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Today, relatively warm Circumpolar Deep Water is melting Thwaites Glacier at the base of its ice shelf and at the grounding zone, contributing to significant ice retreat. Accelerating ice loss has been observed since the 1970s; however, it is unclear when this phase of significant melting initiated. We analyzed the marine sedimentary record to reconstruct Thwaites Glacier’s history from the early Holocene to present. Marine geophysical surveys were carried out along the floating ice-shelf margin to identify core locations from various geomorphic settings. We use sedimentological data and physical properties to define sedimentary facies at seven core sites. Glaciomarine sediment deposits reveal that the grounded ice in the Amundsen Sea Embayment had already retreated to within ~45 km of the modern grounding zone prior to ca. 9,400 y ago. Sediments deposited within the past 100+ y record abrupt changes in environmental conditions. On seafloor highs, these shifts document ice-shelf thinning initiating at least as early as the 1940s. Sediments recovered from deep basins reflect a transition from ice proximal to slightly more distal conditions, suggesting ongoing grounding-zone retreat since the 1950s. The timing of ice-shelf unpinning from the seafloor for Thwaites Glacier coincides with similar records from neighboring Pine Island Glacier. Our work provides robust new evidence that glacier retreat in the Amundsen Sea was initiated in the mid-twentieth century, likely associated with climate variability.

Silt-rich meltwater plume deposits (MPDs) analyzed from marine sediment cores have elucidated relationships that are clearly connected, yet difficult to constrain, between subglacial hydrology, ice-marginal landforms, and grounding-zone retreat patterns for several glacial catchments. Few attempts have been made to infer details of subglacial hydrology, such as flow regime, geometry of drainage pathways, and mode(s) of sediment transport through time, from grain-scale characteristics of MPDs. Using sediment samples from MPD, till, and grounding-zone proximal diamicton collected offshore of six modern and relict glacial catchments in both hemispheres, we examine grain shape distributions and microtextures (collectively, grain micromorphology) of the silt fraction to explore whether grains are measurably altered from their subglacial sources via meltwater action. We find that 75 % of all imaged grains (n = 9400) can be described by 25 % of the full range of measured shape morphometrics, indicating grain shape homogenization through widespread and efficient abrasive processes in subglacial environments. Although silt grains from MPDs exhibit edge rounding more often than silt grains from tills, grain surface textures indicative of fluvial transport (e.g., v-shaped percussions) occur in only a modest number of grains. Furthermore, MPD grain surfaces retain several textures consistent with transport beneath glacial ice (e.g., straight or arcuate steps, (sub)linear fractures) in comparable abundances to till grains. Significant grain shape alteration in MPDs compared to their till sources is observed in sediments from glacial regions where (1) high-magnitude, potentially catastrophic meltwater drainage events are inferred from marine sediment records and (2) submarine landforms suggest supraglacial melt contributed to the subglacial hydrological budget. This implies that quantifiable grain shape alteration in MPDs could reflect a combination of high-energy flow of subglac

Detrital smectite is a ubiquitous clay mineral in marine sediments and has a variable total Fe, Al, and Mg composition depending on the source, i.e., bedrock or unconsolidated sedimentary strata, the smectite is derived from. Analyses of elemental composition of smectite minerals in marine sediments can help to differentiate the smectite sources and, thus, sediment provenance, with potentially far-reaching paleo-environmental implications. In this study, we investigated the smectite compositions of modern-latest Holocene seafloor surface sediments deposited offshore from the Amundsen Sea drainage sector, where the West Antarctic Ice Sheet is currently unstable and losing mass due to ocean-forced melting, to detect and understand the variabilities of their elemental composition and geographical distribution. The smectite composition in continental rise sediments was compared to that of potential source areas on the Amundsen Sea continental shelf and in the sub-Antarctic South Pacific basin. Furthermore, we analyzed the smectite composition of sediments in continental rise cores deposited during the last glacial and last interglacial periods to reconstruct source variations over glacial- interglacial cycles. In particular, high contents of Al–Fe-rich smectite indicate that the glacial sediments had been supplied from the coastal region around Pine Island Bay by the cross-shelf advance of the West Antarctic Ice Sheet during the last glacial period. This clearly contrasts with the smectite minerals in rise sediments deposited during the present and the last interglacial periods that are composed of two distinct smectite types (Mg-rich and Al-rich, respectively), indicating their supply from multiple sources. During interglacials, Mg-rich smectite is probably transported by Circumpolar Deep Water from the sub-Antarctic South Pacific basin to the continental rise, while Al-rich smectite is supplied as part of ice rafted debris and by marine currents from the coasts around

