. F48C). A weaker but significant peak (above 99% confidence level) is detected at a wavelength of 0.73 m. On the basis of the age-depth model, the 16.67 m and 3.89–4.74 m wavelength bands are hypothesized to represent the 405 and ~100 k.y. period bands, respectively, of the orbital eccentricity; the 0.73 m band matches the climatic precession period. The two peaks of closely spaced wavelengths (3.89 and 4.74 m) are perhaps the 98 and 128 k.y. eccentricity components. We hypothesize that orbitally induced oscillations in sediment color that are in tune with precession and eccentricity modulation cycles may reflect detrital input, redox cycles, carbonate content variations, or a combination of these processes.

Orbital cycles in Eocene data series

The Eocene sequences recovered at Sites U1408–U1410 show strong variations of carbonate content (Figs. F49) at a number of depth scales that can be correlated to color reflectance data sets across sites (Fig. F50). Spectral analysis of L* data shows two strong peaks of wavelengths of ~0.5 and ~0.6 m that correspond to the mean thickness of an alternation (or couplet) of greenish–white bands (Fig. F51). The greenish bands are nannofossil clay, whereas the white bands are nannofossil ooze with foraminifers. This prominent contrast of carbonate versus clay contents within these two lithologies is fingerprinted in several physical proxies. These couplets show a modulation by long-wavelength cycles. We hypothesize that the greenish–white couplets are precession in origin, modulated by distorted long-period 405 k.y. eccentricity cycles. At a shorter scale, visual inspection shows bundling of the precession cycles, most likely by ~100 k.y. eccentricity cycles. We suggest that the climatic precession may exert oscillations in marine-surface productivity and detrital flux processes through solar radiation change in the North Atlantic Basin. Multiproxy studies are needed to decipher orbital-scale oceanic and climatic changes and to delineate a depositional model that links climatically driven sedimentary processes to sea level changes during this middle Eocene greenhouse period.

Orbital forcing of Paleocene–Eocene transition: implications for timing and duration of the hyperthermal events

Magnetic susceptibility at Site U1403 of the interval spanning the early Cenozoic hyperthermal events (PETM at ~55.9 Ma and ETM2 at ~54.1 Ma) shows relatively high amplitude variations (Fig. F52). High-frequency magnetic susceptibility oscillations display a cyclic pattern, with a prominent long-wavelength magnetic susceptibility cycle occurring over ~20 m, bounding the two events. Both the ETM2 and PETM show higher magnetic susceptibility values compared to the intervening section. Sediments are greenish gray in this interval, possibly indicating oxygen-poor conditions close to the sediment/water interface and thus fewer ferrimagnetic minerals incorporated in the sediments. Magnetic susceptibility shows no correspondence with carbonate content. The onset of the PETM is placed at the base of the first occurrence of PETM excursion radiolarians (Fig. F52). Magnetic susceptibility variations in Hole U1403A display higher amplitudes than those seen in Hole U1403B and exhibit prominent ~5 m thick cyclicity through the interval between the two events.

Eocene astronomical timescale

The first absolute astronomical timescale was established for the Neogene (Lourens et al., 2004) owing to the validity of astronomical solutions over this time conjointly with high-fidelity climatic proxies. The astronomical calibration of pre-Neogene series is in a floating format, which relies on a calibration of the paleoclimate records to the 405 k.y. stable periodicity of the orbital eccentricity back to ~40 Ma (Laskar et al., 2004; Pälike et al., 2006b). The records we captured during Expedition 342 present an opportunity to extend this approach back to at least middle/early Eocene boundary time. The striking greenish–white sediment alternations observed in the middle Eocene at Sites U1408–U1410 may record primary environmental changes that directly control the lithologic oscillations. Shore-based work will focus on high-resolution multiproxy studies that will, for example,

1. Decipher precession-scale oceanic and climatic changes preserved in multiple proxies;

2. Look for possible differential (nonlinear) responses of the oceanic-climatic system to orbital forcing through the studied proxies;

3. Delineate depositional models that link oceanic and climatic variations to sea level changes in these middle Eocene North Atlantic sediment; and

4. Decipher possible solar-induced cyclicities in carbonate productivity, detrital flux, and redox conditions.

Sedimentation rates

All the sediment drifts drilled during Expedition 342 are lenticular packages of sediment that have much higher sedimentation rates in their mid-sections than at their edges. We found the highest sedimentation rate (~10 cm/k.y.) in the Miocene–Oligocene record at Site U1405. Sedimentation was considerably slower (2–3 cm/k.y.) in the Eocene and early Oligocene but still 2–5 times faster than those of typical ocean records from these time periods. The higher sedimentation rates in the Oligocene–Miocene compared to the early Paleogene may reflect a long-term trend in drift sediment dynamics. It is impressive to compare the massively thick sequences of late Neogene sediment waves in the seismic record of Southeast Newfoundland Ridge with the relatively modest size of the acoustically transparent early Paleogene drifts of J-Anomaly Ridge and the seamount area of Southeast Newfoundland Ridge.

Sites U1403–U1406 comprise a depth transect (4946–3813 mbsl) along the southwestern flank of J-Anomaly Ridge. Figure F53 summarizes linear sedimentation rate histories for Expedition 342 drill sites. The data from the four J-Anomaly Ridge sites (Fig. F53A) suggest two main influences on sedimentation rates, water depth and location within the sediment drift, that are superimposed on a background of slow tectonic subsidence at J-Anomaly Ridge.

All J-Anomaly Ridge sites are characterized by slow rates of sedimentation in the uppermost 10–70 m of the sediment column. At the deep end of our transect, Site U1403, this condensed section spans the last ~40 m.y., whereas at the shallow end of the J-Anomaly Ridge depth transect, Site U1406, the upper sequence of slowly accumulating sediments spans only the last 20 m.y. These two sites were drilled into the edges of the J-Anomaly Ridge sediment drift and record the two most condensed sequences recovered along this depth transect.

In contrast, Sites U1404 and U1405 were drilled into the center of the drift and recovered much more expanded sequences. The sequence penetrated at Site U1405 was positioned close to the mid-section of the drift and captures a remarkably expanded sequence through the late Oligocene and early Miocene, particularly the Oligocene–Miocene transition. Equally striking is the seismic stratigraphic evidence that the lower half of the J-Anomaly Ridge drift not reached by either Site U1404 or U1405 is middle and early Eocene age, implying that drift formation not only began in the early Eocene, but also produced nearly as much sediment as that deposited during the Oligocene–Miocene.

Mass accumulation rates (MAR) at J-Anomaly Ridge sites reach 1–2 g/cm²/k.y. during periods of drift deposition. Significantly lower MAR, <0.5 g/cm²/k.y., are recorded during times of low sedimentation, prior to and following high sedimentation rates during drift formation. Carbonate accumulation rate (CAR) is the predominant sedimentary component during the Paleocene peak in MAR at Site U1406 (Fig. F54). During the Eocene, noncarbonate MAR (nCAR) is predominant across the J-Anomaly Ridge sites.

