ABSTRACT: Pattern and process of sediment delivery from land to sea in two subarctic fjords: Torngat Mountains National Park .

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Abstract

In Saglek and Nachvak fjords within Torngat Mountains National Park, sea?oor morphology, sediment cores, and stream catchment studies have been undertaken to determine rates and patterns of sediment delivery from ocean to sea over timescales of seasons to the past few centuries, and how sediment delivery might be related to environmental changes in stream catchments. During summer cruises in 2008 and 2009, boxcores and gravity cores were collected and sonar surveys undertaken in marine basins proximal to Nakvak Brook (Saglek Fjord) and the McCornick River (Nachvak Fjord). From August 2008 to August 2009, logging pressure sensors were deployed in the beds of both rivers, to measure time series of river stage. Core, sonar, and river stage analyses are being integrated and studied in the context of detailed seabed morphology (collected from the CCGS Amundsen and Heron launch) and catchment characteristics (from SRTM elevation data, processed using RiverTools software). Stage data for both Nakvak Brook and McCornick River show that 2009 peak spring discharge occurred while the fjords were still ice-covered, in late May-early June. If this pattern holds true for most years, then peak sediment delivery to the fjords may occur in the absence of wind and wave-driven marine resuspension, which would be hindered by presence of sea ice. In both fjords, marine sediments deposited since glacial retreat have constructed wedges in proximal marine basins that thin away from each river, suggesting that these rivers are primary sediment sources to these marine basins. Seabed gradients from near river mouths to deeper portions of the basins exceed 40 m/km, steep enough to sustain a variety of gravity-driven transport mechanisms from shallow to deep water. Gradients on basin ?oors in line with river sediment sources may exceed 12 m/km, the minimum gradient required to sustain turbidity currents. Sediment cores from the basins contain evidence of event-layer deposition, in the form of stair-step pro?les of 210Pb activity, and partially bioturbated layers of strati?ed sediment. These observations collectively suggest that sediment-gravity ?ows may form deposits that are thinner than the depth of bioturbation (~10 cm) with decadal to centennial recurrence intervals, and would thus form important depositional processes. The likelihood of such recurring sediment-gravity ?ows could be enhanced by the relatively rapid local deposition of river sediment in quiescent under-ice conditions near river mouths, until a suitable trigger mechanism initiates gravity driven ?ow into the adjacent basin.

Bibliography of Canadian Geomorphology