HOLOCENE STRATIGRAPHY OF LAKES IN GRABEN LAND EAST GREENLAND: IMPLICATIONS FOR AN EXPANDED ICE SHEET

Holocene fluctuations of the Greenland Ice Sheet provide context for its present recession. Clues to changes in the elevations of the Vendue outlet glacier of the Greenland Ice Sheet come from sediment cores retrieved from two glacially fed lakes. Thick (up to 30 cm) sand layers are argued here to be associated with intervals when the ice sheet was at or near present-day extent. The glacier dams an ice-contact lake (Tsunami Lake) which drains 1.5 km along the depression of a graben into two smaller lakes. Black Hole Lake (BHL, 71.17° N, 28.95° E, 943 m) drains via a small stream 300 m into Ice Fall Lake (IFL, 71.17° N, 28.94° E, 942 m). These lakes have depths of 14 m and 9 m, areas of 15,800 m² and 8,400 m², and yielded cores 1.55 m and 2.24 m long, respectively. Sand deposits in BHL comprise grain sizes from 250 to 60 μm in multiple fining upward sequences that are bordered by glacially derived silt. BHL sands contain higher amounts of quartz than mica (3:2 ratio) and are imbricated. IFL sands comprise grain sizes from 375 to 100 μm in fining upward sequences with a lower abundance of glacial silt, have higher amounts of mica than quartz (6:1 ratio), are imbricated, and have high organic content (6%). The imbricated, fining upward sand pulses grade into glacial silt, which are prevalent in BHL but less abundant in IFL, suggest high velocity flow and fallout from suspension. Furthermore, there appears to be a strong correlation between mica/quartz ratio and water velocity. Quartz is more abundant where velocity is assumed to be higher and mica is more abundant where velocity is assumed to be lower. Terrestrial organic remains, found only in mica-rich sand from IFL, also suggest high water velocity and by-passing of BHL. We argue this pattern of sediment properties can be explained by water flow from floods originating when Tsunami Lake, and hence the glacier, is near its present level to drain to BHL and IFL. When the glacier and the water level are lower, no water (or sediment) would exit Tsunami Lake. In this view, glacier expansion is associated with high lake levels, close to that of the present day, and coarse-grained sediments are transported to downstream lakes by floods. Possible sources of this water include surface meltpools, upstream ice contact lakes, or large calving events.