Knowledge of the behaviour of marine-based ice sheets during times of climatic warming, such as the last deglaciation, provides important information to understand how ice sheets respond to external forcing. We analysed swath bathymetric and acoustic sub-bottom profiler data from Wrigley Gulf on the western Amundsen Sea shelf, West Antarctica, to identify glacial features and reconstruct past changes in the extent of the West Antarctic Ice Sheet (WAIS) and ice flow directions. Glacial bedforms mapped within a bathymetric cross-shelf trough include features showing cross-cutting and overprinting relationship and indicate changes in ice-flow orientation. Here, we distinguish at least two phases of different ice-flow patterns on the Wrigley Gulf shelf. During the earlier phase, seaward ice stream flow on the inner shelf was deflected towards the east due to the existence of an ice dome on the middle-outer continental shelf. Retreat of grounded ice towards the centre of this dome is indicated by the asymmetric cross profile of recessional moraines mapped on the middle shelf. The later glaciation phase was characterized by fast, NNW-directed ice flow across the shelf along a broad front and subsequent stepwise landward retreat, which is evident from the common occurrence and orientation of mega-scale glaciation lineations and grounding zone wedges on the middle-inner shelf. It is uncertain whether the two phases of glaciation recorded on the seafloor occurred during the last and penultimate glacial periods or at different times of the last glaciation. Reliable chronological constraints from sediment cores and additional geomorphological information are needed to understand the cause of the changes in WAIS dynamics reflected by the two ice-flow phases.

Understanding the recent history of Thwaites Glacier, and the processes controlling its ongoing retreat, is key to projecting Antarctic contributions to future sea-level rise. Of particular concern is how the glacier grounding zone might evolve over coming decades where it is stabilized by sea-floor bathymetric highs. Here we use geophysical data from an autonomous underwater vehicle deployed at the Thwaites Glacier ice front, to document the ocean-floor imprint of past retreat from a sea-bed promontory. We show patterns of back-stepping sedimentary ridges formed daily by a mechanism of tidal lifting and settling at the grounding line at a time when Thwaites Glacier was more advanced than it is today. Over a duration of 5.5 months, Thwaites grounding zone retreated at a rate of >2.1 km per year—twice the rate observed by satellite at the fastest retreating part of the grounding zone between 2011 and 2019. Our results suggest that sustained pulses of rapid retreat have occurred at Thwaites Glacier in the past two centuries. Similar rapid retreat pulses are likely to occur in the near future when the grounding zone migrates back off stabilizing high points on the sea floor.

How greenhouse forcing affects ice sheets and sea level during peak interglacial times is an unresolved question of great societal importance. Here we examine the development of glacial-interglacial sequence motifs in drill cores from the Pacific margin of Antarctica and explore how the West Antarctic Ice Sheet impacted sedimentary systems under different climate states, including during the Pliocene Climatic Optimum (PCO) ~4.5-4.0 Ma. Hundreds of meters of Pliocene sediments were recovered during IODP Expedition 379 in two drill sites within the Resolution Drift. Site U1533 was positioned near a submarine channel originating from the Amundsen Sea continental slope, and Site U1532 was located on the western upper flank of the hemipelagic drift. Abundance and type of sediment transport structures in the cores change gradually in lower Pliocene strata in both drill sites in concert with changes in sedimentation rate. Pliocene strata are characterized with a repetitive facies stacking pattern composed of greenish gray clast-bearing mud with a biosiliceous component, interbedded with dark brownish gray laminated silty clay. The greenish gray clast-bearing muds are tentatively interpreted as “interglacials”, however, many greenish grey units have sharp upward transitions into “glacial” laminated mud and the units are irregularly spaced with depth. Analysis of the red-green channel (a*) in shipboard reflectance spectroscopy and colorimetry (RSC) data for Site U1532 on the shipboard age model demonstrates the variable orbital cyclicity of the facies assemblage. Sequence motifs, which are further explored in half-core magnetic susceptibility and XRF core-scanner data, highlight how ice growth and decay, downslope sediment transport, and the position and intensity of ocean currents affected the depositional systems. The ultimate goal of this work is to combine facies interpretations with the detrital sedimentology and other data sets to assess the behavior of the West Antarc