A striking observation of sediment accumulation on J-Anomaly Ridge during the Eocene is an anticorrelation between periods of high MAR between the ridge flank (Site U1403) and the ridge crest (Site U1406) (Fig. F54). This observation supports the interpretation from seismic data that drift formation varies not only in time but also in water depth. In contrast, the pattern of sedimentation with depth on J-Anomaly Ridge during the Oligocene and Miocene appears to be in phase among the three sites where peaks in MAR occur at the Oligocene/Miocene boundary (~3.8 to ~4.9 km present water depth) (Fig. F55). Sites U1404–U1406 all exhibit a pronounced double peak in MAR in the latest Oligocene to earliest Miocene sediment sequences. Because the mechanism for drift formation is closely linked to the region of peak sediment transport and deposition in a deep-current system, the pattern of accumulation with depth may serve as a proxy for deep-current dynamics. Using this line of reasoning, it can be tentatively concluded that the deep-current system that draped J-Anomaly Ridge with sediment varied in depth during the Eocene and became more uniform with depth, and perhaps stronger, in the Oligocene and Miocene. This interpretation should inform postcruise sampling for paleocurrent proxy records (i.e., sortable silt and coarse lithic MAR).

Trends in sedimentation rate at the sites drilled on the Southeast Newfoundland Ridge are very similar to those from J-Anomaly Ridge. The key difference is that the main bodies of the various drifts and highest sedimentation rates are older, occurring during the middle and early Eocene at Sites U1407–U1410 and through the EOT at Site U1411. In contrast to the J-Anomaly Ridge sites, there is only 500 m difference in water depth between the shallowest and deepest sites. Consequently, the primary influence of sedimentation rate variations between these sites is location within a given drift.

The three sites within the central parts of sediment drifts yielded remarkably similar sedimentation rate histories. Sites U1408 and U1410 have condensed upper sections spanning 40 m.y. followed by greatly expanded middle Eocene sections in which sedimentation rates reached 2.6–2.8 cm/k.y. The initiation of drift sedimentation is clearly marked by an uphole increase in sedimentation rate from <1 cm/k.y. to these higher rates. Site U1411 shows a very similar trend, although it sampled a younger sequence comprising a condensed Neogene–upper Oligocene section followed by a highly expanded section that spans the Oligocene/Eocene boundary. We did not drill deeply enough at site U1411 to date the onset of drift sedimentation.

Sites U1407 and U1409 are at the head and tail, respectively, of drift bodies. The distinction between head and tail, shallow and deep, or proximal and distal appears to have had little effect on the parallel sedimentation histories at these two sites. Both comprise a condensed upper section that spans 40–45 m.y. followed by a moderately expanded section through the middle to lower Eocene, with maximum sedimentation rates of 1.4–2 cm/k.y., and a lowermost Eocene–Paleocene section in which sedimentation rate decreases progressively. Only Site U1407 penetrated into the Cretaceous sediment of Southeast Newfoundland Ridge. This site shows that the uppermost Cretaceous was very condensed (~0.1 cm/k.y.), but moderately high rates of sedimentation (~0.5 cm/k.y.) are recorded in the Turonian–Albian sequence.

MAR at the drill sites at the head and tail of drift bodies are lower, on average, than those drilled into the main body of drifts (Figs. F56, F57). CAR is higher at the Southeast Newfoundland Ridge sites than at the J-Anomaly Ridge sites, presumably because of the shallower depths.

In summary, these sedimentation rate histories validate the strategy that underpinned this expedition. These drift sediments have been shown to contain greatly expanded sedimentary successions, generally rich in well-preserved microfossils (although seldom all groups in the same interval) that span critical episodes in Earth’s climate history.

Paleogene and Cretaceous carbonate compensation depth in
the northwest Atlantic

One of the primary objectives of Expedition 342 is to provide new insights on the history and dynamics of the Paleogene carbon cycle. A particular goal is to reconstruct the depth history of the carbonate lysocline and CCD of the North Atlantic Ocean. This effort will improve our understanding of changes in carbonate saturation state over Cenozoic time. Before this expedition, records of Cenozoic CCD change in the North Atlantic were of poor stratigraphic and depth resolution because they were compiled largely from dispersed, often spot-cored DSDP sites and a handful of ODP drill sites (van Andel, 1975; Peterson and Backman, 1990). An important result of Leg 208 was the acquisition of a ~2000 m depth transect that permitted estimation of the magnitude of CCD fluctuations at short timescales across the PETM (Zachos et al., 2005). Similarly, the latitudinal and age-depth transects recovered during Leg 199 and Expedition 320/321 have allowed the reconstruction of the long-term behavior of the CCD in the equatorial Pacific at unprecedented resolution (see fig. 2 in Pälike et al., 2012). One aim of Expedition 342 was to test the reproducibility of these findings in the North Atlantic Ocean where changes in ocean circulation are tightly coupled to changes in global climate during the late Pleistocene.

Of particular interest are the multiple transient Eocene CCD deepening events seen in the equatorial Pacific as well the large CCD perturbations associated with the “overshoots” in carbonate ocean chemistry observed during the EOT, PETM, and K/Pg boundary. All three of these extreme events are thought to involve increased deep-sea carbonate burial flux and a rebalancing of oceanic carbonate chemistry following major shocks to the Earth system (Dickens et al., 1997; Coxall, et al., 2005; Zachos et al., 2005; Merico et al., 2008). A key aspect of the Expedition 342 strategy was to target much deeper sites for the bottom end of our depth transect than has been common in modern paleoceanographic transect drilling. The reason for this approach is the need to quantify the full amplitude of these CCD perturbations to better constrain the size of the carbon cycle anomaly involved.

The sedimentary sequence on J-Anomaly Ridge and Southeast Newfoundland Ridge has yielded the following results:

• Upper Cretaceous and Paleocene sediments at J-Anomaly Ridge are markedly carbonate rich, even at our deepest water site (paleodepth 5.5 km at 50 Ma and 4.5 km at 70 Ma).

• The deep Site U1403 contains lower Eocene strata that are carbonate rich, indicating that during a peak interval of sustained Cenozoic warmth the CCD in the North Atlantic Ocean was much deeper than in the equatorial Pacific (by >1.5 km).

• In our mid-depth to deepwater sites (Sites U1403 and U1404), we observed discrete carbonate-rich intervals interspersed throughout strata of middle to late Eocene age. These intervals indicate that the behavior and timing of CCD deepening and shoaling in the North Atlantic is broadly similar to the equatorial Pacific.

• We captured records of the K/Pg boundary, the PETM, and the EOT, and in each case these events are expressed in our deepwater sites by prominent “spikes” in carbonate content and preservation of calcareous microfossils, suggesting large-amplitude CCD overdeepening events during the recovery phases from these major shocks to the carbon cycle and global climate.

• Numerous discrete carbonate-rich beds are also observed clustered around the Oligocene–Miocene transition, and the nannofossil composition of some of these beds suggests unusual surface water, possibly shelf-related conditions.

• The thin veneer of Pliocene–Pleistocene sediment encountered at all of the sites is always more carbonate rich (with superimposed glacial–interglacial cycles) than the immediately underlying sediments with superimposed glacial–interglacial cycles.

Onset and development of Cenozoic glaciation

The canonical view of the onset of Cenozoic glaciation is that it took place in two main steps: (East) Antarctic ice sheets were established around EOT time (~33 Ma), whereas northern hemisphere glaciation was not triggered until ~3–7 Ma (e.g., Miller et al., 1987; Zachos et al., 2001). It is clear that northern hemisphere glaciation underwent a major phase of intensification in the latest Pliocene to earliest Pleistocene around marine isotope Stage G6 (~2.75 Ma) and that large ice sheets grew on multiple continents in the high northern latitudes, triggering sustained iceberg rafting events across the North Atlantic and North Pacific Oceans in glacials from marine isotope Stage 100 (~2.55 Ma) (Shackleton et al., 1984; Lisieki and Raymo 2005; Bailey et al., 2010, 2011), yet little is understood of the timing and chain of events involved in the transition into this climate state from one with a nonglaciated northern hemisphere featuring a genuinely green Greenland.

