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doi:10.2204/iodp.proc.341.101.2014

Site summaries

Site U1417

Setting and objectives

Site U1417 is located in the distal Surveyor Fan in the Gulf of Alaska in 4200 m water depth (Fig. F6). DSDP Site 178 was drilled ~2 km from Site U1417 with ~40% recovery, most of which was Late Pleistocene in age (Fig. F14). Site U1417 is ~60 km from the Surveyor Channel, which delivers sediment to this site via overbank processes.

Site U1417 drilling targets were three major regional seismic boundaries and associated seismic sequences mapped by Reece et al. (2011) using reprocessed USGS, 2004 high-resolution, and 2008 crustal-scale seismic reflection data to correlate stratigraphic changes and fan morphology through time. Sequences I and II exhibit layered, laterally semicontinuous reflectors consistent with turbiditic deposition (Reece et al., 2011) (Fig. F15). Sequence III is thinly layered and contains reflectors that are laterally continuous, flatter, and smoother than those in the other sequences (Fig. F15). Stratal relationships at sequence boundaries are highly variable and greatly influenced by basement topography. Sequence II onlaps Sequence I in areas where Sequence I exhibits topography but is conformable in other locations. Sequence III downlaps Sequence II in parts of the distal fan where both sequences pinch out farther from the Surveyor Channel sediment source (Reece et al., 2011). At Site U1417, Sequences I–III appear to be conformable (Fig. F26).

Site U1417 lies at the intersection of two crustal-scale seismic profiles acquired during the 2011 R/V Langseth U.S. Extended Continental Shelf project for the U.N. Law of the Sea. The shallow primary target for the site is the Sequence III/II boundary from Reece et al. (2011), which has been mapped onto the 2011 profiles. This sequence boundary, which is mappable throughout the Surveyor Fan (Fig. F16), is proposed to represent an increase in sediment supply to the site as a result of an intensification of glaciation at the MPT. The deeper primary target is the Sequence II/I boundary (Reece et al., 2011), which was also mapped onto the 2011 data, and which is proposed to correspond to the onset of tidewater glaciation and the start of Surveyor Fan deposition in the late Miocene. Additional divisions of these sequences for the purpose of core-log-seismic integration are displayed in Figure F24.

Site U1417 was chosen to provide a sedimentary record of Neogene glacial and tectonic processes occurring in the adjacent orogen (Fig. F2). All of Sequences II and III are estimated to correspond to this time period, which would include early tectonic uplift of the St. Elias orogen within the Miocene; preglacial to glacial conditions from the late Miocene to Pleistocene, including the initiation of the NCIS and the INHG; and the potential intensification of glacial extent following the MPT. These sequences may also contain a provenance record reflecting the locus of sediment created during the exhumation and uplift of the St. Elias Mountains.

By drilling into Sequence I, Site U1417 allows us to examine deposition prior to the onset of the sedimentation associated with establishment of the Surveyor Channel. The channel is proposed to have formed in response to increased sediment supply from the adjacent coastal mountains, the timing of which may correspond to the initiation of the NCIS (Reece et al., 2011). Sequence I includes several seismic facies (Fig. F24) that may yield clues to the changes in depositional setting from the late Oligocene (approximate age of basement) through the Miocene, as this site tectonically migrated northwestward from near modern-day British Columbia toward southern Alaska. These sediments may give insight into the initial uplift of the St. Elias orogen, which provided the enhanced elevation needed for later glaciation.

Results

At Site U1417, Holes U1417A, U1417B, U1417C, U1417D, and U1417E were drilled to total depths of 168.0, 358.8, 225.0, 470, and 709.5 m CSF-A, respectively (Fig. F27). In Hole U1417A, only the 9.5 m long (full) advanced piston corer (APC) system was used. In Holes U1417B–U1417D, both the full and the half-length (4.7 m long) APC coring systems were used to refusal. Following refusal of the half-length APC system, the XCB system was used in Hole U1417B to 358.8 m CSF-A and in Hole U1417D to 470.3 m CSF-A. In Hole U1417E, the RCB system was used to interval core from 264 to 302.2 m CSF-A and continuously core from 399.0 to 709.5 m CSF-A. A total of 198 cores were recovered for the site. A total of 811.18 m of core over an 836.5 m interval were recovered using the APC system (97% recovery). The cored interval with the XCB system was 381.8 m with a recovery of 140.77 m (36.9%). The cored interval with the RCB system was 348.7 m with a recovery of 146.92 m (42.1%). The overall recovery percentage for Site U1417 was 70.1% (Fig. F27). Drilling and XCB coring to basement in Hole U1417D was not possible because of repeated mechanical cutting shoe failure during XCB operations, necessitating a separate RCB hole. As a result of time lost during XCB coring in Hole U1417D, the decision was made to limit RCB coring to the upper portion of the deepest seismic unit only and not core all the way to basement.

Real-time stratigraphic correlation at Site U1417 was achievable because of the presence of high-amplitude signals in the physical properties data. The depths of the cores acquired from each hole (in meters CSF-A) cannot be directly correlated without the direct comparison of physical properties data. Thus, a composite depth scale was constructed for Site U1417 from 0.0 to 750.82 m core composite depth below seafloor (CCSF-A) with data from all holes and including core expansion. A splice was chosen of one complete and continuous interval from the mudline to 220.4 m CCSF-D. Deeper than 220.4 m CCSF-D, intervals of correlation were found between holes but were not considered sufficiently continuous to warrant development of a deeper “floating” shipboard splice. To the extent possible, the splice was constructed from Holes U1417C and U1417D because Hole U1417A was sampled at sea and the shallowest 10 cores from Hole U1417B were recovered during heavy weather, when ship heave compromised the quality of many of the cores. The composite depth scale (CCSF-A) and the splice scale (CCSF-D) are based primarily on the stratigraphic correlation of the WRMSL MS and gamma ray attenuation (GRA) density, natural gamma radiation (NGR), split-core color reflectance data, and digital line-scan images. Of these variables, MS and GRA density offered the most reliable tools for correlation at Site U1417; the other variables served primarily as verification data. An additional depth scale (CCSF-B; method: scaled by factor) was created to compress and shift the correlated cores into the correct total drilled interval. This correction was done by applying the relationship between correlated depth and drilled depth using the affine table. Where appropriate, results are reported in this depth-corrected composite depth scale CCSF-B).

The sediment recovered at Site U1417 contains 17 lithofacies that are identified based on variations in the amount of mud, silt, and sand; the style of interbedding of lithologies; and abundances of ash, lonestones, diamict, and biosiliceous material. Based on characteristic facies associations, five major lithostratigraphic units and 12 subunits are defined (Fig. F28). The sedimentary succession cored at Site U1417 extends from the Miocene to Holocene. Mud with varying amounts of biogenic and coarser facies is the dominant lithology observed throughout all Site U1417 holes. There are noticeable variations in the amount of biogenic components, ash, lonestones, diamict, and interbedded mud/sand and mud/silt lithologies. Contacts between lithostratigraphic units at Site U1417 are usually gradational.

Lithostratigraphic Unit I (Fig. F28) is denoted by dark gray mud with subordinate thin interbeds of volcanic ash, primarily of vitric (glass) framework grains and varying amounts of biosiliceous material. Dispersed granule- to pebble-size lonestones (outsized clasts) commonly occur through the unit. Subunit IA contains greenish gray intervals of diatom-bearing mud with a few local intervals of diatom ooze alternating with barren gray mud, whereas Subunit IB includes recurring intervals of diatom ooze ranging in thickness up to 10 cm and rare thin silt and sand beds. Lithostratigraphic Unit II contains gray to greenish gray mud with distinct thin interbeds of fine sand and coarse silt beds that commonly have sharp lower contacts and are massive to normally graded. Relative to Unit I, this unit has fewer lonestones and less frequent occurrences of volcanic ash and diatom-rich intervals. Lithostratigraphic Unit III contains muddy diamict interbedded with bioturbated gray mud. The diamict contains gravel-size subangular to subrounded clasts in a muddy matrix. Diamict intervals have gradational lower and sharp upper contacts. Clast abundance increases uphole in individual diamict beds. Lithostratigraphic Unit IV consists of dark gray to greenish gray well-indurated mud lacking lonestones that is commonly highly bioturbated with common Zoophycos trace fossils. Diatom-bearing intervals are common in the mud, and fine sand beds are rare. Lithostratigraphic Unit V, divided into 10 subunits, is composed of dark gray moderately bioturbated diatom-rich mud, often color-banded, with interbeds of diamict and sand and thick intervals of diatom ooze. Subunits are identified based on the relative abundance of diamict versus diatom ooze intervals. Poorly sorted diamict beds with abundant clasts have sharp lower contacts and gradational upper contacts. Diamict beds contain subrounded to rounded mud clasts, and some beds contain plant debris. Diamict beds have sharp contacts and are dominated by angular black granules of shale and possibly coal (~60% total organic carbon [TOC]). Carbonate-cemented siltstone and sandstone beds are occasionally recovered. Bioturbation in the subunits of diatom ooze is absent to heavy and includes Zoophycos trace fossils.

Overall at Site U1417, there is a strong link between lithofacies types and physical properties, especially MS (Fig. F29). This relationship allows us to use physical properties as a predicative tool for lithofacies identification. Mineralogical characterization from X-ray diffraction (XRD) analysis indicates an overall similar bulk composition for the entire stratigraphic section. Composition from smear slides supports the XRD analyses but is more variable on the bed-level scale.

The sedimentology of lithostratigraphic Units I–V is consistent with mud deposition from hemipelagic/pelagic fallout and overbank levee deposition by turbidity currents. Sediment reworking and redeposition could also be related to temporal variations in bottom current strength and direction. Lonestones are interpreted to be IRD. Thin interbeds of fine sand and coarse silt suggest deposition from sediment gravity flows. Biosiliceous-rich mud and diatom oozes may reflect enhanced primary productivity and/or low terrigenous sedimentation and/or improved biosilica preservation. Lithostratigraphic Unit V is divided into subunits to distinguish periods of higher biological productivity, better preservation, and/or reduced terrigenous supply (diatom ooze) from periods of higher flux of terrigenous sediment (mud) (Fig. F28). Interspersed through both types of lithofacies associations are sandy diamict and breccia deposits. The transport of clasts of coal and wood in intervals of coarse detritus to the distal fan are interpreted as an indication of increasing orogenesis along the southern margin of Alaska, implying a connection between onshore tectonics and the introduction of coarse siliciclastic detritus to the offshore deepwater depocenter. Lonestone composition indicates that the sources for most of the ice-rafted sediment accumulating at Site U1417 are the St. Elias Mountains and Chugach Mountains located along the southern coast of Alaska. There is an extensive record of volcanism in the upper 200 m of the core in the form of ash. The fairly even distribution of volcanic ash throughout the record suggests that the location was proximal enough to the Aleutian and Wrangell volcanic belts to have periodic influxes of pyroclastic detritus.

The biostratigraphy of radiolarians, diatoms, and planktonic and benthic foraminifers indicates that the sediment intervals recovered at Site U1417 are Late Pleistocene to Miocene in age (Fig. F30). The preservation and abundance of all fossil groups vary downhole. For the upper 200 m CCSF-B, both calcareous and siliceous microfossil groups are preserved and vary from poor to good in preservation. From 200 to 280 m CCSF-B, calcareous microfossils are more consistently observed and siliceous microfossils are nearly absent. Calcareous microfossils are rarely observed from 300 to 450 m CCSF-B because of problems with sediment disaggregation, and no samples were analyzed in Hole U1417E because of sediment induration. From 300 to 450 m CCSF-B and from 600 to 708 m CCSF-B, some siliceous microfossils are observed, and between 450 and 600 m CCSF-B, siliceous microfossils are largely absent. Diatom biozones are constrained from 0 to 200 m CCSF-B and indicate a Pleistocene age. Radiolarian datums are constrained from 0 to 250 m CCSF-B and include Pleistocene to late Pliocene fauna. A second zone of well-constrained ages based on diatoms and radiolarians occurs from 325 to 425 m CCSF-B and includes microfossils that are Pliocene in age. Deeper than 600 m CCSF-B, rare occurrences of datum species indicate a Miocene age. Planktonic foraminiferal coiling directions and subarctic radiolarian taxa abundances shallower than 250 m CCSF-B indicate cooler conditions within the Pleistocene than those found deeper in the site. Cool, 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 are derived from shelf to coastal environments.

Paleomagnetic investigations at Site U1417 involved analysis of archive halves from all five holes. NRM intensities at Site U1417 in APC-cored intervals are strong before and after AF demagnetization, and intensities show variability at both the meter scale and tens of meters scale, reflecting lithology and geomagnetic variability for the demagnetized intensity. A clear record of polarity is preserved, including apparently expanded polarity transitions. Deeper than the APC-recovered sequence, magnetic intensities have greater variability, reflecting lithologic changes and variable core quality and recovery of the XCB- and RCB-cored materials. A composite polarity log was correlated to the geomagnetic polarity timescale of Hilgen et al. (2012). Interpretation of polarity chrons and subchrons, based on inclination, can be made.

