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The stability of the West Antarctic Ice Sheet (WAIS), crucial for predicting future sea-level rise, is threatened by ocean-forced melting in the Pacific sector of the Southern Ocean. While some geological records and ice-sheet models suggest WAIS retreat during past warm periods, reliable data constraining the extent of retreat are lacking. Detrital Nd, Sr, and Pb isotope data of sediments recently drilled at International Ocean Discovery Program (IODP) Site U1532 on the Amundsen Sea continental rise manifest repeated alternations in sediment provenance during glacial–interglacial cycles of the Pliocene (5.33 to 2.58 Mya), a time warmer than present. The variations reflect large fluctuations in WAIS extent on the Antarctic continent. A unique high Pb/low ε_Ndsignature of sediments found at the onset of glacial intervals (3.88, 3.6, and 3.33 Ma) is attributed to the supply of detritus sourced from plutonic rocks located in the West Antarctic interior. Its isotopic signature at Site U1532 indicates major inland retreat of the WAIS during the preceding interglacials. During peak interglacials, the ice margin had retreated inland, and icebergs rafted and deposited inland-sourced detritus over 500 km across the Amundsen Sea shelf. Subsequent readvance of grounded ice then “bulldozed” these inland-derived fine-grained sediments from the shelf down to the continental slope and rise, resulting in a high Pb/low ε_Ndpeak in the rise sediments. Our continuous Pliocene records provide conclusive evidence for at least five major inland retreat events of the WAIS, highlighting the significant vulnerability of the WAIS to ongoing warming. 

Sedimentary records can illuminate relationships between the climate, topography, and glaciation of West Antarctica by revealing its Cenozoic topographic and paleoenvironmental history. Eocene fluvial drainage patterns have previously been inferred using geochemical provenance data from an ~44– to 34–million year deltaic sandstone recovered from the Amundsen Sea Embayment. One interpretation holds that a low-relief, low-lying West Antarctic landscape supported a >1500-kilometer transcontinental river system. Alternatively, higher-relief topography in central West Antarctica formed a drainage divide between the Ross and Amundsen seas. Here, zircon U-Pb data from Amundsen Sea Embayment sediments are examined alongside known regional bedrock provenance signatures. These analyses suggest that all observed provenance indicators in the Eocene sandstone derive from West Antarctic rocks. This implies that a local river system flowed off a West Antarctic drainage divide, helping constrain the mid-Late Eocene evolution of West Antarctic topography with implications for the history of rifting and the characteristics of sediments infilling interior basins. 

In 2019, International Ocean Discovery Program (IODP) expeditions to offshore West Antarctica recovered deep ocean sediment cores in the outer Amundsen Sea (Exp. 379) and Dove Basin (Exp. 382). These cores are characterized by numerous ice-rafted detritus (IRD) intervals, including dropstone cobbles released by icebergs calved from past glaciers/ice streams that incised the subglacial bedrock of West Antarctica. We selected nine dropstones from latest Miocene through mid-Pliocene sediment from IODP Sites U1532C, U1533B (Exp. 379) and U1536E (Exp. 382), comprising sandstone, diorite, granitoid, basalt, and rhyolite, for petrologic characterization and multi-method geo-thermochronology. Dating methods applied include U-Pb zircon (UPbZ) geochronology, and apatite fission-track (AFT) and (U-Th)/He (AHe) low-temperature thermochronology, to reveal dates and rates of geologic events with bearing on their crustal provenance and source region bedrock thermal history. Comparison to published data reveal dropstones to be of both local and distant origin. Notable discoveries are: 1) From U1536E, a ~1200 Ma [U-PbZ] diorite cobble, with ca. 130 Ma AFT and 65-50 Ma AHe ages that most resembles cratonic crust of Queen Maud Land (East Antarctica). 2) Three granitoid rocks from U1533B with ca. 174-179 Ma (UPbZ) ages. The only known rocks of similar age and lithology in West Antarctica are described in the Whitmore Mountains (WM). AFT ages of 114 Ma, 91 Ma, and 81 Ma may thus provide the first thermochronology data from the WM. 3) A 27±1 Ma (UPbZ) diorite of from U1533B records 25.6 Ma AFT and 10.6 Ma AHe ages, suggesting origins in the western Antarctic Peninsula. 4) Two very similar distinctive green quartz arenite dropstones were recovered from latest Miocene core at U1533B and U1536E, locations separated by 3270 km. Multivariate statistical comparison of their UPbZ age populations with published data indicates a common provenance in the Ellsworth Mountains (Antarctic interior). When placed within geotectonic and paleoclimate context, discoveries from IRD-dropstones promise to advance understanding of crustal and landscape evolution of evolution of glaciated continents, variations in icesheet extent during warm periods, and ocean/atmospheric current circulation. 

