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Reliable reconstructions of paleoseismicity are useful for understanding, and mitigating, seismic hazard risks. In this study, we apply cosmogenic 36Cl exposure-age dating and concentrations of rare-earth elements and yttrium (REY) to unravelling the paleoseismic history of the Sparta fault, Greece, which is a range-bounding normal fault developed in limestone. Modeling of 36Cl concentrations along two vertical profiles on the Sparta Fault indicates a clustering of four earthquakes within a 1.5 kyr period that culminated with the 464 B.C.E. event that devastated Spartan society. Cumulative uplift was as high as 2.8 mm yr-1 during that period, compared with ~0.6–0.9 mm a-1 over the preceding 2.7–4.4 kyr. Because earthquake activity may shift between faults in extensional settings, a large magnitude earthquake is not necessarily indicated as being overdue by the present ~2.5 kyr quiescent period. More generally, accurate identification of individual earthquakes is presently constrained by spatial variations in 36Cl concentration profiles that reflect neither exposure duration nor imprints of former soil profiles. In cases where this is attributable to mineralogical variations, such as in the Sparta fault scarp, present chemical preparation techniques for AMS measurement of 36Cl may insufficiently account for those variations. The Sparta fault scarp is composed of fault breccia, which contains quartz and clay-lined pores, in addition to host rock-derived clasts of calcite and microcrystalline calcite cement. The exchange of REY between the hanging wall colluvium and the fault scarp calcite, which has been applied to the study of paleoseismicity on other limestone normal faults, is overwhelmed on this fault scarp by REY attached to the breccia pore clays. Holocene earthquakes and their magnitudes, inferred from fault slip lengths, therefore cannot be inferred from REY data for impure limestone faults such as the Sparta fault but, rather, these data may indicate processes of fault evolution in the Earth’s near surface.

Numerical models predict that discharge from the polar ice sheets will become the largest contributor to sea-level rise over the coming centuries. However, the predicted amount of ice discharge and associated thinning depends on how well ice sheet models reproduce glaciological processes, such as ice flow in regions of large topographic relief, where ice flows around bedrock summits (i.e. nunataks) and through outlet glaciers. The ability of ice sheet models to capture long-term ice loss is best tested by comparing model simulations against geological data. A benchmark for such models is ice surface elevation change, which has been constrained empirically at nunataks and along margins of outlet glaciers using cosmogenic exposure dating. However, the usefulness of this approach in quantifying ice sheet thinning relies on how well such records represent changes in regional ice surface elevation. Here we examine how ice surface elevations respond to the presence of strong topographic relief that acts as an obstacle by modelling ice flow around and between idealised nunataks during periods of imposed ice sheet thinning. We find that, for realistic Antarctic conditions, a single nunatak can exert an impact on ice thickness over 20km away from its summit, with its most prominent effect being a local increase (decrease) of the ice surface elevation of hundreds of metres upstream (downstream) of the obstacle. A direct consequence of this differential surface response for cosmogenic exposure dating is a delay in the time of bedrock exposure upstream relative to downstream of a nunatak summit. A nunatak elongated transversely to ice flow is able to increase ice retention and therefore impose steeper ice surface gradients, while efficient ice drainage through outlet glaciers produces gentler gradients. Such differences, however, are not typically captured by continent-wide ice sheet models due to their coarse grid resolutions. Their inability to capture site-specific surface elevation changes appears to be a key reason for the observed mismatches between the timing of ice-free conditions from cosmogenic exposure dating and model simulations. We conclude that a model grid refinement over complex topography and information about sample position relative to ice flow near the nunatak are necessary to improve data-model comparisons of ice surface elevation and therefore the ability of models to simulate ice discharge in regions of large topographic relief.

Funding: This research has been supported by the Swedish Research Council (grant no. 2016-04422), the German Research Foundation (grant no. 1158-365737614), the US National Science Foundation (grant no. PLR-1542930), and the Norwegian Polar Institute/NARE (grant no. 2015/38/7/NK/ihs). Jorge Bernales has been supported by the MAGIC-DML project through DFG SPP 1158 (RO 4262/1-6). The article processing charges for this open-access publication were covered by Stockholm University. Studying the response of the Antarctic ice sheets during periods when climate conditions were similar to the present can provide important insights into current observed changes and help identify natural drivers of ice sheet retreat. In this context, the marine isotope substage 11c (MIS11c) interglacial offers a suitable scenario, given that during its later portion orbital parameters were close to our current interglacial. Ice core data indicate that warmer-than-present temperatures lasted for longer than during other interglacials. However, the response of the Antarctic ice sheets and their contribution to sea level rise remain unclear. We explore the dynamics of the Antarctic ice sheets during this period using a numerical ice sheet model forced by MIS11c climate conditions derived from climate model outputs scaled by three glaciological and one sedimentary proxy records of ice volume. Our results indicate that the East and West Antarctic ice sheets contributed 4.0–8.2 m to the MIS11c sea level rise. In the case of a West Antarctic Ice Sheet collapse, which is the most probable scenario according to far-field sea level reconstructions, the range is reduced to 6.7–8.2 m independently of the choices of external sea level forcing and millennial-scale climate variability. Within this latter range, the main source of uncertainty arises from the sensitivity of the East Antarctic Ice Sheet to a choice of initial ice sheet configuration. We found that the warmer regional climate signal captured by Antarctic ice cores during peak MIS11c is crucial to reproduce the contribution expected from Antarctica during the recorded global sea level highstand. This climate signal translates to a modest threshold of 0.4 °C oceanic warming at intermediate depths, which leads to a collapse of the West Antarctic Ice Sheet if sustained for at least 4000 years. Publisher PDF

Reconstructing the response of present-day ice sheets to past global climate change is important for constraining and refining the numerical models which forecast future contributions of these ice sheets to sea-level change. Mapping landforms is an essential step in reconstructing glacial histories. Here we present a new map of glacial landforms and deposits on nunataks in western Dronning Maud Land, Antarctica. Nunataks are mountains or ridges that currently protrude through the ice sheet and may provide evidence that they have been wholly or partly covered by ice, thus indicating a formerly more extensive (thicker) ice sheet. The map was produced through a combination of mapping from Worldview satellite imagery and ground validation. The sub-metre spatial resolution of the satellite imagery enabled mapping with unprecedented detail. Ten landform categories have been mapped, and the landform distributions provide evidence constraining spatial patterns of a previously thicker ice sheet.