The Matuyama/Brunhes boundary and the upper Matuyama Chronozone containing the Jaramillo (1r.1n) Subchronozone are clearly identified. Two short intervals of normal polarity are observed underlying the Jaramillo Subchronozone, with the older of the two tentatively interpreted as the Cobb Mountain Subchronozone (Fig. F31). The Gauss (2An) to Matuyama (2r) polarity transition is clearly and reproducibly observed, and much of the Gauss is reproduced across holes. Deeper than this transition, however, variable polarity interpretations can be made for the base of the Gauss (2An) and the normal polarity subchrons of the Gilbert (2Ar). A tentative correlation to the oldest of these subchrons (Thvera; 3n.4n) is made. This interval is underlain by extended recovery of material with reversed polarity, consistent with the lower part of the Gilbert Chronozone. Deeper than 500 m CCSF-B, inclination data are derived from a single Hole (U1417E), and therefore caution must be employed with any polarity interpretation made. Using the smoothed and expanded inclination record, an interpretation is made that is generally consistent with the biostratigraphic datums to ~600 m CCSF-B (Fig. F32). Deeper than 600 m CCSF-B, the placement of polarity boundaries is equivocal. Postcruise analysis with discrete measurements may both improve upon and resolve these correlations.

All the available paleomagnetic and biostratigraphic age datums are integrated to construct minimum and maximum preliminary shipboard age models, which together span most of the uncertainty in the datums based on shipboard work (Fig. F32). Based on these initial age models, sedimentation rates increase through time, peaking at values of 112 ± 4 m/m.y. in the interval 0–2.5 Ma. Such a rise in sedimentation rates is consistent with the gradual northward tectonic drift of the site, as well as acceleration of sediment inputs from tectonic and glacial forcing.

The physical properties program for Site U1417 included routine runs on multisensor core loggers and discrete sampling. Whole-round cores were analyzed for GRA bulk density, MS, P-wave velocity, and NGR. Split cores were imaged, and color spectrometry, color reflectance, and MS data were collected. Discrete P-wave velocity and vane shear strength were analyzed, and discrete samples were taken for moisture, density, and porosity measurements from Holes U1417A, U1417D, and U1417E. The NGR data together with the MS and GRA density data were used to correlate the five holes drilled at Site U1417 and to define a composite record. Core logger–based bulk density and P-wave velocity recorded to ~220 m CCSF-B on average with depth from 1.5 to 2.0 g/cm³ and from ~1500 to 1600 m/s, respectively (Figs. F33, F34). Discrete values are similar for the overlapping depths and then show ranges from 2.0 to 2.1 g/cm³ and from ~1600 to >1800 m/s, respectively. Deeper than ~420 m CCSF-B, isolated elevated velocities (~2420–5700 m/s) are correlated to cemented intervals, whereas lower values (<1650 m/s) are associated with diatom oozes. NGR values gradually increase downhole between 0 and 220 m CCSF-B in Holes U1417A–U1417D, interrupted by a short-term decrease in counts between 220 and 360 m CCSF-B and partially returning to higher values deeper than 360 m CCSF-B. Low NGR counts deeper than 220 m CCSF-B may partly be attributed to the smaller diameter of recovered core and/or section voids during XCB coring. Core logger NGR and MS measurements are normalized using the GRA density measurement (Fig. F33). These normalizations reduce ~50% of the variance in the XCB/RCB sections, showing that core logger NGR and MS measurements are affected by core section recovery volume. However, the decrease in NGR observed between 225 and 360 m CCSF-B persists (less pronounced) deeper than 300 m CCSF-B, and we propose that the lower volume-normalized NGR values between 300 and 360 m CCSF-B correspond to a lithology change in the late Pliocene. Discrete moisture and density (MAD) samples indicate that this interval is a zone of low density/high porosity values. There is close agreement between bulk density derived from GRA and MAD. Shear strength increases with depth with a change in slope at ~40 m CCSF-B, with greater scatter in the data deeper than 100 m CCSF-B.

Routine headspace gas analyses were carried out on samples from Holes U1417A, U1417B, U1417D, and U1417E, and 196 samples were taken for analyses of carbonate, carbon, and nitrogen content. Furthermore, 78 interstitial water (IW) samples and 12 rhizon samples were taken for pore water characterization, and splits from the IW samples were analyzed onboard. Solid-phase geochemical results indicate that Site U1417 is a typical oligotrophic, deepwater, subarctic setting with low TOC (generally 0.1–0.6 wt%), total nitrogen (TN) (0–0.3 wt%), and carbonate (0–1.5 wt%) content. Deviations of these values occur in specific lithologies (e.g., diatom oozes, diamict, and cemented siltstones of lithostratigraphic Unit V). The strongest organic matter remineralization occurs in the upper ~40 m CCSF-B and at ~375 m CCSF-B (Fig. F35). Total sulfate depletion is reached around 200 m CCSF-B, possibly linked to authigenic carbonate formation consuming dissolved magnesium and alkalinity. Methane is by far the dominant hydrocarbon gas detected, and concentrations are generally very low. However, they vary by several orders of magnitude (maximum concentration = 5117 ppmv at ~500 m CCSF-B), and methane production is restricted to an interval between 420 and 650 m CCSF-B. An upcore decrease in methane concentration is not related to anaerobic methane oxidation at a sulfate–methane transition. However, a deep sulfate–methane transition exists at the bottom of the methanogenic zone around 650 m CCSF-B, with sulfate likely provided from a deep source of seawater at the basement/sediment contact. Chloride and sodium concentrations are elevated from 10 to 60 m CCSF-B, documenting relict glacial seawater. Barite is dissolved in the sulfate-depleted zone deeper than 200 m CCSF-B and possibly reprecipitates as authigenic barite upon contact with sulfate-containing pore water. Downcore calcium and lithium increases may indicate significant leaching of underlying oceanic basalt by seawater.

Downhole logging measurements in Hole U1417E were made to a total depth of 624 m wireline depth below seafloor (WSF) after completion of RCB coring in the hole. Four tool strings were deployed in the following order: the triple combo, the Formation MicroScanner (FMS)-sonic, the magnetic susceptibility sonde (MSS), and the Versatile Seismic Imager (VSI). The borehole is variable in diameter, ranging from the maximum extent of the logging tool calipers (18 inches) to as small as <5 inches (Fig. F36). Logging data were affected by the rugosity of the borehole wall, particularly in the interval between 87 and 305 m WSF. Despite the rugose borehole, adequate traveltimes were recorded at one depth station during the vertical seismic profile (VSP). Based on hole conditions and characteristic trends and features in the data, two distinct logging units are identified for Hole U1417E. Logging Unit 1 (from the base of the drill pipe to 305 m wireline matched depth below seafloor [WMSF], where the matching is a correlation between logging runs based on the gamma ray measurement on each tool string) is defined primarily by highly variable borehole diameter. Within this unit, gamma ray, MS, and compressional wave velocity (V_P) logs are the most robust to borehole conditions. Logging Unit 2 (305–624 m WMSF) is characterized by improved borehole conditions, and the quality of the logging data is higher throughout this unit. Subunit 2A (305–476 m WMSF) shows elevated gamma ray, density, and velocity values relative to the overlying unit. The MS log is more emergent and generally matches features observed in the gamma ray log. Subunit 2B (476–624 m WMSF) is distinguished by an initial rapid decrease in density, resistivity, and velocity with depth, followed by general increases. The MS log displays a lower signal in this subunit. Together, these data indicate that downhole logs are likely responding to variations in lithology.

Initial core-log-seismic integration was attempted following collection of downhole logging data. Sediment core observations, physical properties data, and downhole logging data were used to (1) evaluate how representative the recovered cores are relative to the portion of the sedimentary sequence that was logged, (2) evaluate the nature and extent of sediment not recovered in the XCB/RCB drilling process, and (3) examine whether observed sedimentary facies can be related to borehole and seismic reflection data. In general, downhole and whole-core MS data exhibit similar trends and scatter when compared over the same measured interval. Transitions between intervals of high and low MS in the downhole log data correspond to lithostratigraphic boundaries in the core (Fig. F37). We also observe an association between described occurrences of sand with relatively high downhole log MS values. Another primary observation is that the highest MS measured in the downhole logs between 350 and 440 m CCSF-B corresponds to the occurrence of muddy diamict in lithostratigraphic Subunit VA and at the Subunit VB–VC transition. In general, we observe multiple examples where volcaniclastic sands/silts correspond to increases in standardized downhole profiles of total gamma ray and K. Sand layers generally have high MS and low gamma ray counts. Overall, the logging data provide additional evidence for widespread distribution of muddy diamict in Unit V that are likely underestimated in the cored intervals.

Shipboard correlations of downhole logging data with core-based physical properties measurements focused on the depth interval from ~305 to 615 m CCSF-B/WMSF. This interval is where the borehole condition is best, and thus the logging data are of higher quality (Fig. F36). The gamma ray log shows good agreement with the core NGR to within a few meters; the latter of these measurements has been corrected for volume using GRA density. Bulk density data also show variable agreement between downhole logging, core logging, and discrete core samples. There is more scatter in the GRA from the core logger; however, the downhole density log values generally correspond to the higher end in the range of GRA values. Discrete MAD measurements overlap with the downhole density log, showing a similar range in values. Discrete P-wave velocity measurements on cores show good correspondence to the P-wave velocity log down to ~430 m CCSF-B/WMSF. At greater depth, the P-wave velocity log shows generally higher velocities than the discrete core data. MS data show reasonable agreement between the log and core measurements.

Seismic Lines MGL1109MSC01 (Fig. F34) and MGL1109MSC14 (Fig. F24), acquired in 2011 aboard the R/V Marcus Langseth, cross at Site U1417 (Fig. F26). The primary seismic sequences on each profile, Sequences I–III, are interpreted after Reece et al. (2011). In preparation for core-log-seismic integration, Sequence I was divided into three subsections: IA, IB, and IC. Subsection IC was further divided into intervals: IC1 and IC2. Two check shots at ~211 m WMSF correlate to the seismic data at 5.87 s TWT within Sequence II, allowing some confidence for integration shallower than this depth (Fig. F34). Sequence III is characterized by smooth, continuous reflectors and limited seismic transparency. At Site U1417, the Sequence II/III boundary (~5.8 ms TWT) is defined by high amplitude variation. Based on a preliminary traveltime-depth relationship, Sequence III corresponds with 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). Smooth, continuous reflectors that are semitransparent in the seismic profile characterize Sequence II. Based on our correlations, this sequence corresponds to gray mud with 1–5 cm thick interbeds of fine sand and coarse silt (Unit II), thick beds of muddy diamict interbedded with gray mud (Unit III), and highly bioturbated gray mud with diatom-bearing intervals (Unit IV). The Unit III/IV boundary correlates generally to the lower part of Sequence II, where a pair of medium-amplitude reflectors lies stratigraphically above the high-amplitude package that defines the Sequence II/I boundary.

The Sequence II/I boundary is located at the top of a prominent grouping of high-amplitude reflectors that are slightly more chaotic and discontinuous than those observed in Sequence II. The lithostratigraphic Unit IV/V boundary at ~350 m CCSF-B may correlate to ~6 s TWT, which is near the top of Sequence I (Fig. F34). This boundary is defined as the top of a series of thick, high-amplitude reflectors that comprise this boundary and may be related to the presence of cemented intervals that inhibited core recovery at this depth. Lithologically, the boundary between Units IV and V represents a change from to highly bioturbated gray mud with diatom-bearing intervals (Unit IV) to gray mud with sandy diamict, interbedded silt and sand, and diatom ooze (Unit V). Because of the lack of check shots deeper than ~211 m WMSF, precise correlation between Sequence I, Units IV and V, and logging Unit 2 will need to be undertaken postcruise; however, some comparisons between lithofacies, seismic facies, and log character can be discussed. In general, the increased velocity and density contrasts within logging Subunit 2A and the upper part of Subunit 2B likely correspond to the series of brighter reflectors that define Sequence IC and in turn may correlate with the various lithofacies of Unit IV to Subunit VC or VD. The boundary between logging Subunits 2A and 2B may be associated with a large negative amplitude reflector that separates seismic Subunits 1B and 1C at ~6.2 s TWT (Fig. F34). A way to check on correlations deeper in the borehole will be to use the basement depth of 780 m from Site 178, located ~2 km away, and create a pinned traveltime/depth boundary at the top of the basement during the creation of a synthetic seismogram.

