Collaborative Research: Constraining the Temporal Resolution and Geochronology of Holocene Sediment Records from the Antarctic Continental Shelf

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This award, provided by the Antarctic Geology and Geophysics Program of the Office of Polar Programs, supports a project to investigate paleoenvironmental proxy records in Antarctic sediments and improve approaches to determine the age of the records. Marine sediment cores from perched basins on the Antarctic continental margin have provided important proxy records of environmental conditions in this region during the late Quaternary. Typically lacking in carbonate microfossils, these sediment cores have primarily been dated by using the carbon-14 (14C) content of bulk organic carbon, measured by accelerator mass spectrometry. However, the non-zero radiocarbon age of organic matter at the sediment-water interface (the so-called carbon reservoir problem) necessitates a "correction" to down-core 14C ages, which may be as much as 1,000-3,000 years. In addition, bioturbation of the upper sediment column by benthic organisms serves to smear the proxy records preserved in sediment cores, thus limiting the temporal resolution of such sediment records. For most of the Antarctic shelf, the radiocarbon age of surface sediments and the depth of mixing by benthic organisms are not known with any certainty.This project will undertake high-resolution down-core analyses of the carbon isotopic composition of bulk organic matter and the activity of unsupported lead-210 in order to establish the radiocarbon age of surface sediments and the depth of the sediment mixed-layer in several Antarctic shelf basins. The work will examine Kasten cores collected in the Scholleart Drift (central Gerlache Strait, west Antarctic Peninsula), the Vega Drift (northwestern Weddell Sea, east Antarctic Peninsula), and the Nielsen Basin and Iceberg Alley (MacRobertson Shelf, East Antarctica). These study areas have been selected because (1) the data collected for these samples will be of direct benefit to ongoing studies of high-quality long sediment cores recovered from these sites, and (2) these sites provide a range of contrasting environments in terms of reservoir age of surface waters, energy of depositional environment, extent and type of benthic biota, and proximity to potential sources of re-suspended and relict organic material, which will allow evaluation of the relative importance of these factors for the radiocarbon age of surface sediments on the Antarctic shelf. The radiochemical determinations will make use of accelerator mass spectrometry for carbon isotope measurements, and high-sensitivity gamma spectrometry for analysis of uranium-series nuclides, two techniques that have been previously used with success for Antarctic shelf sediments. The Antarctic continental shelves are important regions of oceanic deep-water formation, seasonal sea-ice formation, and marine primary production - processes that are likely to be strongly influenced by future climatic change. The ability to predict how this region will be affected by, and modulate, future climate change is critically dependent on an understanding of conditions on the Antarctic margin during past climatic variations. This project will make a significant contribution in this regard, by providing key data that are required to accurately decipher and interpret Holocene sediment-core records from the Antarctic continental shelf. In addition, by improving our understanding of the effects of depositional environment and benthic community on the radiocarbon chronology and resolution of Antarctic sediment records, this work will assist in the selection of future sites that are likely to provide high-quality paleoenvironmental records. This project will also make educational contributions at several levels, including undergraduate and graduate research as well as web-based public outreach.