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Site U1308

Site U1308 is a reoccupation of Site 609, located in the core of the Ruddiman (1977) IRD belt (Fig. F1; Table T1). During DSDP Leg 94 (June–August, 1983), two principal holes (Hole 609A and Hole 609B) were drilled with the variable length piston coring (VLHPC) system and XCB. During that leg, two cores were collected from Hole 609A to recover the mudline, and seven XCB cores were collected from Hole 609C to recover the 123–190 mbsf interval. Samples from Site 609 have played a major role is driving some of the most exciting developments in paleoceanographic research during the last 20 y, such as the recognition and understanding of Heinrich layers, the recognition of the 1500 y pacing in hematite-stained grains and Icelandic glass, and the correlation of ice-core δ18O to SST proxies (Broecker et al., 1992; Bond et al., 1992, 1999, 2001). The majority of the analyses from Site 609 have dealt with the record younger than MIS 6, partly because of the lack of a continuous pristine composite record. A primary objective at Site U1308, the reoccupation of Site 609, was to recover a demonstrably complete composite record and hence considerably enhance the potential for Pliocene–Quaternary climatic records from this site.

The upper Miocene through Quaternary sedimentary succession at Site U1308, which is divided into two units, records variations in the input of terrigenous and biogenic sediments, primarily nannofossil ooze, nannofossil silty clay, and silty clay. Unit I (0–196.85 mcd) comprises a Holocene to upper Pliocene sequence of interbedded biogenic and terrigenous sediments with dropstones. Subunit IIA (196.85–262.14 mcd) is upper Pliocene nannofossil ooze interbedded with terrigenous sediment-rich layers but at a lower frequency than Unit I. Subunit IIB (262.14–355.89 mcd) is entirely composed of lowermost upper Pliocene to uppermost Miocene nannofossil ooze.

Diverse assemblages of calcareous, siliceous, and organic-walled microfossils were recovered at Site U1308. Calcareous microfossils are abundant with good preservation in the upper ~200 mcd, grading to moderate preservation below this depth (Sato et al.; Chiyonobu et al.). Siliceous microfossils are rare to common and moderately preserved above ~255 mcd (upper Pliocene–Pleistocene), with radiolarians locally abundant only in the middle part of the cored sequence. Siliceous microfossils are absent below 255 mcd. The concentration of terrestrial palynomorphs is low. Dinocysts are common to abundant in the upper 200 mcd. Microfossil floral changes observed at Site U1308 document the onset of Northern Hemisphere glaciation, as well as seasonal changes in bioproductivity and the location of hydrographic fronts.

Paleomagnetic directional data document an apparently continuous sequence of polarity transitions. Identification of the Brunhes, Matuyama, and Gauss Chrons are unambiguous (see Channell et al., 2008). The Gilbert Chron is tentatively recognized in the lower part of Hole U1308A. The Jaramillo, Cobb Mountain, Olduvai, Reunion, Kaena, and Mammoth Subchrons are also clearly identified.

Six holes were cored at Site U1308 to ensure complete recovery of the stratigraphic section to 247 mcd. The unusually large number of holes was required because of poor recovery and core disturbance caused by excessive heave and crushed core liners. One problematic interval between ~186 and ~196 mcd contains inclined bedding and sharp lithologic contacts that suggest a possible break in continuity of sedimentation. The mean linear sedimentation rate calculated using magnetostatigraphic datums is ~8.3 cm/k.y. for the last ~3.5 m.y. Prior to that time the mean sedimentation rate was ~3.3 cm/k.y. based on biostratigraphic markers.

Interstitial water sulfate decreases downhole to 9 mM, but complete sulfate reduction is not achieved within the cored interval. Unlike all other Expedition 303 sites, strontium increases with depth to a maximum of 1592 µM at Site U1308. The nearly linear strontium increase and the corresponding increase in Sr/Ca ratios indicates that recrystallization (not dissolution) of biogenic carbonate is occurring. Downhole decreases in magnesium and potassium and increase in calcium below ~100 mbsf are consistent with the alteration of volcanic material and/or basement below the cored interval.

