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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. [ABSTRACT FROM AUTHOR]

Southeastern Texas (United States) recorded one of the largest flooding events in U.S. history during Hurricane Harvey (25-31 August 2017), mobilizing millions of cubic meters of sediment in Houston fluvial-estuarine systems. We conducted an integrated quantitative analysis to determine the net minimum volume of sediment transported during the storm using subaerial elevation change, satellite and ground-based images, and sediment dredging reports along major waterways. The 12 fluvial-estuarine streams and two controlled reservoir drainages in the Houston area transported a minimum of ~2.723 × 107 m3 of sediment. This volume is ~6-51 times larger than the average annual volume of sediment delivered to Galveston Bay in modern times (in the past 200 yr), and ~30-118 times larger when compared to Holocene rates. Nearly ~26% of the measured volume was deposited in Addicks and Barker reservoirs, decreasing holding capacities by ~1.2% and ~1.6%, respectively. In the stream drainages, sediment was mobilized from west-northwest of Houston and pulsed toward Galveston Bay, highlighting the extreme short-term variability in sediment delivery. Sediment flux through the Houston region during Harvey is an example of sediment storage followed by a pulsed delivery of high sediment volume rather than continuous delivery of sediment. Comparison of sediment volumes transported through natural and modified drainages through Houston demonstrates that channel modification resulted in significant bypass of sediment downstream. Urban watershed management is more effective when continual updates are implemented based on regional circumstances rather than based on historical fluxes. [ABSTRACT FROM AUTHOR]

Glacial geomorphology is used in Antarctica to reconstruct ice advance during the Last Glacial Maximum and subsequent retreat across the continental shelf. Analogous geomorphic assemblages are found in glaciated fjords and are used to interpret the glacial history and glacial dynamics in those areas. In addition, understanding the distribution of submarine landforms in bays and the local controls exerted on ice flow can help improve numerical models by providing constraints through these drainage areas. We present multibeam swath bathymetry from several bays in the South Shetland Islands and the western Antarctic Peninsula. The submarine landforms are described and interpreted in detail. A schematic model was developed showing the features found in the bays: from glacial lineations and moraines in the inner bay to grounding zone wedges and drumlinoid features in the middle bay and streamlined features and meltwater channels in the outer bay areas. In addition, we analysed local variables in the bays and observed the following: (1) the number of landforms found in the bays scales to the size of the bay, but the geometry of the bays dictates the types of features that form; specifically, we observe a correlation between the bay width and the number of transverse features present in the bays. (2) The smaller seafloor features are present only in the smaller glacial systems, indicating that short-lived atmospheric and oceanographic fluctuations, responsible for the formation of these landforms, are only recorded in these smaller systems. (3) Meltwater channels are abundant on the seafloor, but some are subglacial, carved in bedrock, and some are modern erosional features, carved on soft sediment. Lastly, based on geomorphological evidence, we propose the features found in some of the proximal bay areas were formed during a recent glacial advance, likely the Little Ice Age. [ABSTRACT FROM AUTHOR]

During the Last Glacial Maximum, grounded ice from the expanded Antarctic Peninsula Ice Sheet extended across the continental shelf. Grounded and flowing ice created a distinctive array of glacial geomorphic features on the sea floor, which were then exposed as the ice sheet retreated. The recent disintegration of the northern parts of the Larsen Ice Shelf (Larsen A and B) have permitted acquisition of marine geophysical data in previously inaccessible and unmapped areas. We present a reconstruction of the evolving ice-flow path and ice sheet geometry of the eastern Antarctic Peninsula, with particular focus paid to newly surveyed areas that shed light on the dynamics of a marine-terminating glacial geomorphic environment, where ice shelves play a major role in grounding line stability. Shifting flow directions were mapped in several areas, including across the Seal Nunataks, which divide Larsen A and B, and offshore of Larsen C, indicating flow reorientation that reflects the changing ice sheet geometry as retreat neared the modern coastline. The measured flow indicators in this area reveal comparatively high elongation ratios (> 20), indicating rapid ice flow. Evidence of possible previous ice-shelf collapses are noted near the shelf break, further illustrating the critical, protective effect that ice shelves impart to marine-terminating glacial environments. Modern ice retreat is governed in part by reorganization of flow patterns accompanying grounding line movement; such reorganizations happened in the past and can aid understanding of modern processes. [ABSTRACT FROM AUTHOR]

