McKay, R.M., De Santis, L., Kulhanek, D.K., and the Expedition 374 Scientists
Proceedings of the International Ocean Discovery Program Volume 374
publications.iodp.org

https://doi.org/10.14379/iodp.proc.374.101.2019

Expedition 374 summary¹

R.M. McKay, L. De Santis, D.K. Kulhanek, J.L. Ash, F. Beny, I.M. Browne, G. Cortese, I.M. Cordeiro de Sousa, J.P. Dodd, O.M. Esper, J.A. Gales, D.M. Harwood, S. Ishino, B.A. Keisling, S. Kim, S. Kim, J.S. Laberg, R.M. Leckie, J. Müller, M.O. Patterson, B.W. Romans, O.E. Romero, F. Sangiorgi, O. Seki, A.E. Shevenell, S.M. Singh, S.T. Sugisaki, T. van de Flierdt, T.E. van Peer, W. Xiao, and Z. Xiong²

Keywords: International Ocean Discovery Program, IODP, JOIDES Resolution, Expedition 374, Site U1521, Site U1522, Site U1523, Site U1524, Site U1525, Ross Sea, West Antarctic, ice sheet history, sea ice, Miocene, Pliocene, Quaternary, Antarctic Bottom Water, Antarctic water masses, turbidites, contourites, glaciomarine sediments, subglacial sediments, ice-rafted debris, paleobathymetry, seismic stratigraphy, paleoclimate, paleoceanography

MS 374-101: Published 10 August 2019

Abstract

The marine-based West Antarctic Ice Sheet (WAIS) is currently locally retreating because of shifting wind-driven oceanic currents that transport warm waters toward the ice margin, resulting in ice shelf thinning and accelerated mass loss. Previous results from geologic drilling on Antarctica’s continental margins show significant variability in ice sheet extent during the late Neogene and Quaternary. Climate and ice sheet models indicate a fundamental role for oceanic heat in controlling ice sheet variability over at least the past 20 My. Although evidence for past ice sheet variability is available from ice-proximal marine settings, sedimentary sequences from the continental shelf and rise are required to evaluate the extent of past ice sheet variability and the associated forcings and feedbacks. International Ocean Discovery Program Expedition 374 drilled a latitudinal and depth transect of five sites from the outer continental shelf to rise in the central Ross Sea to resolve Neogene and Quaternary relationships between climatic and oceanic change and WAIS evolution. The Ross Sea was targeted because numerical ice sheet models indicate that this sector of Antarctica responds sensitively to changes in ocean heat flux. Expedition 374 was designed for optimal data-model integration to enable an improved understanding of Antarctic Ice Sheet (AIS) mass balance during warmer-than-present climates (e.g., the Pleistocene “super interglacials,” the mid-Pliocene, and the Miocene Climatic Optimum). The principal goals of Expedition 374 were to

Evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates;
Reconstruct ice-proximal oceanic and atmospheric temperatures to quantify past polar amplification;
Assess the role of oceanic forcing (e.g., temperature and sea level) on AIS variability;
Identify the sensitivity of the AIS to Earth’s orbital configuration under a variety of climate boundary conditions; and
Reconstruct Ross Sea paleobathymetry to examine relationships between seafloor geometry, ice sheet variability, and global climate.

To achieve these objectives, postcruise studies will

Use data and models to reconcile intervals of maximum Neogene and Quaternary ice advance and retreat with far-field records of eustatic sea level;
Reconstruct past changes in oceanic and atmospheric temperatures using a multiproxy approach;
Reconstruct Neogene and Quaternary sea ice margin fluctuations and correlate these records to existing inner continental shelf records;
Examine relationships among WAIS variability, Earth’s orbital configuration, oceanic temperature and circulation, and atmospheric pCO₂; and
Constrain the timing of Ross Sea continental shelf overdeepening and assess its impact on Neogene and Quaternary ice dynamics.

Expedition 374 departed from Lyttelton, New Zealand, in January 2018 and returned in March 2018. We recovered 1292.70 m of high-quality core from five sites spanning the early Miocene to late Quaternary. Three sites were cored on the continental shelf (Sites U1521, U1522, and U1523). At Site U1521, we cored a 650 m thick sequence of interbedded diamictite and diatom-rich mudstone penetrating seismic Ross Sea Unconformity 4 (RSU4). The depositional reconstructions of past glacial and open-marine conditions at this site will provide unprecedented insight into environmental change on the Antarctic continental shelf during the late early and middle Miocene. At Site U1522, we cored a discontinuous late Miocene to Pleistocene sequence of glacial and glaciomarine strata from the outer shelf with the primary objective of penetrating and dating RSU3, which is interpreted to reflect the first continental shelf–wide expansion of East and West Antarctic ice streams. Site U1523, located on the outer continental shelf, targeted a sediment drift beneath the westward-flowing Antarctic Slope Current (ASC) to test the hypothesis that changes in ASC vigor regulate ocean heat flux onto the continental shelf and thus ice sheet mass balance.

We also cored two sites on the continental rise and slope. At Site U1524, we recovered a Plio–Pleistocene sedimentary sequence from the levee of the Hillary Canyon, one of the largest conduits of Antarctic Bottom Water from the continental shelf to the abyssal ocean. Site U1524 was designed to penetrate into middle Miocene and older strata, but coring was initially interrupted by drifting sea ice that forced us to abandon coring in Hole U1524A at 399.5 m drilling depth below seafloor (DSF). We moved to a nearby alternate site on the continental slope (Site U1525) to core a single hole designed to complement the record at Site U1524. We returned to Site U1524 after the sea ice cleared and cored Hole U1524C with the rotary core barrel system with the intention of reaching the target depth of 1000 m DSF. However, we were forced to terminate Hole U1524C at 441.9 m DSF because of a mechanical failure with the vessel that resulted in termination of all drilling operations and forced us to return to Lyttelton 16 days earlier than scheduled. The loss of 39% of our operational days significantly impacted our ability to achieve all Expedition 374 objectives. In particular, we were not able to recover continuous middle Miocene sequences from the continental rise designed to complement the discontinuous record from continental shelf Site U1521. The mechanical failure also meant we could not recover cores from proposed Site RSCR-19A, which was targeted to obtain a high-fidelity, continuous record of upper Neogene and Quaternary pelagic/hemipelagic sedimentation. Despite our failure to recover a continental shelf-to-rise Miocene transect, records from Sites U1522, U1524, and U1525 and legacy cores from the Antarctic Geological Drilling Project (ANDRILL) can be integrated to develop a shelf-to-rise Plio–Pleistocene transect.