Site U1418

Setting and objectives

Site U1418 is located at 3667 m water depth on a slightly elevated region of the proximal Surveyor Fan. It is located between a modern channel that feeds into the Aleutian Trench and a wide channel-like feature, termed the Bering Channel, that also likely terminated into the trench when active. These two channels appear to originate at the base of the slope seaward of the continental shelf break between Bering Trough and the Pamplona Zone (Fig. F38). This site also lies below the westward-flowing Alaska Current, a boundary current within the Alaska Gyre that commonly contains eddies and meanders (Stabeno et al., 2004). Seasonal high productivity is often associated with these eddies (Ladd et al., 2007). The site likely has been supplied with sediment from hemipelagic settling and sediment gravity flows through these adjacent channels, creating a thick (~1 km) seismically stratified deposit (Fig. F25). A large deposit of chaotic seismic facies interpreted as a MTD is found at ~5.8 s TWT (Fig. F25; Reece et al., submitted). The top of the MTD lies beneath a seismic reflector (Surveyor Fan Sequence II/III boundary; Fig. F15) that can be mapped to Site U1418 (Fig. F16) and is ~1.0–1.2 Ma in age based on magnetostratigraphy, suggesting that this depocenter may contain an expanded Middle–Late Pleistocene sedimentary sequence.

Drilling objectives at Site U1418 are to develop a high temporal resolution, proximal sedimentary record of Late Pleistocene glacial–interglacial dynamics, fan sedimentation and development, and paleoceanography. A primary objective of drilling at Site U1418 is to constrain the timing of glacial events on the Pacific side of the NCIS, testing its relation to dynamics of Northern and Southern Hemisphere ice sheets. Doing so would allow for the examination of the timing of ice rafting, sediment gravity flow processes, and proximal fan strata formation relative to glacial advance–retreat cycles, including the age of the seismically imaged MTD. The observation of an abandoned and buried Bering Channel shallower than the MTD (Fig. F25) suggests that it formed subsequent to the MTD, perhaps as a consequence of Middle–Late Pleistocene changes in glacigenic sediment delivery. An expanded Pleistocene record of glacigenic sediment deposition also allows for the documentation of the spatial and temporal behavior of the geomagnetic field (paleointensity and paleosecular variation) during this time period in an undersampled region of the globe.

Site U1418 lies at the crossing of seismic reflection Profiles GOA3202 and STEEP07, which reveal a seismically stratified depocenter overlying a unit with high-amplitude chaotic reflectors (Figs. F25, F39). Near-horizontal reflectors of varying intensity are mappable over tens of kilometers, implying deposition from suspension and/or overbank flow from turbidity currents. Higher amplitude reflectors may indicate periods of enhanced deposition of ice-rafted sediment and/or coarser sediment gravity flows that originated from melting of the Bering and other tidewater glaciers. Seismic units at this site were identified for Expedition 341 to guide core-log-seismic integration (Fig. F25). The uppermost, stratified seismic units are identified as subunits that are expected to be time-correlative with Sequence III at Site U1417, with the lowermost chaotic unit (seismic Unit II*) interpreted as a MTD (Reece et al., submitted). The lowermost seismic Subunit IIIA at Site U1418 is reflective in the higher resolution GI gun data on Line GOA3201. Based on interpretation of Line F-6-89-GA-26 (a 1989 seismic reflection line with a vertical resolution in between STEEP07 and GOA3201 collected by the USGS ~10 km northeast of Site U1418), these reflective facies may be correlative with deposition associated with the relict Bering Channel (Fig. F25; see Fig. F3 in the “Site U1418” chapter [Jaeger et al., 2014]). The Subunit IIIA/IIIB boundary is at a reflector (5.25 s TWT) interpreted to represent the cessation of sediment transport through the Bering Channel, possibly corresponding with the formation of the eastward extension of the Aleutian Trench (Fig. F38). The higher amplitude reflectors of Subunit IIIC correspond with erosion of the unit at its northern extent, likely by gravity flows moving downslope into the Aleutian Trench. Subunit IIIC consequently is expected to contain evidence of gravity flow deposition. These seismic unit boundaries are therefore primary targets while drilling to the base of Sequence III and the top of Sequence II*.

Results

At Site U1418, Holes U1418A, U1418B, U1418C, U1418D, U1418E, and U1418F were drilled to total depths of 209.9, 17.0, 230.7, 305.8, 181.6, and 948.7 m CSF-A, respectively (Fig. F40). In Holes U1418A–U1418D, both the full- (9.5 m long) and the half-length (4.7 m long) APC coring system were used to refusal. Only the full APC system was used in Hole U1418E, and the RCB system was used in Hole U1418F. Cores from Hole U1418B were collected to cover the sediment section that was disturbed in the first two cores from Hole U1418A. Following a drilled interval to 78 m CSF-A, full-length APC cores were collected from Hole U1418E to fill in gaps identified by the stratigraphic correlators.

Following refusal of the half-length APC system, the XCB system was used in Hole U1418D to 305.8 m CSF-A, after which the hole was terminated because of mechanical cutting shoe failure during XCB operations. In Hole U1418F, the RCB system was used to drill to 260.0 m CSF-A and to continuously core to 948.7 m CSF-A. A total of 185 cores were recovered for the site. A total of 819.08 m of core over an 810.0 m interval were recovered using the APC systems (101% recovery). The cored interval with the XCB system was 48.5 m with a recovery of 22.8 m of core (47%). The cored interval with the RCB system was 688.7 m with a recovery of 498.2 m of core (72%). Overall recovery for Site U1418 was 86% (Fig. F40).

Real-time stratigraphic correlation at Site U1418 was achievable because of the presence of strong signals in the physical properties data. A composite depth scale at Site U1418 was constructed from 0.0 to 948.7 m CCSF-A with data from all holes, and it includes core expansion. A splice was chosen that results in one complete and continuous interval from the mudline to 271.4 m CCSF-D. Deeper than 271.4 m CCSF-D, intervals of correlation were found between holes but were not considered sufficiently continuous to warrant development of a deeper “floating” shipboard splice. To the extent possible, the splice was constructed from Holes U1418C–U1418E because Hole U1418A was sampled at sea and Hole U1418B consisted of only two special-purpose cores for pore water sampling. The composite depth scale (CCSF-A) and the splice (CCSF-D) are based primarily on the stratigraphic correlation of WRMSL MS and GRA density, NGR, and color reflectance data from the Section Half Multisensor Logger. Of these variables, MS and GRA density offered the most reliable tools for correlation at Site U1418; the other variables served primarily as verification data. As with Site U1417, an additional depth scale (CCSF-B) was created to compress and shift the correlated cores to produce the correct total drilled interval. Where appropriate, results are reported in this depth-corrected composite depth scale.

The sediment recovered at Site U1418 contains 15 facies. The dominant facies are gray to dark greenish gray mud and interbedded mud and silt and account for >90% of the core recovered. The remaining minor facies, although volumetrically much less significant, are distinctive and allow us to organize the cores into lithostratigraphic units. Lithofacies include massive mud with lonestones, massive mud without lonestones, laminated mud, silt, interbedded silt and mud, very fine to coarse sand, muddy diamict, interbedded mud and diamict, diatom ooze, biosiliceous ooze, calcareous/carbonate bearing mud, volcanic ash, volcaniclastic mud and sand, rock, and intrastratal contorted mud and diamict. These facies reflect deposition from suspension fallout, sediment gravity flows/large-scale mass wasting, ice rafting, variation in organic productivity/preservation, and subaerial volcanic eruptions. Based on facies associations, four major lithostratigraphic units were defined (Units I–IV; Fig. F41). Unit II is further divided into Subunits IIA–IID based on the characteristics of diamict intervals. The contacts between lithostratigraphic units at Site U1418 are usually gradational.

Lithostratigraphic Unit I is dark gray to dark greenish gray mud with interbedded silt that alternates with intervals of up to 15.5 m thick color-banded dark gray mud. Bioturbation is mostly absent or slight, and diatom ooze and diatom-bearing to diatom-rich mud occurs within numerous cores. Each interval with diatoms has gradational contacts and greater bioturbation intensity, and black mottling is often present. Graded sand beds with sharp lower contacts occur infrequently. Ash beds and volcaniclastic-bearing to volcaniclastic-rich mud occur irregularly. Lonestones appear deeper than ~3 m CSF-A and are present throughout. Unit II is dominated by mud interbedded with intervals of clast-poor diamict that range in thickness from a few centimeters to >1 m. Mud with clast concentrations ranging from dispersed to abundant is also present. Unit II extends over 546 m CCSF-B at Site U1418 and is divided into four subunits based on the relative thickness and occurrence of the diamict facies within the mud facies. Subunits IIA and IIC are dark gray to dark greenish gray muddy clast-poor diamict interbedded with dark gray mud and/or mud with dispersed clasts. The mud is weakly laminated, and coarse sand, granules, and lonestones of variable composition occur within both the diamict and mud. Subunits IIB and IID are dark gray to dark greenish gray laminated mud that is interbedded with mud with dispersed clasts and clast-poor diamict. Some intervals are calcareous bearing, and volcanic ash occurs as burrow fills and in dispersed intervals of moderate to heavy bioturbation. Lonestones occur throughout. Unit III is composed of laminated and bioturbated dark gray mud with thinly bedded, very dark carbonate-cemented gray sandstone and siltstone that are minor but distinctive components. Clast-rich muddy diamict containing rip-up clasts occurs near the base of the unit. A small number of lonestones occurs throughout this unit. Microfossils and ash are also rare. Unit IV is a very dark greenish gray mud mixed with a very dark gray clast-rich muddy diamict. Both facies are characterized by soft-sediment deformation and intrastratal contortions. There is a range of soft-sediment deformation fabrics, and normal faults are common in this unit. Rare lonestones were identified.

Figure F41 shows the lithostratigraphic unit boundaries and core recovery at Site U1418. Unit I consists mainly of dark gray mud with thin silt and sand beds that is interpreted as originating from suspension settling through the water column and mud-rich sediment gravity flows originating at the shelf/slope. The lonestones in Unit I are interpreted to be IRD mostly deposited by icebergs calved from tidewater glaciers and/or from sea ice. The diatom intervals in Unit I might be related to increased biological productivity in the water column, and/or decreased input of terrigenous sediments. The strong influence of icebergs and/or sea ice on the deposition of facies within Unit II is indicated by the presence of IRD in the form of lonestones deposited in massive or laminated mud, as well as the accumulation of diamict. Rare intervals of moderate to heavy bioturbation may suggest a slower sediment accumulation rate than that of the laminated mud intervals. Thin-bedded diamict intervals bounded by thinly laminated packages of mud are similar to gravelly mud beds and laminated mud described from southeast Alaskan glacial fjords deposited by sea ice rafting and turbid meltwater plumes. The thin diamict beds may record the presence of sea ice during glacial periods. Unit III is similar to Unit II with interbedded laminated mud and massive/bioturbated mud being the two dominant facies. This unit additionally contains silt/sand beds and diamict that show evidence of sediment gravity flows. Despite the decrease in the number of lonestones and diamict beds relative to Unit II, their presence in this unit indicates that glacially derived and transported sediment was an important contributor during deposition. The cycles of interbedded massive and bioturbated mud (with dispersed clasts and few diamict beds) and laminated mud may point to periods characterized by a weaker glacimarine signal that alternate with periods of more intense ice rafting. The mixture of gray to dark gray mud and clast-rich muddy diamict characterized by soft-sediment deformation and intrastratal contortions of Unit IV is interpreted as the product of one or more mass transport events, which were interpreted from seismic reflection data (see “Setting and objectives”) (Reece, 2012). The presence of chlorite, mica, and ferromagnesian minerals in the bulk and sand samples in all units indicates the potential contribution from the Yakutat Terrane and/or a metamorphic/mafic source, perhaps the Chugach Metamorphic Complex. Site U1418 contains a more diverse range in lithologies of lonestones than Site U1417. The main lonestone lithologies are, in order of decreasing abundance, argillite, siltstone, basalt, sandstone, granitoid, and limestone. Metasedimentary lonestones may be derived from the St. Elias Mountains, Chugach Mountains, and/or ranges located along the southern coast of Alaska and northwestern British Columbia, but the combination of metagabbro, quartzite, rhyolite, and limestone suggests that the rocks of the Alexander Terrane may be considered as a possible source for some of the IRD. Rare volcanic ash and volcaniclastic-bearing sand at Site U1418 indicate that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have periodic influxes of pyroclastic detritus.

Based on the downcore occurrence of microfossils at Site U1418, two distinct intervals were identified. A rich siliceous and calcareous community was observed in the upper 200 m CCSF-B. Deeper than 200 m CCSF-B, calcareous microfossils are consistently preserved; however, the presence of siliceous microfossils is sporadic (Fig. F42). The rare occurrence of diatoms and radiolarians deeper than 200 m CCSF-B impeded identification of reliable biostratigraphic datums. The oldest datum encountered is the last occurrence of the planktonic foraminifer Neogloboquadrina inglei, which suggests that sediments around 600 m CCSF-B are older than 0.7 Ma (Fig. F43). This age is close to the Matuyama/Brunhes reversal found at ~660 m CCSF-B (Fig. F44).

