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doi:10.2204/iodp.pr.341.2014

Principal results

Continental margin sedimentation in the subarctic northeast Pacific Ocean

Expedition 341 drill sites were arranged in a cross-margin transect to recover sediment supplied from the exhuming mountains formed by the Yakutat terrane convergence with North America during a period of dramatic Neogene climate change (Fig. F6). The recovered sedimentary record extends from the late Miocene through the Pleistocene/Holocene (Figs. F20, F21). Sites U1417 and U1418 recovered distal and proximal deepwater sedimentary records from the Surveyor Fan, respectively. Site U1417 contains a complete and continuous interval from the mudline to 220.4 m core composite depth below seafloor (CCSF-D; splice method), the base of which was dated shipboard to 1.7–1.8 Ma, and additional material was recovered to 709 m core depth below seafloor (CSF-A). Site U1417 contains no apparent hiatuses through the late Miocene based on initial shipboard biostratigraphy and magnetostratigraphy. Site U1418 contains a complete and continuous interval from the mudline to 271 m CCSF-D, which was dated shipboard to 0.2–0.3 Ma, and additional material was recovered to 941 m CSF-A. Site U1418 contains no apparent hiatuses through 1.2 Ma based on initial shipboard chronostratigraphy. Sites U1419 and U1421 sampled the transitional environment along the continental slope. Site U1419 is located on a small ridge at ~780 m water depth in between two large shelf-crossing glacial troughs, whereas Site U1421 is located in a trough-mouth fan downslope of the Bering Trough, in the direct sediment transport pathway associated with the seaward extent of the Bering Glacier. Site U1419 contains a complete and continuous interval from the mudline to 100 m CCSF-D that shipboard chronostratigraphy indicates is younger than 0.3 Ma. At Site U1421, 694 m of sediment was recovered that accumulated in <0.3 m.y. based on shipboard chronostratigraphy. Site U1420 is located proximal to the orogen on the continental shelf, within the Bering Trough. Recovered material consists of drilled rock, lonestones, diamict, and mud that were deposited within <0.78 m.y. based on shipboard biostratigraphy and magnetostratigraphy. The high sediment accumulation rates and preservation of calcareous foraminifers at Sites U1419 and U1421 implies the potential for matching recovered sediments to the global δ¹⁸O stratigraphy, especially at Site U1419, with continuous recovery to ~100 mbsf. Similarly, the incomplete recovery at Site U1421, but very high accumulation rates, may allow use of δ¹⁸O and radiocarbon techniques to establish a chronology and refine the sediment accumulation rates at this site. All sites are associated with notable changes in seismic reflection facies and stratigraphy, some of which correlate to changes in sediment according to preliminary integration of core descriptions, physical properties, and downhole logging data with available seismic profiles. Because of high sediment accumulation rates at shelf, slope, and deepwater sites, Expedition 341 cores have the strong potential to provide a high–temporal resolution record of climate-tectonic interactions.

Shipboard results from Expedition 341 indicate an approximate doubling (>10 cm/k.y.) in sedimentation rates at ~2.56 Ma at Site U1417, which we interpret as the onset of significant glacial coverage in the St. Elias Range after the Pliocene–Pleistocene transition (PPT) (Fig. F21). This result agrees with results from Site 887 in the more distal southwestern Gulf of Alaska. However, the correlation of the PPT with a prominent regional seismic reflector (seismic Unit I/II boundary; Reece et al., 2011) indicates that sediment fluxes to the Surveyor Fan are higher than previous estimates by a factor of 2 after 2.56 Ma. Also, the recovery and discovery of a temporally expanded mid–late Pleistocene section at Sites U1418–U1421 (Fig. F20) is a significant achievement that confirms one of the Expedition 341 hypotheses that the onset of temperate glaciers in the Pleistocene led to increased mass flux from the orogen. The observation that the key angular unconformity penetrated on the shelf at Site U1420 is significantly younger than the MPT was a surprising finding of Expedition 341. According to this observation, rates of sediment flux from onshore to offshore are even higher than expected and potential tectonic responses to climatic forcing through glacial redistribution of sediments occur on unexpectedly short timescales (<10⁶ y).

