Preprints
https://doi.org/10.5194/gchron-2021-21
https://doi.org/10.5194/gchron-2021-21

  29 Jun 2021

29 Jun 2021

Review status: a revised version of this preprint was accepted for the journal GChron and is expected to appear here in due course.

Cosmogenic nuclide exposure age scatter in McMurdo Sound, Antarctica records Pleistocene glacial history and processes

Andrew J. Christ1, Paul R. Bierman1,2, Jennifer L. Lamp3, Joerg M. Schaefer3, and Gisela Winckler3 Andrew J. Christ et al.
  • 1Gund Institute for Environment, University of Vermont, Burlington, VT, 05405, USA
  • 2Rubenstein School of the Environment and Natural Resources, University of Vermont, Burlington, VT, 05405, USA
  • 3Lamont Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA

Abstract. 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 McMurdo Sound, 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 19.6 ka. Cosmogenic exposure ages vary by lithology, suggesting that bedrock source and surface processes prior to, during, and after glacial entrainment explain scatter. 10Be 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. 3He exposure ages of pyroxene in Ferrar Dolerite, which outcrops 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. 3He exposure ages of olivine in basalt from local volcanic bedrock in McMurdo Sound 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 deposits and suggest that the most extensive Pleistocene ice sheet inundated McMurdo Sound during Marine Isotope Stage 8. These results underscore how surface processes operating in the Transantarctic Mountains are expressed in the cosmogenic nuclide inventories held in Antarctic glacial sediments.

Andrew J. Christ et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2021-21', Anonymous Referee #1, 19 Jul 2021
    • AC1: 'Reply on RC1', Andrew Christ, 27 Aug 2021
  • RC2: 'Comment on gchron-2021-21', Ross Whitmore, 11 Aug 2021
    • AC2: 'Reply on RC2', Andrew Christ, 27 Aug 2021

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2021-21', Anonymous Referee #1, 19 Jul 2021
    • AC1: 'Reply on RC1', Andrew Christ, 27 Aug 2021
  • RC2: 'Comment on gchron-2021-21', Ross Whitmore, 11 Aug 2021
    • AC2: 'Reply on RC2', Andrew Christ, 27 Aug 2021

Andrew J. Christ et al.

Andrew J. Christ et al.

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Short summary
Cosmogenic nuclide surface exposure dating is commonly used to constrain the timing of past glacier extents. However, Antarctic exposure age datasets are often scattered and difficult to interpret. We compiled new and existing exposure ages of a glacial deposit with independently known age constraints and identify surface processes that increase or reduce the likelihood of exposure age scatter. Then we present new data for a previously unmapped and undated older deposit from the same region.