Strong variations in environmentally sensitive planktonic foraminifers and radiolarians record the alternation of warming and cooling intervals during the last 1.2 m.y. (Fig. F42). In addition, variations of bottom water oxygenation are suggested by species changes in benthic foraminiferal fauna. Variations in diatom abundance and changes in species composition indicate changes in paleoproductivity, transport from shallow coastal waters, and a period of sea ice influence over Site U1418.

The NRM intensities of the APC-recovered (Holes U1418A–U1418C, Cores 341-U1418D-1H through 32H, and Hole U1418E), XCB-recovered (Cores 341-U1418D-33X through 37X), and RCB-recovered (Hole U1418F) materials were strong before (10^–1 A/m) and after (10^–2 to 10^–3A/m) demagnetization. Intensities show variability at both the meter and tens of meters scale. Inclinations of the RCB sections reveal an almost continuous sequence allowing correlation to the GPTS (Cande and Kent 1995; Hilgen et al., 2012) (Fig. F44). Even though the Matuyama/Brunhes polarity transition was not completely recovered, the transition from reversed polarity in Core 341-U1418F-43R to normal polarity in Core 41R is clear. Polarity transitions into and out of the Jaramillo (1r.1n) Subchronozone are clearly observed. Two short intervals of normal polarity are observed stratigraphically below the Jaramillo Subchronozone, with the older possibly indicating the Cobb Mountain Subchronozone (C1r.2n); this interpretation is consistent with the biostratigraphic datums. Shore-based analyses will allow significant refinement of these interpretations, especially within the APC-recovered sections. Focusing on the splice that contains the best recovered intervals and sampling within the most pristine central part of the core should result in a noticeably improved record. Additionally, discrete sampling through polarity transitions observed within the RCB-recovered interval would likely improve understanding of these features.

All available paleomagnetic and biostratigraphic age datums were integrated to construct minimum and maximum preliminary shipboard age models (Figs. F43, F45), which together span most of the uncertainty in the shipboard datums. All identified paleomagnetic datums (Matuyama/Brunhes boundary, top and base of the Jaramillo Subchron) observed in Hole U1418F are included in the shipboard minimum and maximum age models. Of the biostratigraphic constraints, the depth to the youngest observed datum (last occurrence of the radiolarian Lychnocanoma nipponica sakaii; 0.03 ± 0.03 Ma) is found in Hole U1418D. The shipboard minimum and maximum age models are calculated in increments of 0.2 m.y. between 0 and 1.2 Ma. Within these intervals, inferred sedimentation rates in the past 0.2 m.y. are 128 ± 23 cm/k.y. Over the interval 0.2–1.0 Ma, sedimentation rates are on average a bit lower, ~70 cm/k.y., but uncertainty ranges between 8 and 35 cm/k.y. when the calculation is made at 0.2 m.y. intervals. At ages ~1.0–1.2 Ma (all sediments shallower than the MTD), paleomagnetic constraints suggest even lower sedimentation rates of 45 ± 11 cm/k.y.

The physical properties program for Site U1418 included the same strategy for runs on the multisensor core loggers and sampling for discrete measurements as for Site U1417. Discrete P-wave velocity and vane shear strength were analyzed, and samples were taken for moisture, density, and porosity measurements from Holes U1418A, U1418B, U1418D, and U1418F. NGR data together with MS and GRA density data were used to correlate the six holes drilled at Site U1418 and to define a composite record. Whole-round GRA density averages ~1.96 g/cm³ in the APC cores and displays downhole cyclic variability on the order of ~0.4 g/cm³, abruptly decreasing at ~220 m CCSF-B, corresponding to the transition to XCB coring (Fig. F46). Whole-round GRA density from XCB/RCB cores averages ~1.9 g/cm³ but fluctuates with depth. After Gaussian smoothing of whole-round MS to accommodate for the differing response function of the instruments, we used this relationship to calculate the volume-corrected mass MS (χ). As at Site U1417, >50% of the variance in the uncorrected MS record can be attributed to reduced sediment volume. Mass MS averages ~50 cm³/g downhole (Fig. F47). High-amplitude cyclic variations between 60 and 120 cm³/g are present in the core shallower than ~220 m CCSF-B, transitioning to reduced scatter centered at ~40 cm³/g in the deeper sections. This corresponds with the approximate depth of APC refusal and may reflect a concurrent change in lithology and/or incomplete volumetric correction of the WRMSL volume MS data. A return to a higher MS >60 cm³/g deeper than 810 m CCSF-B may reflect the transition between lithostratigraphic Units III and IV. Compressional wave velocity was measured on the WRMSL P-wave logger (PWL) in Holes U1418A–U1418C at depths to ~200 m CSF-A at a resolution of 2.5 cm. Reasonable measurements could only be obtained for core depths shallower than ~200 m CSF-A because of the development of void spaces within the core liners after switching to XCB coring in Hole U1418D. Between ~110 and 180 m CSF-A, gas expansion prevented P-wave measurement using the PWL, as this sediment interval has high methane contents. PWL velocity values gradually increase downhole, closely following GRA densities, ranging from ~1450 m/s at the seafloor to ~1650 m/s at ~200 m CSF-A. PWL and discrete P-wave measurements using the P-wave caliper (PWC) were taken in Holes U1418A, U1418C, U1418D, and U1418F. Discrete measurements from Holes U1418A and U1418C overlapped PWL measurements deeper than ~200 m CCSF-B. PWC values appear systematically slower than the PWL measurements, but both follow similar downhole trends. PWC values show significant scatter from ~100 to 200 m CCSF-B, likely due to the presence of methane within the cores. Deeper than ~200 m CCSF-B, the range of values narrows and shows a slight increase with depth until ~560 m CCSF-B, where there is an ~20 m/s shift in the value range to ~1700 m/s. There are inflection points at ~700 m CCSF-B (~1750 m/s) and ~800 m CCSF-B (>1800 m/s), where velocities begin to increase at a faster rate, reaching ~2100 m/s at the bottom of the site at ~908 m CCSF-B. NGR values show downcore rhythmic fluctuations between 16 and 45 cps with mean and standard deviations of 33 and 3, respectively. High-frequency variations in NGR values are likely coupled with changes in clay contents and consequently parallel trends in GRA bulk density and P-wave velocity from the core logger measurements, particularly from 0 to 230 m CCSF-B in Holes U1418A and U1418C. A transition to lower volume-normalized NGR values stratigraphically below 810 m CCSF-B corresponds to lithostratigraphic Units III and IV. Shear strength indicates that sediments range from very soft (0–20 kPa) to very stiff (120–180 kPa). Generally, the rate of change of shear strength with depth is constant downhole. Values drop slightly off the trend between ~100 and ~130 m CCSF-B in the interval with high methane concentrations.

Routine headspace gas analyses were carried out on samples from Holes U1418A, U1418C, U1418D, and U1418F, and 117 samples were analyzed for carbonate, carbon, and nitrogen. A total of 105 interstitial water samples were taken for pore water characterization. Solid-phase geochemical results characterize Site U1418 as an oligotrophic, deepwater, subarctic setting. Although both TOC (0.2–1.0 wt%) and TN (0–0.1 wt%) contents are low, their respective accumulation rates are substantially higher than at Site U1417 because of much higher sedimentation rates at Site U1418. The carbonate contents are also low (0–5 wt%) but slightly higher than Site U1417. Chloride, sodium, bromide, and salinity are elevated in the uppermost 30 m CCSF-B, documenting relict glacial seawater (Fig. F48). Higher organic matter accumulation rates at Site U1418 are reflected in the more pronounced diagenetic processes. Strong organic matter remineralization occurs at ~30, ~70, and ~150 m CCSF-B, as indicated by the ammonium, alkalinity, and methane profiles (Fig. F48). Based on the composition of interstitial waters, Site U1418 can be broadly divided into two biogeochemical zones, one reaching from the core top to ~70 m CCSF-B (suboxic-anoxic diagenesis) and the other reaching from ~70 m CCSF-B to the bottom of the record (methanogenesis). Thus, Site U1418 documents a classic catabolic reaction sequence, including the reduction of manganese, iron, sulfate, and methanogenesis. Sulfate depletion is reached at ~76 m CCSF-B, and methane increases deeper than ~80 m CCSF-B (ranging from 5,000 to 63,000 ppmv), marking a distinct sulfate–methane transition zone. Geochemical variability within the expanded methanogenic zone is mostly driven by variable organic matter degradation intensities and by clay mineral adsorption-desorption processes. Elevated ammonium and alkalinity around 500–600 m CCSF-B indicate organic matter degradation by methanogenesis (Fig. F48), probably related to labile marine organic matter availability. Here, preferential adsorption of ammonium to clay mineral surfaces releases barium, lithium, and boron from the adsorption sites. Shallower and deeper than the high-ammonium zone, these cations readsorb to (partly authigenic) clay minerals.

Logging at Site U1418 was done in Hole U1418F and consisted of successful triple combo logs, without the porosity sonde, from 0 to 600 m WSF and FMS-sonic logs from 0 to 582 m WSF. The downhole logged interval in Hole U1418F is assigned to one logging unit (Fig. F46) because the character of the logs changes gradually downhole without major shifts. At the scale of this unit, the NGR signal ranges on average from 35 to 55 gAPI units, with the exception of anomalously low values corresponding to washed-out intervals. The natural radiation signal is generally dominated by potassium and thorium content, with minor uranium contribution. For the most part, the three radioactive elements trend similarly, suggesting that they are mainly responding to clay mineralogy or content. Density measurements do not show specific characteristics in the logged interval. Resistivity data show a slight decrease with depth over the logged interval, which is counter to the expected increasing trend with depth due to compaction; however, lower mean values deeper than 462 m WMSF may simply be a response to the wider borehole diameter in this interval. MS data do not trend noticably downhole, and velocity measurements show a generally increasing trend with depth.

Sediment core descriptions, core physical properties measurements, and downhole logging data obtained from Site U1418 were combined to examine the coherence between the different data sets and evaluate the completeness of the recovered sediment record. High and low MS measurements derived from the sediment core correlate with intervals of high and low MS values in the logging data (Fig. F47). At a finer scale (<10 m), we observe high MS in the logging data associated with diamict and sand layers, whereas low values occur in intervals with diatom ooze, mud with laminations, and bioturbated mud. With the exception of washed-out sections of the borehole, the natural gamma ray log shows reasonable agreement with the volume-corrected core NGR, with a similar range in values and similar features occurring within a few meters offset in depth (Fig. F46). The washouts shown in the caliper log generally correspond to lower gamma ray values in core logger NGR data, which cannot be affected by borehole size, whereas the downhole logging data are. Thus, these washouts appear partly controlled by lithology within the upper part of the borehole, where the dominant lithology is interbedded silt and mud (lithostratigraphic Unit I). Silt or sand beds are more likely to wash out during drilling, leading to thin intervals of enlarged borehole diameter. This interpretation is supported by the lower gamma ray values measured on cores from some of the washed-out intervals (Fig. F46), as would be expected for a transition from muddy to more silty/sandy lithologies. Correspondence was also noted between the lithostratigraphic units, the distribution of diatom ooze and mud with clasts, and downhole changes in spectral gamma ray and total gamma ray. The bulk density log (Fig. F46) is strongly affected by the irregular borehole diameter, giving density values close to water in washed-out intervals. The highest downhole density values, however, show reasonable correspondence with maximum density values in the range of the GRA density data and show good agreement with the trends in discrete MAD measurements. The P-wave velocity log indicates higher formation velocities than discrete P-wave measurements on core over the logged interval (Fig. F46). The separation in velocity estimations increases with depth downhole. The discrete measurements may be biased toward lower velocity matrix material, whereas the downhole log is integrating lower velocity sediment matrix and higher velocity clasts, which were found to be a notable component in all lithostratigraphic units described at this site. Alternatively, the presence of gas in the formation may also play a critical role. Even though gas would affect both core and downhole log measurements of velocity, gas expansion in cores may have caused cracking and the development of void spaces, leading to poor contact with the instrument transducers and resulting in anomalously low velocities.