Lithostratigraphic summary

A major result of Expedition 341 is documentation of a dynamic sedimentary environment in the Neogene as represented by the presence of 22 distinctive lithofacies and subfacies (Table T1). The dominant recovered facies is mud, but the remaining minor facies, although much less volumetrically significant, are distinctive and allow us to organize the cores at each site into lithostratigraphic units (Fig. F20). Lithofacies include massive mud with and without lonestones; laminated mud; silt; interbedded silt and mud; sand; interbedded sand and mud; muddy diamict; interbedded mud and diamict; diatom ooze; biosiliceous ooze; calcareous/carbonate-bearing mud; volcanic ash; volcaniclastic mud, sand, and diamict; sedimentary rock; and intrastratal contorted mud and diamict. These facies reflect deposition from suspension fall out, sediment gravity flows, large-scale mass wasting, ice rafting, variable productivity, and volcanic eruptions.

At all sites, clast-rich mud and/or diamict are prevalent in the Pleistocene (Fig. F20), indicating the persistent transport of glacigenic sediment by icebergs or sea ice rafting. The composition of ice-rafted clasts is different between Site U1417 in the subarctic gyre and the more proximal slope/inner fan/shelf Sites U1418–U1421, which currently are influenced by the Alaska Current and/or the Alaska Coastal Current. An unanticipated observation is the often sharp transitions between these ice-dominated facies and more biogenic rich, often bioturbated muddy lithologies that might be related to increased biological productivity, enhanced nutrient supply, and/or decreased input of terrigenous sediments. Laminated mud and mud interbedded with silt and/or diamict are common lithofacies during what are interpreted to be glacial intervals. At Site U1418, thin-bedded diamict layers bounded by thinly laminated packages of mud are similar to gravelly mud beds and laminated mud described from southeast Alaskan glacial fjords that were deposited by sea ice rafting and turbid meltwater plumes (Cowan et al., 1997, 1999; Jaeger and Nittrouer, 1999). The alternation of interbedded massive and bioturbated mud (with dispersed clasts and few diamict beds) and laminated mud may point to time intervals characterized by a weaker glacimarine signal that alternate with intervals of more intense ice rafting. Interbedded silt-mud and sand-mud facies indicative of deposition from gravity flows are potential indicators of maximum glacial extent to the shelf edge when sediment-rich, subglacial discharge was deposited on the upper slope, feeding into the Surveyor Channel and Aleutian Trench. Postcruise analyses, including the development of higher resolution chronologies, will allow assessment of orbital to millennial controls on variations between glacigenic and biogenic-influenced sedimentation.

A secondary but notable set of lithofacies is interpreted as large-scale, highly episodic gravity-flow deposits derived from the continental shelf and/or slope. These lithofacies include (1) mud and clast-rich muddy diamict characterized by mud rip-up clasts, coal clasts, and woody plant detritus at Site U1417; (2) soft-sediment deformation at Sites U1418 and U1421; and (3) intrastratal contortions at Site U1418. At Site U1418, interpretation of seismic reflection profiles suggests that a chaotic seismic unit was a mass transport deposit (MTD) (Reece et al., 2012). The coring results confirm this interpretation, with the MTD likely occurring at ~1.0–1.2 Ma based on a tentative interpretation of the Cobb Mountain Subchronozone (C1r.2n) just above this unit. The second type of gravity-flow deposit found in the late Miocene to Pliocene at Site U1417 indicates direct input from terrestrial sources, as noted by the freshwater diatoms, coal, and plant detritus, which are tentatively interpreted to have been derived from the coal-bearing Eocene Kulthieth Formation, which has been exhumed and is exposed in the Chugach-St. Elias mountains onshore (e.g., Plafker, 1987).