Introduction

International Ocean Discovery Program (IODP) Expedition 374 recovered Ross Sea outer continental shelf-to-rise sedimentary sequences of Neogene to Quaternary Antarctic Ice Sheet (AIS) evolution that will improve understanding of climate forcings/feedbacks on past ice sheet variability. These sequences, in combination with model sensitivity tests, tectonic considerations, and the well-developed seismic stratigraphic framework of the Ross Sea, will enable researchers to determine the contribution of Antarctic ice volume to far-field Neogene sea level variations (20–60 m) (cf., Miller et al., 2005, 2012; Kominz et al., 2008; Naish and Wilson, 2009; John et al., 2011) (Figures F1, F2, F3). These sequences, recovered from an area of demonstrated climate sensitivity (Figure F4), allow for improved understanding of ocean–ice sheet interactions on orbital to million-year timescales.

Figure F1. Data reconstructions.

Figure F2. Bathymetric map with site locations.

Figure F3. Ross Sea seismic stratigraphy and previous drilling.

Figure F4. Ice sheet responses.

The onset of the Neogene (23 Ma; Oligocene/Miocene [O/M] boundary) is associated with a transient, abrupt increase in Antarctic ice volume attributed to changes in Earth’s orbital parameters (Zachos et al., 1997; Naish et al., 2001) and declining atmospheric CO₂ (Figure F1) (Pagani et al., 2005; Foster and Rohling, 2013). Following the relatively cool early Miocene, proxy records indicate a period of sustained global warmth (on average ~3°C warmer than present; Flower and Kennett, 1994; You et al., 2009) and a significant carbon cycle reorganization (e.g., Vincent and Berger, 1985; Foster et al., 2012) referred to as the Miocene Climatic Optimum (MCO; ~17–14.5 Ma) (Flower and Kennett, 1994; Shevenell et al., 2004). During the MCO, significant polar amplification of temperature occurred (Shevenell et al., 2004, 2008; Lewis et al., 2008; Warny et al., 2009; Feakins et al., 2012; Sangiorgi et al., 2018), the origins of which are unknown. Immediately following the MCO, Antarctic ice growth and cooling, termed the Middle Miocene Climate Transition (MMCT; 14.2–13.8 Ma), is inferred from both deep-sea benthic foraminifer δ¹⁸O records and ice-proximal data (Figure F1B) (Kennett, 1977; Flower and Kennett, 1994; Zachos et al., 2001; Shevenell et al., 2004, 2008; Holbourn et al., 2007; Cramer et al., 2009; John et al., 2011; Pierce et al., 2017; Sangiorgi et al., 2018). However, far-field evidence suggests ice growth may have initiated during the MCO (Shevenell et al., 2004, 2008). The MMCT resulted in the loss of the Antarctic tundra vegetation at high elevations in the Transantarctic Mountains (Lewis et al., 2008). Although ice expansion has traditionally been inferred in East Antarctica, Ross Sea seismic evidence also suggests West Antarctic Ice Sheet (WAIS) expansion during the MMCT (Bart, 2003). However, the timing of the Ross Sea event, WAIS development, and forcings and feedbacks involved in the MMCT remain enigmatic, as does the subsequent climate and ice sheet history of the relatively late Miocene (e.g., Kennett, 1977; De Santis et al., 1995; Zachos et al., 2001; Ohneiser et al., 2015; Herbert et al., 2016). During the mid-Pliocene, global sea levels are estimated to have been ~20 ± 10 m above present-day levels, indicating a reduction/collapse of both the Greenland Ice Sheet and the WAIS (Miller et al., 2012). Ice-proximal sedimentary facies indicate WAIS advances and retreats from the early Pliocene to ~1.0 Ma (Figure F4) (Naish et al., 2009). However, equivocal evidence exists for WAIS collapse as recently as the last interglacial (Scherer et al., 1998; Kopp et al., 2009; Naish et al., 2009; McKay et al., 2012b; Dahl-Jensen et al., 2013; Kingslake et al., 2018). Thus, obtaining late Pleistocene paleoclimate records from the Ross Sea was a high priority for Expedition 374.

Background

Oceanographic setting

At present, the abyssal ocean is filled with cold, dense seawater produced in the large polynyas of the Weddell and Ross Seas (Orsi et al., 1999; Johnson, 2008). Thus, regional changes in temperature and/or meltwater input to the Ross Sea are expected to have a pronounced impact on global meridional overturning circulation (Jacobs et al., 2002; Orsi and Wiederwohl, 2009; Purkey and Johnson, 2010). Over the past 40 y, Ross Sea–derived Antarctic Bottom Water (AABW) has freshened as a result of increased meltwater input to the Amundsen and Bellingshausen Seas from melting ice shelves/glacial systems (Jacobs et al., 2002, 2011; Johnson, 2008; Jacobs and Giulivi, 2010; Purkey and Johnson, 2010, 2012).

Unlike the Amundsen and Bellingshausen Seas, where the Antarctic Circumpolar Current (ACC) impinges the continental shelf and cross-shelf bathymetry encourages the presence of relatively warm Circumpolar Deep Water (CDW) on the inner shelf (Rignot and Jacobs, 2002), the eastern limb of the Ross Gyre brings cooler modified Circumpolar Deep Water (mCDW) to the Ross Sea along the lower continental slope (Whitworth et al., 1995; Orsi and Wiederwohl, 2009). The strong westward-flowing Antarctic Slope Current (ASC), which has a sharp subsurface front (Antarctic Slope Front), separates Antarctic Surface Water (AASW) on the shelf from mCDW on the lower continental slope (Figure F5). This front serves as a dynamic barrier that limits the transfer of CDW and mCDW onto the Ross Sea continental shelf (Ainley and Jacobs, 1981). Thus, ASC vigor and the formation of fresh AASW regulate the volume of mCDW on the Ross Sea continental shelf.

Figure F5. Ross Sea bathymetric map.

Geological setting and previous drilling

The breakup of Gondwana during the Middle Jurassic began with the initiation of the West Antarctic Rift, which led to the opening of the Ross Sea (Behrendt et al., 1991) and the development of three sedimentary basins (Figures F2, F3) (Cooper et al., 1991). The westernmost Victoria Land Basin (VLB) was the focus of previous regional geological drilling (e.g., Dry Valley Drilling Project, McMurdo Sound Sediments and Tectonics Studies (MSSTS), Cenozoic Investigations in the Western Ross Sea [CIROS-1 and CIROS-2], Cape Roberts Project [CRP], and the Antarctic Geological Drilling Project [ANDRILL]). Expedition 374 focused on the Eastern Basin, which encompasses the Glomar Challenger Basin and contains as much as 6 km of Cenozoic sediment infill (Figure F3).