This paper examines the variability in foraminiferal assemblages in surface samples from coastal settings along the northwestern Antarctic Peninsula, as well as in the Pine Island Bay area. Material comes from water-depths ranging between 20 and 1257 m, latitudes between 62° and 73°S, and bottom water temperatures from − 1.5 to 1.2 °C. The microfossil analysis within this broad geographical and environmental context provides a basis for paleo-environmental studies. Environmental affinities of the assemblages were interpreted based on a wide array of ecological conditions, including CTD data, as well as on faunal indices and ecological affinities of the key taxa. Six foraminiferal assemblages are dominated by calcareous taxa. Among other factors, they correspond to variable food supply, terrigenous sedimentation, and water mass properties. Another two assemblages are dominated by arenaceous foraminifera and reflect conditions corrosive to carbonate. Our findings indicate that preserved fossil assemblages in cores from these locations may not be accurate representations of living assemblages. [ABSTRACT FROM AUTHOR]

Abstract: Three Holocene core sections from Maxwell Bay, South Shetland Islands, West Antarctica are investigated, using MS, grain size, TOC, foraminifera, δ18O and δ13C, 14C, and 210Pb, in order to examine Holocene oceanographic, glacial and climatic changes in the region. They come from 190, 221, and 333m water-depths and the bay margin-proximal locations, complementing earlier studies from the deep central part. The three records vary between locations and reflect combined marine and terrigenous influence from the open ocean and surrounding islands, respectively. The longest and best dated composite section comes from the inner part of the bay (core JTC/JPC-17) and extends back to 6.3calkyr BP, close to the termination of the Holocene Climatic Optimum. A combination of dating problems and mixed foraminiferal assemblages prevented reliable interpretation of core JPC-15 from the outer part of the bay. Core JPC-16, collected from the inner bay near the inlet between King George and Nelson Islands, revealed repeated highs in planktonic Neogloboquadrina pachyderma occurrence during last 3000yr. These highs are interpreted as indicating periods of decreased meltwater flow into the bay, and thus greater influence of open-marine surface-waters. Core JTC/JPC-17, located closest to King George Island and the Collins Ice Cap, records increased glacial conditions at ~2000calyr BP. This event is recorded by turbidite deposition at core site JPC-16. Major components of the benthic foraminiferal assemblage indicate decreasing tolerance to near-glacier or near-shore conditions, however, local climate variability appears to have overall little impact on benthic foraminiferal biota. Thus, it is unlikely that, in similar settings, recent warming would be strongly manifest as changes in benthic foraminiferal communities. Stable isotopic analysis is based on Cassidulinoides parkerianus and Globocassidulina biora. Subtle decrease in δ18O at ~3500calyr BP, followed by episodic increases at ~1000calyr BP and in the youngest part of the record, appear to correlate with the period prior to the beginning of the Neoglacial, with Medieval Warm Period, and with post Little Ice Age warming, respectively. The latest event is also marked by a significant increase in sediment accumulation rate in the second half of the 20th century. [Copyright &y& Elsevier]

The detailed seafloor topography around Antarctica was virtually unknown until RVIB Nathaniel B. Palmer and other ships started to collect multibeam swath measurements in the 1990s. These new, high-resolution bathymetric data allowed the creation of topographic maps of the Antarctic continental shelf and rise with unprecedented detail. The multibeam data revealed glacial morphological features that allowed reconstruction of paleo-ice flow and maximum ice extent of the formerly glaciated continental margin. Furthermore, these data provided the basis for new insights into the continent's tectonic evolution and defined the nature of marine biological habitats in polar regions. [ABSTRACT FROM AUTHOR]

