Benjamin A. Keisling

Basal conditions and ice dynamics inferred from radar-derived internal stratigraphy of the northeast Greenland ice stream

Abstract We analyze the internal stratigraphy in radio-echo sounding data of the northeast Greenland ice stream to infer past and present ice dynamics. In the upper reaches of the ice stream, we propose that shear-margin steady-state folds in internal reflecting horizons (IRHs) form due to the influence of ice flow over spatially varying basal lubrication. IRHs are generally lower in the ice stream than outside, likely because of greater basal melting in the ice stream from enhanced geothermal flux and heat of sliding. Strain-rate modeling of IRHs deposited during the Holocene indicates no recent major changes in ice-stream vigor or extent in this region. Downstream of our survey, IRHs are disrupted as the ice flows into a prominent overdeepening. When combined with additional data from other studies, these data suggest that upstream portions of the ice stream are controlled by variations in basal lubrication whereas downstream portions are confined by basal topography.

Greenland‐Wide Seasonal Temperatures During the Last Deglaciation

The sensitivity of the Greenland ice sheet to climate forcing is of key importance in assessing its contribution to past and future sea level rise. Surface mass loss occurs during summer, and accounting for temperature seasonality is critical in simulating ice sheet evolution and in interpreting glacial landforms and chronologies. Ice core records constrain the timing and magnitude of climate change but are largely limited to annual mean estimates from the ice sheet interior. Here we merge ice core reconstructions with transient climate model simulations to generate Greenland-wide and seasonally resolved surface air temperature fields during the last deglaciation. Greenland summer temperatures peak in the early Holocene, consistent with records of ice core melt layers. We perform deglacial Greenland ice sheet model simulations to demonstrate that accounting for realistic temperature seasonality decreases simulated glacial ice volume, expedites the deglacial margin retreat, mutes the impact of abrupt climate warming, and gives rise to a clear Holocene ice volume minimum. Plain Language Summary The Greenland ice sheet could contribute 7 m (23 feet) of sea level rise if it were to melt completely. For future sea level rise predictions we need to know how the Greenland ice sheet will respond to rising temperatures. We can figure out how sensitive Greenland is by studying a natural period of warming (called the last deglaciation) that happened at the end of the last Ice Age 18,000 years ago. During the last Ice Age the Greenland ice sheet was much larger than it is today, and as the climate warmed it shrunk to its present size. We combine ice core data and climate models to reconstruct Greenland-wide temperatures for all seasons over the last 22,000 years. This reconstruction makes it possible to simulate Greenland ice loss during the last deglaciation in ice sheet models. The model output can be compared to data on past ice sheet volume, for example, from moraines left behind in the landscape as the ice melted. Our reconstruction provides a critical step in learning from the past behavior of the Greenland ice sheet in order to predict its future.