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Abstract Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice‐sheet models projecting future sea‐level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre‐Oligocene, subaerial environment. Detrital hornblende⁴⁰Ar/³⁹Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian‐Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high¹⁴³Nd/¹⁴⁴Nd and low⁸⁷Sr/⁸⁶Sr ratios. Detecting this “fingerprint” in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming. 

List of sample locations and fission track data produced at the University of Arizona under NSF Proposal 1917009 "Collaborative Research: ICe sheet erosional Interaction with Hot geotherm (ICI-Hot) in West Antarctica 

List of supplementary data produced at the University of Arizona under NSF Proposal 1917009 from publication Marschalek, J.W., Thomson, S.N., Hillenbrand, C.-D., Vermeesch, P., Siddoway, C., Carter, A., Nichols, K., Rood, D.H., Venturelli, R.A., Hammond, S.J., Wellner, J., Van De Flierdt, T., 2024. Geological insights from the newly discovered granite of Sif Island between Thwaites and Pine Island glaciers. Antarctic Science 36, 51–74. https://doi.org/10.1017/S0954102023000287 

List of supplementary data produced at the University of Arizona under NSF Proposal 1917009 from publication Marschalek, J.W. et al., 2024a, Byrd Ice Core Debris Constrains the Sediment Provenance Signature of Central West Antarctica: Geophysical Research Letters, v. 51, p. e2023GL106958. 

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.