For preliminary correlation between lithostratigraphic units, logging units, and seismic features, lithostratigraphic and logging unit boundaries were converted from the CCSF-B (for core data) or WMSF (for logging data) scales to TWT using the average velocities of each unit. Average P-wave velocity was derived from physical properties measurements, using data from the PWC shallower than ~257 m (CCSF-B) and the downhole sonic logs at depths less than ~548 m (WMSF/CCSF-B). Deeper than ~548 m WMSF/CCSF-B, we used values calculated from the linear trendline of the downhole sonic log, though detailed correlations will require postcruise research. Seismic Subunit IIIC features a prominent reflection at ~5.04 s TWT (Fig. F25), separating the seismically more transparent facies at the top of the unit from the continuous horizons underlying the reflector. Line GOA3202 resolves an additional high-amplitude package of approximately four reflectors at ~5.1 s TWT (Fig. F39). Lithologically, these high-amplitude features could be associated with thick beds of mud separated by silt-rich intervals observed within lithostratigraphic Unit I. The boundary between seismic Subunits IIIB and IIIC at ~5.26 s TWT is located at the top of the youngest aggradational package that comprises the northwest flank of the Bering Channel (Fig. F25). At Site U1418, this boundary marks a subtle downsection change from higher to lower amplitude horizons, which is clearly resolved on the high-resolution Profile GOA-3202 (Fig. F39). Based on our traveltime-depth conversions, this boundary coincides with the boundary between lithostratigraphic Units I and II at ~257 m CCSF-B. Within seismic Subunit IIIB, two high-amplitude packages are resolved on the GOA3202 profile at ~5.31 and ~5.41 s TWT (Fig. F39). Each of these packages corresponds to a subtle change in brightness within the unit on the regional seismic Profile STEEP07 (Fig. F25). The boundary between lithostratigraphic Subunits IIA and IIB (~335 m CCSF-B) correlates to a reflector that lies between these two high-amplitude packages. The seismic Subunit IIIB/IIIA boundary correlates with the boundary between logging Subunits 1D and 1E at ~500–510 m WMSF. Both the density and velocity log values decrease at this boundary (Fig. F39). At the top of seismic Subunit IIIA, starting at ~5.54 s TWT, we observe both an increase in amplitude and a shift into slightly more chaotic facies. Deeper than ~5.75 s TWT, the seismic reflections are less stratified and more heterogeneous. The strata here are no longer parallel, and internal truncations and lobate geometry indicate either a more energetic depositional environment or deformation. Lithologic boundaries separating lithostratigraphic Subunits IIB–IID and Unit III are all located within seismic Subunit IIIA. Seismic Unit II* is defined by chaotic seismic facies starting at ~5.88 s TWT (Fig. F39). Some internal structure can be observed within the upper ~100 ms of Unit II*. The transition out of this 100 ms interval into the underlying noncoherent facies may be equivalent to the lithostratigraphic Unit III/IV boundary and the transition into an MTD.

Site U1419

Setting and objectives

Site U1419 is located at 687 m water depth on a relatively flat bank on the continental slope above Khitrov Basin and Ridge (Fig. F38). We informally refer to the feature as Khitrov bank (KB). The site is within the influence of the near-surface Alaska Coastal Current (Stabeno et al., 2004; Weingartner et al., 2005) and at a water depth possibly influenced by deeply rooted surface eddies and North Pacific Intermediate Water.

The drilling objectives at the site sought to capitalize on good preservation of carbonate microfossils and associated geochronologic methods that can be used to develop a high temporal resolution, proximal sedimentary record of Late Pleistocene glacial dynamics and paleoceanography (Davies et al., 2011; Addison et al., 2012). A primary objective was to constrain the timing of glacial events of the Pacific side of the NCIS to test its relation to changes of the larger ice sheets of the Northern and Southern Hemispheres and other global-scale climate effects. An allied goal is to understand the role of North Pacific sea-surface temperatures as a possible control on the glacial system over Late Pleistocene time, with the potential for decadal-to-centennial resolution in the glaciated and laminated intervals. The proximity of Site U1419 to regions of seasonally high surface productivity (Ladd et al., 2007) offers an opportunity to assess the relative roles of productivity and intermediate water circulation on hypoxia in the northeast Pacific and their importance in the global carbon cycle. The potential for a highly resolved radiocarbon and oxygen isotopic chronology (Davies et al., 2011) permits documentation of the interrelationship between paleomagnetic intensity and secular variation in the Pacific sector in comparison with records from the Atlantic sector.

Coring results and high-resolution CHIRP and MCS data reveal a complex depositional setting. The drill site is ~30 km west of the of Bering Trough mouth that may have been the terminus of the Bering Glacier at the LGM (Carlson and Bruns, 1997; Berger et al., 2008a) (Fig. F38). The site was surveyed during Cruise EW0408, resulting in a jumbo piston core (~11.5 m long) that contains a record of late glacial (~17.5 k.y.) to Holocene hemipelagic and glacimarine sedimentation (Barron et al., 2009; Davies et al., 2011; Addison et al., 2012). Seismic reflection data reveal variations in seismic facies that likely reflect the time-varying input of glacigenic sediment interspersed with biogenic-rich, potentially laminated hemipelagic facies (Fig. F13A). Processed CHIRP images (Fig. F13B), coincident with MCS profiles, reveal that an upper acoustic transparent layer on the profile corresponds to the upper ~8 m of postglacial sediment in Core EW0408-85JC, which dates to younger than 14.7 ka (Davies et al., 2011). The strong reflections in the CHIRP line and near the sediment/water interface to ~0.03 s (~8–25 m) in MCS Line GOA3201 likely represent the glacimarine sediments associated with the local LGM deposited at ~15–30 ka, at an average sedimentation rate of ~120 cm/k.y. (although with extremely high rates for brief intervals; Davies et al., 2011) (Fig. F13). It is hypothesized that the underlying, less reflective layered sediments in the MCS profile represent finer grained interglacial or interstadial events when the Bering Glacier terminus retreated relative to the shelf break and ice rafting of coarse sediment was reduced or absent. In contrast, the highly reflective intervals likely indicate times when ice rafting was active and there were higher accumulation rates of coarser glacigenic sediment.

Seismic Units A–K (Fig. F49) were selected at Site U1419 based on seismic facies possibly reflecting changes in glacial proximity. Active faulting is imaged in high-resolution seismic Profile GOA3101, showing surface deformation indicative of significant amounts of extension or transtension (Fig. F13A) (Worthington et al., 2008).

Results

At Site U1419, Holes U1419A, U1419B, U1419C, U1419D, and U1419E were drilled to total depths of 193, 113, 107.1, 103.7, and 75.5 m CSF-A, respectively (Fig. F50). Core recovery and refusal depths were significantly affected by frequent lonestones and diamict. For Holes U1419A, U1419B, U1419C, U1419D, and U1419E, full-length (9.7 m) APC refusals were encountered at 98.4, 81.8, 78.7, 80.16, and 69.0 m drilling depth below seafloor (DSF), respectively; half-length (4.7 m) APC refusals were encountered at 118.5, 114.03, 108.6, 114.15, and 98.76 m DSF, respectively. XCB coring using the soft-formation cutting shoe was used only in Hole U1419A from 118.5 to 189.68 m DSF. A total of 101 cores were recovered at the site. A total of 473 m of core over a 517.9 m interval was recovered using the APC systems (91% recovery). The cored interval with the XCB system was 74.4 m with a recovery of 14.86 m of core (20%). APC shot depths in Holes U1419B–U1419E were carefully planned with the aid of the stratigraphic correlators, but poor recovery required five holes to attain essentially complete recovery to ~100 m CCSF-B.

The composite depth scale at Site U1419 was constructed from 0.0–205.79 m CCSF-A. The shipboard splice extends from the mudline to 112.10 m CCSF-D. Weather was calm and ship heave was negligible while coring Site U1419, but the relatively lonestone rich ice-proximal environment proved challenging for core recovery and for interhole correlation. Intervals of core disturbance, incomplete recovery, and use of the half-length APC coring system presented complications in the development of composite depths and a spliced record. In particular, we observed many instances of flow-in, generally (but not always) near the base of cores, and some fall-in of rocky debris in core tops. Some parts of the splice are unique in an individual hole and are not well verified. We urge caution in using the splice from Site U1419 until it can be verified with postcruise data. An additional depth scale (CCSF-B) was created to compress and shift the correlated cores to produce the correct total drilled interval. Where appropriate, results are reported in this depth-corrected composite depth scale.

The sediment recovered at Site U1419 contains 14 facies. The dominant facies are dark gray to dark greenish gray mud and diamict. They account for >95% of the core recovered. Specific lithofacies include massive mud with and without lonestones, silt, interbedded silt and mud, very fine to coarse sand, interbedded sand and mud, interbedded mud and diamict, clast-poor diamict, clast-rich diamict, diatom ooze, biosiliceous ooze and biosiliceous rich/bearing mud, calcareous/carbonate-bearing mud, volcanic ash, and volcaniclastic mud and sand. These facies are inferred to reflect deposition from suspension fallout, sediment gravity flows, ice rafting, variations in productivity/preservation of biogenic material, and subaerial volcanic eruptions.

The main lithologies of the diamict clasts and lonestones recovered at Site U1419 are, in order of decreasing abundance, siltstone, argillite, sandstone, basalt, granitoid, and chert. The petrology of lonestones is similar to that found in the St. Elias and Chugach Mountains located along the southern coast of Alaska. The rare volcanic ash and volcaniclastic-bearing sand at Site U1419 indicate that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have some influxes of pyroclastic detritus. Smear slides and XRD analysis indicate similar bulk mineralogies downhole, although there are some variations in relative XRD diffraction peak intensities that may indicate slight variations in mineral content.

Based on facies associations, two lithostratigraphic units were defined at Site U1419 (Fig. F51). The contact between these units is gradational. Unit I lithologies largely consist of olive-gray to dark greenish gray diatom ooze to diatom-rich mud, dark gray to dark greenish gray mud with lonestones, and clast-rich diamict. The clast content increases deeper than ~90 m CCSF-B, forming intervals of mud with abundant clasts. Subordinate lithologies in Unit I include dark gray interbedded sand and mud, thin sand beds with sharp lower contacts, interbedded silt and mud, and volcanic ash. Unit II is dark gray muddy clast-poor diamict interbedded with dark gray laminated mud and thin coarse sand beds. A few meter-thick intervals of diatom and bisosiliceous ooze also are observed. Detailed description of Unit II is limited by incomplete recovery deeper than 114 m CCSF-B.

Both lithostratigraphic units are interpreted as reflecting sedimentation in a dynamic setting primarily influenced by glacimarine processes with secondary intervals of biogenic accumulation. The difference between the units may reflect the relative weighting of these different styles of sedimentation. The lonestones in Unit I are interpreted to have been rafted by icebergs calved from tidewater glaciers. The presence of diamict suggests that icebergs contributed large quantities of debris, larger than sand size, to this site. Some sediment gravity flows are inferred from the presence of sand beds and intervals with interbedded sand and mud. These sand beds typically have sharp lower contacts and are normally graded. Other thin sand beds have less definite contacts and may be deposited from turbid meltwater plumes and/or sea ice. The biosiliceous-rich intervals more prevalent in Unit I might reflect either increased biological productivity and/or decreased dilution with terrigenous sediment. The frequent alternation of diatom ooze and diatom-rich sediments with intervals dominated by mud with lonestones and diamict suggests a system influenced by changes in climate and glacier dynamics.

The diamict facies most prominent in Unit II likely has a glacigenic origin. Gradational contacts between facies and the observation of clast-rich diamict interstratified with mud (including mud with diatoms) indicate fluctuating sediment supply typical of a glacimarine environment. Massive diamict beds may indicate periods of more intense iceberg rafting with a high flux of sand and coarser sediment. Intervals of interbedded mud and thin diamict beds suggest the possibility of sea ice rafting in addition to iceberg rafting. Sediment gravity flows are indicated by interbedded sand and mud and thicker sand beds. Three prominent intervals of diatom/biosiliceous ooze, interbedded with diamict in Unit II, roughly similar to modern sedimentation at the site, imply some intervals of reduced glacial sediment supply. The exact timing of the observed lithologic and compositional transitions requires the development of a detailed chronology at Site U1419.

Microfossil abundance and preservation at Site U1419 varies depending on skeletal composition. Calcareous microfossils (planktonic and benthic foraminifers) and nannofossils are well preserved and continuously abundant in Site U1419 cores (Fig. F52). Fourteen planktonic foraminifer species were encountered at Site U1419, with faunal assemblages dominated by polar to subpolar species. Within the interval from ~80 to 90 m CCSF-B, diatom resting spores and planktonic and benthic foraminifers have an abundance peak and the benthic foraminifer Eubuliminella exilis dominates the assemblage, suggesting a strong environmental change in the water column and at the seafloor during the time represented by that interval.

Siliceous microfossil preservation and abundance is less consistent. Radiolarian fauna are moderately preserved in the upper 115 m CCSF-B, and their abundances fluctuate from rare to abundant (Fig. F52). Generally cold-water radiolarian taxa dominate the assemblages. Deeper than 115 m CCSF-B, most samples examined were barren of radiolarians. The diatom preservation and abundance trends are similar, but their preservation and abundance are lower than the radiolarians in the upper 115 m CCSF-B. Cold-water diatom species dominate the assemblage at Site U1419 and are most abundant in the uppermost 10 m CCSF-B and in the interval from 75 to 90 m CCSF-B (Fig. F53).