While tidal environments are ubiquitous around the planet, limited work on these systems, particularly those that are in microtidal settings, hinders our understanding of tidal paleo-environments. In this study, we analyze the modern sediment dynamics of the San Luis Pass area, a microtidal system located on the Texas Gulf Coast. The focus of this work is to characterize sediment distribution and establish a quantitative relationship between sedimentary structure geometry, water depth, sediment grain size, and current velocities. A total of 160 seafloor sediment samples were collected in the modern San Luis Pass tidal delta between July 2017 and August 2018; this time span coincided with the landfall of Hurricane Harvey in the area in August 2017. Ripple geometry was measured in 46 locations along the flood-tidal delta. There is a rapid transition in grain size, from proximal (fine sand) to distal (silt and very fine sand) sediments at about 4.3 km from the mouth of the tidal inlet. This change is likely due to a decrease in current velocities away from the inlet entrance, resulting in a change in sediment transport from a combined bedload/suspension to suspension. Post Hurricane Harvey sediments are finer grained compared to those collected before the hurricane landfall, most likely due to high runoff during the storm event. In addition, increased water depths were measured after the hurricane landfall, indicating a significant erosion in the San Luis Pass area. Current ripples were only present in the proximal (sandier) area. We found a positive correlation between ripple height/ripple wavelength and water depth. However, the correlation is much stronger for ripple wavelength, even when compared to other published ripple measurements. We, therefore, propose to use ripple wavelength as a more reliable means to estimate paleo-water depths instead of ripple height. Nevertheless, more measurements from modern environments are necessary to establish a more robust geometric relation from ripples in a variety of paleoenvironments. • Shear stress and divergent current velocities control the grain size distributions. • Current ripple wavelength is a better proxy for water depth than ripple height. • Flood-tidal deltas are crucial coastal features as sediment sinks and erosion buffers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article focuses on drilling operations in 2005 and 2006 (SHALDRIL I and II) conducted from the icebreaking research vessel Nathaniel B. Palmer in the northern Antarctic Peninsula region. The operations were able to obtain complete climate records from a section of Holocene strata. It relates the recovery of a record of the initial onset of alpine glaciation in the northern Antarctic Peninsula in SHALDRIL II.

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 FROM AUTHOR]

Postglacial fluctuations of Southern Patagonian Icefield (SPI) glaciers are well constrained on the leeward side of the Andes, but they remain mostly unknown on the windward side of the icefield, where most glaciers are marine-terminating. Here, we reconstruct the postglacial fluctuations of the HPS19, Penguin, and Europa glaciers along the hyperhumid western side of the SPI using a multi-proxy sedimentological and geochemical analysis of a 12.2 m long sediment core from Wide Channel (50°S). Results show that the glaciers retreated into Penguin and Europa fjords by 11.2 cal kyr BP and that they were relatively stable and marine-terminating between 11.2 and 5.8 cal kyr BP. Thereafter, they fluctuated rapidly, with four marked episodes of glacier shrinkage at 5.8–4.8, 3.9–2.4, 1.0–0.2 cal kyr BP, and during the 20th century. Although the HPS19, Penguin, and Europa glaciers were calving into Penguin and Europa fjords during most of the Holocene, our data suggest that they retreated to land-based positions between 5.8 and 4.8 cal kyr BP. The comparison of our sediment record with geological archives from both sides of the Patagonian icefields (46°–56°S) suggests synchronous glacier variability on multi-centennial timescales during the Neoglacial period, which is particularly clear after 2.5 cal kyr BP. We conclude that western SPI outlet glaciers remained relatively stable during the first half of the Holocene but fluctuated considerably during the Neoglacial period, and that they retreated to locations further inland than today during the first retreat of the Neoglacial period between 5.8 and 4.8 cal kyr BP. • SPI outlet glaciers retreated into Penguin and Europa fjords by 11.2 cal kyr BP. • Glaciers were relatively stable between 11.2 and 5.8 cal kyr BP. • High glacier variability during the last 5.8 kyr. • Glacier shrinkage occurred at 5.8–4.8, 3.9–2.4, 1.0–0.2 cal kyr BP, and during the 20th century. • Glaciers on both sides of the Patagonian Andes fluctuated synchronously on multi-centennial timescales during the last 5.8 kyr. [ABSTRACT FROM AUTHOR]

