ABSTRACT: Late Quaternary glacial influence on coastal sedimentation, paleoproductivity and sedimentary redox, southwest British Columbia, Canada.

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Abstract

We present a high-resolution, 50 ka marine record documenting the effects of the Cordilleran ice sheet on sedimentation, paleoproductivity and sedimentary redox conditions on the Vancouver Island margin. A multiproxy sedimentological and geochemical approach was applied to core MD02-2496 (48.97°N, 127.04°W; 1243 m water depth; 38 m long), which was dated with 46 AMS radiocarbon dates. Glaciomarine sedimentation from Fraser glaciation began at 30.5 ka, however sedimentation rates did not increase dramatically (<3 m/kyr) until 19.5 ka when cyclic sedimentation suggests the abrupt initiation of glacial outburst flooding. Terrigenous organic matter contents were elevated with respect to marine organic carbon, and opal concentrations as a proxy of marine diatom production was low, suggesting that primary productivity was sluggish during the cooler climate. Bulk sedimentary d15N was also lighter, suggesting that isotopically heavy source waters from the Eastern Tropical North Pacific brought north by the poleward flowing California Undercurrent were less denitrifying or that isotopically heavier waters were not being upwelled along the coast. Concentrations of trace elements Ag, Cd, Re, U and Mo were at background levels indicating that export productivity was low, which led to lower oxidant demand in the water column, and more oxygenated sedimentary pore waters. The number of grains with a diameter of >250 µm (ice-rafted debris or IRD) increased from background values of 0–1 grains to a maximum of 467 grains at 16.2 ka. A second IRD event occurred at 14.8 ka. Similar sediment characteristics (except the presence of IRD) were recorded between 12.8 and 11.5 ka. In comparison, during warmer, non-glaciated intervals (~44–30 kyr BP, Bølling/Allerød warm period, Holocene), trends are opposite. Sedimentation rates were lower (~0.5 m/kyr) and deposition was dominated by hemipelagic sediments. No IRD grains were present. Marine organic carbon contents were elevated, as was the concentration of opal, indicating that primary and export productivity were enhanced during the warmer climate. Bulk sedimentary d15N was heavier, suggesting enhanced northward advection of isotopically heavier waters from the south, coupled with upwelling of such waters along the coast. Concentrations of trace elements were all elevated above background levels showing that increased export productivity led to increased oxidant demand as sinking organic matter decayed. Sedimentary pore waters became reducing, leading to suboxic to anoxic conditions. The records from 50.4–44 kyr BP have characteristics similar to those displayed during the Fraser glaciation, suggesting that a tidewater glacier was proximal to the coring site. However, there is no known terrestrial analogue of a glaciation event at this time.

Bibliography of Canadian Geomorphology