Dropstones have been reported from the Arctic (IODP Expedition 303) in sediment of ~45 Ma age (Backman 2005; Moran et al., 2006). Discontinuous δ¹⁸O records in bulk and benthic foraminiferal calcite from the Pacific Ocean have been interpreted to indicate extensive ice sheet development in both hemispheres together with a huge (>150 m) eustatic sea level fall around 42 Ma (Tripati et al., 2008). This interpretation is controversial (Lear et al., 2004; Miller et al., 2005; Pekar et al., 2005; Edgar et al., 2007; DeConto et al., 2008). In contrast to early interpretations (Lear et al., 2000), it is now also clear that the amplitude of the δ¹⁸O increase across the EOT (~33.5 Ma) is impossibly large to reflect ice growth on Antarctica alone (Coxall et al., 2005) and must incorporate some component of global cooling (Lear et al., 2004, 2008; Eldrett et al., 2009; Liu et al., 2009).

Yet, there is little evidence that conditions were cold enough to develop a substantial ice sheet in the northern hemisphere, even on southeast Greenland, until the initiation of the major phase of northern hemisphere glaciation at ~2.75 Ma (e.g., Larsen et al., 1994; Bailey et al., 2012). Winter sea-ice formation appears to have been initiated in the Arctic by the start of the middle Eocene, and isolated alpine outlet glaciers are thought to have existed on southeast Greenland by the EOT (Eldrett et al., 2007; Stickley et al., 2009). This picture agrees with the results of coupled global climate–ice sheet model experiments that suggest that the northern hemisphere is likely to have contained glaciers and small isolated ice caps at high elevations through much of the Cenozoic, especially during favorable orbital periods, but that major continental-scale glaciation is unlikely prior to the Miocene (DeConto et al., 2008). Only by the Miocene do paleo-CO₂ records (Pagani et al., 2005, 2011; Pearson et al., 2009) suggest that Cenozoic carbon dioxide levels first intercept the model threshold for the growth of large ice sheets in the northern hemisphere. In fact, if the proxy CO₂ records and the climate–ice sheet models are representative, then transient northern hemisphere ice sheets growing and disappearing on orbital timescales might help to explain the pronounced variability we see in the Neogene deep-sea benthic δ¹⁸O record regardless of the strong hysteresis (Pollard and DeConto, 2005) in the model Antarctic ice sheet systematics. Particularly notable in this context is the earliest Miocene glacial maximum that corresponds to the Mi1 event in the terminology of Miller et al. (1991) and the multiple, relatively short lived (~100 to ≤400 k.y.) subsequent glaciation “events” (Flower et al., 1997; Zachos et al., 1997, 2001; Paul et al., 2000; Billups et al., 2002, 2004; Pälike et al., 2006b; Liebrand et al., 2011). These events have been largely interpreted to reflect shifts between complete and partial ice coverage of Antarctica, but we must consider the possibility of a northern hemisphere contribution to the global ice budget.

Expedition 342 provided an opportunity to shed new light on these aspects of Paleogene climate in the high northern latitudes at unprecedented stratigraphic detail. The middle Eocene through lower Miocene sections obtained will allow us to generate the high-resolution records of changes in sedimentation rate, clay mineral assemblage, and occurrence and provenance of possible IRD that are needed to test for early ice in the northern hemisphere. A potential problem exists with differentiating between putative Paleogene IRD and sediment eroded from the continental margin. In Pleistocene strata, IRD is often identified because of its coarse grain size (sand sized and larger) and its distinctive provenance (e.g., “red-coated grains” from Labrador, basalt from Iceland) (Bond and Lotti, 1995; Hemming et al., 1998). IRD in the silt fraction has been identified by a number of techniques including modeling of end-member components using a wide spectrum of grain-size analyses (Weltje, 1997; Prins et al., 2002; Weltje and Prins, 2003). IRD might also be identified by combining grain size (especially sand content) and mineralogical information during “cold” events inferred from light stable isotope studies and geochemical assessment of provenance. Finally, ice-transported sediment has been identified using geochemical markers such as Sr/Ca ratios in core-scanning X-ray fluorescence data sets and provenance studies by lead isotope work (e.g., Bailey et al., 2012; Channel et al., 2012).

Preliminary results indicate the consistent presence across multiple sites of abundant lithic grains in the 63–150 µm size fraction in some of the intervals drilled, most notably in lower Miocene and particularly in Oligocene strata (Fig. F25). In some cases (such as at Site U1411), these grains can be resolved to be parts of larger, coarse sand-sized carbonate-cemented lithics that disaggregate easily into their component silt-sized grains (see “Lithostratigraphy” in the “Site U1411” chapter [Norris et al., 2014b]). Quartz grains are by far the most commonly observed of these particles, many of which are angular and some of which are stained a distinctive purple-red color. Metamorphic (e.g., schist) and mafic rock fragments are also present but are much less abundant. A second pulse of sand-sized lithic grains occurs within the Oligocene–Miocene transition and are indistinguishable from those documented at the base of the Oligocene sequence. At nearly all sites, lithics are observed for the first time in the early Oligocene (Zone NP22/NP23). The distribution of these grains in sediment throughout the post-Eocene sediment column at J-Anomaly and Southeast Newfoundland Ridges indicates the presence of granitic or clastic sedimentary sources, presumably located north of the Newfoundland ridges, and possibly indicating ice-rafted transportation. The relatively coarse grain size of some of the lithics is consistent with iceberg transport as early as the basal Oligocene.

Clay minerals are excellent tracers of provenance in the modern North Atlantic and should have broadly similar distributions in the Paleogene. Continental-sourced clays should dominate the west Greenland and Canadian margins, whereas volcanic-sourced clays should be more typical of the Paleogene flood basalt province in east Greenland. In the modern North Atlantic, smectite, illite, chlorite, and kaolinite constitute the major proportion of the clay fraction. Smectite has been regarded as a major tracer for Iceland-Faeroe–derived material (Fagel et al., 2001) and could be expected from the Greenland volcanic province in the Paleogene. Where smectites are lacking, chlorite is the typical high-latitude clay from old metamorphic sources. A lack of volcanogenic and weathered basalt–derived clays (nontronite and amorphous minerals) suggest a rather restricted contribution of east Greenland and mid-ocean-ridge sources. Clay mineral type (e.g., montmorillonite versus beidellite) and percentage data and Sm/Nd ratios in the clay fraction of Reykjanes Ridge sediment indicate a dominant terrigenous contribution from young continental crust that may derive from Europe and/or the Arctic (Innocent et al., 2000; Fagel et al., 2001). Today, the clay fraction may also contain terrigenous calcites and dolomites derived from glacier milk in the Hudson Bay area. Such a signal could be transported for much longer distances than sand-sized IRD.

Oligocene–Miocene transition

The Oligocene–Miocene transition includes the last large, transient glaciation (Mi1) before the onset of widespread northern hemisphere glaciation around 3–7 Ma (see “Onset and development of Cenozoic glaciation”). The Oligocene–Miocene sediment recovered from J-Anomaly Ridge holds great promise for addressing outstanding questions regarding climatic and paleoceanographic dynamics of this time through a combination of highly expanded sedimentation rates, a depth transect, and exceptional calcareous microfossil preservation.