We use extensive sedimentary and marine geophysical data to derive sediment volume--based millennial time-scale glacial erosion rates (Ē) from glacially influenced fjords and bays across a broad latitudinal transect, from central Patagonia (46°S) to the Antarctic Peninsula (65°S), and to determine how glacial erosion rates change with increasing latitude and decreasing atmospheric temperatures. We also calculate million-year time-scale erosion rates for the western Antarctic Peninsula cordillera and inner continental shelf from seismic stratigraphic analysis of the continental margin. These results are complemented by erosion rates derived from existing thermochronology data sets (apatite fission-track and apatite [U-Th]/He) for both Patagonia and the Antarctic Peninsula regions. Despite considerable regional variability, our results show a clear trend of decreasing Ē with increasing latitude. Millennial Ē values span two orders of magnitude, from 0.02 mm/yr for Illiad glacier on Anvers Island, Antarctica (~64.5°S), to 0.83 mm/yr for San Rafael glacier in northern Patagonia (~46.5°S). Regional averages are three times higher for the Patagonian areas than the Antarctic Peninsula areas. This trend is interpreted to result from a general decrease in temperature and water availability at the ice-bedrock interface. For the Antarctic Peninsula study sites, erosion rates are highly clustered around 0.1 mm/yr, with the exception of Maxwell Bay, for which the Ē value is 0.36 mm/yr. In Patagonia, erosion rates are more variable than in the Antarctic Peninsula, with Ē ranging between 0.14 mm/yr (Europa glacier area) and 0.83 mm/yr (San Rafael glacier area). This regional variability in Ē is interpreted as due to differences in hypsometry and bedrock resistance to erosion. Million-year time-scale Ē values derived from thermochronology ages also decrease with latitude, with maximum values decreasing from ~0.9-1.1 mm/yr north of 46°S to ~0.1-0.2 mm/yr south of 48°S in Patagonia, and reaching ~0.2-0.3 mm/yr in the Antarctic Peninsula. The sediment-based million-year time-scale Ē estimates for the western Antarctic Peninsula cordillera indicate that glacial erosion rates increased by 25%-30% after 5.3 Ma, from ~0.09 mm/yr (5.3-9.5 Ma) to ~0.11-0.12 mm/yr (<5.3 Ma). For Patagonia, the decrease in long-term erosion rates south of ~46°S is interpreted to result from relatively long periods of slow glacial erosion associated with the ice masses having been colder (subpolar) on the southern Patagonian cordillera, and having eroded at rates comparable to those we obtained for the Antarctic Peninsula. These long-term erosion rates are 1-2 orders of magnitude lower than estimates based on recent sediment yields, highlighting the transient nature of high-sediment-flux events. However, our sediment volume--derived millennial time-scale Ē closely approximates the maximum values of tectonic time-scale Ē values derived from thermochronology ages. Our combined millennial and million-year time-scale glacial erosion data quantify the significant decrease in rates of glacially driven denudation at geological (tectonic) and millennial time scales with increasing latitude from Patagonia to the Antarctic Peninsula, highlighting the influence of climate on mountain denudation. [ABSTRACT FROM AUTHOR]