The basement geology of the Ross Sea Central High adjacent to the Eastern Basin was penetrated at Deep Sea Drilling Project (DSDP) Site 270 and consists of high-grade Paleozoic calcareous metamorphic rocks (Ford and Barrett, 1975) that were mylonitized during Late Cretaceous uplift (Siddoway et al., 2004). Upper Cretaceous mylonites were also recovered from the western part of the Eastern Basin (Luyendyk et al., 2001). The lack of basement younger than Devonian at CRP-3 and Site 270 suggests that younger Permian–Triassic Beacon Supergroup strata are likely absent in the Ross Sea. This interpretation is supported by paleogeographic maps from Barrett (1981) that, although poorly constrained, suggest that the Ross Sea was likely an area of uplift and erosion through much of the Permian–Triassic. The lower part of the Beacon Supergroup (Devonian) is dominated by coarse clastics (Barrett, 1981). The tectonic models for the formation of the Ross Sea, through the thinning of thickened (~40 km) crust that extends across the entire Transantarctic Mountain (TAM)–Ross Sea–Marie Byrd Land region (Karner et al., 2005; Decesari et al., 2007), would also tend to argue against the preservation of upper Beacon Supergroup strata because uplift and erosion of these strata is predicted during early rifting. Consequently, rifting would result in a very different provenance signature for glacial deposits transported from the western compared with the eastern Ross Sea.

The Antarctic Offshore Stratigraphy (ANTOSTRAT) seismic stratigraphic scheme defines eight seismic units (RSS-1 to RSS-8) in the sediment infill above the basement. These seismic units are bounded by six major shelf-wide seismic unconformities (RSU1–RSU6) (Figures F3, F6; Table T1) (Brancolini et al., 1995; De Santis et al., 1995) whose ages are only partially constrained by drilling (see synthesis by Bart and De Santis, 2012). In the western Ross Sea, CIROS-1 and CRP-3 indicate marine-terminating glaciation in the TAM by the earliest Oligocene (Barrett, 1989, 2007). Seismic-based reconstructions suggest ice caps nucleated on subaerially exposed basement highs in the central Ross Sea during the Oligocene. The adjacent deep-water basins in the outer Ross Sea appear to have remained free of grounded ice until the late Oligocene, although they were probably influenced by voluminous sediment-laden meltwater plumes associated with more temperate glacial sedimentation, resulting in progressive shoaling of the Eastern Basin as accommodation space filled (Shipboard Scientific Party, 1975a; De Santis et al., 1995). A major goal of Expedition 374 is to constrain the ages of RSU4 through RSU1 to understand how these features relate to the marine-based WAIS evolution.

Figure F6. Chronostratigraphic summary of Ross Sea drilling.

Strata (Unit RSS-1) underlying RSU6 are the oldest and deepest basin-infill sedimentary packages in the central Ross Sea. Unit RSS-1 is divided into lower and upper packages. The lower part of Unit RSS-1 has not been drilled, whereas the upper part was drilled in the western Ross Sea (CRP and CIROS-1) and consists of upper Eocene to Oligocene high-energy fluvial, deltaic, and shallow-shelf rift-fill strata, the uppermost of which were glacially influenced (Fielding et al., 2000; Galeotti et al., 2016). The lithology and age of the lowermost strata below RSU6 are uncertain because this interval has not been sampled, but it likely consists of coarse-grained fluvial facies deposited in the initial phases of Late Cretaceous to Paleocene rifting in the central Ross Sea (Wilson and Luyendyk, 2009).

The units overlying RSU6 have all been partially sampled by drilling, making the stratigraphic architecture in the Ross Sea relatively well constrained for this time period. However, most drill holes are located in isolated basins in the VLB of the western Ross Sea (Figures F2, F6), and thus basin-to-basin correlation remains uncertain. Direct correlation of RSU6 from across the various Ross Sea basins is not possible because the unconformity onlaps basement flanks (Figure F3). However, an inferred correlation can be made based on the distinctive acoustic character of the unconformity, which is a high-amplitude continuous reflector that is very distinct from the underlying seismic facies.

Upper Oligocene (~28 Ma) to lower Miocene (~20 Ma) strata at Site 270 consist of a 365 m thick sequence of lithified glaciomarine mudstone with ice-rafted debris (IRD) and common macro- and microfossils (Hayes et al., 1975; Leckie and Webb, 1986), suggesting a shallow continental shelf environment with mudstone facies indicative of times with abundant terrestrial runoff. Shallower than 100 meters below seafloor (mbsf), many of the units (originally classified as mudstone) are lower Miocene diamictite (i.e., >25% sand, according to the classification scheme used in this volume) that were eroded and transported by glacial ice (Barrett, 1975).

Two ANDRILL sites drilled on the inner continental shelf of the western Ross Sea (98% recovery) contain unprecedented records of marine-based ice sheet variability (Naish et al., 2009; Fielding et al., 2011; Wilson et al., 2012; Patterson and Ishman, 2012; Levy et al., 2016) over the past 20 My, although both sites are influenced by the East Antarctic Ice Sheet (EAIS). Drilling at Antarctic Geological Drilling Project (AND) Site AND-2A recovered a ~20–14 Ma sequence interpreted to reflect TAM outlet glaciers overriding and/or calving near the site (Fielding et al., 2011; Passchier et al., 2011; Levy et al., 2016). A diatomite (dated to 15.7 Ma) with abundant dinoflagellate cysts (dinocysts), foraminifers, pollen, and other biomarkers suggests a warmer-than-present (~11°C mean surface temperature) and hydrologically active climate during the MCO (Warny et al., 2009; Feakins et al., 2012).

At 300 mbsf in Core AND-2A, a 300 ky disconformity equivalent to RSU4 (middle Miocene) suggests a shelf-wide advance of grounded marine-based ice originating from the EAIS during the MMCT (Figures F6, F7, F8) (De Santis et al., 1999; Passchier et al., 2011; Levy et al., 2016). This unconformity is the first unequivocal seismic evidence for a glacially carved trough in the central Ross Sea and is a key target of Site U1521 (Figures F7, F8) (De Santis et al., 1995; Ten Brink et al., 1995; Anderson, 1999; Anderson et al., 2018; Colleoni et al., 2018a). At DSDP Site 272, a ~400 m thick early–middle Miocene sequence of glaciomarine mudstone was recovered (Shipboard Scientific Party, 1975b) (Figure F6). Approximately 250 m of till foreset and aggrading bottomset strata above RSU4 (De Santis et al., 1997) is associated with numerous outwash channels, suggesting the region was influenced by large volumes of erosive sediment-laden meltwater during the middle Miocene (~14 Ma; Figure F8) (Anderson and Bartek, 1992; Chow and Bart, 2003) that may have been associated with extensive channel-levee systems above RSU4 on the continental slope and rise (De Santis et al., 1995, 1999).