Physical property records at Site U1308 document long-term changes in sediment composition, which likely reflect fundamental changes in North Atlantic climate. The NGR and lightness (L*) records from lithologic Unit I (0–197 mcd) show a strong glacial–interglacial variability. In Subunit IIA (197–262 mcd), magnetic susceptibility and NGR values decrease both in absolute value and variability and L* increases. NGR shows a fairly abrupt change in absolute values and variability at the Unit II/I boundary at ~197 mcd (~2.74 Ma). In the white nannofossil ooze of lithologic Subunit IIB (262–356 mcd), magnetic susceptibility and NGR values are significantly lower and less variable than in Subunit IIA.

The six holes at Site U1308 have been pieced together to produce a complete composite section to ~247 mcd. The base of the composite section correlates to the middle part of the Gauss Chron at ~3.2 Ma. A discontinuous record of white upper Miocene nannofossil ooze was recovered below this level down to 356 mcd. The upper Pliocene to Quaternary composite section provides a means of studying the evolution of NADW, the extension of the Central Atlantic detrital layer (Heinrich-type) stratigraphy beyond the last glacial cycle, and the 1500 y cycle in the petrologic characteristics of IRD. The mean sedimentation rate for the composite section (7.6 cm/k.y.) indicates that these studies can be carried out at moderately high resolution.

At Site U1308, a detailed benthic oxygen isotope record has been resolved for the upper 110 mcd, equivalent to the last 1.5 m.y. (Hodell et al., 2008). The age model was based on the optimal fit of the benthic oxygen isotope record to the oxygen isotope stack of Lisiecki and Raymo (2005), augmented by correlations to Core MD952042 (Shackleton et al., 2004) for the 35–60 ka interval, and by 14C ages in the 14–35 ka interval that were transferred from Site 609 using reflectance records (Hodell et al., 2008). The resulting sedimentation rate map indicates interval sedimentation rates in the range of 3.8–16.0 cm/k.y., with minima in glacial MIS 10, 12, and 48 and the maximum in interglacial MIS 31. Although other interglacial stages, such as MIS 13 and 17, have relatively high sedimentation rates, other prominent interglacial stages such as MIS 11 do not have particularly elevated sedimentation rates relative to neighboring glacial intervals.

Heinrich-type detrital layers at Site U1308 can be detected using magnetic susceptibility, GRA density measurements, δ18O of bulk carbonate, and Ca/Sr ratios derived from XRF core scanning (Figs. F3, F4) (Hodell et al., 2008). The Ca/Sr ratio provides a means of distinguishing detrital carbonate from biogenic carbonate (and hence detecting Heinrich-type detrital layers) because of the relatively high concentration of Sr in biogenic carbonate relative to detrital carbonate dominated by inorganic carbonate and dolomite. The δ18O of bulk carbonate is also a useful proxy for Heinrich-type detrital layers because of the different values of δ18O for (inorganic) detrital carbonate and foraminifer calcite (Hodell and Curtis, 2008). The Si/Sr ratio from XRF scanning was used to detect layers that are poor in biogenic carbonate (low Sr and Ca) but relatively rich in detrital silicate minerals (Hodell et al., 2008). The Si/Sr ratio mimics counts of the ratio of lithics to foraminifers from Site 609 (Bond et al., 1992) (Fig. F3).

The record of “Hudson Strait” Heinrich-type detrital layers, based on the Ca/Sr ratio, has been placed in a precise chronostratigraphic framework based on benthic oxygen isotope data, indicating the existence of detrital carbonate-rich layers during glacial stages to MIS 16 (Hodell et al., 2008). Hudson-Strait–derived Heinrich-type events, characterized by detrital carbonate, appear in MIS 8, 10, 12, and 16 but not apparently in MIS 6 or 14 (Fig. F4). At Site U1313 (see below), located at the southern edge of Ruddiman’s IRD belt, Heinrich-type layers characterized by lithics with detrital carbonate (dolomite) appear in glacial MIS 10, 12, and 16 as well as at Terminations V and VII (Stein et al., 2009; Voelker et al., 2010).