We present the first study which directly compares carbonate radiocarbon (14C) dates with the Ramped PyrOx (RP) radiocarbon dating technique within a single sediment core, and we confirm the utility for the latter constructing chronologies of high latitude, Holocene marine margin sediment successions. Historically, the heavily detrital nature of Antarctic margin sedimentary organic material and lack of carbonate preservation have made these sediments difficult to date accurately. Here, we use archived cores with existing foraminiferal ages to compare with RP dates at equal or similar depth intervals. The lowest temperature RP splits were integrated over narrower intervals than in previous studies to reduce the amount of mixing with older, more thermochemically stable end-members during pyrolytic decomposition. Ages of the low-temperature RP splits coincide with their corresponding carbonate counterparts, suggesting that the RP 14C dating method is a reliable alternative to carbonate dates in sediments where carbonates are absent or not sufficiently preserved for 14C dating. The rarity of calcareous material in most Antarctic sediments often obligates the use of the bulk acid insoluble organic (AIO) fraction of the sediment, which can be problematic because of contamination by older carbon. The bulk AIO 14C ages, which are calculated using the weighted arithmetic mean of all RP splits of individual samples, show that age reversals and biases can occur using bulk AIO dates for age models because of variable proportions of pre-aged organic matter down-core. The application of the RP dating method can reduce these effects to produce a more reliable chronology that is statistically identical to the foraminiferal dating chronology. [ABSTRACT FROM AUTHOR]

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 the Amundsen Sea embayment. Our research demonstrates that analysis of the elemental composition of smectite minerals on the Amundsen Sea continental margin provides a valuable tool to trace variations in sources for detrital sediment components and their pathways throughout glacial-interglacial cycles. • Various types of smectite appeared in the continental margin sediments in the Amundsen Sea. • Elemental composition of smectite can differentiate sediment sources and their pathways during glacial-interglacial cycles. • New mineralogical proxy to trace the sediment provenance using smectite composition has been suggested. [ABSTRACT FROM AUTHOR]

The article presents an overview of oceanographic research aboard two Antarctic research vessels, namely, Research Vessel Icebreaker (RVIB) Nathaniel B. Palmer and the Antarctic Research and Supply Vessel (ARSV) Laurence M. Gould. It introduces a series of papers published in September 2012 issue of "Oceanography" devoted to Antarctic oceanography research. Information is provided on the predecessors of Palmer and Gould.

