Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> The preservation of cosmogenic nuclides that accumulated during periods of prior exposure but were not subsequently removed by erosion or radioactive decay complicates interpretation of exposure, erosion, and burial ages used for a variety of geomorphological applications. In glacial settings, cold-based, non-erosive glacier ice may fail to remove inventories of inherited nuclides in glacially transported material. As a result, individual exposure ages can vary widely across a single landform (e.g., moraine) and exceed the expected or true depositional age. The surface processes that contribute to inheritance remain poorly understood, thus limiting interpretations of cosmogenic nuclide datasets in glacial environments. Here, we present a compilation of new and previously published exposure ages of multiple lithologies in local Last Glacial Maximum (LGM) and older Pleistocene glacial sediments in the McMurdo Sound region of Antarctica. Unlike most Antarctic exposure chronologies, we are able to compare exposure ages of local LGM sediments directly against an independent radiocarbon chronology of fossil algae from the same sedimentary unit that brackets the age of the local LGM between 12.3 and 19.6â<span class="inline-formula">ka</span>. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source and surface processes prior to, during, and after glacial entrainment explain scatter. <span class="inline-formula"><sup>10</sup>Be</span> exposure ages of quartz in granite, sourced from the base of the stratigraphic section in the Transantarctic Mountains, are scattered but young, suggesting that clasts entrained by sub-glacial plucking can generate reasonable apparent exposure ages. <span class="inline-formula"><sup>3</sup>He</span> exposure ages of pyroxene in Ferrar Dolerite, which crops out above outlet glaciers in the Transantarctic Mountains, are older, which suggests that clasts initially exposed on cliff faces and glacially entrained by rock fall carry inherited nuclides. <span class="inline-formula"><sup>3</sup>He</span> exposure ages of olivine in basalt from local volcanic bedrock in the McMurdo Sound region contain many excessively old ages but also have a bimodal distribution with peak probabilities that slightly pre-date and post-date the local LGM; this suggests that glacial clasts from local bedrock record local landscape exposure. With the magnitude and geological processes contributing to age scatter in mind, we examine exposure ages of older glacial sediments deposited by the most extensive ice sheet to inundate McMurdo Sound during the Pleistocene. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide inventories held in Antarctic glacial sediments.
Highlights
In situ cosmogenic nuclides, which accumulate in nearsurface materials during exposure to cosmic radiation, can be measured across a wide range of environments and timescales to quantitatively describe earth surface processes, including quantifying erosion rates (Portenga et al, 2019), dating landforms and exposed bedrock surfaces (Christ et al, 2021; Wells et al, 1995), and reconstructing changes in climate over millions of years (Bierman et al, 2016; Schaefer et al, 2016; Shakun et al, 2018), among many other applications
Christ et al.: Cosmogenic nuclide exposure age scatter in McMurdo Sound the decay of radioactive nuclides – can produce results that are difficult to interpret in surface exposure dating samples
Exposure ages of glacial deposits in the McMurdo Sound region record multiple surface processes operating in the Transantarctic Mountains prior to, during, and following glacial entrainment throughout the Pleistocene
Summary
In situ cosmogenic nuclides, which accumulate in nearsurface materials during exposure to cosmic radiation, can be measured across a wide range of environments and timescales to quantitatively describe earth surface processes, including quantifying erosion rates (Portenga et al, 2019), dating landforms and exposed bedrock surfaces (Christ et al, 2021; Wells et al, 1995), and reconstructing changes in climate over millions of years (Bierman et al, 2016; Schaefer et al, 2016; Shakun et al, 2018), among many other applications. Glacial sediments can be entrained, transported, and deposited by both warm- and cold-based glacial ice and produce scattered cosmogenic surface exposure ages (Hein et al, 2014; Heyman et al, 2011). During past glacial periods when southerly EAIS outlet glaciers expanded into the Ross Sea, grounded ice overrode some or all of the volcanic features in the McMurdo Sound region, impounding the flow of Koettlitz Glacier and other portions of the McMurdo Dry Valleys (Christ and Bierman, 2020; Denton and Marchant, 2000; Greenwood et al, 2018; Hall et al, 2015; Stuiver et al, 1981) (Fig. 1) These incursions of grounded ice into McMurdo Sound transported and deposited lithologies from the Transantarctic Mountains, including Granite Harbor Intrusives, Ferrar Dolerite, Koettlitz Group metamorphic rocks, Beacon Sandstone, and other rocks from the East Antarctic interior (Christ and Bierman, 2020; Denton and Marchant, 2000; Talarico et al, 2012, 2013) (Fig. 1b).
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