Figure F7. TWT depth maps.

Figure F8. Composite seismic cross section.

Post-MCO cooling and glaciation caused multiple erosional episodes across the shallow central Ross Sea, which resulted in poor preservation of the Site 272–recovered foreset strata that thicken toward the Central High within a thick, gently seaward-dipping section, suggesting that middle Miocene glaciation records may be preserved in the eastern Ross Sea (De Santis et al., 1997; cf. Figures F7, F8). Evidence for glacial erosion and deposition in the southeasternmost Ross Sea in the Oligocene suggests the presence of an ice cap on Marie Byrd Land (Sorlien et al., 2007). However, in the mid- to late Miocene, the eastern Ross Sea was still a wide, gently sloping seaward embayment surrounded by grounded ice that was delivering sediments to the slope from the Central High and Marie Byrd Land.

RSU3, a key target of Site U1522, provides the first evidence for a major cross-shelf paleotrough eroded by an expanded WAIS, although the age of this event(s) is poorly constrained (~14–4 Ma; Figures F6, F7) (De Santis et al., 1995, 1999; Bart, 2003). Large meltwater and outwash features are absent, and laminated seismic facies are progressively thinner/less common in strata younger than RSU3, suggesting sediment starvation and a transition to a colder glacial regime. The inner–continental shelf Site AND-1B sediments indicate that this transition may not have occurred until the Pliocene (McKay et al., 2009), although evidence for meltwater outburst features is lacking in the TAM after 12.4 Ma (Lewis et al., 2006).

High-velocity seismic units above RSU3 and RSU2 suggest overcompaction by ice-loading during WAIS expansion (Böhm et al., 2009). Bathymetric reconstructions suggest that overdeepening, with a transition to a landward-deepening continental shelf, occurred by the RSU2 event during the early Pliocene to early Pleistocene(?) (Figure F7) (De Santis et al., 1995, 1999). A trough-mouth fan on the upper slope and a sediment-starved continental rise (typical of the Plio–Pleistocene Antarctic margin) coincided with overdeepening (Bart et al., 1999; Cooper and O’Brien, 2004; Rebesco et al., 2006; Bart and Iwai, 2012).

At Site AND-1B, ~58 sedimentary cycles of ice sheet advance and retreat are observed in the Ross Embayment over the past 13 My (McKay et al., 2009). Diatomites indicate frequent collapses of the WAIS in the Pliocene (5.3–2.6 Ma), during which diatom assemblages and geochemical paleothermometry indicate ocean temperatures were as much as 4°C warmer than present (Naish et al., 2009; McKay et al., 2012a). However, sedimentary lithofacies are dominated by pelagic diatom ooze, indicating that meltwater discharge was reduced during Pliocene interglacials compared with thick glaciomarine mudstone deposited during the late Miocene (11–5.3 Ma). By the mid-Pleistocene (1.0 Ma), the Ross Ice Shelf persisted through most interglacials (McKay et al., 2009).

Seismic facies above RSU2 are indicative of till sheets bound by erosional unconformities in an aggrading shelf margin caused by shelf-wide advances of the WAIS (Figure F7) (Alonso et al., 1992; Brancolini et al., 1995; Bart et al., 2011). Unlike other sectors of the Antarctic, the eastern Ross Sea trough mouth contains thick (~2000 m) sedimentary sequences on the shelf and upper continental slope that may contain a detailed WAIS history. Above RSU1 (0.7? Ma), shelf-edge sediments are aggrading or backstepping (rather than prograding; Bart et al., 2018), indicating that most sediment delivered from land was sequestered on the outer shelf or bypassed the continental shelf to be transported directly into the deep sea.

Site survey data

Multichannel seismic (MCS) and single-channel seismic (SCS) profiles have been collected in the Ross Sea by several nations since 1980. MCS data are available through the Antarctic Seismic Data Library System, which works under the auspices of the Scientific Committee on Antarctic Research and the Antarctic Treaty (ATCM XVI-12). Prestack data are available only from the Italian cruises (1988, 1989, 1991, 1994, and 2006) and recently also from the BGR80 cruise.

We selected sites based on maximum thickness of target sequence, potential for dating sediments, and acoustic facies and geometry of deposits, which usually can be seen more clearly on high-resolution profiles. In most cases, high-resolution profiles are single channel, or in the case of the TAN060_08 profile, they have a very short streamer (200 m) relative to the water depth (>1000 m). In some cases, crossing lines are not available; the remoteness of Antarctic water and the presence of sea ice prevents the easy collection of new site survey data. New MCS Profiles KSL14-02 and KSL14-04 (Kim et al., 2018) were collected in February 2013 and in 2015 by the Korea Polar Research Institute (KOPRI) with the aim of providing cross-lines for several proposed sites. Another SCS survey cruise was conducted in 2017 (EU/FP7 EUROFLEETS2 ANTSSS project and Programma Nazionale di Ricerche in Antartide WHISPERS and ODYSSEA projects) with the aim to collect additional cross-lines for proposed sites and to identify alternate sites.

SCS data, collected by National Science Foundation (NSF) Cruises in 1990 and 1994–1995 were made available by John Anderson (Rice University, TX) as digital SEGY data (Cruise PD90) and paper copies (Cruise NBP 94-95). We made the conversion of the TIFF or JPG image to SEGY format to depth-convert the sections and load the data in the IHS Kingdom interpretation software with the other available data sets. The supporting site survey data for Expedition 374 are archived in the IODP Site Survey Data Bank (https://ssdb.iodp.org/SSDBquery/SSDBquery.php; select proposal number P751).

Scientific objectives

Evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates.

Far-field benthic foraminifer δ¹⁸O and sequence stratigraphic records suggest that large global ice volume and sea level (20–60 m) variations occurred during the Miocene and Pliocene (Figures F1, F6) (Zachos et al., 2001; Miller et al., 2005, 2012; Cramer et al., 2009; Raymo et al., 2011). Miocene–Pliocene sea level reconstructions could potentially be reconciled without invoking large Northern Hemisphere contributions if Antarctica’s ice sheets expanded to the continental shelf edge (~14 m sea level equivalent [SLE]) (Figure F4). The modeled difference between the glacial maxima states and loss of the marine-based WAIS (assuming present bathymetry) represents ~21 m SLE (with some minor loss of the EAIS), although changes in Ross Sea bathymetry could increase this value (Colleoni et al., 2018a) (Figure F4) (see Objective 5). Expedition 374 records will help constrain the timing of the first WAIS advances to the shelf edge, and integration with ANDRILL records will allow assessment of the WAIS contribution to Neogene sea level estimates.

