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,
geodetic observations, 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, and isostatic subsidence. With independent age control, ice structure revealed by radar can be used to infer past changes in ice
flow and geometry, and therefore potential readvance. Since ice cores capture changes in surface mass balance, elevation, and 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.
Abstract. We describe new Last Glacial Maximum (LGM) ice
thickness constraints for three locations spanning the Weddell Sea Embayment
(WSE) of Antarctica. Samples collected from the Shackleton Range, Pensacola
Mountains, and the Lassiter Coast constrain the LGM thickness of the Slessor
Glacier, Foundation Ice Stream, and grounded ice proximal to the modern
Ronne Ice Shelf edge on the Antarctic Peninsula, respectively. Previous
attempts to reconstruct LGM-to-present ice thickness changes around the WSE
used measurements of long-lived cosmogenic nuclides, primarily 10Be. An
absence of post-LGM apparent exposure ages at many sites led to LGM
thickness reconstructions that were spatially highly variable and
inconsistent with flow line modelling. Estimates for the contribution of the
ice sheet occupying the WSE at the LGM to global sea level since
deglaciation vary by an order of magnitude, from 1.4 to 14.1 m of sea level
equivalent. Here we use a short-lived cosmogenic nuclide, in situ-produced
14C, which is less susceptible to inheritance problems than 10Be
and other long-lived nuclides. We use in situ 14C to evaluate the
possibility that sites with no post-LGM exposure ages are biased by
cosmogenic nuclide inheritance due to surface preservation by cold-based ice
and non-deposition of LGM-aged drift. Our measurements show that the Slessor
Glacier was between 310 and up to 655 m thicker than present at the LGM. The
Foundation Ice Stream was at least 800 m thicker, and ice on the Lassiter
Coast was at least 385 m thicker than present at the LGM. With evidence for
LGM thickening at all of our study sites, our in situ 14C measurements
indicate that the long-lived nuclide measurements of previous studies were
influenced by cosmogenic nuclide inheritance. Our inferred LGM
configuration, which is primarily based on minimum ice thickness constraints
and thus does not constrain an upper limit, indicates a relatively modest
contribution to sea level rise since the LGM of < 4.6 m, and
possibly as little as < 1.5 m.
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