Abstract: A high-resolution record of Holocene deglacial and climate history was obtained from a 77m sediment core from the Firth of Tay, Antarctic Peninsula, as part of the SHALDRIL initiative. This study provides a detailed sedimentological record of Holocene paleoclimate and glacial advance and retreat from the eastern side of the peninsula. A robust chronostratigraphy was derived from thirty-three radiocarbon dates on carbonate material. This chronostratigraphic framework was used to establish the timing of glacial and climate events derived from multiple proxies including: magnetic susceptibility, electric resistivity, porosity, ice-rafted debris content, organic carbon content, nitrogen content, biogenic silica content, and diatom and foraminiferal assemblages. The core bottomed-out in a stiff diamicton interpreted as till. Gravelly and sandy mud above the till is interpreted as proximal glaciomarine sediment that represents decoupling of the glacier from the seafloor circa 9400cal. yr BP and its subsequent landward retreat. This was approximately 5000yr later than in the Bransfield Basin and South Shetland Islands, on the western side of the peninsula. The Firth of Tay core site remained in a proximal glaciomarine setting until 8300cal. yr BP, at which time significant glacial retreat took place. Deposition of diatomaceous glaciomarine sediments after 8300cal. yr BP indicates that an ice shelf has not existed in the area since this time. The onset of seasonally open marine conditions between 7800 and 6000cal. yr BP followed the deglacial period and is interpreted as the mid-Holocene Climatic Optimum. Open marine conditions lasted until present, with a minor cooling having occurred between 6000 and 4500cal. yr BP and a period of minor glacial retreat and/or decreased sea ice coverage between 4500 and 3500cal. yr BP. Finally, climatic cooling and variable sea ice cover occurred from 3500cal. yr BP to near present and it is interpreted as being part of the Neoglacial. The onset of the Neoglacial appears to have occurred earlier in the Firth of Tay than on the western side of the Antarctic Peninsula. The Medieval Warm Period and Little Ice Age were not pronounced in the Firth of Tay. The breadth and synchroneity of the rapid regional warming and glacial retreat observed in the Antarctic Peninsula during the last century appear to be unprecedented during the Holocene epoch. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Major ice loss in the Amundsen Sea sector of the West Antarctic Ice Sheet (WAIS) is hypothesized to have triggered ice sheet collapses during past warm periods such as those in the Pliocene. International Ocean Discovery Program (IODP) Expedition 379 recovered continuous late Miocene to Holocene sediments from a sediment drift on the continental rise, allowing assessment of sedimentation processes in response to climate cycles and trends since the late Miocene. Via seismic correlation to the shelf, we interpret massive prograding sequences that extended the outer shelf by 80 km during the Pliocene through frequent advances of grounded ice. Buried grounding zone wedges indicate prolonged periods of ice‐sheet retreat, or even collapse, during an extended mid‐Pliocene warm period from ∼4.2–3.2 Ma inferred from Expedition 379 records. These results indicate that the WAIS was highly dynamic during the Pliocene and major retreat events may have occurred along the Amundsen Sea margin. Plain Language Summary: Collapses of the West Antarctic Ice Sheet (WAIS) during past warm times are suggested to begin in the Amundsen Sea sector. During a drilling expedition of the International Ocean Discovery Program (IODP), deep‐sea sediments were retrieved from the Amundsen Sea. These sediment cores contain records of colder and warmer periods in the Pliocene (5.3–2.6 million years ago) which relate to the behavior of the WAIS. By analyzing seismic images that allow correlation of sediment layers from the drill sites to the continental shelf, we show that the shelf grew oceanward by 80 km due to the erosion of sediments below the WAIS and their deposition at the shelf break as the result of frequent Pliocene advances of grounded ice across the shelf. The shelf sediment layers contain grounding zone wedges predominantly formed during ice sheet retreat. The preservation of these grounding zone wedges testifies that they were not eroded by subsequent ice advances, with their burial requiring periods of prolonged WAIS retreat during warm intervals. This interpretation is consistent with our IODP drill core observations of a prominent decrease in terrigenous sedimentation between 4.2 and 3.2 million years ago, which indicates a highly dynamic WAIS during the Pliocene. Key Points: Seismic stratigraphic continental rise‐to‐shelf link from International Ocean Discovery Program (IODP) Expedition 379 drill sites on the Amundsen Sea riseMajor prograding shelf growth in the Amundsen Sea Embayment by frequent grounded ice advances in the early PlioceneExtended period of ice sheet retreat in mid‐Pliocene (4.2–3.2 Ma) from buried grounding zone wedges on shelf and core records from rise [ABSTRACT FROM AUTHOR]

Seismically-imaged clinoforms, constrained by well data, provide a unique opportunity to evaluate sedimentary processes using geometric considerations. The Lower Cretaceous of the Alaskan North Slope is a well-known example of large-scale (600 - 2500 m depositional relief), constructional, siliciclastic clinoforms. This study has depth-converted and decompacted a regional seismic profile in this area to evaluate the relationships among depositional slope angles, shelf-edge trajectories, seismic facies, sediment volume ratios, graded/erosional profiles, and lithologies. Insights from this analysis refute several assumptions about clinoform formation, and could help establish a quantitative methodology for clinoform analysis. Few published examples have been decompacted to estimate original depositional relief. In this study, seismically-imaged slopes of about 1.5° - 3.5° have approximately doubled in magnitude after backstripping, owing to their fine-grained nature and deep burial history. These slopes exceed the largest published estimates of constructional (“graded”) margins. North Slope clinoforms that show more evidence of slope erosion and bypass (out-of-grade) have, on average, lower slope angles than graded clinoforms. This observation is contrary to the assumption that higher slope angles correlate with more extensive erosion. In this study, we hypothesized that erosional modification has lowered the clinoform slope. On average, erosionally-modified clinothems have lower angle shelf-edge margin trajectories and thinner topsets relative to foresets/bottomsets. Additionally, erosionally-modified clinoforms are characterized by higher amplitude seismic facies. Well calibration indicates that the higher amplitude response results from interbedded sandstones and shales; low amplitude seismic facies, associated with graded margins, are more mud-prone. Out-of-grade clinoforms likely facilitate additional bypass/deposition of sediment downslope via sediment gravity flows. Flatter shelf margin trajectories indicate sediment supply exceeds accommodation; this may drive greater slope erosion and bypass. A predictive model, employing these clinoform geometric criteria, can assist in delineating intervals of enhanced bypass/submarine fan development. However, wide applicability beyond this example would require additional calibration and further testing. [ABSTRACT FROM AUTHOR]

