Abstract
Widespread existing geological records from above the modern ice-sheet surface and outboard of the current ice margin show that the Antarctic Ice Sheet (AIS) was much more extensive at the Last Glacial Maximum (~20 ka) than at present. However, whether it was ever smaller than present during the last few millennia, and (if so) by how much, is known only for a few locations because direct evidence lies within or beneath the ice sheet, which is challenging to access. Here, we describe how retreat and readvance (henceforth “readvance”) of AIS grounding lines during the Holocene could be detected and quantified using subglacial bedrock, subglacial sediments, marine sediment cores, relative sea-level (RSL) records, radar data, and ice cores. Of these, only subglacial bedrock and subglacial sediments can provide direct evidence for readvance. Marine archives are of limited utility because readvance commonly covers evidence of earlier retreat. Nevertheless, stratigraphic transitions documenting change in environment may provide support for direct evidence from subglacial records, as can the presence of transgressions in RSL records. With independent age control, past changes in ice structure and flow patterns revealed by radar can be used to infer ice volume changes commensurate with readvance. Since ice cores capture changes in surface mass balance, elevation, and changes in atmospheric and oceanic circulation that are known to drive grounding-line migration, they also have potential for identifying readvance. A multidisciplinary approach is likely to provide the strongest evidence for or against a smaller-than-present AIS in the Holocene.
Highlights
Improving our understanding of the glacial history of the Antarctic Ice Sheet (AIS) during the Holocene (11.7 ka to present) is essential for determining the likely ice-sheet response to future ocean and atmospheric warming
Despite the promise of low elevation sites and coastal domes for detecting Holocene retreat and readvance, there are limitations: (i) The high snow accumulation at coastal sites reduces the number of years that the record contains, (ii) the high concentration of marine aerosols can make dating, the use of volcanic tie-points, challenging (e.g., Winstrup et al, 2019; Moser et al, 2021), (iii) low elevation sites can be susceptible to surface melting (Moser et al, 2021; Thomas et al, 2021), which may compromise the preservation of ice core proxies (Moser et al, 2021), and (iv) complex orography at the ice-sheet margin may limit sites to capturing regional, rather than large-scale, changes in ice-sheet dynamics
There are several complementary approaches that can be used in Antarctica to obtain geological and glaciological records for determining whether, where, and when grounding-line readvance occurred during the Holocene
Summary
Improving our understanding of the glacial history of the Antarctic Ice Sheet (AIS) during the Holocene (11.7 ka to present) is essential for determining the likely ice-sheet response to future ocean and atmospheric warming. We describe a range of glacial and geological records that have the potential to provide direct or circumstantial evidence for retreat of the Antarctic grounding line upstream of its present position during the past few millennia. These records are situated either below or within the modern ice sheet, or in currently exposed areas. Overall, existing exposure age data are (i) consistent with the hypothesis that late Holocene readvance took place in the Weddell and Amundsen Sea embayments and at outer Ross Sea sites, (ii) not obviously consistent with the hypothesis that readvance took place in the inner Ross Sea, and (iii) provide minimal information for East Antarctica
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