Sea ice–related warm and temperate diatom species are often noted when diatoms are observed, but these species are a relatively minor component of the assemblage and range between present and rare. The only diatom biozone recognized at Site U1419 is NPD 12 (present–[0.3 ± 0.1] Ma). The last occurrence of Proboscia curvirostris (Jousé) Jordan et Priddle (D120; 0.3 ± 0.1 Ma) is not observed. The last occurrence datum of radiolarian Stylocontharium acquilonium Hays (0.4 Ma) is also not encountered.

NRM intensities of the APC (Holes U1419A–U1419E) and XCB (Cores 341-U1419A-21X through 29X) cores were strong before AF demagnetization (10^–1 A/m) but were significantly weaker after demagnetization (10^–2 to 10^–4 A/m). Intensities vary at both the meter and decameter scales, with discrete intervals of very low intensities (Fig. F54) corresponding to intervals of biosiliceous ooze and diatom-rich mud. Because inclinations indicate generally normal polarity, it is thought that the recovered sediment is exclusively within the Brunhes Chronozone and younger than 0.781 Ma (Cande and Kent, 1995; Hilgen et al., 2012).

Normal geomagnetic polarity in all cores implies that all sediments recovered are younger than 781 ka. The uppermost part of the sequence can be correlated with site survey Core EW0408-85JC, a radiocarbon-dated piston core from the same site (Davies et al., 2011), based on MS. The interval of relatively low MS from 0 to 6.4 m CCSF-A (0 to 5.5 m CCSF-B) at Site U1419 is assigned to Holocene and Late Pleistocene ages (0–15 ka), implying an average interglacial sedimentation rate of 40 cm/k.y. Oxygen isotopes of foraminifers picked at sea from the core catcher sample and analyzed immediately postcruise at Oregon State University, indicate that the maximum age of at the base of the site (205.79 m CCSF-A) is <60,000 years, implying an average glacial sedimentation rate of 380 cm/k.y.

Physical properties analyses included measurements on the multisensor core loggers and sampling for discrete measurements. Whole-round GRA density averages ~1.93 g/cm³ in the APC cores and displays downhole recurring variability on the order of ~0.2 g/cm³ (Fig. F55). Two intervals of relatively low GRA density (<1.8 g/cm³) occur between ~0 and 6 and between ~80 and 87 m CCSF-B. After corrections for variable sediment volume within the core liner, mass MS averages around ~68 cm³/g downhole (Fig. F55). An interval of high-amplitude variability between 55 and 90 cm³/g is found shallower than 80 m CCSF-B. Whole-round loop MS trends largely parallel GRA bulk density trends from ~0 to 6 and ~80 to 90 m CCSF-B, but a shift to the opposite trends at finer scales is observed from 6 to 80 and 90 to 120 m CCSF-B, suggesting fundamental changes in lithofacies between these intervals. Compressional wave velocity was measured on the WRMSL. A near-continuous record of velocities was captured in Holes U1419C–U1419E between ~5 and 25 m CCSF-B and showed velocities <1600 m/s. At depths greater than ~25 m CCSF-B, WRMSL PWL velocity measurements were compromised because of the high gas content of the recovered sediment.

Discrete P-wave velocity and vane shear strength were analyzed, and samples were taken for moisture, density, and porosity measurements from Hole U1419A. Void spaces within the unconsolidated sections, gas expansion, and gaps in core recovery result in only a limited characterization of P-wave velocity, although all discrete measurements at this site were taken within the dominant lithology of the recovered interval. Velocity values show no significant overall trend with depth, though a region of higher velocity is observed at ~120 m CCSF-B where values increase to ~1800–2100 m/s (Fig. F51).

NGR measurements show recurring downhole variations between 14 and 43 cps with mean and standard deviations of 32 and 4, respectively. Downcore variability in raw NGR values closely parallels changes in GRA bulk density. Two notable intervals of reduced raw NGR (<30 cps) occur between ~0 and 6 and between ~80 and 87 m CCSF-B, corresponding to the uppermost and lowermost parts of lithostratigraphic Unit I, respectively (Fig. F56). MAD discrete values correspond well to GRA bulk densities measured on the WRMSL for Hole U1419A. MAD densities increase downhole from ~1.5–1.6 g/cm³ at the seafloor to ~2.2–2.4 g/cm³ at ~120 m CCSF-B. The interval between ~120 and ~140 m CCSF-B was not recovered. Deeper than ~140 CSF-A, density increases from ~2.0–2.1 to 2.2 g/cm³ by ~180 m CCSF-B (Fig. F51). Porosity (percent pore space of wet sediment volume) measured on discrete samples generally decreases with depth. Between ~55 and 100 m CCSF-B, the values show more scatter than seen in grain density values in comparable depth intervals. Shear strength indicates that sediments range from very soft (0–20 kPa) to stiff (up to 79 kPa). Generally, shear strength increases with depth at a constant rate. An anomalous interval of elevated shear strength (~60–79 kPa) is present at ~115 m CCSF-B. This interval coincides with an increase in both P-wave velocity and density and a decrease in porosity (Fig. F51).

Routine headspace gas analyses were carried out on samples from Hole U1419A, and 20 samples were analyzed for carbonate, carbon, and nitrogen. A total of 22 interstitial water samples were taken for pore water characterization. TOC (0.4–1.0 wt%), TN (0–0.1 wt%), and carbonate (1.5–4.0 wt%) contents are within the range exhibited by Holocene to late glacial sediments at this location (Addison et al., 2012) (Fig. F57). Organic carbon/TN (C/N) ratios range between 10 and 22 (Fig. F57), consistent with a contribution from both marine and terrigenous organic matter and within the range exhibited by recent sediments in the area (Walinsky et al., 2009). Chloride, salinity, and sodium profiles document a significant freshening of pore waters with depth, related to gas hydrate dissociation or buried glacial meltwater. Biogeochemical processes at Site U1419 follow the catabolic sequence, with the exception of manganese reduction, which is apparently completed shallower than the depth of the uppermost pore water sample. Total sulfate depletion is reached at ~20 m CCSF-B, and methane production occurs deeper than 20 m (mostly 10,000–40,000 ppmv), indicating a sulfate–methane transition zone at ~20 m CCSF-B. Ammonium and alkalinity both exhibit broad maxima between ~20 and 90 m CCSF-B, suggesting that most organic matter degradation is occurring within the methanogenic zone. This process also seems to release organic matter–derived bromide, boron, and phosphate to the pore waters. At Site U1419, both ammonium and alkalinity show relatively high concentrations, indicating intense diagenesis likely driven by the input of more reactive organic matter, possibly reflecting the location of Site U1419 adjacent to the coastal Alaskan high-productivity zone.

Seismic Lines GOA3101 and GOA3102 cross Site U1419 (Fig. F13). At a gross scale, seismic character across both profiles changes from laterally continuous subhorizontal reflectors (seismic Units A–B) to generally more chaotic facies with intermittent semicontinuous units (seismic Units C–F) (Fig. F51). Unit A is defined by two high-amplitude, continuous reflectors between the seafloor and ~0.937 s TWT. This seismic unit correlates with a gray diatom ooze interval that comprises the top ~6 m CCSF-B of the core. This interval also displays relatively low MS, P-wave velocity, and density. Unit B (~0.937 to ~.0.997 s TWT) features conformable, subhorizontal, continuous reflectors. The corresponding interval of the cores (~5–55 m CCSF-B) consists of gray mud with lonestones. Some of the peaks in MS within this interval may be associated with occasional sand layers, whereas other peaks may be associated with individual clasts. The presence of diatomaceous material within this interval correlates with lows in MS at ~16 and ~60 m CCSF-B. Unit C (~0.997–1.045 s TWT) is defined by high-amplitude, semichaotic reflectors with some truncations. According to our depth conversion, the lower portion of Unit C correlates to an interval of low MS and density from ~80 to 90 m CCSF-B composed of diatom ooze and mud with lonestones. The boundary between seismic Units C and D correlates to the boundary between lithostratigraphic Units I and II, which is defined by a change from mud with lonestones (Unit I) to muddy clast-poor diamict interbedded with laminated mud and thin coarse sand beds (Unit II). Seismic Unit D (~1.045–1.095 s TWT) is composed of two packages of high-amplitude, semicontinuous reflectors. The lower portion of Unit D correlates with an interval of high density and velocity between ~113 and ~120 m CCSF-B, which consists of muddy clast-poor diamict that is interbedded with laminated mud and thin coarse sand beds (part of lithostratigraphic Unit II).

Site U1420

Setting and objectives

A fundamental hypothesis to be tested with results from Expedition 341 is that the St. Elias orogen has undergone perturbation that has markedly changed the patterns and rates of deformation and exhumation in the orogenic wedge. It is proposed that enhanced glacial erosion associated with the MPT and the establishment of highly erosive ice streams lead to substantial mass redistribution in the wedge, shutting down regions of active deformation and refocusing the deformation and exhumation patterns of the orogen (Berger et al., 2008a; Worthington et al., 2008, 2010; Chapman et al., 2008).

Site U1420 lies within the Bering Trough (Fig. F10B), a shelf-crossing trough likely formed by the Bering Glacier advancing across the shelf during glacial maxima (Carlson, 1989) at ~250 m current water depth. The site penetrates through an angular unconformity (Horizon H1; Fig. F9) at shallow subseafloor depths (180–200 mbsf expected) that has been suggested to be the first occurrence of grounded glaciers reaching as far as the modern shelf edge (Berger et al., 2008a). Underlying this angular unconformity are the waning folds of a portion of the Pamplona Zone, which is a fold-and-thrust belt that has accommodated some of the tectonic shortening caused by the underthrusting of the Yakutat Terrane beneath North America to form the St. Elias orogen (Worthington et al., 2010). Previous mapping suggests that the folding within the Pamplona Zone beneath the Bering Trough waned by the time of seismic Horizon H2 (Fig. F9) (Berger et al., 2008a; Worthington et al., 2010). Lack of significant deformation in the sequences stratigraphically above Horizon H2 indicates that the underlying faults were abandoned before the angular unconformity of Horizon H1 formed, possibly due to loading by sediments (Berger et al., 2008a; Worthington et al., 2010). The overall architecture of the continental margin is thus the product of coupled depositional and tectonic processes.

The drilling objective at the site was to address the hypothesis that the onset of ice streams has resulted in correspondingly high erosion rates that could markedly alter orogenesis (Berger et al., 2008a). Drilling targets are Horizons H1 and H2. Retrieval of sediments from the seafloor to Horizon H2 will provide age control, which is lacking for this location, thereby establishing sediment accumulation rates that can be used to test the hypothesis. Cored sediments can also provide provenance records that can be used to test the hypothesis that the locus of erosion is within the windward side of the orogen (Berger et al., 2008a).

Results

Cores 341-U1420A-1R through 106R (1020.8 m DSF) were drilled with the RCB. Recovery over the entire hole was problematic because of the presence of numerous rock clasts that continually jammed the core catchers and prevented core recovery. A total of 106 rotary cores were taken over a 1020.8 m interval with a total recovery of 139.91 m of core (14%) (Fig. F58). Because only a single hole was drilled/cored at Site U1420, no stratigraphic correlation was performed at this site. All results are presented on the CSF-A scale.

The limited sediment recovered at Site U1420 contains seven facies. Recovery from 58.2 to 448.5 m CSF-A was <10% but improved deeper than 448.5 m CSF-A, where several cores were collected with recoveries between 30% and 94%. Numerous drilled rocks and washed clasts were recovered in the hole without a supporting sediment matrix. The dominant facies are very dark gray to dark gray clast-rich and clast-poor diamict (Fig. F59). Additional facies include massive mud with and without lonestones, mud with diatoms/biosilica, calcareous/carbonate-bearing mud, and volcaniclastic mud and diamict. These facies are inferred to reflect deposition from suspension fallout, sediment gravity flows, ice rafting, and variable productivity/preservation of biogenic material.

The main lithologies of the drilled rocks, washed pebbles, clasts within the diamict, and rare lonestones contained in the sediment are, in order of decreasing abundance, sandstone, siltstone, basalt, and granitoids. The granitoid group includes intermediate and felsic intrusive rocks. Argillite, rhyolite, and metasandstone represent minor lithologies. The petrology of lonestones is similar to that found in the St. Elias and Chugach Mountains located along the southern coast of Alaska. The rare volcaniclastic-bearing mud at Site U1420 indicates that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have influxes of pyroclastic detritus. Smear slides and XRD indicate similar bulk mineralogies downhole, though there are some variations in relative XRD diffraction peak intensities, which may indicate slight variations in mineral content.