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 3400–1800 yr 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 FROM AUTHOR]

The Antarctic Peninsula is considered to be the last region of Antarctica to have been fully glaciated as a result of Cenozoic climatic cooling. As such, it was likely the last refugium for plants and animals that had inhabited the continent since it separated from the Gondwana supercontinent. Drill cores and seismic data acquired during two cruises (SHALDRIL I and II) in the northernmost Peninsula region yield a record that, when combined with existing data, indicates progressive cooling and associated changes in terrestrial vegetation over the course of the past 37 million years. Mountain glaciation began in the latest Eocene (approximately 37-34 Ma), contemporaneous with glaciation elsewhere on the continent and a reduction in atmospheric CO2 concentrations. This climate cooling was accompanied by a decrease in diversity of the angiosperm-dominated vegetation that inhabited the northern peninsula during the Eocene. A mosaic of southern beech and conifer-dominated woodlands and tundra continued to occupy the region during the Oligocene (approximately 34-23 Ma). By the middle Miocene (approximately 16-11.6 Ma), localized pockets of limited tundra still existed at least until 12.8 Ma. The transition from temperate, alpine glaciation to a dynamic, polythermal ice sheet took place during the middle Miocene. The northernmost Peninsula was overridden by an ice sheet in the early Pliocene (approximately 5.3-3.6 Ma). The long cooling history of the peninsula is consistent with the extended timescales of tectonic evolution of the Antarctic margin, involving the opening of ocean passageways and associated establishment of circumpolar circulation. [ABSTRACT FROM AUTHOR]

Abstract: The marine geological record of the glacial-retreat history of the South Shetland Islands is derived from the integration of seismic data, core lithofacies, radiocarbon ages and geomorphological data from two study areas. On the northern shelf, we present one of the first published multibeam bathymetric images of the seafloor. This new image combined with descriptions of new and existing short sediment cores and high-resolution seismic data from the continental shelf north of King George Island are used to map a grounding zone wedge that marks the minimum seaward extent of the South Shetland Ice Cap during the Last Glacial Maximum. Seismic data, multibeam bathymetry, and sediment cores from Maxwell Bay, on the opposite side of King George Island, are used to provide further spatial constrains on the history of grounding and retreat of the ice cap along the southern margin of the South Shetland Islands. During the Last Glacial Maximum the South Shetland Ice Cap extended onto the outer continental shelf, some 50 km north of its present location. The ice grounded on the shelf in troughs in present water depths of almost 400 m. It had a minimum thickness of ∼570 m and was also grounded in the deepest part of fjords and straits between the islands. To the south, the ice cap extended seaward of the mouths of fjords and straits to the steep northern boundary of the Bransfield Strait. Sediment eroded from the fjords and straits was deposited in prominent fans that extend into the Bransfield Strait and show evidence for an ice shelf that covered the Bransfield Strait during the Last Glacial Maximum. A long drill core (SHALDRIL) in Maxwell Bay and several jumbo piston cores yielded carbonate material used to constrain the timing and spatial extent of glacial retreat within Maxwell Bay. The oldest glacimarine sediments in the fjord date back to ∼14.1 to ∼14.8 ka and suggest an initial deglaciation earlier than most previous studies of the South Shetland Islands. Upper Maxwell Bay was ice free by 9.1 ka and most of Maxwell Bay proper was ice free by 5.9 ka except for the smaller tributary fjords of Maxwell Bay. These smaller tributary fjords were ice free by 1.7 ka, but may have been ice free earlier but reoccupied by a neoglacial advance that ended approximately 1.7 ka. [Copyright &y& Elsevier]