The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet because of the great water depth and retrograde slope at the grounding line, the observed incursion of warm Circumpolar Deep Water onto the shelf, and the lack of substantial buttressing ice shelves. Notably, ice flowing into the Amundsen Sea embayment has been undergoing rapid changes over recent decades. International Ocean Discovery Program (IODP) Expedition 379 accomplished two successful drill sites on the continental rise of the Amundsen Sea in January-March 2019, the first from this sector, despite significant logistical limitations, including persistent sea ice that prevented access to all proposed continental shelf sites and abundant mobile icebergs that forced loss of ~50% of drilling time. Site U1532 is located on a large sediment drift and penetrated to a depth of 794 m below seafloor with 90% recovery. Nearly continuous cores were collected from the Pleistocene into the upper Miocene. Site U1533 reached 383 m below seafloor (70% core recovery) in a more condensed sequence down to the upper Miocene at the lowermost flank of the same sediment drift. The cores from both sites contain unique records to study the cyclicity of ice sheet advance and retreat processes as well as ocean-bottom circulation and water mass changes. In particular, Site U1532 revealed a sequence of Pliocene lithofacies, with an excellent paleomagnetic record for very high-resolution, sub-orbital scale climate change studies of the previously sparsely sampled region. Coarse-grained sediments, interpreted as ice-rafted debris, were identified throughout all time periods recovered. Proximal sources in West Antarctica are confirmed for crystalline rock detritus in some intervals. Cyclicity interpreted to represent relatively warmer periods, variably characterized by higher microfossil abundance and higher counts of ice-rafted debris, alternating with transitional and col

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Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Quaternary Science Reviews 177 (2017):265-275, doi:10.1016/j.quascirev.2017.10.029., Most outlet glaciers of the Cordillera Darwin Icefield (CDI; Patagonia, 54⁰S) are currently transitioning from calving to land-based conditions. Whether this situation is unique to the modern climate or also occurred during the Holocene is entirely unknown. Here, we investigate the Holocene fluctuations of outlet glaciers from the northern flank of the CDI using a multi-proxy sedimentological and geochemical analysis of a 13.5 m long sediment core from Almirantazgo fjord. Our results demonstrate that sedimentation in Almirantazgo fjord started prior to 14,300 cal yr BP, with glacier-proximal deposits occurring until 13,500 cal yr BP. After 12,300 cal yr BP, most glaciers had retreated to land-locked locations and by 9800 cal yr BP, Almirantazgo fjord was a predominantly marine fjord environment with oceanographic conditions resembling the present-day setting. Our sediment record shows that during the first part of the Holocene, CDI glaciers were almost entirely land-based, with a possible re-advance at 7300–5700 cal yr BP. This is in clear contrast with the Neoglaciation, during which CDI glaciers rapidly re-advanced and shrank back several times, mostly in phase with the outlet glaciers of the Southern Patagonian Icefield (SPI). Two significant meltwater events, indicative of rapid glacier retreat, were identified at 3250–2700 and 2000–1200 cal yr BP, based on an increase in grain-size mode and related inorganic geochemical parameters. This interpretation is additionally supported by concomitant decreases in organic carbon of marine origin and in Cl counts (salinity), reflecting higher terrestrial input to the fjord and freshening of the fjord waters. Overall, our record suggests that CDI outlet glaciers advanced in phase with SPI glaciers during the Neoglaciation, and retreated far enough into their valleys twice to form large outwash plains. Our results also highlight the potential of fjord sediments to reconstruct glacier variability at high resolution on multi, This research was supported by an EU Marie Curie FP6 postdoctoral fellowship to S.B., by National Geographic Grant 8379-07 (to S.B.), by COPAS Center FONDAP Grant 150100007 and COPAS Sur-Austral CONICYT PIA PFB31 (to C.L and S.P), and by IDEAL Center FONDAP Grant 15150003 (to C.L.).

Author Posting. © The Oceanography Society, 2012. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 25, no. 3 (2012): 14-17, doi:10.5670/oceanog.2012.68., "Antarctic Oceanography in a Changing World" commemorates the twentieth anniversary of the commissioning of Research Vessel Icebreaker (RVIB) Nathaniel B. Palmer and the fifteenth anniversary of Antarctic Research and Supply Vessel (ARSV) Laurence M. Gould. The addition of these two Antarctic research vessels to the US fleet in the 1990s ushered in a new era of Antarctic oceanographic research for US scientists and their international collaborators. Although several US Coast Guard icebreakers in the Arctic and Antarctic waters conduct oceanographic research, their primary mission is icebreaking to facilitate access to land-based stations. The Palmer was, and remains to this day, the first and only purpose-built US research icebreaker in Antarctic service and has been serving sea-going scientists in all areas of Antarctica's seas for two decades. The Gould has afforded reliable year-round access to Palmer Station and has conducted oceanographic research in the Antarctic Peninsula area since 1997.