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1. Continuous Simulations over the Last 34 Million Years with a Coupled Antarctic Ice Sheet and Sediment Model

   Pollard, D. ( December 2018 , AGU Fall Meeting)

   Much of the knowledge of Antarctic Ice Sheet variations since its inception ~34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. If coupled ice-sheet and sediment models can simulate these deposits explicitly, direct comparisons with the sediment record would be valuable in linking it to Cenozoic ice and climate history. Here we apply an existing 3-D ice sheet and sediment model to the whole period of late Cenozoic Antarctic evolution. The ice-sheet model uses local parameterizations of grounding-line flux, ice-shelf hydrofracture and ice cliff failure. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing is determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core d18O, and orbital insolation variations. Initial ice-free and sediment-free bedrock topography is prescribed from the 34 Ma reconstruction of Wilson et al., Palaeo3, 2011, and their estimated rate of tectonic subsidence is applied in West Antarctica. The model is run continuously from 34 Ma to the present, to capture the entire post-Eocene Antarctic landscape evolution and off-shore sediment packages in a single self-consistent simulation. In order to make these long simulations feasible, the model resolution is very coarse, 80 km. However the ice model's use of local parameterizations for fine-scale dynamical processes yields results that are not seriously degraded compared to finer resolutions in short tests. The primary goals are (1) to reproduce major recognized ice-sheet trends and fluctuations from the Eocene to today, and (2) to produce a 3-D model map of modern sediment deposits. "Strata" are tracked by recording times of deposition within the model sediment stacks. Unconformities in these strata occur in the model that can be compared with observed profiles. Initial results are presented, and preliminary overall comparisons are made with observed sediment packages, focusing on sensitivities to climate forcing, quarrying rates, and sediment parameters that stand in for alternate sediment rheologies. 

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2. Continuous simulations over the last 40 million years with a coupled Antarctic ice sheet-sediment model

   https://doi.org/10.1016/j.palaeo.2019.109374  

   Pollard, D. ; DeConto, R. ( January 2020 , Palaeogeography palaeoclimatology palaeoecology)

   null (Ed.)

   Much of the knowledge of Antarctic Ice Sheet variations since its inception ~34 Ma derives from marine sediments on the continental shelf, deposited in glacimarine or sub-ice environments by advancing and retreating grounded ice, and observed today by seismic profiling and coring. Here we apply a 3-D coupled ice sheet and sediment model from 40 Ma to the present, with the goal of directly linking ice-sheet variations with the sediment record. The ice-sheet model uses vertically averaged ice dynamics and parameterized grounding-line flux. The sediment model includes quarrying of bedrock, sub-ice transport, and marine deposition. Atmospheric and oceanic forcing are determined by uniform shifts to modern climatology in proportion to records of atmospheric CO2, deep-sea-core δ18O, and orbital insolation variations. The model is run continuously over the last 40 Myr at coarse resolution (80 or 160 km), modeling post-Eocene ice, landscape evolution and off-shore sediment packages in a single self-consistent simulation. Strata and unconformities are tracked by recording times of deposition within the model sediment stacks, which can be compared directly with observed seismic profiles. The initial bedrock topography is initialized to 34 Ma geologic reconstructions, or an iterative procedure is used that yields independent estimates of paleo bedrock topography. Preliminary results are compared with recognized Cenozoic ice-sheet variations, modern sediment distributions and seismic profiles, and modern and paleo bedrock topographies. 

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3. Expedition 379 Preliminary Report: Amundsen Sea West Antarctic Ice Sheet History

   https://doi.org/10.14379/iodp.pr.379.2019  

   Gohl, K. ; Wellner, J.S. ; Klaus, A. ( May 2019 , Preliminary report)

   null (Ed.)