The lead (Pb) isochron method applied to detrital carbonate grains extracted from Heinrich-type detrital layers detected through the Ca/Sr ratio yields Ordovician depositional/diagenetic ages consistent with a Hudson Strait (Laurentide) source (Nielsen and Hodell, submitted). Furthermore, the Heinrich-type detrital layers were clearly identified by a unique biomarker association of “petrogenic” compounds such as benzohopanes, D-ring monoaromatic 8,14-secohopanes, rearranged diasterenes, mono- and triaromatic steranes, and isorenieratene derivatives, as well as characteristic pristane/n-C17 and pristane/phytane ratios (Hefter et al., 2007). This biomarker association provides circumstantial evidence for derivation from a relatively immature Paleozoic marine rock deposited under occasional photic zone anoxic conditions, which is today located on the Laurentide/Canadian shield (Rashid and Grosjean, 2006). Reinvestigation of available geologic and organic geochemical data narrowed this assumed source to an Ordovician oil shale close to Hudson Strait, which bears a striking resemblance in terms of biomarker distributions when compared to the specific association of compounds in Heinrich-type detrital events (Hefter et al., 2007).

Interestingly, MIS 6 (and MIS 14) differs from other glacial stages in the MIS 2–16 interval as being apparently devoid of Heinrich-type detrital layers. This observation is consistent with the rarity or absence of the radiolarian species Amphimelissa setosa during MIS 6, indicating relatively warm sea-surface conditions relative to other Brunhes-aged glacial stages (see K.R. Bjørklund et al., unpubl. data). In addition to the Ca/Sr ratio, Heinrich-type detrital layers rich in dolomite can be effectively detected using a Fourier transform infrared spectrophotometer (Balsam et al., 2007), and this method has been put to use in the uppermost 29 m of the sediment recovered at Site U1308 (Ji et al., 2009).

Oxygen isotope (δ18O) values from detrital carbonate associated with Heinrich-type layers are ~9‰ lower than coexisting foraminifer calcite. Hodell and Curtis (2008) have shown that the δ18O of bulk carbonate at Sites U1308 and U1302/U1303 provides a means of detecting detrital carbonate associated with Heinrich-type layers. The δ18O of bulk carbonate can therefore be used to recognize Heinrich-type layers rich in detrital carbonate, and hence provide a clue to provenance. This observation has implications for oxygen isotope data from (planktonic) foraminifers within carbonate-rich detrital layers that may be “contaminated” by adhered detrital carbonate resulting in spuriously low δ18O values, previously interpreted as meltwater pulses (e.g., Hillaire-Marcel et al., 1994).

Benthic stable isotope data and lithic grain counts in the MIS G4 and 100 (2.5–2.6 Ma) interval at Site U1308, close to the Gauss/Matuyama boundary, indicate large amplitude suborbital variations in ice rafting at the onset of Northern Hemisphere Glaciation (NHG) (Bailey et al., 2010). As benthic δ18O maxima were ~0.45‰ lower in this interval than for the late Pleistocene, it was supposed that the benthic oxygen isotope “ice-volume threshold” at the onset of NHG, required to trigger ice sheet instability, was lower than for the late Pleistocene, implying differences in ice sheet dynamics and morphology at the onset of NHG (Bailey et al., 2010).

Stratigraphic control at Site U1308 has been enhanced by a RPI record that has been calibrated using benthic oxygen isotope data for the last 1.5 m.y. (Fig. F5) (Channell et al., 2008). The Site U1308 RPI and δ18O records have higher resolution and fidelity than most coupled RPI/δ18O records. They are therefore used as the templates for RPI and δ18O stacks for the last 1.5 m.y., through correlation to Site U1308 of 12 globally distributed (but mainly North Atlantic) sites that have yielded both RPI and δ18O data (Channell et al., 2009b). The coupled correlations of isotope and RPI data are accomplished using the Match algorithm (Lisiecki and Lisiecki, 2002). The simultaneous match reduces the degree of freedom associated with correlations using RPI or oxygen isotope records alone. The overall compatibility of RPI and oxygen isotopes indicates a dominant global component in both signals. The so-called PISO-1500 stacks represent a powerful new stratigraphic tool that can be used to correlate among marine sediment records and link them to polar ice cores through variations in cosmogenic nuclide production. Scaling the RPI stack to values for virtual axial dipole moment (VADM) indicate maxima at ~15 × 1022 Am2 and minima that imply a threshold of ~2.5 × 1022 Am2, below which values are associated with either polarity reversals or magnetic excursions. The resulting RPI/isotope templates are part of an overall goal of enhancing stratigraphic resolution by combined use of oxygen isotope and RPI.