Sedimentation in arctic proglacial lakes: Mittivakkat Glacier, south‐east Greenland

Several sediment cores were collected from two proglacial lakes in the vicinity of Mittivakkat Glacier, southeast Greenland, in order to determine sedimentation rates, estimate sediment yields and identify the dominant sources of the lacustrine sediment. The presence of varves in the ice-dammed Icefall Lake enabled sedimentation rates to be estimated using a combination of X-ray photography and down-core variations in 137 Cs activity. Sedimentation rates for individual cores ranged between 0.52 and 1.06 g cm ˇ2 year ˇ1 , and the average sedimentation rate was estimated to be 0.79 g cm ˇ2 year ˇ1 . Despite considerable down-core variability in annual sedimentation rates, there is no significant trend over the period 1970 to 1994. After correcting for autochthonous organic matter content and trap eAciency, the mean fine-grained minerogenic sediment yield from the 3. 8k m 2 basin contributing to the lake was estimated to be 327 t km ˇ2 year ˇ1 . Cores were also collected from the topset beds of two small deltas in Icefall Lake. The deposition of coarse-grained sediment on the delta surface was estimated to total in excess of 15 cm over the last c. 40 years. In the larger Lake Kuutuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedimentation rates determined for six cores collected from the centre of the lake, based on their 137 Cs depth profiles, were estimated to range between 0.05 and 0.11 g cm ˇ2 year ˇ1 , and the average was 0.08 g cm ˇ2 year ˇ1 . The longer-term (c. 100‐150 years) average sedimentation rate for one of the cores, estimated from its unsupported 210 Pb profile, was 0.10‐0.13 g cm ˇ2 year ˇ1 , suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100‐150 years. The average fine-grained sediment yield from the 32. 4k m 2 catchment contributing to the lake was estimated to be 13 t km ˇ2 year ˇ1 . The 137 Cs depth profiles for cores collected from the topset beds of the delta of Lake Kuutuaq indicate that in excess of 27 cm of coarse-grained sediment had accumulated on the delta surface over the last approximately 40 years. Caesium-137 concentrations associated with the most recently deposited (uppermost) fine-grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fine-grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debris, is the most likely source of the sediment deposited in the lakes. This finding is confirmed by the 137 Cs concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to those for proglacial material. Copyright # 2000 John Wiley & Sons, Ltd.