The Cenozoic tectonic history of Marie Byrd Land (MBL), West Antarctica, is dominated by uplift of the MBL dome, a ~800 by ~300 km topographic swell thought to be supported by a hot mantle anomaly, and normal faulting accompanying extension of the West Antarctic rift system (WARS). Additionally, glaciation beginning at 34 – 20 Ma resulted in deeply incised glacial troughs with up to 5km of relief. This study investigates the timing, magnitude, and spatial relationships of these tectonic and erosional events by determining a regional exhumation history of western MBL through thermo-kinematic modeling of low-temperature thermochronologic data. New apatite (U-Th)/He (AHe) analyses include ages between 46 – 63 Ma, significantly younger than previously determined ages between 80 – 100 Ma. 3D thermo-kinematic modeling reveals focused glacial incision alone is incapable of producing this young population of AHe ages, indicating additional exhumation processes have been at work since ~80 Ma. Differential exhumation across western MBL is required to produce the range of observed AHe ages, with laterally variable exhumation ranging from little to none on the Edward VII Peninsula to ~0.04 km/myr in the eastern Ford Ranges. This spatial pattern is consistent with enhanced exhumation related to uplift of the MBL dome in the eastern Ford Ranges, with this effect diminishing westward to the Edward VII Peninsula. A sharp change in exhumation rate in the western Ford Ranges suggests recent motion on inferred normal faults consistent with WARS extension and down-dropping of the Edward VII Peninsula. Models based on available bedrock data provide little insight into the timing and magnitude of glacial incision due to the present inability to directly sample bedrock in deep glacial troughs. However, model predictions of bedrock low-temperature age distributions within glacial troughs are useful as a point of comparison for detrital age distributions. New detrital AHe ages from Sulzberger Bay, offshore western MBL, range from 49 – >100 Ma and are consistent with model age distributions. These model results support a complex, spatially heterogeneous exhumation history for western MBL tied to its position between the MBL dome and the WARS and provide insight into the impact of glacial incision across the regional landscape. 

Marie Byrd Land (MBL), West Antarctica, is poorly studied geologically due to its ice cover and remoteness. As a result, the timing and magnitude of tectonic and erosional events, such as the tectonic uplift of the Marie Byrd Land dome and the incision of the DeVicq Glacial Trough, are debated. When faced with problems difficult to study and solve through in-person field work, it becomes necessary to turn to modeling. Pecube is a thermo-kinematic modeling program that uses topographic and crustal thermal data to calculate thermochronologic ages across a landscape. Thermochronology uses radiometric dating of mineral systems that are sensitive to specific temperatures and can be used to track cooling related to the tectonic and exhumation history of a region. Model predictions can be compared to observed ages to test the viability of tectonic or geomorphic scenarios. Observed ages used here include dates derived from apatite fission track analysis (AFT; closure temperature ~ 110 °C) and apatite (U-Th)/He dating (AHe; closure temperature ~ 60 °C) of detrital material recovered from offshore MBL that presumably originated from the DeVicq Trough region of MBL. Ongoing modeling efforts will determine how closely calculated bedrock ages compare to new detrital AHe ages, ranging from 23.5-82.8 Ma, and AFT ages, ranging from 49.7-83.6 Ma. These ages broadly correspond to late breakup of Gondwana (~100-85Ma), erosion during and after the uplift of the Marie Byrd Land dome (~30Ma), and glacial incision (beginning at 34 or 20Ma). In light of these new data, alterations were made to existing Pecube models for the DeVicq Trough region to rule out and narrow down the timings and rates possible for both glacial incision at the DeVicq Glacial Trough and exhumation of the Marie Byrd Land dome. Preliminary results suggest that varying glacial incision start time between 34 and 20 Ma, dates proposed for the initiation of the West Antarctic Ice Sheet, does not change resulting bedrock ages significantly. However, varying background exhumation rates results in ages that are broadly consistent with observed ages. Ongoing modeling efforts seek to refine this range further to give insight on the exhumation history and tectonic processes of this region. 

The stability of the West Antarctic Ice Sheet (WAIS), crucial for preventing major future sea-level rise, is threatened by ocean-forced melting in the Pacific sector, especially in the Amundsen Sea. So far, direct evidence of the extent and rate of WAIS retreat during past warm periods has been lacking. Here, we analyzed detrital Nd, Sr, and Pb isotope data of sediments ( 18.93 for 206Pb/204Pb) and low eNd (< –5 eNd) values. This distinct isotopic signature suggests long-distance supply of detritus sourced from plutonic rocks located in the continental interior. The presence of this material at Site U1532 indicates major inland retreat of the WAIS during the immediately preceding interglacials, which allowed icebergs to transport and deposit the detritus on the Amundsen Sea shelf. Our Pliocene records reveal multiple major inland retreats of the WAIS, highlighting the extent of possible WAIS response to ongoing global warming.  