   The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet (WAIS) because of the great water depth at the grounding line and the absence of substantial ice shelves. Glaciers in this configuration are thought to be susceptible to rapid or runaway retreat. Ice flowing into the Amundsen Sea Embayment is undergoing the most rapid changes of any sector of the Antarctic Ice Sheet outside the Antarctic Peninsula, including changes caused by substantial grounding-line retreat over recent decades, as observed from satellite data. Recent models suggest that a threshold leading to the collapse of WAIS in this sector may have been already crossed and that much of the ice sheet could be lost even under relatively moderate greenhouse gas emission scenarios. Drill cores from the Amundsen Sea provide tests of several key questions about controls on ice sheet stability. The cores offer a direct record of glacial history offshore from a drainage basin that receives ice exclusively from the WAIS, which allows clear comparisons between the WAIS history and low-latitude climate records. Today, warm Circumpolar Deep Water (CDW) is impinging onto the Amundsen Sea shelf and causing melting of the underside of the WAIS in most places. Reconstructions of past CDW intrusions can assess the ties between warm water upwelling and large-scale changes in past grounding-line positions. Carrying out these reconstructions offshore from the drainage basin that currently has the most substantial negative mass balance of ice anywhere in Antarctica is thus of prime interest to future predictions. The scientific objectives for this expedition are built on hypotheses about WAIS dynamics and related paleoenvironmental and paleoclimatic conditions. The main objectives are 1. To test the hypothesis that WAIS collapses occurred during the Neogene and Quaternary and, if so, when and under which environmental conditions; 2. To obtain ice-proximal records of ice sheet dynamics in the Amundsen Sea that correlate with global records of ice-volume changes and proxy records for atmospheric and ocean temperatures; 3. To study the stability of a marine-based WAIS margin and how warm deep-water incursions control its position on the shelf; 4. To find evidence for earliest major grounded WAIS advances onto the middle and outer shelf; 5. To test the hypothesis that the first major WAIS growth was related to the uplift of the Marie Byrd Land dome. International Ocean Discovery Program (IODP) Expedition 379 completed two very successful drill sites on the continental rise of the Amundsen Sea. Site U1532 is located on a large sediment drift, now called Resolution Drift, and penetrated to 794 m with 90% recovery. We collected almost-continuous cores from the Pleistocene through the Pliocene and into the late Miocene. At Site U1533, we drilled 383 m (70% recovery) into the more condensed sequence at the lower flank of the same sediment drift. The cores of both sites contain unique records that will enable study of the cyclicity of ice sheet advance and retreat processes as well as bottom-water circulation and water mass changes. In particular, Site U1532 revealed a sequence of Pliocene sediments with an excellent paleomagnetic record for high-resolution climate change studies of the previously sparsely sampled Pacific sector of the West Antarctic margin. Despite the drilling success at these sites, the overall expedition experienced three unexpected difficulties that affected many of the scientific objectives: 1. The extensive sea ice on the continental shelf prevented us from drilling any of the proposed shelf sites. 2. The drill sites on the continental rise were in the path of numerous icebergs of various sizes that frequently forced us to pause drilling or leave the hole entirely as they approached the ship. The overall downtime caused by approaching icebergs was 50% of our time spent on site. 3. An unfortunate injury to a member of the ship's crew cut the expedition short by one week. Recovery of core on the continental rise at Sites U1532 and U1533 cannot be used to precisely indicate the position of ice or retreat of the ice sheet on the shelf. However, these sediments contained in the cores offer a range of clues about past WAIS extent and retreat. At Sites U1532 and U1533, coarse-grained sediments interpreted to be ice-rafted debris (IRD) were identified throughout all recovered time periods. A dominant feature of the cores is recorded by lithofacies cyclicity, which is interpreted to represent relatively warmer periods variably characterized by higher microfossil abundance, greater bioturbation, and higher counts of IRD alternating with colder periods characterized by dominantly gray laminated terrigenous muds. Initial comparison of these cycles to published records from the region suggests that the units interpreted as records of warmer time intervals in the core tie to interglacial periods and the units interpreted as deposits of colder periods tie to glacial periods. The cores from the two drill sites recovered sediments of purely terrigenous origin intercalated or mixed with pelagic or hemipelagic deposits. In particular, Site U1533, which is located near a deep-sea channel originating from the continental slope, contains graded sands and gravel transported downslope from the shelf to the abyssal plain. The channel is likely the path of such sediments transported downslope by turbidity currents or other sediment-gravity flows. The association of lithologic facies at both sites predominantly reflects the interplay of downslope and contouritic sediment supply with occasional input of more pelagic sediment. Despite the lack of cores from the shelf, our records from the continental rise reveal the timing of glacial advances across the shelf and thus the existence of a continent-wide ice sheet in West Antarctica at least during longer time periods since the late Miocene. Cores from both sites contain abundant coarse-grained sediments and clasts of plutonic origin transported either by downslope processes or by ice rafting. If detailed provenance studies confirm our preliminary assessment that the origin of these samples is from the plutonic bedrock of Marie Byrd Land, their thermochronological record will potentially reveal timing and rates of denudation and erosion linked to crustal uplift. The chronostratigraphy of both sites enables the generation of a seismic sequence stratigraphy not only for the Amundsen Sea rise but also for the western Amundsen Sea along the Marie Byrd Land margin through a connecting network of seismic lines. 

