Paper No. 138-2
Presentation Time: 9:15 AM
IMPACT OF THE CORDILLERAN ICE SHEET ON COASTAL SEDIMENTOLOGY, MARINE PRODUCTIVITY AND SEDIMENTARY GEOCHEMISTRY ON THE VANCOUVER ISLAND MARGIN DURING THE LAST GLACIAL MAXIMUM
The Cordilleran Ice Sheet (CIS) had a large impact on marine sedimentology, productivity, and geochemistry on the Vancouver Island continental margin during the last glacial (Fraser Glaciation, ~14.7–30.5 kyr BP). High resolution, multi-proxy geochemical records from the ~50-kyr long core MD02-2496 (48˚58.47’ N, 127˚02.14’ W, water depth of 1243 m), illustrate the sensitivity of the Vancouver Island margin to the presence of the CIS, which approached within 35 km of the coring site during the last glacial maximum (LGM, ~17–20 kyr BP). During intervals when the ice sheet was absent from the coast (e.g., Holocene and Marine Isotope Stage 3), marine productivity was relatively elevated (higher concentrations of organic carbon, opal, Ag, Cd), leading to the deposition of organic-rich hemipelagic sediments under suboxic (elevated Cd, Re and U concentrations) to intermittently anoxic (elevated Mo) sedimentary pore-water conditions. However, during the LGM and subsequent ice retreat, sedimentation rates on the margin rapidly increased from 20–80 cm/kyr to up to 460 cm/kyr with the deposition of lithogenous sediments, rock flour, ice-rafted debris, and terrigenous organic material sloughed off by the ice sheet. At times, sedimentation was cyclical due to glacial outburst floods. The resultant turbidity of the waters, along with inclement conditions for phytoplankton growth (reduced coastal upwelling, cool sea surface temperatures from a cool climate and/or melting ice) greatly reduced marine productivity (lower marine organic carbon, opal and/or biogenous barium concentrations). The reduced productivity led to lowered oxidant demand in sedimentary pore waters and changed redox conditions within the sediments from suboxic and anoxic conditions before the LGM to weakly suboxic and oxygenated conditions that allowed for the preservation of oxyhydroxides at multiple times, which was not seen in non-glaciated sediments throughout the MD02-2496 record. A similar set of proxy responses in an older part of the record (~44–50 kyr BP) suggests that an ice sheet also reached the coast before the LGM.