Sedimentological and micropaleontological analyses at continental shelf Sites U1521 and U1522, combined with seismic stratigraphic correlations, will identify deposition under grounded-ice, glaciomarine, and open-marine conditions following ANDRILL/CRP methodology (Fielding et al., 2000, 2011; Powell and Cooper, 2002; McKay et al., 2009; Passchier et al., 2011). Magneto-, bio-, and tephrochronology will enable identification of orbital-scale ice sheet variations and have been employed in discontinuous Antarctic margin sequences (Florindo et al., 2003, 2005; Tauxe et al., 2012; Wilson et al., 2012) (Figure F4). Furthermore, new quantitative techniques have greatly enhanced the biostratigraphic framework of the Southern Ocean (Cody et al., 2008, 2012; Crampton et al., 2016). Glacially reworked volcanic clasts (Wilson et al., 2012) and radiometrically datable felsic ashes from Marie Byrd Land may be used to provide maximum ages (Wilch et al., 1999). Climate snapshots near magnetic reversals will be targeted (e.g., the M2 glacial in Figure F4) because these events can be traced to more continuous records from the continental rise (Site U1524) and global sea level records. Sediment provenance studies (clast/sand petrology, mineral thermochronology, and radiogenic isotope analysis) at Sites U1521 and U1522 will enable understanding of the changes in the origin of sediments (e.g., local ice caps versus ice sheet expansion and provenance from East versus West Antarctic sources) (Figure F7) (Licht et al., 2005, 2014; Talarico et al., 2012; Cook et al., 2013; Perotti et al., 2017; Simões Pereira et al., 2018). As with all objectives, data integration with modeling studies will be undertaken (Figure F4) (Golledge et al., 2012; Wilson et al., 2013; DeConto and Pollard, 2016; Gasson et al., 2016; Colleoni et al., 2018b).

Reconstruct ice-proximal atmospheric and oceanic temperatures to identify past polar amplification and assess its forcings/feedbacks.

Obtaining atmospheric and ocean temperatures from the Expedition 374 drill sites will enable the paleoclimate community to address the following key scientific questions:

Were polar temperatures sensitive to the low-amplitude variations in Neogene atmospheric pCO₂?
Were Neogene ocean and atmospheric temperatures at the Antarctic margin amplified relative to the global mean, and if so, what were the forcings?
How did ocean temperatures evolve as Antarctica’s ice sheets expanded and contracted during major Neogene climate transitions and on orbital timescales?

Although the ANDRILL records provide important archives of high-latitude oceanic conditions, they only provide snapshots of temperature and sea ice conditions through interglacials, when these sites were not covered by ice (Warny et al., 2009; McKay et al., 2012a; Levy et al., 2016). In contrast, Sites U1521 and U1522 provide intervals of more continuous sedimentation (albeit with some periods of erosion during large glaciations) because they are in paleobathymetric depressions that provided the accommodation space needed for preservation after ice expansion (De Santis et al., 1995). These sites are also farther out on the continental shelf, near the paleoshelf break (Figure F7), and are therefore less likely to be frequently overridden by grounded ice (Pollard and DeConto, 2009; Bart et al., 2011). More continuous records of oceanographic change were recovered at Sites U1523–U1525, providing high-latitude information on late Neogene and Quaternary ocean and atmospheric temperatures, meltwater input, and bottom water production.

Lithofacies analysis will be used to reconstruct glacial thermal regimes and glacial cyclicity (cf. McKay et al., 2009; Naish et al., 2009), whereas diatom and radiolarian census counts, marine and terrestrial palynology, organic biomarkers (e.g., TEX₈₆, branched and isoprenoid tetraether [BIT] index, and methylation of branched tetraether [MBT]/cyclization of branched tetraether [CBT] proxy; cf. McKay et al., 2012a; Levy et al., 2016; Sangiorgi et al., 2018), and redox-sensitive metals (e.g., Mn, U, Re, and Mo) provide records for paleoproductivity, high-latitude marine and terrestrial climate, and oceanography. Lithofacies analysis is also relevant to monitoring alteration of the biomarkers by methanogenesis and shifting redox boundaries. Biogenic carbonate (e.g., calcareous nannofossils/foraminifers) is also present in Expedition 374 Neogene and upper Pleistocene interglacial sequences and at other circum-Antarctic sites (e.g., Theissen et al., 2003; Scherer et al., 2008; Villa et al., 2008; Expedition 318 Scientists, 2011). In the lower to middle Miocene, biogenic carbonate is more common in the Southern Ocean and Antarctic margins (Hayes et al., 1975; Kennett and Barker, 1990; Shipboard Scientific Party, 2001; Shevenell et al., 2004; Expedition 318 Scientists, 2011; Fielding et al., 2011), making stable isotope (δ¹⁸O and δ¹³C), trace element (e.g., Mg/Ca, Li/Ca, U/Ca, Ba/Ca, and B/Ca), and clumped isotope analyses possible. However, for all of the proxies discussed above, their strengths/weaknesses in a marginal marine setting should be considered.

Assess the role of oceanic forcing (e.g., sea level and temperature) on WAIS stability/instability.

WAIS retreat events during past warmer-than-present climates may have been the consequence of intensified ocean–cryosphere interactions (Naish et al., 2009; Pollard and DeConto, 2009). Interactions between the wind-driven upwelling of warm CDW and the ice shelves that buttress the WAIS appear to play a significant role in modern ice mass loss in West Antarctica (e.g., Mercer, 1978; Joughin et al., 2012; Pritchard et al., 2012; Shepherd et al., 2012). Observations and numerical ice sheet models suggest that changes in ocean heat flux are a key factor influencing the stability/instability of the WAIS (Golledge et al., 2012; Pritchard et al., 2012; DeConto and Pollard, 2016; Rintoul et al., 2018). We postulate that changes in either the formation of AASW and AABW or the vigor of the wind-driven ASC control incursions of CDW onto the Ross Sea continental slope and shelf and the resultant retreat of the WAIS (Figure F5). The records from Expedition 374 will allow us to test this hypothesis by simultaneously assessing AASW/AABW/ASC changes from grain size and lithofacies analysis of sediment mounds on the continental shelf bank, slope, and rise (e.g., Sites U1523–U1525) and ice sheet extent in the Ross Embayment (Sites U1521 and U1522, DSDP Site 272, and ANDRILL sites).

The Ross Sea is also one of the three main sources of AABW that feed the abyssal ocean (to become Southern Component Water [SCW]). In the middle Miocene, benthic foraminifer δ¹³C indicates changes in the relative input of SCW and Northern Component Water (NCW) into the global ocean (Woodruff and Savin, 1985; Wright et al., 1991; Shevenell et al., 2004; Cramer et al., 2009). Newer proxies, such as Nd isotopes, are now used to further refine the geographic source of deep-water production (e.g., van de Flierdt et al., 2004; Scher and Martin, 2006; Newkirk and Martin, 2009; Huck et al., 2017). Thus, the records obtained during Expedition 374 and comparison with far-field records will provide insight into temporal changes in SCW production through the Neogene (Vincent and Berger, 1985; Flower and Kennett, 1994; Hodell and Venz-Curtis, 2006).

