A COMPARISON OF 45 YEARS OF REGIONAL RIVER DISCHARGE RECORDS AND SUBANNUAL EVENTS IN VARVED SEDIMENTS FROM TWO LAKES IN WHITE PASS, BRITISH COLUMBIA/ALASKA
COCKBURN, JACLYN . Department of Geography, Queen's University.
Lamoureux, Scott F. Department of Geography, Queen's University.
Annually laminated (varved) sediments are potentially useful proxies for watershed hydrometeorological behaviour. Varve formation in a lake with primarily clastic, allochthonous inputs is controlled by seasonal and subseasonal sediment delivery. Process studies have correlated meteorological (temperature and precipitation) and river discharge records to varve thickness, demonstrating the climatic influences on annual sediment accumulation (Hardy et al., 1996). Individual hydrological events are also identifiable in detailed examinations of subannual rhythmites within a varve (Lamoureux, 2000). In this study, varves from two adjacent catchments located at White Pass, British Columbia are used to evaluate the preservation and reproducibility of subannual sedimentary events based on a comparison with 45 years of daily river discharge records. Specifically, the ability to discern subannual sedimentary events within each varve record is examined and the hydrometeorological thresholds that produce subannual rhythmites are estimated.
The inflow to Summit Lake (59°39’N, 135°08’W, 28 km^2 watershed, 17% glacial cover) is dominated by substantial spring nival melt, summer glacial melt and autumn rainstorms although limited field observations suggest that sediment transport and deposition occurs primarily during glacial melt and autumn rainstorms. The sediment cores used in this study are from the main proximal basin and include a long vibracore and surface core (water depth 27 m). The cores were subsampled for thin sections and grain size determination.
Meadow Lake (unofficial name, 59°41’N, 135°05’W) is an isolated basin of Summit Lake, northeast and distal to the main basin. The creek flowing into Meadow Lake drains a 14 km^2 watershed with 7% glacial cover. Meadow Lake is much smaller than Summit Lake (Meadow is 300 x 50 m and 10 m deep compared to Summit 7.5 x 0.3 km and 40 m deep) and field observations indicate that the majority of sediment deposition occurs during spring melt. A long vibracore and a surface core (water depth 10 m) were retrieved and subsampled for thin sections and physical characterization.
The varve chronologies developed for each lake were compared year-by-year with discharge records from several rivers in the region in order to correlate river discharge events with subannual laminae in each varve. A comparison of the results for each lake record confirmed that there were differences in the types of sedimentary events preserved in each lake. Temperature and precipitation records from stations in British Columbia, Yukon and Alaska, were also matched to the combined river discharge and sedimentary event data to determine the meteorological conditions associated with the preserved river discharge events.
The expectation was that both lakes would preserve nival and glacial melt and late autumn rainstorm events as subannual laminae within a varve. However, because Meadow Lake is smaller, shallower and has a short residence time, it does not trap clay efficiently, as is evident from core and trapped sediments. Therefore, the Meadow varve is typically a simple couplet composed of an initial coarse silt layer and clay cap with few subannual laminae. Hence the signal from Meadow Lake is at best, an indication of nival melt intensity for that watershed.
By contrast, in Summit Lake the presence or absence of a discharge event within the varve depends on the intensity and duration of both nival and glacial peak discharges, as well as late season river discharge events. A Summit Lake varve typically consists of multiple graded units ranging from coarse to fine silt and clay. The thin clay cap is frequently interrupted by a unit of coarse sediment likely deposited during major autumn rainstorms.
In summary, this study demonstrates that a watershed can record discernable sedimentary events related to three different types of runoff. It is possible to compare the sensitivity to nival melt intensity between the two lakes through comparison of runoff units preserved in the respective varves. Regional river discharge, air temperature and precipitation records can further decipher the series of subannual events preserved in varves from Summit Lake.
Hardy, D. R., Bradley, R. S., and Zolitschka, B., 1996. The climatic signal in varved sediments from Lake C2, northern Ellesmere Island, Canada: Journal of Paleolimnology, v. 16, 227-238.
Lamoureux, S. F., 2000. Five centuries of interannual sediment yield and rainfall-induced erosion in the Canadian High Arctic recorded in lacustrine varves, Water Resources Research: v. 36, p. 309-318
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