ABSTRACT: Controls on recent sedimentation and lamina characteristics in two adjacent High-Arctic lakes.

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Abstract

Laminated sediments from Upper and Lower Murray Lakes, Ellesmere Island, Nunavut, Canada (81°20’N, 69°30’W), provide an ideal opportunity to examine recent sedimentation in two adjacent lake basins. The close proximity of the two lakes (separated by approximately 100m) subjects them to similar external (climatic) forcings. However, actual sedimentation in the individual lakes is influenced by the unique characteristics of their basins and within-lake processes. Therefore the question remains: is there a consistent climatic signal preserved in the sedimentary records of High-Arctic lakes, or do local processes dominate sedimentation? Upper and lower Murray Lakes occupy a north-south trending valley within the Hazen Plateau. The long (8 & 6 km respectively), narrow (~1 km) lakes share a combined drainage basin of approximately 251 km2. Roughly 70% of the watershed drains directly into the upper lake before leaving through a broad (~35m), shallow (<30cm) stream that drains into the lower lake. Although this stream likely contributes a significant portion of the total runoff to the lower lake, the accompanying sediment flux between the lakes is most likely very low, as the upper lake acts a sediment trap, and only the lowest density overflows would be capable transporting sediment over the shallow spillway. The drainage basin is characterized by considerable relief, with the majority of the watershed located on upland plateaus that drain into the lakes via short, high gradient streams. Approximately 6% of the watershed is glaciated with a large unnamed ice cap to the southwest contributing runoff to the upper Lake, and two small, stagnant ice caps east of the lakes (Murray and Simmons ice caps) contributing runoff directly to the lower lake. The short, steep streams draining the watershed should enable rapid routing of melt water into channels, and minimize retention of early runoff behind snow dams. In combination with the essentially unlimited supply of meltwater from the ice caps, these characteristic should help maintain the connection between climatic conditions, snow/ice melt production, and runoff into the lakes. The maximum depth of Upper and Lower Murray Lakes are ~80 m and ~45 m, respectively. Both lakes are depleted in oxygen near the sediment water interface, and clastic laminations are well preserved in the sedimentary record. Short (<30 cm) surface cores from each of the lakes contain well preserved records of recent sedimentation. Lamina characteristics (including thickness and grain size) and sedimentary structures are analyzed by creating thin sections from epoxy-impregnated sediment slaps. Digital images are then acquired using an optical scanner and scanning electron microscope. Lower Murray Lake shows a pattern of relatively consistent sedimentation over the past ~100 years, with predominantly thin (~0.06mm) laminae. In contrast, recent sedimentation in Upper Murray Lake is dominated by thick (10mm), coarse grained layers interspersed with periods of finer laminae. Preliminary results indicate that local factors significantly influence the sedimentary records of the individual lakes, and these features must be identified before an accurate climate signal can be interpreted from the long term record.

Bibliography of Canadian Geomorphology