The three sites at J-Anomaly Ridge with an Oligocene–Miocene transition lie along a depth transect from the deep Site U1404 (4745 mbsl) to the mid-depth Site U1405 (4286 mbsl) to the shallow Site U1406 (3813 mbsl). All three sites occur in a single sediment drift. Sites U1404 and U1405 are close to the middle of the drift and capture the highly expanded Oligocene–Miocene sequence with much higher sedimentation rates (up to 10 cm/k.y.) than is typical in pelagic settings (~1 cm/k.y.). Site U1406 is near the nose of the same drift where sedimentation rates are ~2.5 to ~1.5 cm/k.y. Taken together, these three sites record a near-continuous sequence of the early Miocene to late Oligocene interval (~22.0 to ~26.5 Ma), coincident with drift formation and the deposition of IRD (see “Onset and development of Cenozoic glaciation”). The calcareous microfossil preservation at Site U1405 is exceptional—planktonic foraminifers appear glassy (i.e., see-through) and small, delicate coccoliths abound (see “Exceptional preservation in drift clay-rich sediments”). The preservation at Site U1406 is similar. Both Sites U1405 and U1406 have a high clay/CaCO₃ ratio, a factor known to be crucial for this type of preservation (Norris and Wilson, 1998; Sexton et al. 2006b).

The occurrence of the nannofossil Sphenolithus delphix at Sites U1405 and U1406 marks the time period immediately preceding the Mi1 glaciation interval. The range of S. delphix is in line with the paleomagnetic placement of the Oligocene/Miocene boundary at the Chron C6Cn.2n/C6Cn.2r reversal (Fig. F58). The exact position of the Oligocene/Miocene boundary at Site U1404 is currently unclear because the preliminary magnetostratigraphy is ambiguous and S. delphix is absent in shipboard samples. Rare, light-colored Braarudosphaera beds are recognized at Sites U1405 and U1406 in at least four intervals within ~40 m of the boundary (Fig. F59). The presence of Braarudosphaera beds is surprising because they are typically associated with older sediments (Oligocene aged), particularly from the South Atlantic.

The high sedimentation rates and outstanding calcareous microfossil preservation of the Oligocene and Miocene successions recovered at Sites U1404–U1406 have the potential to resolve climate changes to a millennial scale. The Miocene succession presents the opportunity to generate the first pre-Pliocene suborbital deep-sea record and promises to greatly increase our understanding of climate dynamics during the onset and demise of the Mi1 glaciation.

Eocene–Oligocene transition

The EOT is one of the most profound climate shifts of the Cenozoic and the final stage of the switch from greenhouse to glacial climates. Although our understanding of this transition has improved in recent years, many of the key sections around the globe have only discontinuous carbonate or contain one or more hiatuses (e.g., Coxall et al., 2005; Coxall and Wilson, 2011; Wade et al., 2012). No complete Eocene/Oligocene boundary record currently exists for the northern North Atlantic.

Coring during Expedition 342 recovered the EOT (Fig. F60) at Sites U1404 (4746 mbsl), U1406 (3813 mbsl), and U1411 (3299 mbsl). The EOT is identified on the basis of biostratigraphic and magnetostratigraphic observations and is characterized by a series of changes in calcium carbonate content and lithostratigraphy above the Eocene/Oligocene boundary (Chron C13r/C13n, nannofossil Zone NP21, and, at Site U1411, the extinction level of the planktonic foraminifer Hantkenina alabamensis). The Site U1411 EOT section is particularly notable in that it falls within a drift deposit with sedimentation rates of ~3 cm/k.y. The EOT at Sites U1404 and U1406 both lie beneath upper Oligocene–lower Miocene drift sequences and are relatively condensed, with sedimentation rates of 0.3 and 0.9 cm/k.y., respectively. However, the lithologic expression of the EOT is broadly similar at Sites U1404, U1406, and U1411, with intersite differences in background carbonate and clay content affecting the observed magnitude of change in carbonate content and sedimentary brightness (L*). At all sites, carbonate content increases markedly between the latest Eocene and earliest Oligocene, from 5 to 50 wt% at Site U1404, from 40 to 80 wt% at Site U1406, and from 15 to 55 wt% at Site U1411. The different values are likely predominantly the effect of proximity to the CCD/lysocline at Sites U1404 and U1406 and the combined effects of the CCD and clay dilution at Site U1411. Lithostratigraphically, sediment in the high-carbonate interval is lighter than that in underlying and overlying sediment at the sites, with the greatest changes in color contrast observed across the EOT at Site U1404. At all three sites, sand-sized lithic grains, mostly composed of angular quartz grains in the 63 to 150 µm size fraction, are observed in lowermost Oligocene sediment and possibly uppermost Eocene sediment at Site U1411 (green triangles in Fig. F60).

These findings appear to be broadly in line with those of Coxall et al. (2005), suggesting that the CCD deepening event associated with the EOT is not restricted to the equatorial Pacific Ocean but also occurs in the North Atlantic Ocean. However, these new Expedition 342 sites contain continuous carbonate across the EOT and, in particular at Site U1411, host well-preserved and glassy planktonic foraminifers, and therefore provide an unprecedented opportunity to generate a full range of paleoceanographic proxies at high temporal resolution at a location in the North Atlantic. Such a record will allow us to investigate the relationships between paleotemperatures, ice volume, and CO₂ through this major climate shift. Furthermore, the high quality of calcareous microfossil preservation will enable us to study the faunal and floral response to this major climate shift event and specifically to examine the relative timing of a sequence of plankton extinction events (e.g., Pearson et al., 2008).

Middle Eocene climatic optimum and carbonate accumulation events

The long-term global cooling through the middle–late Eocene is punctuated by a series of (at least seven) carbonate accumulation events (CAEs) documented in the equatorial Pacific that reflect major depressions of the Pacific CCD (Lyle, Wilson, Janecek, et al., 2002; Lyle et al., 2005; Pälike et al., 2010, 2012). The late middle Eocene is also marked by a brief (~500 k.y.) interval of global warming and CCD shoaling (MECO) at ~40 Ma (Bohaty and Zachos, 2003; Bohaty et al., 2009). CCD shoaling and coincident global warming are consistent with a transient rise in atmospheric CO₂ concentrations during the MECO, although the mechanisms driving this rise are still debated (Bohaty et al., 2009). Regardless, the MECO had a marked effect on plankton communities in terms of biogeographic range shifts, evolutionary turnover, and changes in community structure (e.g., Edgar et al., 2010, 2013; Toffanin et al., 2011; Agnini et al., 2011).

Expedition 342 recovered spectacular records of the middle Eocene as sediment drift deposits on the Southeast Newfoundland Ridge composed of light grayish green nannofossil clay interlayered with nannofossil ooze. The recovered sequences are notable for the quality of magnetostratigraphic and biostratigraphic age control, the extremely high quality of preservation of calcareous microfossils, and high sedimentation rates (up to 3 cm/k.y.). The middle Eocene records all show intervals of relatively high carbonate concentrations and mass accumulation rates suggestive of a North Atlantic equivalent to the “carbonate accumulation events” of the equatorial Pacific (e.g., Fig. F50). These sequences hold great promise for refining the Eocene timescale and addressing outstanding questions of climatic and biotic dynamics during the middle Eocene. In particular, data generated on these sequences will

• Help refine the timing, global extent, and cause of the CAEs;

• Provide a high northern hemisphere Atlantic record across the MECO; and

• Enable orbital- to suborbital-scale studies of the climatic, paleoceanographic, and biotic response to climate change in a nonicehouse climatic regime.