The presence of chlorite, mica, and ferromagnesian minerals in bulk and sand samples from all units indicates the potential contribution from the Yakutat terrane and/or a metamorphic/mafic source, perhaps the Chugach Metamorphic Complex. A more diverse range in lithologies of clasts was found at the more proximal sites (U1418-U1421) relative to Site U1417, along with some suggestion of greater numbers of sedimentary clasts at Sites U1420 and U1421 (Fig. F22). The main clast lithologies are, in order of decreasing abundance, sedimentary, igneous, and metamorphic. Clasts may be derived from the St. Elias Mountains, the Chugach Mountains, and/or coastal ranges located along the southern coast of Alaska and northwestern British Columbia. Volcanic ash and volcaniclastic-bearing sand at all sites indicates that the locations were proximal enough to either the Aleutian or Wrangell volcanic belts to have periodic influxes of pyroclastic detritus.

Paleoproductivity and paleoclimate indicators

One of the primary objectives of Expedition 341 was to understand the dynamics of productivity and circulation of surface and intermediate water masses in the northeast Pacific and their role in the global carbon cycle. Addressing this objective requires retrieval of well-preserved microfossils suitable for isotope geochemistry and faunal and floral studies. All Expedition 341 sites contained variable preservation of biosiliceous and calcareous microfossils that indicate dynamic water column productivity and/or variable sediment geochemistry. Previous work in the southern Gulf of Alaska at Site 887 (Barron et al., 1995; Rea and Snoeckx, 1995) showed that a robust Neogene biosiliceous biostratigraphy was achievable and that opal and calcareous accumulation rates varied at submillion-year timescales. At Expedition 341 sites, higher sediment accumulation rates of silt-clay–rich terrigenous sediment preserved both calcareous and siliceous groups, allowing us to examine this Neogene variability at higher biostratigraphic temporal resolution.

At deeper water Sites U1417 and U1418, both calcareous and siliceous microfossil groups were observed and vary from poor to good in preservation. In the middle to late Pleistocene, both microfossil groups are present in the upper 200 m CCSF-B at Site U1417 and the occurrence of diverse siliceous and calcareous assemblages are observed in the upper 200 m CCSF-B at Site U1418. Calcareous microfossils were present in variable abundance and degree of preservation through the Pleistocene at Site U1417, after which sediments became too indurated for separation. Biosiliceous microfossil abundance and preservation at Site U1417 varies in time and is well preserved in the middle to late Pleistocene, Pliocene, and late Miocene but absent in the early Pleistocene. At the more proximal Site U1418, calcareous microfossils are consistently preserved between 200 and 600 m CCSF-B, although numerous intervals are barren of calcareous microfossils. Below 600 m CCSF-B, sediments are mostly barren of siliceous microfossils, although the more consistent occurrence of planktonic and benthic foraminifers suggests that accumulation of calcareous taxa persisted. At shelf and slope sites, microfossil abundance and preservation varies depending on skeletal composition. Calcareous microfossils (planktonic and benthic foraminifers and calcareous nannofossils) are well preserved and nearly continuously abundant at all sites, with only a few barren samples noted. However, siliceous microfossil preservation and abundance is less consistent, but it tends to be highest in distinct intervals of generally clast free mud.

At deepwater Sites U1417 and U1418, strong variations in environmentally sensitive planktonic foraminifers, radiolarians, and diatoms record the alternation of warming and cooling intervals during the Pleistocene. In addition, variations of bottom water oxygenation are suggested by species changes in the benthic foraminiferal fauna. At Site U1417, planktonic foraminifers and radiolarian taxa abundances indicate cooler conditions in the upper 250 m (within the Pleistocene) than are found deeper (pre-Pleistocene). Temperate radiolarian taxa are present throughout the record but have higher relative abundance in the Pliocene and Miocene. Rare occurrences of coastal diatoms, diatom-resting spores, and shallow-water benthic foraminifers indicate at least some component of the sediments is derived from shelf and neritic settings. At Site U1418, variations in diatom abundance and changes in species composition likely reflect changes in paleoproductivity, transport from shallow coastal waters, and intermittent sea ice influence.

