ABSTRACT: Sedimentary evidence for changes in the dissolved oxygen content of a High-Arctic lake and its impact on varve preservation.

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Annually laminated or varved sediments provide a valuable paleolimnological tool for reconstructing environmental changes with high temporal resolution. The formation and preservation of annual laminations necessitates both a strong seasonal cycle in lake processes and a depositional environment free from subsequent disturbances. Lakes with oxygen depleted bottom waters are particularly favorable for the preservation of annual laminations because the lack of oxygen restricts or excludes the presence of benthic organisms that would otherwise result in bioturbation of sediments. This study examines the mechanism responsible for the formation and preservation of annual laminations in a high arctic lake with particular emphasis on the role of hypoxia on varve preservation. The focus of the study is Lower Murray Lake (81°20’N, 69°30’W) on northern Ellesmere Island in the Canadian High Arctic. Varve deposition in Lower Murray Lake results from the extreme seasonality of temperatures in the High Arctic and its control over meltwater production, streamflow, and consequently clastic sediment transfer into the lake. A total of 13.4 m of sediment were recovered from Lower Murray Lake providing a nearly complete record of the evolution of the lake basin following the retreat of the Innuitian Ice Sheet ca. 6000 14C years BP. Early lake sedimentation is characterized by extremely rapid deposition of coarse clastic material, followed by a transitional period marked by decreasing grain size and sediment accumulation. These changes in the physical characteristics of the sediments occur simultaneously with an apparent decrease in the oxygen content of bottom waters evident in ITRAX scanning X-ray fluorescence (XRF) measurements of the manganese and iron content of the sediments. Specifically, decreasing oxygen content is inferred by a decrease in Mn:Fe ratio of the sediments and a decrease in the level of covariance between manganese and iron. The transition to varve deposition occurred promptly ca. 5200 varve years BP and coincides with the onset of hypoxic conditions. Oxygen depletion is interpreted to be the result of enhanced productivity and organic matter decay following a reduction in clastic sediment input into the lake, reduced turbidity, and increased light availability. Varve deposition has continued up to the present. Recent (2005) dissolved oxygen profiles indicate hypoxic conditions are limited to the bottom 1-2 m of the water column. The importance of hypoxia on varve preservation is further supported by several cores that were collected at water depths slightly shallower than the deepest point in the lake and consequently lack annual laminations. In addition, an apparent decrease in the preservation quality of the most recently deposited varves may be linked to a recently documented increase in the duration of ice free conditions on the lake and the likely impact of a longer open water period on lake mixing processes and the oxygenation of bottom waters.

Bibliography of Canadian Geomorphology