Age of the Fjord Stade moraines in the Disko Bugt region, western Greenland, and the 9.3 and 8.2 ka cooling events

Abstract

Retreat of the western Greenland Ice Sheet during the early Holocene was interrupted by deposition of the Fjord Stade moraine system. The Fjord Stade moraine system spans several hundred kilometers of western Greenland's ice-free fringe and represents an important period in the western Greenland Ice Sheet's deglaciation history, but the origin and timing of moraine deposition remain uncertain. Here, we combine new and previously published 10Be and 14C ages from Disko Bugt, western Greenland to constrain the timing of Fjord Stade moraine deposition at two locations ∼60 km apart. At Jakobshavn Isfjord, the northern of two study sites, we show that Jakobshavn Isbræ advanced to deposit moraines ca 9.2 and 8.2–8.0 ka. In southeastern Disko Bugt, the ice sheet deposited moraines ca 9.4–9.0 and 8.5–8.1 ka. Our ice-margin chronology indicates that the Greenland Ice Sheet in two distant regions responded in unison to early Holocene abrupt cooling 9.3 and 8.2 ka, as recorded in central Greenland ice cores. Although the timing of Fjord Stade moraine deposition was synchronous in Jakobshavn Isfjord and southeastern Disko Bugt, within uncertainties, we suggest that Jakobshavn Isbræ advanced while the southeastern Disko Bugt ice margin experienced stillstands during the 9.3 and 8.2 ka events based on regional geomorphology and the distribution of 10Be ages at each location. The contrasting style of ice-margin response was likely regulated by site-specific ice-flow characteristics. Jakobshavn Isbræ's high ice flux results in an amplified ice-margin response to a climate perturbation, both warming and cooling, whereas the comparatively low-flux sector of the ice sheet in southeastern Disko Bugt experiences a more subdued response to climate perturbations. Our chronology indicates that the western Greenland Ice Sheet advanced and retreated in concert with early Holocene temperature variations, and the 9.3 and 8.2 ka events, although brief, were of sufficient duration to elicit a significant response of the western Greenland Ice Sheet.