We recovered particularly high quality MECO records at Southeast Newfoundland Ridge Sites U1408 and U1410 (Fig. F61). Both sites yielded expanded sequences (sedimentation rates of 1–2 cm/k.y.) with exceptionally well preserved calcareous microfossils. The glassy preservation of benthic and planktonic foraminiferal calcite at Site U1408, in particular, offers unprecedented potential for geochemical investigations of the MECO. Sediment spanning the MECO was also recovered at J-Anomaly Ridge Sites U1403, U1404, and U1406. Site U1403, at the deepest end of the expedition transect (4946 mbsl), is barren of carbonate throughout the middle Eocene interval and provides a constraint on the maximum depth of the local CCD.

We identified the MECO at Sites U1403, U1404, U1406, U1408, and U1410 on the basis of calcareous nannofossil and planktonic foraminiferal biostratigraphy. In particular, at Site U1408 the onset of the MECO is pinned to Section 342-U1408A-8H-2 by the base of Dictyococcites bisectus and the top of Sphenolithus furcatolithoides (calcareous nannofossil species) (Toffanin et al., 2011; Agnini et al., 2011). The evolutionary rise of Sphenolithus predistentus, associated with the peak in abundance of Sphenolithus spiniger and its subsequent extinction, point to the relative completeness of our record across the MECO (Fig. F61). The lithologic and geochemical structure of the Site U1410 record suggests another complete record of the MECO. Prominent changes in calcareous nannofossil assemblages starting just below the Chron C18r/C18n.2n boundary are thought to mark the warmest interval of the MECO, a ~50 k.y. pulse of additional warming following several hundred thousand years of prior warming. Orbulinoides beckmanni, a planktonic foraminiferal marker species with a total range closely coincident with the MECO (Edgar et al., 2010), first appears immediately after the remarkable rise in abundance of S. spiniger and the base of S. predistentus. The MECO also coincides with a peak in the dominance of infaunal benthic foraminifers at Site U1408, possibly indicating increasing export productivity coincident with warming. However, barite and carbonate accumulation rates from the equatorial Pacific have been interpreted to indicate the opposite; that is, a peak in export productivity during the CAEs and relatively low export productivity during the MECO (Griffith et al., 2010).

The relative duration and spacing of the intervals of low versus high color cyclicity suggest that the long-period, million-year variability in cyclicity may correspond to the middle Eocene CAEs documented in the Pacific (Pälike et al., 2012). At Southeast Newfoundland Ridge sites, we tentatively identified four intervals of increased color contrast between layers that are broadly correlative across Sites U1408–U1410 (Fig. F50). These intervals are broadly coincident with four Pacific CAEs in magnetochrons C20r through C18n.2n; however, at finer resolution there are indications that the Southeast Newfoundland Ridge CAEs may be diachronous with equatorial Pacific CAEs. Regardless of their exact temporal relationship to the Pacific, the expanded middle Eocene sequences from Southeast Newfoundland Ridge promise to provide key information on the timing, extent, and mechanisms of changes in the CCD and carbonate accumulation through the middle Eocene in the North Atlantic.

Early to middle Eocene transition

Ocean circulation and the onset of sediment drift deposition

During Expedition 342, the lower to middle Eocene transition was recovered at four sites (U1403, U1407, U1409, and U1410) (Figs. F62, F63). This is a major and unprecedented achievement because the lower/middle Eocene boundary has, for over 40 y of deep-sea drilling, been almost universally represented in sediment sequences by a hiatus of ~1 to 2 m.y. duration (Aubry, 1995; Norris et al., 2001a). This boundary also correlates closely in age to seismic Reflector A_C, which is widespread throughout the North Atlantic and represents either a diagenetic boundary between cherts and biosilica-rich oozes (Tucholke and Mountain, 1986) or an unconformity (Norris et al., 2001a). Consequently, we know very little of the climatic and oceanographic significance of this boundary, despite indications that it represents a critical threshold transition in Earth’s climate evolution. For example, the onset of the long-term Cenozoic global climatic cooling trend appears to commence close to this boundary (Sexton et al., 2006a; Zachos et al., 2008). Significant reorganization of global overturning circulation is suggested by the expansion of interbasin δ¹³C gradients from the early to middle Eocene (Sexton et al., 2006a), suggesting either a switch to a single dominant source of deep water, a fundamental increase in biological export production (and consequent deep-sea remineralization), or both.

At all four Expedition 342 sites spanning the lower/middle Eocene boundary, sedimentary calcium carbonate content shows a pronounced drop in Chron C21n to >80 wt% in the earliest middle Eocene from the Southeast Newfoundland Ridge sites and ~40 wt% in the later middle Eocene (Fig. F62). Multiple physical properties, including magnetic susceptibility, NGR, and color reflectance, show a distinct step change across this interval (Figs. F19, F20, F21, F22, F62). These changes in physical properties are associated with pronounced uphole increases in MAR at all sites, which correlate with increases in nCAR, primarily clay but probably including a significant biosiliceous component, particularly in the radiolarian-rich sediments at Site U1403 (Figs. F54, F56). These observations suggest that the major uphole decreases in carbonate content at least partly represent dilution of carbonate by enhanced clay input. The increase in MAR across this transition may be symptomatic of an intensification of bottom water currents that can carry greater sediment loads.

Enhanced vigor of deep ocean currents across this critical transition in Earth’s climate is supported by the contemporaneous onset of sediment drift deposition within a small basin dissecting the Greenland-Scotland Ridge, a key gateway for Arctic Ocean overflow into the North Atlantic (Hohbein et al., 2012). Furthermore, on Blake Nose in the subtropical North Atlantic, winnowed and silicified foraminiferal sands are found associated with the lower/middle Eocene boundary unconformity (Norris et al., 2001a). These observations, combined with the initial findings from Expedition 342 Southeast Newfoundland Ridge drilling, suggest that a major change in ocean circulation occurs across the lower–middle Eocene transition. The continuous sequence recovered across Southeast Newfoundland Ridge will allow postcruise studies to explore the response of ocean overturning to this critical switch in Earth’s long-term climate evolution.

Climate-carbon cycle perturbations in the early to middle Eocene

The early/middle Eocene boundary coincides with the onset of global cooling following the Early Eocene Climatic Optimum (EECO; ~51–53 Ma) when Earth reached the warmest temperatures of the entire Cenozoic (e.g., Sexton et al., 2006a; Zachos et al., 2008). A series of hyperthermals, or acute carbon cycle–driven global warming events, are superimposed on the ~5 m.y. interval leading up to the EECO as well as the subsequent onset of cooling approaching the early–middle Eocene transition (e.g., Cramer et al., 2003; Zachos et al., 2010; Sexton et al., 2011). The total number of these dramatic perturbations to Earth’s carbon cycle and climate remains unclear, particularly across the EECO, though their occurrence appears to wane following the early/middle Eocene boundary (e.g., Sexton et al., 2006a; Zachos et al., 2008), until later in the middle Eocene when two more isolated events are encountered in Chron C19n (Edgar et al., 2007) and at the Chron C18r/C18n.2n boundary (Bohaty et al., 2009).

Sites U1404, U1409, and U1410 together recovered a nearly complete lower Eocene sedimentary sequence, whereas Sites U1407 and U1403 recovered expanded lower–middle Eocene sequences (sedimentation rates up to 3.5 cm/k.y.) with excellent cyclostraphic and paleomagnetic control. An example of the imprint of hyperthermals upon deep-sea sediments is shown in Figure F63, which highlights several “clay layers” marked by increases in magnetic susceptibility and a darkening of sediment color, both likely caused by intensified dissolution of CaCO₃ upon deep-sea acidification. Collectively, the lower and middle Eocene sequences recovered during Expedition 342 will allow a detailed reconstruction of the frequency and total number of these carbon-cycle perturbations from the height of Cenozoic warmth through the onset of middle Eocene global cooling, thereby providing a detailed framework with which to evaluate the boundary conditions and forcing mechanisms required for their genesis.

