ABSTRACT: Sedimentary evidence of climate and landscape changes from Upper and Lower Murray Lakes in the Canadian High-Arctic.

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Abstract

Here we present preliminary results from a series of sediment cores recovered from two adjacent High Arctic lakes. Upper and Lower Murray Lakes (81°20’N, 69°30’W) occupy a glacially carved valley on northeastern Ellesmere Island, Nunavut Canada. The lakes are similar in size with surface areas of ~6 km2 and a combined drainage basin covering ~269 km2. Approximately 70% of the watershed drains directly into Upper Murray Lake, which is connected to Lower Murray Lake by a short (~100m), shallow (<0.5m) stream. Two small (~2.5 km2), stagnant ice caps are located to the east of Lower Murray Lake. While there are several small streams leading from these ice caps directly into Lower Murray Lake, at present the majority of runoff drains outside of the Murray Lakes watershed. However, during periods of increased glacial expanse in the past (e.g. during the Little Ice Age), it is likely that considerably higher runoff was directed into the Murray Lakes Watershed, and the two lakes (now separate) may have formed one larger system. A larger ice cap (~15 km²), located to the southwest, drains into Upper Murray Lake. 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. Lower Murray Lake was first cored in 2000 when two short, 55 cm long, gravity cores were recovered. Image analysis of thin sections made from these cores indicated that the sediments were annually laminated and produced a record of sediment flux into Lower Murray Lake spanning (at a minimum) the past 935 years. Two long vibracores (3.2 and 5.1m in length) were recovered in 2001, and in 2005, two adjacent, overlapping cores extending 13.2m were collected using a Uwitec percussion corer. The long cores contain laminations similar to those observed in the 2000 short cores that extent to a depth of 2.2m. Below this depth the distinct silt-clay couplets transition into more uniform clay containing faint laminations. Assuming an accumulation rate of 0.06 cm/yr (based on a varve count of the 2000 short core) the presumed annual laminations observed in the upper 2.2m may span the last ~3,600 years. Based on an increase in the apparent thickness of the laminations (to approximately ~0.5-1.0cm) and the fact that the valley in which the Murray Lakes are located was glaciated until sometime between 5,000 and 7,000 years BP, it is believed that the rate of sediment accumulation in the lowest 11m of the record was significantly greater than the accumulation in the first 2.2m. Results from bulk density, loss-on-ignition (LOI), and scanning X-ray Fluorescence (XRF) measurements further highlight the distinct transition that occurred at 2.2m depth. The upper 2.2m are characterized by wide variability in bulk density and LOI, but the average values remain relatively constant. Below, 2.2m variability in both records decreases, bulk density begins to increase, and LOI begins to decrease. Furthermore the 2.2m transition is characterized by a decrease in the concentration of Fe and K in the sediments and an in increase in the concentration of Ca. We hypothesize that this abrupt change in sedimentation likely reflects a change in glacial conditions within the watershed (possibly a change in glacial extent or a redirection of glacial outflow out of the drainage basin). The high resolution scanning XRF data also illustrates seasonal changes in sediment accumulation recorded in a short core from Upper Murray Lake. The upper most sediments from this lake contain distinct, alternating dark-colored silt and light-colored clay couplets approximately 1.0-1.5cm thick. It is believed that these layers reflect annual cycles of sediment accumulation, with coarser sediment input during the melt season followed by the accumulation of a clay-cap. XRF measurements across these laminae show peak levels of Fe and K at the bottom of the coarse layers, followed by minimum values at the top of the coarse layers (with Ca content showing an inverse relationship). These results illustrate the potential of high-resolution XRF data to capture both short term cycles in seasonal sediment accumulation and long term trends associated with changes in landscape evolution. Continued analysis of the sedimentary archives from Upper and Lower Murray Lakes will provide an important opportunity to compare paleoclimate records produced from two systems subjected to similar external forcing mechanisms but influenced by unique local controls within their individual catchments. In order to identify likely mechanisms responsible for the variations recorded in these records, future work will include thin section analysis, grain-size measurements, and elemental analysis of the total and organic C and N content of the sediments. Age control will be provided through 210Pb and 137Cs profiles of the upper sediments, radiocarbon dating where available, and correlation of paleomagnetic variations with independently dated records.

Bibliography of Canadian Geomorphology