Uranium-series isotopes as tracers of physical and chemical weathering in glacial sediments from Taylor Valley, Antarctica

Abstract

The McMurdo Dry Valleys of Antarctica formed by extensive glacial erosion, yet currently exhibit hyperarid polar conditions canonically associated with limited chemical and physical weathering. Efficient chemical weathering occurs when moisture is available, and polythermal subglacial conditions may accommodate ongoing mechanical weathering and valley incision. Taylor Valley, one of the MDV, hosts several Pleistocene glacial drift deposits that record prior expansions of Taylor Glacier and sediment redistribution, if not sediment production. We present U-series isotopics of fine-grained sediments from these drifts to assess the timescales of physical weathering and subsequent chemical alteration. The isotopes ²³⁸U, ²³⁴U, and ²³⁰Th are sensitive to both chemical and physical fractionation processes in sedimentary systems, including the physical fractionation of daughter isotopes by energetic recoil following radioactive decay. By comparing U-series isotopic measurements with models of U-series response to chemical weathering and physical fractionation processes, we show that Pleistocene drift sediments record histories of significant chemical alteration. However, fine-grained sediments entrained in the basal ice of Taylor Glacier record only minor chemical alteration and U-series fractionation, indicating comparatively recent sediment comminution and active incision of upper Taylor Valley by Taylor Glacier over the Pleistocene. In addition, the results of this study emphasize the utility of U-series isotopes as tracers of chemical and physical weathering in sedimentary and pedogenic systems, with particular sensitivity to radionuclide implantation by $\alpha$-recoil from high-U authigenic phases into lower-U detrital phases. This process has occurred extensively in $>200$ ka drifts but to a lesser degree in younger deposits. U-series $\alpha$-recoil implantation is an important physicochemical process with chronometric implications in other hyperarid and saline sedimentary systems, including analogous Martian environments.