Paleocene/Eocene Thermal Maximum

During Expedition 342, drilling operations penetrated the Paleocene/Eocene boundary at Sites U1403 and U1406–U1409. At Site U1406, sediments of middle Eocene age unconformably overlie the upper Paleocene, whereas at Site U1407, the lower Eocene and upper Paleocene were drilled without recovery because of thick chert beds. Sequences of lower Eocene to upper Paleocene (including nannofossil Zones NP9 and NP10) were recovered at Sites U1403, U1408, and U1409 (Fig. F64), providing a depth transect of 3022 to 4946 mbsl. PETM calcareous nannofossil excursion taxa (nannofossil Subzone NP9b) were recovered at Sites U1403 and U1409 but not at Site U1408, where presumably the Paleocene/Eocene boundary falls within a hiatus.

A notable feature common to each of the Expedition 342 Paleocene–Eocene sequences is the presence of siliceous sediments at or near the Paleocene/Eocene boundary, including siliceous claystone, siliceous limestone, porcellanite, and chert. These lithologies are likely also present in the Paleocene/Eocene boundary interval sediments at Site U1407, which had poor recovery in the lower Eocene chert-rich interval, and Site U1410, which was terminated in lower Eocene sediments because of slow and poor recovery, with fragments of chert present in the deepest core. Chert is also common as frequent centimeter-scale beds higher up in the lower Eocene at Sites U1406–U1410. The occurrence of abundant well-preserved radiolarians in upper Paleocene sediments at Sites U1403 and U1407–U1409 and the persistence of poorly preserved assemblages through the Paleocene–Eocene transition is consistent with the derivation of cherts through diagenetic alteration of biogenic silica. The presence of chert at the Paleocene/Eocene boundary and in the lower Eocene, an interval of previously documented hyperthermal events (e.g., Cramer et al., 2003; Lourens et al., 2005; Zachos et al., 2010), suggests an association between hyperthermal events and siliceous sedimentation or alteration in the Newfoundland drifts. An association between warm climates and chert deposition in the North Atlantic has been previously identified for the middle and early Eocene (McGowran, 1989; Muttoni and Kent, 2007), but not for the PETM.

Site U1407 and U1408 sediment is carbonate-rich above and below the siliceous PETM interval (>70 wt% CaCO₃), suggesting a paleodepth above the CCD during most of the Paleogene. High-resolution shipboard sampling at Site U1408 (not performed at Site U1407) shows a temporary decline in carbonate content from ~70 wt% to a low of ~30 wt% within the siliceous claystone of the PETM interval (Fig. F64). This decrease in carbonate content is similar to other pelagic PETM records (Colosimo et al., 2006; Zachos et al., 2005), where it is interpreted as documenting shoaling of the CCD in response to the rapid injection of thousands of gigatons of carbon into the ocean-atmosphere system.

Site U1403, however, is carbonate-free below the Paleocene/Eocene boundary, suggesting a depositional depth below the late Paleocene CCD. The exact placement of the Paleocene/Eocene boundary is uncertain because of a lack of reliable upper Paleocene biostratigraphic markers but may be represented by a prominent siliceous claystone layer at 182.2 mbsf in Hole U1403A. At ~30 cm above this layer, carbonate content increases rapidly from <1 to ~30 wt% (Fig. F64), driven mostly by an abundance of calcareous nannofossils representing Subzone NP9b. This may be direct evidence of a CCD overshoot during the PETM recovery phase, a global ocean carbonate oversaturation relative to pre-event steady-state conditions. This effect is predicted by carbon cycle models (e.g., Dickens et al., 1997) in which enhanced rock weathering in response to elevated atmospheric CO₂ increases the supply of alkalinity and dissolved inorganic carbon to the ocean, causing carbonate oversaturation and deepening of the CCD. The result of this carbonate neutralization is greater preservation of calcium carbonate in sediments where the excess carbon is ultimately preserved. Carbonate oversaturation has previously been suggested by increased carbonate mass accumulation rates during the PETM recovery at ODP Site 690 (Farley and Eltgroth, 2003) and ODP Site 1266 (Kelly et al., 2010); however, direct evidence of CCD deepening has remained elusive. Site U1403, carbonate-barren before the PETM and carbonate-rich shortly afterward, places important constraints on the evolution of the Atlantic CCD during the PETM recovery phase that promise to guide understanding of the carbon cycle perturbation during the PETM and the processes involved in restoring steady state.

Paleocene and Danian–Selandian transition

The Paleocene is generally overshadowed by the major boundary events at either end: the K/Pg boundary mass extinction and the PETM. Our understanding of major biotic, paleoceanographic, and geochemical changes between these well-studied intervals is limited by a lack of sites and records spanning the ~6 m.y. from the late Danian to late Thanetian. Outstanding issues for this broad sweep of the Paleocene include

• The mechanisms leading to the diversification and turnover of calcareous plankton during the Danian–Selandian transition (Coxall et al., 2006; Fuqua et al., 2008);

• The rediversification of assemblages dominated by photosymbiotic planktonic foraminifers following the K/Pg boundary mass extinction (Norris, 1996; Berggren and Norris, 1997; Quillévéré et al., 2001);

• The relationship between paleoceanographic change and the global carbon cycle (e.g., Kurtz et al., 2003; Hilting et al., 2008); and

• The nature of short-lived carbon isotope and/or biotic events in the Paleocene (e.g., Quillévéré et al., 2002; Petrizzo, 2005; Bornemann et al., 2009; Westerhold et al., 2011).

During Expedition 342, sediment of late Danian to late Selandian age was recovered from J-Anomaly Ridge Site U1403 (4946 mbsl) and Southeast Newfoundland Ridge Site U1407 (3074 mbsl). Biostratigraphy and preliminary magnetostratigraphic data indicate that the Danian–Selandian transition (nannofossil Zones NP4–NP6) is largely complete at Site U1407 (Fig. F65). Site U1403 suffers from poor calcareous microfossil preservation and poor core recovery but also may span this interval based on preliminary evidence from nannofossils and radiolarians.

Our new records span two short-lived climatic and biotic events in the Paleocene, the Latest Danian Event (LDE) at the Chron C27n/C26r boundary (Westerhold et al., 2008) and the Mid-Paleocene Biotic Event (MPBE) at ~58.9 Ma (Bernaola et al., 2007; Petrizzo, 2005). Both events coincide with changes in calcareous microfossil assemblages (Bernaola et al., 2007; Petrizzo, 2005; Fuqua et al., 2008). Whereas the LDE is associated with a ~200 k.y. hyperthermal excursion and an inferred perturbation to the carbon cycle (Bornemann et al., 2009; Westerhold et al., 2011), the only tenuous evidence for climatic or paleoceanographic change at the MPBE is a dissolution horizon coincident with this event (Petrizzo, 2005).

The Danian–Selandian succession at Site U1403 may be as thick as ~30 m, although it includes substantial recovery gaps (Fig. F65). Sediment is composed of carbonate-free grayish brown siliceous clay in the upper part of the transition and shifts toward predominantly light reddish brown clayey nannofossil ooze with radiolarians below 198 mbsf, covering mainly nannofossil Zone NP4. At Southeast Newfoundland Ridge, Site U1407 includes ~32 m of thick pinkish to white nannofossil chalk with radiolarians in the upper part of the Danian–Selandian interval, gradually changing to light greenish gray nannofossil chalk below 175 mbsf. Calcareous microfossil preservation is considered to be moderate to good at this site. As the northernmost long Paleocene section drilled to date, the J-Anomaly Ridge and Southeast Newfoundland Ridge sites have the potential to unravel the mechanisms leading to both events.

