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Abstract 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. 

Abstract. Benthic foraminiferal assemblages are useful tools for paleoenvironmental studies but rely on the calibration of live populations to modern environmental conditions to allow interpretation of this proxy downcore. In regions such as the region offshore of Thwaites Glacier, where relatively warm Circumpolar Deep Water is driving melt at the glacier margin, it is especially important to have calibrated tracers of different environmental settings. However, Thwaites Glacier is difficult to access, and therefore there is a paucity of data on foraminiferal populations. In sediment samples with in situ bottom-water data collected during the austral summer of 2019, we find two live foraminiferal populations, which we refer to as the Epistominella cf. exigua population and the Miliammina arenacea population, which appear to be controlled by oceanographic and sea ice conditions. Furthermore, we examined the total foraminiferal assemblage (i.e., living plus dead) and found that the presence of Circumpolar Deep Water apparently influences the calcite compensation depth. We also find signals of retreat of the Thwaites Glacier Tongue from the low proportion of live foraminifera in the total assemblages closest to the ice margin. The combined live and dead foraminiferal assemblages, along with their environmental conditions and calcite preservation potential, provide a critical tool for reconstructing paleoenvironmental changes in ice-proximal settings. 

Abstract. 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 subglacial meltwater, persistent sediment entrainment, and/or long sediment transport distances through subglacial drainage pathways. Integrating grain micromorphology into analysis of MPDs in site-specific studies could therefore aid in distinguishing periods of persistent, well-connected subglacial discharge from periods of sluggish or disorganized drainage. In the wider context of deglacial marine sedimentary and bathymetric records, a grain micromorphological approach may bolster our ability to characterize ice response to subglacial meltwater transmission through time. This work additionally demonstrates that glacial and fluvial surface textures are retained on silt-sized quartz grains in adequate amounts for microtexture analysis, which has heretofore been conducted exclusively on the sand fraction. Therefore, grain microtextures can be examined on silt-rich glaciogenic deposits that contain little to no sand as a means to evaluate sediment transport processes. 

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. 

Abstract To increase inclusivity, diversity, equity and accessibility in Antarctic science, we must build more positive and inclusive Antarctic field work environments. The International Thwaites Glacier Collaboration (ITGC) has engaged in efforts to contribute to that goal through a variety of activities since 2018, including creating an open-access ‘Field and Ship Best Practices’ guide, engaging in pre-field season team dynamics meetings, and surveying post-field season reflections and experiences. We report specific actions taken by ITGC and their outcomes. We found that strong and supported early career researchers brought new and important perspectives regarding strategies for transforming culture. We discovered that engaged and involved senior leadership was also critical for expanding participation and securing funding to support efforts. Pre-field discussions involving all field team members were particularly helpful for setting expectations, improving sense of belonging, describing field work best practices, and co-creating a positive work culture. 

Abstract 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. 

Abstract Terrestrial climate records for Antarctica, beyond the age limit of ice cores, are restricted to the few unglaciated areas with exposed rock outcrops. Marine sediments on Antarctica's continental shelves contain records of past oceanic and terrestrial environments that can provide important insights into Antarctic climate evolution. The SHALDRIL II (Shallow Drilling on the Antarctic Continental Margin) expedition recovered sedimentary sequences from the eastern side of the Antarctic Peninsula of late Eocene, Oligocene, middle Miocene, and early Pliocene age that provides insights into Cenozoic Antarctic climate and ice sheet development. Here, we use biomarker data to assess atmospheric and oceanic temperatures and glacial reworking from the late Eocene to the early Pliocene. Analyses of hopanes andn‐alkanes indicate increased erosion of mature (thermally altered) soil biomarker components reworked by glacial erosion. Branched glycerol dialkyl glycerol tetraethers from soil bacteria suggest similar air temperatures of 12°C ± 1°C (1σ,n = 46) for months above freezing for Eocene, Oligocene, and Miocene timeslices but much colder (and likely shorter) periods of thaw during the Pliocene (5°C ± 1°C,n = 4) on the Antarctic Peninsula. TEX₈₆‐based (Tetraether index of 86 carbons) sea surface temperature estimates indicate ocean cooling from 7°C ± 3°C (n = 10) in the Miocene to 3°C ± 1°C (n = 3) in the Pliocene, consistent with deep ocean cooling. Resulting temperature records provide useful constraints for ice sheet and climate model simulations seeking to improve understanding of ice sheet response under a range of climate conditions. 

