Abstract
Abstract. To date the final stage in deglaciation of the Greenland shelf, when a contiguous ice sheet margin on the inner shelf transitioned to outlet glaciers in troughs with intervening ice-free areas, we generated cosmogenic 10Be dates from bedrock knobs on six outlying islands along a stretch of 300 km of the southwestern Greenland coast. Despite 10Be inheritance influencing some dates, the ages generally support a Greenland Ice Sheet (GrIS) margin that retreated off the inner shelf during the middle Younger Dryas (YD) period. Published 10Be- and 14C-dated records show that this history of the GrIS margin is seen in other parts of Greenland but with large variations in the extent and speed of retreat, sometimes even between neighbouring areas. Areas with a chronology extending into the Allerød period show no marked ice margin change at the Allerød–YD transition except in northernmost Greenland. In contrast, landforms on the shelf (moraines and grounding zone wedges) have been suggested to indicate YD readvances or long-lasting ice margin stillstands on the middle shelf. However, these features have been dated primarily by correlation with cold periods in the ice core temperature records. Ice margin retreat during the middle and late YD is explained by advection of warm subsurface water at the ice margin and by increased seasonality. Our results therefore point to the complexity of the climate–ice margin relation and to the urgent need for direct dating of the early deglaciation history of Greenland.
Highlights
The Younger Dryas (YD) cold climate oscillation from 12.8 to 11.7 ka began after the Allerød warm period, with a 200-year-long period of cooling, and culminated with a 60-year-long period of abrupt warming, as recorded in Greenland ice cores (Steffensen et al, 2008)
During the YD, it seems that the Greenland Ice Sheet (GrIS) in most areas had its margin on the shelf, and earlier work concentrated on the behaviour of ice streams in transverse troughs on the shelf (e.g. Larsen et al, 2016); newer references are discussed below
10Be dates on bedrock surfaces in the glacially eroded archipelago on the inner shelf of southwestern Greenland are affected by 10Be inherited from earlier exposure, but clustering of ages from each site suggest that the ice margin here was retreating and close to the coast at least from mid-YD times
Summary
The Younger Dryas (YD) cold climate oscillation from 12.8 to 11.7 ka (thousand years before present) began after the Allerød warm period, with a 200-year-long period of cooling, and culminated with a 60-year-long period of abrupt warming, as recorded in Greenland ice cores (Steffensen et al, 2008). During the YD, it seems that the GrIS in most areas had its margin on the shelf, and earlier work concentrated on the behaviour of ice streams in transverse troughs on the shelf A belt of shallow banks with a gently undulating surface is composed of younger stratified marine and fluvial sediments These banks are dissected by 400 to 500 m deep transverse troughs that are a continuation of the major fjords inland (Holtedahl, 1970; Sommerhoff, 1975; Roksandic, 1979; Sommerhoff, 1981; Ryan et al, 2016). At a distance of 10–15 km beyond the coast, the inner trough forms an archipelago, with a multitude of small glacially sculptured rocky islands and skerries reflecting intensive but uneven glacial erosion From these rocky islands we collected our samples (Fig. 2)
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