A significant pattern of radiolarian occurrence is noted across the Danian/Selandian boundary. At all sites, radiolarians are absent in the lower Paleocene but become very abundant and well preserved from the upper part of nannofossil Zone NP4 (radiolarian Zone RP6) to the uppermost Paleocene (see “Paleocene/Eocene Thermal Maximum”). At Site U1407, they are accompanied by diatoms, suggesting that the Selandian–Thanetian interval was a period of high biosiliceous productivity.

Cretaceous/Paleogene boundary

The mass extinction at the K/Pg boundary is one of the five largest extinction events in Earth history, with a loss of ~75% of Late Cretaceous marine species (Sepkoski et al., 1981; Norris, 2001). The Chicxulub impact (Yucatan, Mexico) at the K/Pg boundary is now widely considered to be the primary cause of the end-Cretaceous mass extinction (Alvarez et al., 1980; Hildebrand et al., 1991; Schulte et al., 2010). The asteroid, estimated to be 10–13 km in diameter (Hildebrand et al., 1991; Morgan et al., 1997), would have caused immediate devastation through massive earthquakes, tsunamis, and wildfires, as well as longer lasting effects including ejecta-induced solar dimming and acid rain (Toon et al., 1997; D’Hondt, 2005).

Ocean drilling has been critical to resolving various debates regarding the cause and consequences of the K/Pg boundary extinction, including

• Whether an impact occurred at the K/Pg boundary (e.g., Officer and Drake, 1983; Alvarez et al., 1984; Michel et al., 1981, 1985; Claeys et al., 2002);

• Whether the impact was temporally and mechanistically linked to the mass extinction (Pospichal, 1994; Huber, 1996; Huber et al., 1994; Keller, 1993; Norris et al., 1999); and

• Whether the magnitude and duration of environmental perturbation caused by the K/Pg boundary impact was greater than that of Deccan volcanism, another hypothesized cause of the K/Pg boundary extinction (Ravizza and Peucker-Ehrenbrink, 2003; Robinson et al., 2009; Schulte et al., 2010).

At J-Anomaly Ridge Site U1403, coring recovered two K/Pg boundary sections (Holes U1403A and U1403B) with an ~0.5–1 cm thick graded impact spherule bed (Fig. F66). Of the >24 K/Pg boundary sites drilled during the history of ocean drilling (summarized from the supplemental table in Schulte et al., 2010), only about nine contain discrete ejecta beds with a thickness of 1 cm or more (Fig. F67). Thus, this new K/Pg boundary section, with a distinct sequence of impact markers, has potential to provide detailed chronostratigraphic and geochemical records that will work to further resolve details related to the impact proper and to test the relative chronology, magnitude, and competing importance of the K/Pg boundary impact and Deccan volcanism. The impact spherule bed in Section 342-U1403B-28X-1 provides a better target than Core 342-U1403A-26X for such studies because it is less fragmented by drilling and preserves the original horizontal orientation and spherule bed grading across the K/Pg boundary.

The K/Pg boundary cores (342-U1403A-26X and 342-U1403B-28X) are biostratigraphically complete across the K/Pg boundary and indicate sedimentation rates of 1.08 cm/k.y. in the latest Maastrichtian and 0.31 cm/k.y. in the earliest Paleocene. Cores 342-U1403A-26X and 342-U1403B-28X both capture the same general lithostratigraphic sequence (Fig. F66), including (from bottom to top):

1. Moderately bioturbated splotchy pink and white-gray latest Cretaceous chalk with abundant, diverse Late Cretaceous nannoplankton assemblages;

2. A pale green ~0.5 cm thick unbioturbated bed of chalk immediately below the ejecta horizon;

3. A ~0.5 cm thick bed of green sand to silt-sized impact spherules topped by a ~0.5 cm thick bed of light greenish gray chalk with abundant calcispheres and early Paleocene biomarkers; and

4. A distinctly bioturbated pink bed topped by light brown chalk containing early Danian planktonic foraminifers and nannoplankton.

Biogeographically, the K/Pg boundary at Site U1403 is of great interest because it captures the highest northern latitude, open-ocean K/Pg boundary site drilled to date. The abundant and well-preserved calcareous microfossils will allow us to explore recent ideas concerning plankton extinction, recovery, and survivorship (e.g., Bown, 2005a; Coxall et al., 2005; Jiang et al., 2010; Hull and Norris, 2011). Specifically, we will be able to work toward addressing three unresolved questions:

1. Does the change in oceanic export productivity across the K/Pg boundary vary geographically (e.g., Hollis et al., 1995; Alegret and Thomas, 2009; Hull and Norris, 2011)?

2. Is there diachroneity in pelagic recovery (e.g., Coxall et al., 2006; Jiang et al., 2010)?

3. Is the recovery of export productivity within one site tied to the recovery of pelagic foodwebs (e.g., D’Hondt, 2005; Coxall et al., 2006; Hull et al., 2011b)?

Site U1403 has the potential to provide valuable multiproxy records (e.g., stable isotopes, biogenic barium, faunal assemblages, and organic proxies and biomarkers) to these ends.

Late Cretaceous sedimentation

Warm mid-Cretaceous supergreenhouse climates were followed by long-term climate cooling in the Late Cretaceous (Huber et al., 2002; Friedrich et al., 2012) that culminated in the latest Campanian to Maastrichtian. However, rapid short-term climate shifts are superimposed on this long-term cooling trend, indicated by short stable isotope excursions, including the Campanian/Maastrichtian Boundary Event (Voigt et al., 2010) and the Mid-Maastrichtian Event (e.g., Frank et al., 2005).

Both of these events have been linked with changes in Earth’s climate and ocean circulation and explained by changes in intermediate to deepwater circulation (e.g., Barrera et al., 1997; Frank and Arthur, 1999; Koch and Friedrich, 2012) or by temporary build-up of ice sheets on Antarctica (e.g., Barrera and Savin, 1999; Miller et al., 2003, 2005). Both of these explanations remain controversial, not least because our understanding of Cretaceous oceanic circulation is limited. It has been suggested that Atlantic Ocean circulation was driven by multiple intermediate to deepwater sources during the mid-Cretaceous greenhouse climate (e.g., MacLeod et al., 2008; Martin et al., 2012) and that the Campanian interval saw significant changes in Atlantic deepwater circulation, potentially including the encroachment of southern component waters (e.g., Frank and Arthur, 1999; Friedrich et al., 2009; Robinson et al., 2010) and northern North Atlantic Deep Water (MacLeod et al., 2011). But even with these new data, the mode of ocean circulation during the latest Cretaceous to Paleocene times (75–55 Ma) is still poorly understood, and possible sites of deepwater formation relative to the opening of the Atlantic Ocean are hotly debated.

One of the major barriers to better understanding Cretaceous ocean circulation is the lack of truly deep ocean sites that provide a record of changes in deepwater masses. In this respect, the Expedition 342 sequences of late Campanian to Paleocene age (especially Site U1403) provide the opportunity to fill the gap in our understanding of the evolution of ocean circulation during this time interval. Site U1403 provides a ~43 m thick sequence of late Campanian to Maastrichtian sediment and is located in the pathway of a potential North Atlantic Deep Water current (a possible end-member of the latest Cretaceous ocean circulation). Paleoceanographic data from this site will therefore provide critical information concerning the role of North Atlantic Deep Water during the latest Cretaceous and Paleocene interval.

