Project Outcomes Report

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This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

Understanding the history of West Antarctic Ice Sheet (WAIS) glaciation is important for assessing the ice sheet’s response to climate forcing, its contribution to deglacial sea-level change, and for satellite- and GPS-based estimates of current ice-sheet mass loss. A radiocarbon-based chronology for WAIS deglaciation in the McMurdo Sound area of the Ross Sea suggests that grounded ice was at its maximum extent from ~18,700 years ago to ~12,800 years ago, with subsequent retreat through the early-to-middle Holocene, consistent with reconstructions of late grounding line retreat in the western Ross Sea. The late retreat for this sector of WAIS is similar to late retreat reconstructed for some sectors of the East Antarctic Ice Sheet (EAIS), but significantly later than onset of retreat identified for other sectors of WAIS, EAIS, and the Antarctic Peninsula. Many of these radiocarbon ages, however, represent minimum limiting ages or are subject to uncertain reservoir ages and reworking of preexisting organic matter.

To evaluate this radiocarbon-based chronology and its uncertainties, we sampled erratic boulders for cosmogenic ¹⁰Be dating from the Ross Sea Drift (RSD) in the McMurdo Sound area of the western Ross Sea, focusing on the Taylor Valley where the majority of previous work on land-based chronologies has been done. There is a pronounced tendency for cosmogenic-nuclide inheritance in erratics from high latitudes, which is likely due to transport by less-erosive, cold-based ice. We addressed this issue by sampling a large number of the largest available boulders from each landform, and screening for inheritance by measuring ³He in quartz prior to dating boulders with ¹⁰Be.Our proposal supported the measurement of 70 new ¹⁰Be ages and 121 ³He ages on quartz to address these chronological uncertainties. The use of paired 3He and 10Be measurements proved to be an effective method to assess samples with prior exposure, thus improving the overall quality of the data.

Three samples from a drift surface above the RSD limit near Blue Glacier have a mean age of 139.1 ± 10.5 ka, while two samples above the RSD limit on Mount Discovery have a mean age of 97.7 ± 8.8 ka, confirming that this upper drift represents a significantly older and more extensive glaciation than represented by the RSD. The large spread in ³He and ¹⁰Be ages from this drift, however, precludes associating it with any specific glaciation phase before or after the last interglaciation.    

Our cosmogenic ¹⁰Be ages of erratics deposited by grounded ice in the McMurdo Sound region record onset of sustained retreat from the local Last Glacial Maximum (LGM) at 18.6±1.1 ka, and that sustained retreat occurred throughout the ¹⁴C-derived LGM interval. The age of the local LGM interval in McMurdo Sound (~28 ka to ~18.6 ka) is in good agreement with that defined for the Antarctic Peninsula Ice Sheet (>28.6 ka to 18.2-20.3 ka) and the EAIS in the Weddell Sea (>25.7 ka to 19 ka). Moreover, the age for onset of deglaciation is similar to when WAIS began to retreat in the Amundsen Sea and to the first episode of increased iceberg flux from the Antarctic Ice Sheet 19-20 ka, with seven additional millennial-scale episodes occurring throughout the deglaciation until ~9 ka corresponding to the period of sustained retreat identified by our record. We attribute most of this retreat to temperature and radiative forcing of an ablation zone in McMurdo Sound, with only the final stages of retreat possibly influenced by Holocene grounding line retreat. Results from in situ cosmogenic ¹⁴C on 11 selected boulders are broadly consistent with this chronology but indicate complex exposure is a common issue in the region.