At shelf and slope Sites U1419–U1421, cold-water diatom and radiolarian species dominate the assemblage when present. Specific radiolarian taxa were also encountered at shelf/slope sites that previously have been observed occurring in association with high terrigeneous sediment supply. In spite of low total abundances, sea ice related species and warm and temperate diatom species are intermittently present downhole. The planktonic foraminiferal assemblages are also dominated by species associated with cold-water conditions. Changes in the composition of the benthic foraminiferal fauna at Site U1420 suggest changes in water depth ranging from inner neritic to upper bathyal, and the observation of inner neritic benthic species at the bottom of Sites U1419 and U1420 and throughout Site U1421 probably reflects downslope reworking. Variations of bottom water oxygenation are suggested by species changes in benthic foraminiferal fauna.

Given the high accumulation rates, these sediments will allow for high-resolution studies of paleoproductivity proxies that are dependent upon siliceous microfossils (diatoms and radiolarians), foraminiferal assemblages, and biomarkers. Especially high sedimentation rates during the late Pleistocene hold promise for future studies related to suborbital-scale variability in paleoproductivity. Another valuable aspect of the recovered Expedition 341 sediment is the possibility to study changes in bottom water oxygenation and paleoproductivity across the drilled depth transect that spans from the more productive shelf-slope break to the less productive HNLC subarctic gyre.

Paleomagnetism and magnetic stratigraphy

Expedition 341 recovered an exceptional paleomagnetic record at five sites that spans a range of depositional environments with varying biogenic and terrigenous input. At all sites, the natural remanent magnetization (NRM) intensities of recovered materials were generally strong before (>10^–1 A/m) and after (10^–2 to 10^–3 A/m) alternating field (AF) demagnetization, with only some intervals at Site U1419 having significantly weaker intensities (10^–4 A/m) after demagnetization. In general, well-resolved paleomagnetic records could be extracted from the recovered material at each site, adding a key chronostratigraphic tool to interpret the sedimentary record.

At Site U1417, we recovered a long, continuous record with outstanding magnetic properties. The upper 150 m CCSF-B contains a continuous record of interpretable polarity with well-defined magnetic transitions. Initial results suggest that normalized remanence should primarily reflect the strength of the geomagnetic field and therefore is applicable for paleointensity studies and paleointensity-assisted chronological development for at least the last 1.2 m.y. Polarity transitions (Brunhes/Matuyama, upper and lower Jaramillo) show substantial structure with a temporal resolution comparable to the best records from the North Atlantic. Several Matuyama polarity excursions are well preserved, particularly an event tentatively interpreted as the Punaruu polarity excursion (~1092 ka; e.g., Channell et al., 2008), where two complete and two partial copies are preserved. The extended core barrel (XCB)/rotary core barrel (RCB) part of the Site U1417 record also obtained well-resolved polarity transitions, including what appears to be the Gauss/Matuyama polarity transition (recovered in Holes U1417B, U1417D, and U1417E). Older polarity transitions are also well resolved, though further postcruise analyses are required for unambiguous interpretation.

At Site U1418, well-resolved Brunhes/Matuyama and upper and lower Jaramillo polarity transitions are observed at exceptional depths below seafloor (>600 m CCSF-B), attesting to the rapid middle to late Pleistocene sedimentation rates (~1 m/k.y.) at this location. The RCB-recovered material appears to preserve a high-fidelity paleomagnetic record that is likely to provide long records of paleosecular variation and potentially RPI time series. The upper and lower Jaramillo polarity transitions were recovered in extended records at exceptionally high temporal resolution that will build upon the records of these same transitions from multiple holes at Site U1417 that are preserved at lower but still considered high resolution.

The slope and shelf Sites U1419–U1421 contain strata with normal polarity interpreted as being entirely within the Brunhes Chron extending in some cases below 1000 m CCSF-B, indicating exceptionally high sedimentation rates (1–2 m/k.y.). At all sites, variations in magnetic properties are likely to prove valuable for ascertaining sediment source changes.