Paleocurrent strength associated with past ASC changes and the history of AABW transport through the Hillary Canyon will be reconstructed by examining the sedimentological (e.g., lithofacies analysis and grain size) and magnetic characteristics of Sites U1523–U1525 (e.g., Bianchi et al., 1999; Hall et al., 2001; Prins et al., 2002; Joseph et al., 2004). Micropaleontological, geochemical, and sedimentological records from drill cores from all Expedition 374 sites will provide reconstructions of changing regional surface and seafloor conditions (e.g., sea ice, surface stratification, sea-surface temperatures [SSTs], polynya mixing, glacial meltwater discharge, nutrient uptake, and supercooling of dense water by ice shelves) proximal to the AISs (e.g., Shevenell et al., 2011; McKay et al., 2012a; Houben et al., 2013; Levy et al., 2016; Sangiorgi et al., 2018) and thus AABW (and SCW) formation. Additionally, downslope currents resulting from the transfer of High-Salinity Shelf Water into the abyssal ocean can also be assessed (and distinguished from ASC flow) by integrated lithofacies analysis, geochemistry, micropaleontology, and seismic-reflection profiles (e.g., Hepp et al., 2006; Lucchi and Rebesco, 2007; Caburlotto et al., 2010; Kim et al., 2018) at Sites U1523–U1525. Carbonate-based paleotemperature and carbonate ion proxies (e.g., foraminiferal Mg/Ca, Li/Ca, U/Ca, and clumped isotopes) will also be applied where appropriate species are preserved (see Objective 2).

Identify the sensitivity of WAIS to Earth’s orbital configuration under a variety of climate boundary conditions.

Fundamental questions remain about the orbital pacing of AIS development and variability. The Ross Sea Expedition 374 sequences will shed light on the absence of the 20 ky precession cycle in benthic δ¹⁸O records (Figure F4) (Lisiecki and Raymo, 2005; Huybers, 2006; Raymo et al., 2006) and the origin of transient shifts in the sensitivity of Earth’s climate system to orbital forcing (e.g., 40–100 ky–dominated frequencies) in the middle Miocene (Shevenell et al., 2004) and Plio–Pleistocene (Tzedakis et al., 2017).

A recent hypothesis suggests that the last such shift in Earth history (the mid-Pleistocene transition) was initiated by an abrupt increase in Antarctic ice volume (Elderfield et al., 2012). This hypothesis may be tested by identifying and dating grounding events on the outer Ross Sea continental shelf. If the dominant frequency of AIS advance and retreat shifted from 40 to 100 ky at 0.8 Ma, records from this sensitive region will likely record this transition.

Recent evidence from ice-proximal drill sites indicates that AISs did advance and retreat with 40 and 100 ky cyclicity in the Neogene (Grützner et al., 2003; Williams and Handwerger, 2005; Naish et al., 2009; Patterson et al., 2014). We envision a more complete picture of the forcings and feedbacks involved with ice advance and retreat from our outer shelf to slope/rise transect. Sedimentological analyses (complemented by downhole logs) will enable development of an orbital-scale continental shelf-to-rise sequence stratigraphy of glacial advance and retreat (all Expedition 374 sites; see Objective 1). Additional micropaleontologic data, inorganic and organic geochemistry (e.g., δ¹⁸O, δ¹³C, Mg/Ca, and ε_Nd), minor and trace elements (X-ray fluorescence and discrete samples), and organic biomarkers may be used to assess associated frequencies of change in the outermost continental shelf, slope, and rise sites (U1523–U1525).

Reconstruct eastern Ross Sea bathymetry to examine relationships between seafloor geometry, ice sheet stability/instability, and global climate.

The transition from a terrestrial (or shallow marine) West Antarctica with a seaward-dipping shallow continental shelf to that of the modern overdeepened (i.e., landward dipping) continental shelf would have had a first-order control on AIS volume and mass balance (Wilson et al., 2013; Gasson et al., 2016). The cooling threshold for the development of a terrestrial-based ice sheet is lower than that of a marine-based ice sheet, which is highly sensitive to changes in oceanic heat flux (Figure F4) (Pollard and DeConto, 2009; Golledge et al., 2012; Colleoni et al., 2018a). A terrestrial (or shallow marine) West Antarctica may have supported a larger ice sheet in warmer-than-present climates, whereas overdeepening of the continental shelves may have resulted in a smaller ice sheet with less frequent ice sheet advances, as hypothesized for the Antarctic Peninsula (e.g., Bart and Iwai, 2012). Ice sheet models indicate that a largely terrestrial West Antarctica could accommodate an extra ~13 million km³ of grounded ice in the warmer-than-present climate of the Eocene (~30 m SLE; Figure F4) (Wilson et al., 2013). Therefore, constraining the timing of overdeepening in the Ross Sea is critical to reconcile far-field records of eustatic sea level variance into the late Neogene and Quaternary (see Objective 1).

The timing of Ross Sea shelf overdeepening is currently unconstrained. However, the Ross Sea has the most developed seismic framework in Antarctica and the highest resolution history of WAIS variability currently available (ANDRILL), making this location ideal for achieving this objective. Dating the sedimentary sequence and providing information about the depositional environment at the time of formation of RSU3 and RSU2 (Figure F6) at Sites U1521 and U1522 in the eastern Ross Sea (by methodologies in Objective 1) will constrain the timing of this overdeepening (Figure F7).

Site summaries

Site U1521

Background and objectives

Site U1521 (proposed Site EBOCS-01D) is located in the Pennell Basin at 75°41.0351′S, 179°40.3108′W at 562 m water depth (Figure F9). The site is on the continental shelf near a northeast–southwest-oriented Miocene paleotrough identified by mapping regional unconformities in seismic-reflection profiles. The primary objective at Site U1521 was to sample and date strata above and below RSU4, which was previously identified in seismic stratigraphic studies (Hinz and Block, 1984; Anderson and Bartek, 1992; Cooper et al., 1997) (Figure F10). RSU4 is a regional angular unconformity (Figure F8) that has been mapped over most of the Ross Sea and is interpreted to represent a cross-shelf expansion of the EAIS in the central regions of the Ross Sea (Brancolini et al., 1995; De Santis et al., 1995). Coring through RSU4 should improve the broad age constraints provided for the unconformity by DSDP Sites 272 and 273 (Bart, 2003).

