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

Several sediment cores were collected from two proglacial lakes in the vici nity of Mittivakkat Glacier, southeast Greenland, in order to determine sed imentation rates, estimate sediment yields and identify the dominant source s of the lacustrine sediment. The presence of varves in the ice-dammed Icef all Lake enabled sedimentation rates to be estimated using a combination of X-ray photography and down-core variations in Cs-137 activity. Sedimentati on rates for individual cores ranged between 0.52 and 1.06 g cm(-2) year(-1 ), and the average sedimentation rate was estimated to be 0.79 g cm(-2) yea r(-1). Despite considerable down-core variability in annual sedimentation r ates, there is no significant trend over the period 1970 to 1994. After cor recting for autochthonous organic matter content and trap efficiency, the m ean fine-grained minerogenic sediment yield from the 3.8 km(2) basin contri buting to the lake was estimated to be 327 t km(-2) year(-1). Cores were al so 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 Kuu tuaq, which is located about 5 km from the glacier front and for which the glacier represents a smaller proportion of the contributing catchment, sedi mentation rates determined for six cores collected from the centre of the l ake, based on their Cs-137 depth profiles, were estimated to range between 0.05 and 0.11 g cm(-2) year(-1), and the average was 0.08 g cm(-2) year(-1) . The longer-term (c. 100-150 years) average sedimentation rate for one of the cores, estimated from its unsupported Pb-210 profile, was 0.10-0.13 g c m(-2) year(-1), suggesting that sedimentation rates in this lake have been essentially constant over the last c. 100-150 years. The average fine-grain ed sediment yield from the 32.4 km(2) catchment contributing to the lake wa s estimated to be 13 t km(-2) year(-1). The Cs-137 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 as sociated with the most recently deposited (uppermost) hue-grained sediment in both Icefall Lake and Lake Kuutuaq were similar to those measured in fin e-grained sediment collected from steep slopes in the immediate proglacial zone, suggesting that this material, rather than contemporary glacial debri s, is the most likely source of the sediment deposited in the lakes. This f inding is confirmed by the Cs-137 concentrations associated with suspended sediment collected from the Mittivakkat stream, which are very similar to t hose for proglacial material. Copyright (C) 2000 John Wiley & Sons, Ltd.