Based on characteristic facies associations, three lithostratigraphic units were defined, the contacts between which are not observed because of poor core recovery. Lithostratigraphic Unit I consists of very dark gray, muddy, clast-rich diamict interbedded with clast-poor diamict with angular to rounded clasts ranging in size from granule to pebble. The diamict beds are massive and have mainly silt and clay matrixes with some outsized sand grains. Recovery in Unit II ranged from 0% to 8%. The major lithology was not recovered, and cores mostly contain washed pebbles and cylindrical-shaped drilled rocks with abraded surfaces. Small amounts (<10 cm) of usually heavily disturbed sediment were occasionally recovered. Unit III consists of very dark gray clast-rich, muddy diamict and very dark gray to dark gray clast-poor, muddy diamict interbedded with very dark gray to dark greenish gray mud with or without clasts. Drilled rocks occasionally occur coincident with low core recovery.

These lithostratigraphic units are interpreted as reflecting sedimentation in a dynamic setting primarily influenced by glacimarine processes with intervals of biogenic deposition. Unit I diamict is interpreted as either deposited subglacially or by deposition from intense iceberg rafting along with mud from meltwater plumes. Both tentative interpretations for Unit I require additional sediment fabric analyses for verification. In Unit II, it is impossible to interpret the depositional environment of these isolated clasts without the context provided by the matrix and other “missing” finer grained lithologies. The cylindrical drilled rocks within Unit II are interpreted as being recovered from boulders within the formation; such boulders are observed in outcrops of the glacimarine Yakataga Formation.

The interbedded diamict and mud in Unit III is interpreted to represent fluctuations in the glacial grounding-line position relative to Site U1420, as well as variations in the supply of IRD and meltwater in a proglacial setting. The sources for clasts at Site U1420 are interpreted to be the St. Elias and Chugach Mountains located along the southern coast of Alaska.

Microfossil abundances are generally low at Site U1420 (Fig. F60). Diatoms and radiolarians are only observed in three intervals. Thus, age constraints are few, but they suggest an age younger than 0.7 Ma. Benthic and planktonic foraminifers are better preserved and occur throughout the record. Diatom occurrence is mostly low, and preservation is mostly poor. The diversity of the diatom community at Site U1420 is low and mainly consists of Pleistocene to Holocene species. Cold-water species are the most abundant, and coastal and benthic diatoms are observed. Radiolarians are mostly barren except for two intervals, and the fauna is mostly marked by the presence of cold shallow-water radiolarians, but deeper than ~750 m CSF-A, the assemblage is marked by relatively high abundances of deepwater species. Planktonic foraminifers are present, sometimes in high abundance, and the preservation is generally good. Planktonic foraminifers are mostly associated with cold-water conditions. Benthic foraminifers are present to abundant, and preservation varies between good and poor. Changes in the composition of the benthic foraminiferal fauna suggest changes in water depth ranging from inner neritic to upper bathyal.

The NRM intensities of the cores were strong before AF demagnetization (10^–2 ~ 10^–1 A/m), but were weaker after demagnetization (10^–2 to 10^–3 A/m). Because inclinations indicate generally normal polarity, it is thought that the recovered sediment is exclusively within the Brunhes Chronozone and younger than 0.781 Ma.

The biostratigraphy indicates that the cored sediments are in modern biozones. The recovered cores are all normal magnetic polarity. Because the Matuyama/Brunhes geomagnetic polarity boundary was not observed, we can conclude that the entire sedimentary sequence recovered at Site U1420 is younger than 0.781 Ma.

Physical properties analyses included measurements on the multisensor core loggers and sampling for discrete measurements. Whole-round GRA density averages ~1.8 g/cm³ in the RCB cores and displays downhole variability on the order of ~0.3 g/cm³ (Fig. F59). Densities measured with the WRMSL should be regarded as minimum estimates, as observed densities are likely influenced by the highly variable diameter of the recovered sediment in the cores. Mass MS averages ~54.3 cm³/g downhole, with several successions of variability between 25 and 70 cm³/g deeper than 550 m CSF-A that appear to increase deeper than ~940 m CSF-A, although limited core recovery hinders interpretation. Because of poor core recovery, only P-wave measurements using the PWC were taken from 0 to 449 m CSF-A. Deeper than ~449 m CSF-A, PWC values show no significant overall trend with depth and vary widely, sometimes within the same core section, likely because of the varying amounts and lithology of clasts in the diamict. Though values generally range from ~1500 to ~2000 m/s, occasional high values with velocities >2200 m/s are also observed (Fig. F61). Low-frequency variability in mass-normalized NGR activity is limited in the recovered cores, although an increase in activity deeper than 940 m CSF-A parallels changes in normalized MS (?) and may reflect a change in lithology.

MAD bulk density values range from 2.0 to 2.4 g/cm³ between ~40 and 440 m CSF-A in recovered intervals (Fig. F62). Deeper than ~450 m, MAD values vary from ~2.0–2.4 to ~2.1 g/cm³ with no obvious overall trend with depth. Grain density values consistently display wide scatter from ~2.7 to 3.0 g/cm³ and do not appear to correspond with lithology. Porosity measured on discrete samples generally decreases with depth, with some ~10–20 m thick intervals of wide scatter (~22–45 vol%) with higher values corresponding with muddy lithologies. Porosity is ~25–30 vol% in clast-rich diamict facies. Void ratio values mimic porosity with depth with typical values around 0.4. Figure F62 shows these density, porosity, and void ratio values in relation to lithologic facies. Only four shear strength measurements were obtained, all shallower than 40 m CSF-A. All measurements were taken in the dominant lithology of the recovered sediment, which were soft (~20 kPa).

Routine headspace gas analyses were carried out on 34 samples, 41 samples were analyzed for carbonate, carbon, and nitrogen, and 20 interstitial water samples were taken for pore water characterization. Because of the highly variable core recovery and the sandy to gravelly composition of the sediment, the sampling resolution was overall low and irregular (Fig. F63), resulting in discontinuous profiles of chemical parameters that should be interpreted with caution.

TOC content at Site U1420 ranges between 0.4 and 0.9 wt% with no overall downcore trend, with the highest contents in the shallowest 40 m and between ~600 to 800 m CSF-A. TN content at Site U1420 ranges between 0 and 0.1 wt%, with highest values observed at ~790 m CSF-A. C/N ratios range between 11 and 77, consistent with dominantly terrigenous organic matter input. CaCO₃ values range between 1.4 and 4.2 wt%, with slightly elevated (4.2 and 3.8 wt%) values in two biosiliceous mud-rich samples. At Site U1420, low alkalinity (<2.5 mM) and phosphate (<4 µM) concentrations contrast with relatively high ammonium concentrations (up to 2.5 mM) because of the precipitation of authigenic carbonates and phosphate but limited adsorption of ammonium to clay minerals (Fig. F63). Sulfate at Site U1420 ranges from 2.2 to 7.6 mM, with the highest concentration in the shallowest sample (10.7 m CSF-A). Total sulfate depletion is not observed, although sulfate reduction likely occurs within the uppermost 40 m of the sediment, and methane production occurs mostly deeper than 410 m CSF-A (1,700–33,000 ppmv). At Site U1420, chloride, salinity, bromide, and sodium profiles document a significant freshening of pore waters in all recovered samples with respect to the overlying seawater. The freshening is particularly extreme in the shallowest 40 m and between ~760 and 900 m CSF-A (Fig. F63) and may be related to gas hydrate dissociation or burial of glacial meltwater.

Because of concerns about borehole stability based on poor core recovery and challenging coring conditions, only one logging tool string was deployed in Hole U1420A. This sonic-induction tool string was designed to provide the highest priority measurements to meet science objectives with the lowest risk to logging tools. The string comprised the Enhanced Digital Telemetry Cartridge (EDTC), Hostile Environment Litho-Density Sonde (HLDS) without neutron source, Dipole Shear Sonic Imager (DSI), and phasor dual induction–spherically focused resistivity tool (DIT) and measured total gamma radiation, borehole diameter, sonic velocity, and resistivity. Because of a bridge or collapsed borehole, the tool string was only able to record data from ~90 to 290 m WSF. Figure F64 shows a summary of the main logging data recorded in Hole U1420A. The caliper measurement indicates that the borehole diameter exceeded 18 inches, the limit of the HLDS caliper arm, in the upper and lower sections of the logged interval. Borehole size was smaller (~15 inches) between ~140 and 200 m WMSF. Even with this large aperture, the data seem to be of good quality, as the measurements show relatively consistent variability throughout the logged interval.

The logged interval in Hole U1420A is assigned to a single logging unit, based on the minimal measurements recorded and the limited depth interval of the logging data in the context of the entire drilled depth. However, on the basis of distinctive changes in resistivity and velocity measurements, logging Unit 1 is divided into five subunits (Fig. F64). Logging Subunit 1A is characterized by a relatively constant mean trend in all data types, with little net downhole variation. Subunit 1B is distinguished by abrupt decreases in gamma ray and resistivity measurements. The deep resistivity curve likely measured formation, whereas the shallow and medium curves most likely reflect the resistivity of the borehole fluid, given the large borehole diameter. Gamma radiation, resistivity, and V_P all increase at the Subunit 1B/1C boundary, whereas gamma radiation decreases and resistivity and V_P increase across the Subunit 1C/1D boundary. All three of these logs then decrease and the caliper increases in Subunit 1E. Overall, resistivities in Hole U1420A are generally >3.0 Ωm, with the deepest resistivity curve showing values >8 Ωm in Subunit 1D. The relatively high sonic velocity values (~1700 to >2500 m/s) measured within this shallow logged interval from ~92 to 282 m WMSF support the notion that high resistivities are not simply because of reduced pore water salinity.

Each of the seismic profiles that cross Site U1420 exhibits a distinct change in stratal architecture across the regional unconformity marked by Horizon H1 (Fig. F9). At Site U1420, the seismic packages overlying Horizon H1 are acoustically semitransparent and semichaotic. Three subpackages are present, bounded by Subhorizons H1A and H1B (Fig. F61). Truncations shallower and deeper than each of these subhorizons indicate that these are erosional surfaces likely related to glacial advance–retreat cycles. According to the TWT-depth conversion using both the PWC and extrapolated sonic log values, Subhorizon H1A likely corresponds with the boundary between lithostratigraphic Units I and II. Lithologically, Unit I is characterized by a massive clast-rich diamict, whereas Unit II consists primarily of washed pebbles and drilled clasts of varying lithologies. Using velocities from the sonic log for preliminary shipboard TWT-depth conversions, we appear to have logged a portion of Unit II that starts between Subhorizons H1A and H1B and continues across the Horizon H1 unconformity that underlies H2A. Logging Subunits 1A–1D show changing velocity, natural gamma radiation, and resistivity that appear to correlate with changes in seismic facies. Subunit 1E coincides with the uppermost aggradational packages that are truncated by Horizon H1.

Deeper than logging Subunit 1E (~282 m WMSF), we observe increasing disparity between P-wave velocities measured by the PWC and those extrapolated from the sonic log. This velocity discrepancy creates potential errors in TWT calculation as large as ~0.2 s. Figure F61 includes both sets of correlations, but postcruise analysis will be essential for further interpretations. Current results indicate that the boundary between lithostratigraphic Units II and III lies somewhere within the package of bright, continuous reflectors deeper than Subhorizon H2B. Core recovery increased at the top of Unit III, and the sediments within this unit consist primarily of clast-poor and clast-rich diamict with occasional intervals of mud and bioturbation.

Site U1421

Setting and objectives

Site U1421 is located at 721 m water depth on the continental slope seaward of the Bering Trough mouth (Figs. F9, F10). Whereas Site U1420 targeted strata overlying an inactive thrust fault, Site U1421 was positioned to sample correlative strata on the limb of an actively deforming structure, where more deeply buried seismic sequences on the shelf are observed slightly closer to the seafloor. The site is located downslope of the Bering Trough stratigraphically above the youngest two thrusts of the Pamplona Zone where they cut obliquely across the slope. The slope sediments trapped behind these folds are seismically reflective, and sequences within them are mappable onto the shelf. These sequences appear to aggrade up through seismic Horizon H1, roughly parallel with the modern slope surface. They include thinned distal extents of shelf sedimentary sequences as well as slope sequences that are correlative with upslope/shelf sequences truncated by the Horizon H1 unconformity (Fig. F9). Overlying Horizon H1, the slope sequence at Site U1421 is more uniform in thickness and includes higher amplitude reflections that may represent the formation of a glacial trough-mouth fan associated with glacier termini at the shelf edge during glacial maxima, which is first suggested to have initiated during the MPT (Fig. F9) (Berger et al., 2008a). Also, as with Site U1419, this site is within the influence of the near-surface Alaska Coastal Current (Stabeno et al., 2004; Weingartner et al., 2005).