Figure F9. Site U1521 location.

Figure F10. Seismic-reflection profile and interpretation, Site U1521.

Records from this site will directly address all five primary Expedition 374 objectives. The record of ice advance and retreat from Site U1521, when integrated with seismic stratigraphy, provenance studies, and numerical modeling, will allow for a more complete assessment of Antarctica’s contributions to numerous variations in far-field Miocene sea level (Objective 1) (Miller et al., 2005; John et al., 2011). The magnitude of polar amplification during the MCO will be assessed through proxies for oceanic and atmospheric temperatures derived from both in situ marine organic matter and meltwater-transported terrestrial organic matter (e.g., micropaleontology and lipid biomarkers) (Objective 2). Insights into the oceanic drivers of early Miocene retreat of marine-terminating ice sheets and their subsequent middle to late Miocene readvances can be achieved by detailed lithofacies analysis. This information can be paired with micropaleontological and geochemical proxies to identify the magnitude of the oceanic change and to understand the effect of ocean heat on ice sheet variability (Objective 3). Similarly, these methods, when combined with downhole logging results to complete the stratigraphy, will identify the sensitivity of middle Miocene ice sheets to orbital forcing based on the presence of cyclicity in the open-marine, glaciomarine, and glacial proximal sediments (Objective 4). Finally, a “backstripped” paleobathymetry established with accurate dating and depth estimates of the RSU4 surface and other unconformities at Site U1521, in combination with previous drilling constraints from other locations in the Ross Sea, will allow for a fuller assessment of bathymetric controls on AIS evolution.

Operations

After a 2014 nmi transit from Lyttelton, the vessel arrived at Site U1521 at 0954 h (UTC + 13 h) on 16 January 2018. The original operations plan for Site U1521 called for a single rotary core barrel (RCB) hole to 950 m drilling depth below seafloor (DSF) and downhole logging with the triple combination (triple combo), Formation MicroScanner (FMS)-sonic, and Vertical Seismic Imager (VSI) tool strings. We ultimately cored Hole U1521A to 650.1 m DSF (Cores 374-U1521A-1R through 71R) (Table T2). Coring proceeded without incident through Core 37R (0–352.5 m DSF); however, after poor recovery in Cores 32R through 37R (14.49 m; 25%) caused by jamming of the core barrel with clasts, we switched to cutting half-length cores to improve core recovery. Cores 38R through 43R (352.5–381.3 m DSF) were collected using a 4.8 m advance (instead of 9.6 m), which significantly improved recovery (80%). We switched back to 9.6 m advances after the formation became less indurated and continued to core with the RCB system from Core 44R through Core 71R (381.3–650.1 m DSF). We terminated coring at that depth after meeting the primary scientific objectives. At the end of coring operations, we displaced the hole with heavy (10.5 lb/gal) mud and logged with all three tool strings, all of which reached to within ~1 m of the bottom of the hole. Operations at Site U1521 concluded at 2245 h on 21 January. A total of 132.75 h (5.5 days) were spent at Site U1521. RCB coring in Hole U1521A penetrated to 650.1 m DSF, recovering 411.50 m of core (63%).

Principal results

The 648 m succession of lower Miocene to recent sediment recovered at Site U1521 is divided into seven lithostratigraphic units (I–VII). Unit VI is further divided into Subunits VIA–VIC (Figure F11). Contacts between lithostratigraphic units at Site U1521 range from sharp to gradational. Mudstone and diamictite account for ~90% of the recovered core, and minor lithofacies include chert and conglomerate (Figure F11). The assemblage of facies reflects open-marine to ice-proximal depositional environments at this location since at least the early Miocene (~18 Ma).

Figure F11. Site U1521 summary.

Lithostratigraphic Unit I (0–7.4 m core depth below seafloor, Method A [CSF-A]) consists of a sequence of unconsolidated olive to dark grayish brown diatom-rich mud with dispersed clasts and gray diatom-bearing sandy mud. Unit II is ~80 m thick and consists of dark gray massive clast-poor muddy diamictite interbedded with bioturbated diatom-bearing/rich mudstone with dispersed clasts. There are several intervals of physically intermixed diatom-bearing/rich mudstone and diamictite with microfaulting and soft-sediment deformation features. Unit III (~124 m thick) consists of bioturbated olive gray to greenish gray diatom-bearing/rich mudstone. Some intervals are faintly laminated, and whole bivalve shells and shell fragments are present throughout. Unit IV consists of ~70 m of dark gray massive diatom-bearing clast-poor sandy diamictite with basalt clasts. Mudstone and diamictite are interbedded at the top and bottom of the unit, and a conglomerate composed of elongated mudstone intraclasts is observed at the base of the unit. Unit V is a 40 m interval of poorly recovered chert nodules and silica cemented mudstone. Unit VI (~245 m thick) is characterized by interbedded mudstone and diamictite that is divided into three subunits based on the style of interbedding. Subunit VIA (~56 m thick) consists of interbedded dark gray massive diamictite with silica-cementing and mudstone, whereas Subunit VIB (~60 m thick) consists of massive to stratified dark gray sandy diamictite with shell fragments, calcium carbonate nodules, and three intervals of clast-supported conglomerate. Subunit VIC (~128 m thick) is a sequence of interbedded mudstone with dispersed clasts and sandy diamictite with carbonate concretions and shell fragments. Unit VII consists of ~70 m of interbedded massive to stratified sandy diamictite.

Micropaleontological investigation included examination of core catcher samples and additional samples from split core sections for diatoms, radiolarians, foraminifers, calcareous nannofossils, and palynomorphs (including dinocysts, pollen, and spores) to obtain biostratigraphic ages and preliminary paleoenvironmental information. The presence and abundances of the microfossil groups vary significantly throughout the recovered sediment column (Figure F11). Rich assemblages of diatoms are present in the upper ~280 m CSF-A. Below that depth, diatoms are altered to opal-CT or are poorly preserved and offer minimal biostratigraphic constraint. Radiolarians are rare in the upper ~30 m CSF-A, and their abundance decreases significantly downhole. Foraminifers and palynomorphs are generally sparse throughout the hole, and calcareous nannofossils are only present in trace numbers deeper than ~525 m CSF-A. All microfossil group assemblages represent a combination of in situ and reworked taxa of different ages.