Ocean Anoxic Event 2

OAE 2 is characterized by widely distributed deposition of organic matter–rich sediments and ¹³C enrichment in carbonate and organic carbon (Arthur et al., 1987; Schlanger et al., 1987; Tsikos et al., 2004). Over a period of ~400 to 600 k.y. (Sageman et al., 2006; Voigt et al., 2008), the ocean was characterized by widespread anoxia linked to major faunal turnover in marine plankton (Leckie et al., 2002) and perturbations of the carbon, sulfur, nitrogen, and phosphorus cycles on a global scale (e.g., Arthur et al., 1988; Kuypers et al., 2004; Junium and Arthur, 2007; Mort et al., 2007; Adams et al., 2010; Barclay et al., 2010). The trigger for OAE 2 is thought to be the rapid addition of carbon dioxide to the ocean-atmosphere system associated with large igneous province volcanism (Kuroda et al., 2007; Turgeon and Creaser, 2008; Adams et al., 2010). In this sense, OAE 2 is similar to the Paleogene ocean acidification and hyperthermal events in that they are a direct response to addition of carbon to the Earth system. However, the net result of CO₂ addition in the mid-Cretaceous was distinctly different from the Eocene dissolution of calcium carbonate followed by high rates of carbonate production (overshoot). During OAE 2, carbon dioxide was rapidly sequestered as organic matter and is expressed by the quasi-global distribution of organic matter–rich rocks known as black shales (for review see Jenkyns, 2010).

Newfoundland ridges record of OAE 2

The recovery of Cenomanian–Turonian black shales deposited during OAE 2 at Site U1407 was an unexpected but fortuitous discovery that fills a gap in existing OAE 2 geologic records. The presence of well-constrained biostratigraphy from a pelagic depositional setting will help better define the temporal transience of black shale deposition that is recognized through the OAE 2 interval but poorly understood (Tsikos et al., 2004). Additionally, the Site U1407 record is one of the most shallowly buried OAE 2 black shales yet recovered through ocean drilling and holds great promise for geochemical studies.

Drilling operations recovered an OAE sequence in all three holes drilled at Site U1407 that is defined on the basis of lithology and calcareous nannofossil biostratigraphy. The significant differences in the lithostratigraphic sequence and thickness of beds that exist between Holes U1407A, U1407B, and U1407C are due in part to drilling disturbances and mass wasting indicated by slump features in the overlying Turonian strata (Figs. F68, F69, F70).

Shipboard nannofossil stratigraphy from Holes U1407A and U1407B indicate a relatively complete yet highly condensed OAE 2 sequence. Elsewhere, the last occurrence of Corollithion kennedyi at 232.6 and 232.8 mbsf in Holes U1407A and U1407B, respectively, occurs after the initial rise in δ¹³C of carbonate and organic carbon reservoirs that defines the initiation of OAE 2 (Arthur et al., 1987; Sageman et al., 2006). This suggests that the OAE 2 interval begins in the underlying nannofossil chalk and that black shale deposition at Site 1407 lags onset of the global increase in the fractional burial of organic carbon defined by the δ¹³C excursion as observed elsewhere (e.g., Wunstorf, Germany; Voigt et al., 2008). The first occurrence of Quadrum intermedium occurs in greenish white nannofossil chalk ~80 cm above the black shale at Site U1407 and recorded elsewhere within the δ¹³C excursion plateau that defines the heart of OAE 2. The termination of OAE 2 elsewhere (Sageman et al., 2006) occurs shortly after the first occurrence of Quadrum gartneri in the early Turonian (Tsikos et al., 2004; Hardas and Mutterlose, 2006) (Fig. F68). The lack of well-developed pink chalk in Hole U1407B suggests the presence of a hiatus or slumping at 230.64 mbsf in this hole.

Global context

The pelagic style of sedimentation is distinctly different from the proximal turbiditic OAE 2 sequence in the Newfoundland Basin (Site 1276) and the carbonate-free sequence from Hatteras Rise (DSDP Hole 603B) (Dean and Arthur, 1987).

The lithologic expression of black shale deposition at J-Anomaly Ridge is more suggestive of the “black band” sequences from the Tethys and England where black shales occur principally in a narrow interval of time within the heart of the OAE δ¹³C excursion, nested in calcareous sediments (Tsikos et al., 2004). The carbonate-poor black bands in Holes U1407A–U1407C are interbedded with dark gray claystone that is relatively organic carbon poor but almost exclusively laminated and free of preserved benthic foraminifers. Despite the lower organic carbon content, the presence of laminations and lack of benthic foraminifers indicates that suboxic bottom water conditions remained through the whole black band interval. Significant variability in organic carbon content in the black bands is also a characteristic of other locations (Tsikos et al., 2004; Erbacher et al., 2005; Jenkyns et al., 2007; Voigt et al., 2008) and may reflect temporary fluctuations in productivity, water column ventilation, or winnowing of organic matter.

The color progression of greenish white to black to pink through the OAE 2 interval at Site U1407 is very similar to Cenomanian/Turonian boundary sequences from the Umbria-Marche Basin of Italy. The greenish white to pink nannofossil chalk is reminiscent of the Scaglia Bianca and Scaglia Rossa limestones that bound the siliceous Bonarelli horizon (Arthur and Premoli Silva, 1982). Associated lithologies include the presence of radiolarian sand interbedded with the black shales and chert and silicified limestone with radiolarians. This stratigraphic progression is also very similar to the Italian sequences. However, the preliminary biostratigraphy at Site U1407 indicates that the δ¹³C excursion must predate black shale deposition and, in this sense, is more similar to continental records from England, North Germany (Tsikos et al., 2004; Voigt et al., 2008), and DSDP Site 551 (Goban Spur) (de Graciansky and Bourbon, 1985).

Albian reef and beginning of pelagic sedimentation

Aptian–Albian sediment along the easternmost margin of North America has long been recognized as an extensive carbonate depositional environment (Mountain and Tucholke, 1985). At Site U1407, we recovered sediment of Albian age at the bottom of Hole U1407A (Fig. F71). The topmost interval of this sediment is a current-laminated fine sand with common belemnite rostra. The paleowater depth of these deposits is inferred to be tens of meters deep (shelf sediment). Beneath this sandy sediment of Albian age lies an unconformity between the overlying pelagic sediment and the top of the main shallow-water (neritic) carbonate facies. The top of the neritic shallow-water carbonate unit corresponds to a ubiquitous reflector on the North American eastern margin, called Reflector β by Mountain and Tucholke (1985). In Hole U1407A, this “ringing” reflector is a coarse-grained sand, cemented and coated by iron manganese oxides (Sample 342-U1407A-31X-CC, 17–18 cm) (Fig. F71A). From a neritic carbonate facies standpoint, lithology transitions downhole from heavily cemented (iron manganese cement) beach sediment into back-reef sediment that includes large Orbitolina benthic foraminifers (found in Section 342-U1407A-33X-CC) (Fig. F71E). Deeper within the hole we recovered pelleted carbonate mud (Sample 342-U1407A-32X-CC, 0–3 cm) (Fig. F71B) consistent with a deeper lagoonal environment. Finally, at the base of the hole, sediment is composed of grainstone and framestone more typical of a back-reef environment, examples of which include molds of scleractinian corals, rudist bivalves, and other mollusks (Fig. F71C, F71D, F71F).

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