Similar to Site U1420, sedimentation at this site is affected by the combination of antecedent topography created by active faulting and the progressive input of glacigenic sediments. At the southeastern end of the STEEP09 seismic profile (Fig. F10A), two currently active faults (BT1 and BT2) are present at the continental slope, exhibiting less burial than the structures on the shelf. Scarps ~750 and ~300 m high associated with the active slope structures are visible on high-resolution bathymetry of the continental slope (Worthington et al., 2008). Site U1421 is located just landward of Fault BT2, which may have initiated after the PPT (Worthington et al., 2010), given the lack of growth strata observed deeper than Horizon H3 (Fig. F10). Between Horizons H1 and H2, the angle of the observed growth strata becomes less pronounced, indicating either a gradual decrease in fault growth rate during the early to Middle Pleistocene (Worthington et al., 2010) or an increase in accumulation rate. Overlying Horizon H1, sediments are truncated by the anticline and are very slightly tilted toward the shelf, indicating either minimal deformation on Fault BT2 since Horizon H1 or high accumulation rates.

The drilling objectives at Site U1421 seek to capitalize on good preservation of carbonate microfossils and associated geochronologic methods that can be used to develop a chronostratigraphy of a proximal sedimentary record of Late Pleistocene glacial dynamics. A secondary objective is to recover material for provenance studies. The key objective is to establish depositional ages of seismic sequences correlative with shelf units drilled at Site U1420. The target depth at this site is designed to penetrate seismic reflector Horizon H2 (expected to lie at ~1 km depth), which is mapped from the shelf where it marks the latest growth strata associated with a now-inactive thrust fault (Figs. F9, F10A). Determining the age of this horizon (hypothesized to be younger than the Pliocene/Pleistocene boundary) can allow us to infer the timing of when loading by increasing sediment accumulation forced accommodation of collisional stresses to be shifted elsewhere in the orogen (Worthington et al., 2010). Crossing the slope equivalent of the angular unconformity Horizon H1 will occur while drilling to the depth of Horizon H2, allowing for a second opportunity beyond Site U1420 to constrain the timing of Horizon H1 formation. The expected lithofacies are alternating diamict (IRD and debris flow deposits), turbidites, and hemipelagic mud (Fig. F65). This site is expected to provide a proximal provenance record of sediment supply from the Bering Glacier that can be used to locate the temporal and spatial loci of glacial erosion in the St. Elias orogen.

Results

At Site U1421, Holes U1421A, U1421B, and U1421C were drilled to total depths of 702.7, 6.2, and 38.2 m CSF-A, respectively (Fig. F66). Core recovery and refusal depths were significantly affected by frequent lonestones and diamict. For Hole U1421A, full-length (9.7 m) APC refusal was encountered at 65.5 m DSF; half-length (4.7 m) APC refusal was encountered at 96.4 m DSF. XCB coring using the soft-formation and hard-formation cutting shoes was used in only Hole U1421A from 96.4 to 702.7 m DSF. In Holes U1421B and U1421C, full-length APC cores were collected. A total of 92 cores were recovered at the site. A total of 114.47 m of core over a 140.8 m interval was recovered using the APC systems (81% recovery). A total of 66 XCB cores were drilled over 606.3 m with a recovery of 61.54 m (10%). The overall recovery for Site U1421 was 23.6%. APC shot depths in Holes U1421B and U1421C were carefully planned with the aid of the stratigraphic correlators.

The composite depth scale at Site U1421 is constructed from 0.0 to 695.72 m CCSF-A (the base of Core 341-U1421A-85X). The splice consists of one complete and continuous interval from the mudline to 33.21 m CCSF-D (the base of Core 341-U1421C-5H). Weather was calm and ship heave was negligible while coring Site U1421, but the relatively lonestone and gravel rich ice-proximal environment proved challenging for core recovery and for interhole correlation. Intervals of core disturbance, incomplete recovery, and use of the half-length APC coring system all presented complications in the development of composite depths and a spliced record. In particular, we observed many instances of flow-in. We urge caution in using the splice from Site U1421 until it can be verified with postcruise data.

The sediment recovered at Site U1421 contains nine facies. The dominant facies are dark gray to dark greenish gray mud and diamict. They account for >95% of the core recovered. Specific lithofacies include massive mud with and without lonestones, interbedded mud and diamict, clast-poor diamict, clast-rich diamict, diatom ooze, mud with diatoms/biosilica, calcareous/carbonate-bearing mud, and mud and diamict with volcanic ash. These facies are inferred to reflect deposition from suspension fallout, sediment gravity flows, ice rafting, and variations in productivity/preservation of biogenic material.

The main lithologies of the diamict clasts and lonestones recovered at Site U1421 are, in order of decreasing abundance, siltstone, sandstone, basalt, and argillite. The petrology of clasts is similar to that found in the St. Elias and Chugach Mountains located along the southern coast of Alaska. Smear slides indicate similar bulk mineralogies downhole.

Based on facies associations, two lithostratigraphic units were defined. Unit I from 0 to 57 m CSF-A is very dark gray (N 3) to dark greenish gray (10Y 4/1) mud interbedded with diatom-bearing mud and diatom-rich mud. The number of lonestones varies from dispersed to abundant deeper than 6.4 m CSF-A. One interval of olive-colored thin laminae was deposited stratigraphically above the uppermost lonestones. Unit II from 57 to 702.7 m CSF-A is very dark gray (N 3) clast-rich diamict interbedded with clast-poor diamict and mud with abundant clasts. Most common clast lithologies are siltstone, sandstone, basalt, and argillite.

The Unit I/II boundary is marked by an increase in GRA bulk density from 1.9 to 2.2 g/cm³, and it remains high throughout the clast-rich intervals within Unit II. Vane shear strength of 78.8–69.9 kPa was measured from interval 341-U1421A-8H-2, 26 cm, to 8H-4, 42 cm, which is more than twice as high as the measurements in Unit I. This increased shear strength possibly represents overconsolidated sediment and suggests that either a coherent block of subglacial sediment was transported downslope from the Bering Trough or deposition occurred by glacigenic debris flows. Other evidence of downslope transport within Unit II includes erosive lower boundaries of diamict and soft-sediment deformation in intervals of laminated diatom ooze. Muddy intervals with biogenic silica and low clast abundance indicate reduced ice rafting and/or increased productivity. Biosilica-rich or diatom-rich mud occurs in Cores 341-U1421A-22X, 41X, and 55X through 57X. Diatom ooze occurs in Cores 61X through 63X and 75X. Dark greenish gray (10Y 4/1) finely laminated diatom ooze occurs in Section 63X-1.

Siliceous microfossils occur infrequently at Site U1421, whereas calcareous microfossils are continuously present (Fig. F67). Based on microfossil biostratigraphic data, the oldest collected sediments are between 0.03 and 0.3 Ma. Benthic foraminifers appear to be a mixture of transported specimens, largely shallow-water (neritic; <100 m) Elphidium spp., and in situ specimens comprised primarily of Epistominella pacifica, Eubuliminella exilis, and Islandiella norcrossi, which are expected to be abundant at middle neritic to upper bathyal water depths (~100–1000 m) in the Gulf of Alaska. Elphidium spp. specimens are often fragmented, further suggesting that they have a transport history. When the siliciclastic sand fraction is low and foraminiferal group abundances increase, the deeper water fauna dominates and Elphidium spp. are rare to absent, suggesting periods of lower downslope transport. However, sandy samples with Elphidium spp. dominate in the core catchers. Relatively more radiolarian and diatom taxa typically found in shallow water (neritic) are observed at the lower part of the section. Neritic radiolarian and planktonic foraminiferal species are typical for subarctic faunas. The ratio of cold-water to temperate-water planktonic foraminiferal species fluctuates, suggesting some changes in temperature in this record.

The NRM intensities of the APC (Holes U1421A–U1421C) and XCB (Cores 341-U1421A-20X through 85X) cores were relatively strong before AF demagnetization (10^–2 ~ 10^–1 A/m) but were weaker after demagnetization (10^–2 to 10^–3 A/m). Intensities were higher in the APC- than in the XCB-recovered intervals (Fig. F68). Development of a splice and placement of all holes on a CCSF-A depth scale allows comparisons to be made of the APC-recovered intervals, and intensities are consistent between holes and vary at both the meter and decimeter scale. Because inclinations indicate generally normal polarity, it is thought that the recovered sediment is exclusively within the Brunhes Chronozone and younger than 0.781 Ma (Cande and Kent, 1995, Hilgen et al., 2012).

Physical properties were measured on all cores from Holes U1421A–U1421C. All routine core logger measurements, including MS, GRA bulk density, P-wave velocity, and NGR, were collected. GRA bulk density increases downcore within the depths of APC coring, with slight drops at ~20 and ~50 m CSF-A. NGR measurements varied between 14 and 41 cps, illustrating downhole increases from 0 to 96 m CSF-A. P-wave velocity values from the track delineated between ~1500 and 2200 m/s. We could not measure P-wave velocity of the core sections on the core logger deeper than ~96 m CSF-A in Hole U1921A after switching from APC to XCB coring. Discrete P-wave measurements measured on the working-half sections ranged from ~1450 to 1975 m/s. P-wave values show significant scatter, with a slight increasing downhole trend. Shear strength measurements ranged from ~5 to 80 kPa and generally increase downhole, with increasing scatter observed starting at Core 341-U1421A-6H. We measured multiple instances of elevated (~50–80 kPa) shear strength at ~50, ~60, ~80 and ~110 m CSF-A in Hole U1421A. MAD measurements show generally increasing bulk density and decreasing porosity from ~0 to 50 m CSF-A in Hole U1421A. Deeper than ~50 m CSF-A in Hole U1421A, bulk density values range from 1.9 to 2.4 g/cm³ and porosities range from 28% to 45% with no significant trend.

At Site U1421, 64 headspace samples were analyzed for hydrocarbon gases; 44 discrete sediment samples were analyzed for carbon, nitrogen, and carbonate; and 26 interstitial water samples were taken, with sampling resolution in the deeper part of the hole becoming irregular because of spotty core recovery (Fig. F69). TOC contents are low overall, but accumulation rates are high, and C/N ratios indicate variable contributions from terrigenous organic matter. Carbonate contents are <2 wt% in most samples, but there are a few isolated peaks (3–13 wt%) that appear to be related to mud-rich lithologies. Downcore profiles of ammonium, alkalinity, and methane reflect moderate organic matter remineralization overall, with zones of higher alkalinity and ammonium (~30 and ~450 m CSF-A) likely related to the increased presence of more labile marine organic matter. Methane concentrations are highly variable, ranging between quantification limit and 34,000 ppmv, whereas in large parts of the record concentrations are <5,000 ppmv. Both chloride and salinity are low (340–530 mM and 20–25, respectively) throughout the record, with minima recorded at ~40–70 and ~485 m CSF-A, likely related to gas hydrate dissociation or burial of glacial meltwater.

Two tool strings (sonic-induction tool string and VSI tool string) were deployed in Hole U1421A on the basis of potentially unstable borehole conditions and limited time at the end of the expedition. The sonic-induction tool string, initially deployed in the previous hole (U1420A), was composed of the EDTC, HLDS without neutron source, DSI, and DIT, respectively measuring total gamma ray, borehole diameter, sonic velocity, and resistivity. The second tool string was the VSI tool string, run without the Hostile Environment Natural Gamma Ray Sonde because of concerns about borehole instability. With the exception of some thin washouts, borehole diameter varied smoothly and rarely exceeded 18 inches, the limit of the HLDS caliper arm (Fig. F70). The character of the borehole wall in Hole U1421A on the slope is a distinct change from the rugose character observed at the deeper water sites (U1417 and U1418). Shallower than 500 m WMSF, borehole diameter ranges from 10 to 17 inches, with an average diameter of ~15 inches. There is a distinct separation between the shallow resistivity curve and the medium and deep resistivity curves through much of the logged interval. Given that the borehole diameter is within the depth of investigation of all three curves, this could indicate that the shallow borehole wall was invaded by logging mud, which has relatively low resistivity. The only exception is one narrow spot indicated by the caliper log at ~300 m WMSF. The hole was nearly in-gauge deeper than ~500 m WMSF, with the exception of a washed-out zone between 583 and 591 m WMSF. High coherence in sonic waveforms was observed in the compressional velocity (V_P) and flexural velocity (V_S) logs throughout the entire borehole (Fig. F70). Gamma ray was measured through the entire hole, through the drill pipe from the seawater/seafloor boundary to the base of the pipe, and in the open hole to total depth. The gamma ray measurement is highly attenuated when the tool was inside the bottom-hole assembly (0–96 m WMSF). However, although the log signal is attenuated and there is a slight vertical offset, there is still a reasonable agreement between gamma ray from downhole logs and core logs, and similar trends are shown in both data sets. The VSP in Hole U1421A provides data for establishing a link between core and log data (recorded in depth) and seismic surveys (recorded in TWT) at the location of the borehole. Six out of nine stations yielded TWTs ranging from 1.278 s TWT below sea level at 284.7 m WMSF to 1.641 s TWT at 687 m WMSF.

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