Although all investigated microfossil groups provide valuable age and/or paleoenvironmental information, diatoms provide the best age constraints, particularly in the upper ~280 m CSF-A (Figure F11). Shallower than 75 m CSF-A, age assignment is difficult because of reworking; however, diatom and radiolarian assemblages suggest a late Pliocene to Pleistocene/recent age for the uppermost 25 m CSF-A. An unconformity at ~25 m CSF-A separates Pliocene sediment from middle Miocene sediment. Age-diagnostic diatoms present between ~75 and 280 m CSF-A allow assignment of that interval to the Actinocyclus ingens and Denticulopsis maccollumii Zones (middle to early Miocene). Dinocysts and other aquatic palynomorphs confirm this age. Between ~280 and 380 m CSF-A, siliceous microfossils are altered to opal-CT. From ~380 to 580 m CSF-A, diatoms are present but preservation is poor, with valves occurring as silicified casts. Despite this lithology, the presence of Thalassiosira sp. cf. Thalassiosira bukryi and absence of Thalassiosira praefraga indicate an early Miocene age. Dinocysts are present over this interval, but the assemblage contains a mix of in situ and reworked taxa. Siliceous microfossils are absent deeper than 580 m CSF-A, whereas dinocysts and calcareous nannofossils are present and could indicate an early Miocene age if not reworked.

Foraminifers are generally sparse, and the assemblages are dominated by calcareous benthic species. The foraminifer assemblages represent distinct biofacies that vary in parallel with the lithofacies recovered at Site U1521. Well-preserved diatoms and dinocysts indicate a highly productive and seasonally stratified water column between ~75 and 280 m CSF-A (early to early middle Miocene). All microfossils will greatly contribute to constraining paleoceanographic and paleoproductivity changes that occurred through the early to middle Miocene in the Ross Sea.

Paleomagnetic investigations focused on measurements of archive-half core sections and oriented discrete (cube) samples. Natural remanent magnetization (NRM) measurements of archive-half core sections were conducted before and after progressive alternating field (AF) demagnetization, commonly in 5 mT increments up to 20 mT peak AF. After 20 mT peak AF demagnetization, NRM intensity usually ranges between ~10⁻⁴ and 10⁻²A/m and varies with lithology. Selected discrete samples were also used for initial NRM measurement, followed by NRM measurements after progressive AF demagnetization in 2–10 mT increments up to a peak AF of 80 mT. Principal component analysis (PCA) of these measurements reveals a stable characteristic remanent magnetization (ChRM) component in most of the samples, yielding normal and reversed polarities based on inclination data. Steep upward or downward inclinations are also present in most intervals of archive-half core data and agree with directions from discrete samples, suggesting a stable and reliable magnetic carrier that can be used for magnetostratigraphic interpretation. The identified normal and reversed polarity zones can be correlated to the geomagnetic polarity timescale (GPTS) with the aid of biostratigraphic datums.

Discrete samples were also used to measure the anisotropy of magnetic susceptibility (AMS) and mean bulk magnetic susceptibility. The AMS agrees well with variability in the lithostratigraphic units. For instance, AMS shows an oblate fabric that reflects sedimentary compaction in the diatom-bearing/rich mudstone of lithostratigraphic Unit III.

Physical property measurements were conducted on whole-round cores, split core sections, and discrete samples. In general, the whole-round core gamma ray attenuation (GRA) bulk density and magnetic susceptibility measurements show trends similar to those from discrete moisture and density (MAD) samples and point measurements of magnetic susceptibility on the section halves (Figure F11). A downhole increase in P-wave caliper (PWC) values generally coincides with an increase in bulk density and a decrease in porosity. Downhole variations in natural gamma radiation (NGR) also correlate well to changes in bulk density. Downhole changes in physical property characteristics are in good overall agreement with the lithostratigraphic units, which are defined based on sedimentological characteristics. In general, diatom-bearing/rich mud(stone) lithofacies (lithostratigraphic Units I–III and V) correspond with low magnetic susceptibility, NGR, thermal conductivity (<1 W/[m·K]), and bulk density and high porosity. The lowest grain density and highest porosity correspond with diatom-bearing/rich mudstone, which is the dominant facies in lithostratigraphic Unit III (Figure F11). In contrast, intervals that are predominantly diamictite (Units II, IV, VI, and VII) are characterized by increased NGR; high bulk density, P-wave velocity, and thermal conductivity; moderate to high magnetic susceptibility; and low to moderate porosity. Variations in magnetic susceptibility for clast-poor diamictite (low to moderate) and clast-rich diamictite (variable and generally high) suggest that magnetic susceptibility is an indicator of the abundance of glacially transported clasts or coarse sand in the sediment (Figure F11).

Headspace gas analyses show low methane and ethane concentrations in the upper 40 m CSF-A at Site U1521, and concentrations increase downhole to 67,000 ppmv methane and 264 ppmv ethane. Interstitial water analyses on 15 samples from the uppermost ~360 m CSF-A show a typical diagenetic profile with sulfate reduction and increasing ammonium concentration in the uppermost 76 m CSF-A. Alkalinity follows a profile similar to that of ammonium, and barium concentration increases deeper than ~76 m CSF-A. Total organic carbon (TOC), total nitrogen (TN), and carbonate (CaCO₃) contents are generally low (<1.2, 0.1, and <6 wt%, respectively) (Figure F11). TOC content is slightly higher in the mudstone facies in the upper 300 m CSF-A and decreases in the diamictite between ~330 and 640 m CSF-A. The TOC/TN ratio suggests mixed input of terrestrial- and marine-derived organic matter with no clear downhole trend. Elevated CaCO₃ content is only observed in the sandy diamictite of lithostratigraphic Unit IV, Subunit VIA, and Unit VII, which also have higher magnetic susceptibility. Elemental ratios obtained by handheld portable X-ray fluorescence (pXRF) scans show good agreement with major lithostratigraphic units but also suggest some deviations between chemical and lithologic boundaries.

Downhole logging consisted of three separate tool runs (triple combo, FMS-sonic, and VSI), all of which reached the bottom of the hole and proceeded without issue because of favorable borehole conditions. The triple combo measured NGR, density, porosity, resistivity, magnetic susceptibility, and borehole diameter. The caliper data show that the borehole was in excellent condition, with only one washed out interval between 221 and 226 m wireline matched depth below seafloor (WMSF). The NGR, bulk density, and porosity data match well with the data from core measurements, whereas the magnetic susceptibility data do not correlate as well, possibly as a result of temperature drift or other issues with the Magnetic Susceptibility Sonde (MSS). The second logging run consisted of the FMS-sonic tool string to measure resistivity borehole images and acoustic velocity. The latter shows good correlation with the data measured with the PWC, especially shallower than ~300 m CSF-A. The FMS images are extremely well resolved and clearly show clasts in the diamictite intervals and a poorly recovered layered interval of chert and mudstone in lithostratigraphic Unit V. The third logging run was with the VSI tool string (geophone) to conduct a vertical seismic profile (VSP) experiment. Raw check shot data were used in preliminary velocity models for an initial seismic-core-log correlation.