The Perseverance Drift, located in the Joinville-D' Urville Trough, northwestern Weddell Sea, records changes in ocean and sea ice conditions throughout the middle to late Holocene, with a record extending back to ca. 3400 yr BP. The 2562-cm composite record collected from a water depth of 806 m, documents the uppermost section of the 90-m thick sediment drift. Spring-blooming diatoms (Chaetoceros subg. Hyalochaete) are abundant through the sedimentary record. The greater proportion of Chaetoceros vegetative valves compared to resting spores indicates that the marine environment is highly productive, and nutrients generally are not limiting. Epiphytic diatoms, dominated by Cocconeis spp., are observed throughout JKC36, suggesting transport of algal detritus from shallower regions to the benthos. Three foraminiferal assemblages (FAs): Miliammina spp., Globocassidulina spp., and Paratrochammina bartami/Paratrochammina lepida/Portatrochammina antarctica characterize the benthic foraminiferal fauna and reflect affinities with water masses circulating across the Perseverance Drift and tolerance to corrosive bottom waters. The interval 3400–1800yr BP is marked by high abundances of Globocassidulina spp., indicating incursions of Weddell Sea Transitional Water over the drift site. This interval implies a period of “freshening” of the water column, coinciding with an open-marine or seasonally open-marine environment during the middle-to-late Holocene Climatic Optimum. The interval 1800 yr BP to the present displays characteristics of slightly colder conditions, as indicated by the absence of the calcareous Globocassidulina spp. FA, and the pronounced presence of agglutinated P. bartami/P. lepida/P. antarctica FA, along with other agglutinated species that are indicative of the presence of sea ice. Therefore, this interval is interpreted to represent the onset of Neoglaciation at the northeastern tip of the Antarctic Peninsula. The consistent presence of Miliammina spp. FA corroborates that the sedimentary record represents a productive, open-marine setting with seasonally variable sea ice extent. The Drift is a unique geologic archive that provides an excellent target for future coring based on the preservation of abundant carbonate material for radiocarbon dating and the potential to develop a multi-proxy data set that could offer a robust understanding of the Holocene depositional and paleoclimatic conditions of the northwestern Weddell Sea. 

Abstract Three drivers of subsidence are recognized in the Western Interior Basin: Mesozoic–early Cenozoic flexure adjacent to the thin‐skinned, eastward propagating Sevier Orogeny, Late Cretaceous–Eocene flexure associated with thick‐skinned Laramide Uplifts and Late Cretaceous dynamic subsidence. This study combines outcrop lithofacies, palaeocurrent measurements, detrital zircon geochronology, biostratigraphy, stratigraphic correlations and isopach maps of Coniacian–Maastrichtian (89–66 Ma) units to identify these subsidence mechanisms impact on basin geometry and stratigraphic architecture in the northern Utah to southwestern Wyoming segment of the North American Cordillera. Detrital zircon maximum depositional ages and biostratigraphy support that the Maastrichtian Hams Fork Conglomerate was deposited above the Moxa unconformity in the wedgetop and foredeep depozones. The Moxa unconformity underlies the progradational Ericson Formation in the distal foredeep. The Hams Fork, however, is younger than the Ericson Formation, and instead equivalent to upper Almond Formation. Therefore, the hiatus associated with the Moxa unconformity continued for several million years longer in the fold belt and proximal basin than in the distal foredeep, with Ericson Formation‐equivalent strata onlapping the Moxa unconformity towards the west. Regional thickness patterns record and constrain the timing of the transition from Sevier to Laramide‐style tectonic regimes. From 88 to 83 Ma (upper Baxter Formation) a westward‐thickening stratigraphic wedge characterized the foredeep developed by lithospheric flexure by thrust‐belt loading. Nevertheless, the presence of >500 m of subsidence >200 km from the thrust front suggests a long‐wavelength subsidence mechanism consistent with dynamic subsidence. By 83 Ma (Blair Formation) the long‐wavelength depocentre shifted away from the thrust belt, with no evidence of a Sevier foredeep. This depocentre continued migrating eastward during the early‐mid Campanian (ca. 81–77 Ma). The late Campanian–Maastrichtian (ca. 74–66 Ma) is marked by narrow sedimentary wedges adjacent to the Wind River, Granite and Uinta Mountain uplifts and attributed to flexural loading by Laramide deformation.