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4. Amundsen Sea West Antarctic Ice Sheet History

   https://doi.org/10.14379/iodp.proc.379.2021  

   Gohl, K. ; Wellner, J.S. ; Klaus, A. ( February 2021 , Proceedings of the International Ocean Discovery Program)

   The Amundsen Sea sector of Antarctica has long been considered the most vulnerable part of the West Antarctic Ice Sheet (WAIS) because of the great water depth at the grounding line, a subglacial bed seafloor deepening toward the interior of the continent, and the absence of substantial ice shelves. Glaciers in this configuration are thought to be susceptible to rapid or runaway retreat. Ice flowing into the Amundsen Sea Embayment is undergoing the most rapid changes of any sector of the Antarctic ice sheets outside the Antarctic Peninsula, including substantial grounding-line retreat over recent decades, as observed from satellite data. Recent models suggest that a threshold leading to the collapse of WAIS in this sector may have been already crossed and that much of the ice sheet could be lost even under relatively moderate greenhouse gas emission scenarios. Drill cores from the Amundsen Sea provide tests of several key questions about controls on ice sheet stability. The cores offer a direct offshore record of glacial history in a sector that is exclusively influenced by ice draining the WAIS, which allows clear comparisons between the WAIS history and low-latitude climate records. Today, relatively warm (modified) Circumpolar Deep Water (CDW) is impinging onto the Amundsen Sea shelf and causing melting under ice shelves and at the grounding line of the WAIS in most places. Reconstructions of past CDW intrusions can assess the ties between warm water upwelling and large-scale changes in past grounding-line positions. Carrying out these reconstructions offshore from the drainage basin that currently has the most substantial negative mass balance of ice anywhere in Antarctica is thus of prime interest to future predictions. The scientific objectives for this expedition are built on hypotheses about WAIS dynamics and related paleoenvironmental and paleoclimatic conditions. The main objectives are: 1. To test the hypothesis that WAIS collapses occurred during the Neogene and Quaternary and, if so, when and under which environmental conditions; 2. To obtain ice-proximal records of ice sheet dynamics in the Amundsen Sea that correlate with global records of ice-volume changes and proxy records for atmospheric and ocean temperatures; 3. To study the stability of a marine-based WAIS margin and how warm deepwater incursions control its position on the shelf; 4. To find evidence for the earliest major grounded WAIS advances onto the middle and outer shelf; 5. To test the hypothesis that the first major WAIS growth was related to the uplift of the Marie Byrd Land dome. International Ocean Discovery Program (IODP) Expedition 379 completed two very successful drill sites on the continental rise of the Amundsen Sea. Site U1532 is located on a large sediment drift, now called the Resolution Drift, and it penetrated to 794 m with 90% recovery. We collected almost-continuous cores from recent age through the Pleistocene and Pliocene and into the upper Miocene. At Site U1533, we drilled 383 m (70% recovery) into the more condensed sequence at the lower flank of the same sediment drift. The cores of both sites contain unique records that will enable study of the cyclicity of ice sheet advance and retreat processes as well as ocean-bottom water circulation and water mass changes. In particular, Site U1532 revealed a sequence of Pliocene sediments with an excellent paleomagnetic record for high-resolution climate change studies of the previously sparsely sampled Pacific sector of the West Antarctic margin. Despite the drilling success at these sites, the overall expedition experienced three unexpected difficulties that affected many of the scientific objectives: 1. The extensive sea ice on the continental shelf prevented us from drilling any of the proposed shelf sites. 2. The drill sites on the continental rise were in the path of numerous icebergs of various sizes that frequently forced us to pause drilling or leave the hole entirely as they approached the ship. The overall downtime caused by approaching icebergs was 50% of our time spent on site. 3. A medical evacuation cut the expedition short by 1 week. Recovery of core on the continental rise at Sites U1532 and U1533 cannot be used to indicate the extent of grounded ice on the shelf or, thus, of its retreat directly. However, the sediments contained in these cores offer a range of clues about past WAIS extent and retreat. At Sites U1532 and U1533, coarse-grained sediments interpreted to be ice-rafted debris (IRD) were identified throughout all recovered time periods. A dominant feature of the cores is recorded by lithofacies cyclicity, which is interpreted to represent relatively warmer periods variably characterized by sediments with higher microfossil abundance, greater bioturbation, and higher IRD concentrations alternating with colder periods characterized by dominantly gray laminated terrigenous muds. Initial comparison of these cycles to published late Quaternary records from the region suggests that the units interpreted to be records of warmer time intervals in the core tie to global interglacial periods and the units interpreted to be deposits of colder periods tie to global glacial periods. Cores from the two drill sites recovered sediments of dominantly terrigenous origin intercalated or mixed with pelagic or hemipelagic deposits. In particular, Site U1533, which is located near a deep-sea channel originating from the continental slope, contains graded silts, sands, and gravels transported downslope from the shelf to the rise. The channel is likely the pathway of these sediments transported by turbidity currents and other gravitational downslope processes. The association of lithologic facies at both sites predominantly reflects the interplay of downslope and contouritic sediment supply with occasional input of more pelagic sediment. Despite the lack of cores from the shelf, our records from the continental rise reveal the timing of glacial advances across the shelf and thus the existence of a continent-wide ice sheet in West Antarctica during longer time periods since at least the late Miocene. Cores from both sites contain abundant coarse-grained sediments and clasts of plutonic origin transported either by downslope processes or by ice rafting. If detailed provenance studies confirm our preliminary assessment that the origin of these samples is from the plutonic bedrock of Marie Byrd Land, their thermochronological record will potentially reveal timing and rates of denudation and erosion linked to crustal uplift. The chronostratigraphy of both sites enables the generation of a seismic sequence stratigraphy for the entire Amundsen Sea continental rise, spanning the area offshore from the Amundsen Sea Embayment westward along the Marie Byrd Land margin to the easternmost Ross Sea through a connecting network of seismic lines. 

