Comment on tc-2022-252

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> During the interglacial and interstadials of Marine Isotope Stage 5 (MIS 5e, 5c, 5a), outlet and alpine glaciers in the Dry Valleys region, Antarctica, appear to have advanced in response to increased precipitation from enhanced open ocean conditions in the Ross Sea. We provide further evidence of this antiphase behaviour through retreat of a peripheral lobe of Taylor Glacier in Pearse Valley, a region that was glaciated during MIS 5. We measured cosmogenic ¹⁰Be and ²⁶Al in three granite cobbles from thin, patchy drift (Taylor 2 Drift) in Pearse Valley to constrain the timing of retreat of Taylor Glacier. Assuming simple continuous exposure, our minimum, zero erosion, exposure ages suggest Taylor Glacier partially retreated from Pearse Valley no later than 65&ndash;74 ka. Timing of retreat after 65 ka and until the Last Glacial Maximum (LGM) when Taylor Glacier was at a minimum position, remains unresolved. The depositional history of permafrost sediments buried below Taylor 2 Drift in Pearse Valley was obtained from ¹⁰Be and²⁶Al depth profiles to ~3 metres in permafrost in proximity to the cobble sampling sites. Depth profile modelling gives a depositional age for near-surface (&lt; 1.65 m) permafrost at Pearse Valley of 180 ka ⁺²⁰/_&minus;40 ka, implying deposition of permafrost sediments predate MIS 5 advances of Taylor Glacier. Depth profile modelling of deeper permafrost sediments (&gt; 2.09 m) indicates a depositional age of &gt; 180 ka. The cobble and permafrost ages reveal Taylor Glacier advances during MIS 5 were non-erosive or mildly erosive, preserving the underlying permafrost sediments and peppering boulders and cobbles upon an older, relict surface. Our results are consistent with U/Th ages from central Taylor Valley, and suggest changes in moisture delivery over Taylor Dome during MIS 5e, 5c and 5a appear to be associated with the extent of the Ross Ice Shelf and sea ice in the Ross Sea. At a coastal, lower elevation site in neighbouring Lower Wright Valley, ¹⁰Be and ²⁶Al depth profiles from a second permafrost core exhibit near-constant concentrations with depth, and indicate the sediments are either vertically mixed after deposition, or are sufficiently young and post-depositional nuclide production is negligible relative to inheritance. ²⁶Al/¹⁰Be concentration ratios for both depth profiles range between 4.0 and 5.2 and are all lower than the nominal surface production rate ratio of 6.75 indicating that prior to deposition, these sediments experienced a complex exposure-burial history. Assuming a single cycle exposure-burial scenario, the observed ²⁶Al/¹⁰Be ratios are equivalent to a total minimum exposure-burial history of ~1.2 Ma. Our new data corroborates antiphase behaviour between outlet and alpine glaciers in the Dry Valleys region and ice extent in the Ross Sea. We suggest a causal relationship of cold-based glacier advance and retreat that is controlled by an increase in moisture availability during retreat of sea ice and perhaps the Ross Ice Shelf, and conversely, a decrease during times of sea ice and Ross Ice Shelf expansion in the Ross Sea.