A high-latitude continental margin geochemical depth-time transect

The completion of a high-latitude continental margin depth transect provides a unique opportunity to examine a time-varying Neogene sedimentary geochemical record that spans from a deepwater gyre to a continental shelf, examining the relative role of glacial dynamics, surface productivity, and volcanigenic input on the geochemical flux to the seafloor and diagenetic processes within the sediments. A major result of Expedition 341 is the documentation of substantially different geochemical environments between the deeper water fan sites and the more proximal slope and shelf sites. In general, all sites have low organic carbon (<1 wt%) and carbonate (<5 wt%) contents, typical of a high-latitude oligotrophic water column with periods of intermittent productivity and/or terrigenous sedimentation. However, organic matter burial rates at proximal sites are much higher because of higher bulk sediment accumulation rates. In terms of organic matter preservation and degradation, organic matter remineralization increases significantly from distal to proximal drill sites, as reflected in increasing ammonium, alkalinity, and methane concentrations but decreasing sulfate penetration depths. Evidence of authigenic carbonate formation is observed at all sites, mostly in relation to sulfate–methane transitions and sometimes resulting in carbonate-cemented sand or silt layers, especially at the deepwater sites. In addition, at the deepwater sites dissolution of volcanic ash and/or biogenic opal likely leads to the liberation of dissolved calcium and silica, whereas the possible formation of authigenic clay minerals results in the removal of magnesium and potassium from the pore waters. Although pore water profiles above the sulfate/methane interface are generally associated to different organic matter degradation pathways, the pore water composition in the methanogenic zone is mostly controlled by desorption from, and adsorption to, clay minerals. Chlorinity and salinity at the distal sites decrease with burial depth in the sediments, most likely due to clay mineral dehydration. The slope and shelf have greatly reduced pore water chlorinity and salinity values relative to seawater at a few tens of meters below the seafloor and at subsurface minima at several hundred meters depth. The source of the freshwater is unknown at this time, but it likely related to freshwater infiltration to coastal sediments from glacial sources and its storage within the shelf and slope deposits.

Physical properties

Collection of Whole Round Multisensor Logger (WRMSL) physical properties data allows us to correlate lithologic variability between holes, aiding in the generation of complete stratigraphic splices at Sites U1417–U1419. The whole-round and discrete physical properties data also allow us to tentatively correlate intervals between downhole log data and recovered cores at Sites U1417, U1418, and U1421. Changes in magnetic susceptibility (MS) and bulk density follow large-scale lithologic variability; intervals in which those parameters are high generally reflect coarse-grained facies, whereas low MS and density indicate a shift to more biogenic lithofacies. Changes in porosity and shear strength within and between sites indicate that the lithostratigraphic units recovered on the shelf and slope are largely underconsolidated and that sediment accumulation rates are high. Additionally, consolidation curves (porosity vs. depth) show that distal Sites U1417 and U1418 exhibit a well-constrained exponential relationship, but there is increased variance in the shallow-water sites with no apparent overall trend (Fig. F23).

Physical properties data were also used to integrate lithostratigraphic interpretation with seismic profiles at each site. For Sites U1417, U1418, U1420, and U1421, both core and logging data were available for integration. Spectral and total gamma ray and MS data from logging have been correlated to lithostratigraphic units, in particular to distinguish diamict from biosiliceous intervals. Where velocities are well constrained, lithostratigraphic features often correlated with major seismic units defined by changes in acoustic facies or a truncation surface as seen in seismic reflection profiles. Changes in physical properties at these sites seem to be related to lithologic indicators of proximity to glacial sediment sources.