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5. Inventory of ice-rafted clasts and sediment constituents that track with dynamic ice-margin processes and biological productivity, Amundsen Sea region, Antarctica

   Siddoway, Christine S. ( April 2023 , EGU General Assembly 2023)

   Marine sediments, obtained from cores and captures from deep sea and continental shelf sites of West Antarctica, contain rich records of latest Miocene to Present glacial and deglacial processes and conditions at the margin of the West Antarctic ice sheet (WAIS). The materials we are investigating were recovered from a) Resolution Drift on the Amundsen Sea continental rise (water depths >3900m), b)the continental shelf in the Amundsen Sea, Wrigley Gulf, and Sultzberger Bay (water depths <1000m). Resolution Drift cores were drilled by IODP Expedition 379 (Gohl et al., doi:10.14379/iodp.proc.379.2021) in sediments dominated by compacted clay and silty clay, with conglomeratic intervals of ice-rafted detritus (IRD) and downslope deposits. The shelf sediments were recovered by piston core, trigger core, and Smith McIntyre Grab (SMG) during USA research cruises of the RVIB Nathaniel B Palmer (1999, 2000, 2007) and USCGC Glacier (1983). The shelf samples are non-compacted clay, containing abundant cobbles, pebbles and biogenic fragments. Our research focuses upon rock clasts, detrital apatite and zircon, felsic volcanic tephra, and micro-manganese nodules separated from marine and glaciomarine clay. The rock clasts and detrital minerals represent samples of continental crust that we characterise according to rock type, petrology, geochemistry, and geo-thermochronology [U-Pb, (U-Th)/He, and fission track methods]. These characteristics illuminate solid Earth processes, including the development of subglacial topography . We compared clasts’ petrology and age data to the exposed onshore geology and thermochronology of bedrock, and determined that ≥90% of clasts likely originated in West Antarctica. Therefore the materials can be used to assign roughness, erodibility, and heat production factors for subglacial bedrock, which constitute boundary conditions used by ice sheet modelers. Rhyolite ash and fragments provide new evidence for explosive eruptions (dated ca. 2.55 to 2.92 Ma; feldspar 40Ar/39Ar) delivered to sea as airfall, IRD, and possible subglacial water transport. Silicic eruptions produce ash and aerosols that may screen solar energy, and provide bio-available nutrients that produce phytoplankton blooms leading to sequestration of carbon. The rhyolite dates coincide with the end of a Pliocene warm period recorded in IODP379 cores (Gille-Petzoldt et al., 10.3389/feart.2022.976703). Our work in progress seeks to obtain higher resolution geochronology in order to determine whether silicic continental volcanism occurred in response to ice unloading due to deglaciation (cf. Lin et al., 10.5194/cp-18-485-2022) and whether erupted products contributed to latest Pliocene significant cooling and WAIS re-glaciation. Another distinctive sediment constituent is micro-manganese nodules of unusual form. Whereas typical micro-MN nodules are dark, formed of concentric layers, this form is pale in color, ‘barbell’ shaped, and transparent in transmitted light. Scanning electron microscopy shows these to be microcrystalline Mn-oxide with embedded grains of quartz and feldspar, which likely served as seed material. Mn-oxides form by authigenesis at/near the seafloor surface, requiring high oxygen concentrations in the bottom water and low sedimentation rates, generally associated with the end of glacials/during interglacials (Hillenbrand et al. 2021, 10.1029/2021GL093103). Work is in progress to determine whether Mn oxides formed through passive accretion upon seed grains or microbially-mediated precipitation from Mn-oxyhydroxides or colloids, of possible relevance for coastal carbon budgets. https://doi.org/10.5194/egusphere-egu23-9728 

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