Brittle faults are widespread but rarely exposed in Marie Byrd Land, a part of the West Antarctic rift system, owing to enhanced erosion of zones of cataclasis by the regional ice sheet. Tourmaline-mineralized faults discovered at three locations in the Ford Ranges constitute a new record of fluid-rock interactions in this region of extended crust. Tourmaline resists re-equilibration, even during metamorphism, thus strongly aligned tourmaline from high-angle faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak likely contain direct records of fault-hosted fluids and timing of fault movements. The faults form an array oriented NNW-SSE and WNW-ESE, which displays brittle kinematic criteria indicating normal-oblique and strike-oblique displacement. Mirrored fault surfaces suggest formation during seismic slip. Tourmaline is concentrated within a 2 to 4 mm zone bordering the fault planes. Petrography and EMPA analyses show unzoned tourmaline , with the dravite variety at Lewissohn Nunatak and schorl at the other two sites. Fluid inclusions in dravite are tubular (A-axis-parallel), 10 to 15 um, and up to 25 um, in length, containing gas and fluid phases. Fluid inclusions in schorl are C-axis-parallel and breached. Tourmaline ∂18O ratios (n=4) range from 9.2 to 10.4 ± 0.1 ‰ VSMOW (average 9.7‰, s.dev. = 0.7). Paired quartz yield ∂18O values of 11.1 to 10.3 ± 0.1 ‰, and ∆Qtz-Trm values between 1.3 and 2.0. Brittle microfractures in parallel arrays, evident in thin section, indicate tensile opening along ENE- WSW axes, in accordance with outcrop evidence. The strong preferred orientation and uniform mineral composition of tourmaline indicate syntectonic growth of tourmaline along fault planes. ∆Qtz-Trm values suggest equilibration between host-rock quartz and tourmaline was not achieved, likely due to rapid tourmaline precipitation. Relative isotopic homogeneity between sites suggests similar fluid conditions across the region, for crust underlying a minimum area of 2000 km2. Preliminary results of tourmaline 40Ar/39Ar dating indicate broadly Cretaceous timing for fault-related fluid flow. Ongoing work seeks to determine the temperature of mineralizing fluids and evaluate whether the brittle array localizes geothermal heat beneath the contemporary icesheet. 

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 

Large-scale geological structures have controlled the long-term development of the bed and thus the flow of the West Antarctic Ice Sheet (WAIS). However, complete ice cover has obscured the age and exact positions of faults and geological boundaries beneath Thwaites Glacier and Pine Island Glacier, two major WAIS outlets in the Amundsen Sea sector. Here, we characterize the only rock outcrop between these two glaciers, which was exposed by the retreat of slow-flowing coastal ice in the early 2010s to form the new Sif Island. The island comprises granite, zircon U-Pb dated to ~177–174 Ma and characterized by initial ɛ_Nd,⁸⁷Sr/⁸⁶Sr and ɛ_Hfisotope compositions of -2.3, 0.7061 and -1.3, respectively. These characteristics resemble Thurston Island/Antarctic Peninsula crustal block rocks, strongly suggesting that the Sif Island granite belongs to this province and placing the crustal block's boundary with the Marie Byrd Land province under Thwaites Glacier or its eastern shear margin. Low-temperature thermochronological data reveal that the granite underwent rapid cooling following emplacement, rapidly cooled again at ~100–90 Ma and then remained close to the Earth's surface until present. These data help date vertical displacement across the major tectonic structure beneath Pine Island Glacier to the Late Cretaceous. 

Brittle faults in the southern Ford Ranges of Marie Byrd Land, West Antarctica, provide unique opportunity to study fluid-rock interactions in the West Antarctic Rift System and the role of crustal fluids during regional-scale faulting. This fault array contains steep, NNW-striking, normal-oblique slip faults and sub-vertical WNW-ESE strike-oblique faults. \ Faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak display strongly aligned tourmaline, indicating syntectonic mineralization; surfaces in one location feature distinctive mirror surfaces, suggestive of formation during seismic slip. Tourmaline has been demonstrated to resist chemical and isotopic re-equilibration during even high-temperature metamorphism, and to maintain a record of conditions during formation, therefore oxygen isotope compositions of tourmaline and quartz pairs may elucidate crustal conditions (e.g. temperatures and fluid-rock ratios) and fluids sources. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces; host rocks are tourmaline-free. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰; ∆Qtz-Trm values between 1.3 and 2.0 may reflect an inability of quartz to equilibrate during tourmaline crystallization. Equilibrium between quartz and tourmaline would suggest temperatures of formation in excess of 550°C. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. Geometric and kinematic relationships suggest a link to deeper level shears hosting muscovite, and sillimanite with garnet. On-going investigation includes O isotope analyses of these shears, as well as analysis of H and B isotopes in tourmaline, which will better characterize the relationship between the deeper crustal shears with the brittle fault array, and the fluid sources and metasomatic processes of regional fault systems. Furthermore, temporal constraints on tourmaline mineralization will establish whether faulting is associated with Cretaceous intracontinental extension of the West Antarctic rift system (Siddoway 2008) or a crustal response to Neogene mantle delamination beneath the South Pole region (Shen et al 2018).