Sedimentation and seismic facies

Expedition 341 provided new constraints on the ages of the major seismic units on the Surveyor Fan (Reece et al., 2011), which in turn significantly alter our interpretation of the geologic evolution of the St. Elias orogen and continental margin. Sequence III is characterized by smooth, continuous reflectors and limited seismic transparency and is mappable throughout the Surveyor Fan. At Site U1417, the Sequence II/III boundary (~5.8 ms TWT) is defined by high-amplitude variation (Fig. F24). Based on a preliminary shipboard traveltime-depth relationship, Sequence III corresponds to dark gray mud with thin beds of volcanic ash (lithostratigraphic Subunit IA) and gray mud with thin beds of volcanic ash and diatom ooze (Subunit IB) (Fig. F20). Magnetostratigraphy places this sequence boundary near the top of the Jaramillo Subchronzone (C1r.1n ~ 0.988 Ma). At Site U1418, Sequence III is underlain by the Surveyor MTD (Reece et al., 2012) (Fig. F25), and a transition was observed in the cores from laminated mud, diamict, and lonestones (lithostratigraphic Unit II) to laminated and bioturbated mud, normal faulting, and beds of clast-rich muddy diamict containing rip-up clasts (Unit III) to intrastratal contorted sediments (Unit IV). Within Sequence III at Site U1418, a subsequent seismic boundary is observed at the top of the youngest aggradational seismic package that comprises the northwest flank of the Bering Channel and corresponds both to a change in seismic amplitude and to the boundary between lithostratigraphic Unit I (mud with silt and sand beds and lonestones) and Unit II.

Smooth, continuous reflectors that are semitransparent in seismic data characterize Sequence II throughout the Surveyor Fan. Based on our shipboard correlations, this sequence corresponds to several lithostratigraphic units. These include gray mud with few centimeter-thick interbeds of fine sand and coarse silt (lithostratigraphic Unit II); thick beds of clast-poor and clast-rich diamict, both indicative of IRD, interbedded with gray mud (Unit III); and highly bioturbated gray mud with diatom-bearing intervals (Unit IV) (Fig. F20). The lithostratigraphic Unit III/IV boundary, which marks the first arrival of IRD material to Site U1417, tentatively correlates to the lower part of Sequence II and to the Matuyama/Gauss boundary at 2.56 Ma.

Sequence I, which lies beneath the regionally mappable Sequence I/II boundary (Figs. F15, F24), includes the Pliocene and Miocene sections recovered during Expedition 341. These intervals contain a series of lithologic changes within lithostratigraphic Unit V, including mud with sandy diamict, interbedded silt and sand, and diatom ooze. Some diamict displays evidence of terrigenous input, including wood fragments and coal, indicative of Miocene–Pliocene tectonic exhumation and subsequent transport to the deep basin.

Sites U1420 and U1421 are linked by mappable seismic sequences and unconformities associated with the Bering Trough (Fig. F9). Seismic sequences above angular unconformity Horizon H1 are acoustically semitransparent and semichaotic packages that, where recovered (lithostratigraphic Unit I at Site U1420), correspond to massive clast-rich diamict. Lithostratigraphic Unit II at Site U1420 consists primarily of washed pebbles and drilled clasts of varying lithologies. Using velocities from the sonic log recorded in Hole U1420A for preliminary shipboard TWT-depth conversions, we appear to have logged across the Horizon H1 unconformity and into the uppermost aggradational seismic packages that are truncated by Horizon H1. Core recovery increased at the top of lithostratigraphic Unit III at Site U1420, and the sediments within this unit consist primarily of clast-poor and clast-rich diamict with occasional intervals of mud and bioturbation (Fig. F20), demonstrating the dominance of glacially related seismic and sedimentary facies. Growth strata marking the cessation of local thrusting at Site U1420 were likely not reached. Based on the sediment interval recovered at Site U1420, it appears likely that these growth strata are significantly younger than originally proposed (Worthington et al., 2010). At slope Site U1421 ~700 m of diamict, mud, laminated diatom ooze, and interbedded sand and mud were drilled and interpreted shipboard to represent trough-mouth fan deposits recording mass wasting, deposition from surface, and ice-rafted sediment transport. Given their location directly downslope of the Bering Trough mouth, the interpreted seismic sequences may distinguish individual thick glacial packages and thinner interstadial or interglacial periods (Fig. F9).

In summary, sediments recovered during Expedition 341 mark key transitions that can be mapped regionally using seismic reflection profiles. These transitions include the intensification of Northern Hemisphere glaciation (INHG) at 2.56 Ma at Site U1417, the MPT at Sites U1417 and U1418, and transitions in sedimentary and seismic facies within the middle Pleistocene at Sites U1418, U1420, and U1421 that need further study into their exact timing.

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