History of Cordilleran Ice Sheet activity recorded in core MD02-2496: Vancouver Island, British Columbia.

Abstract

MD02-2496, a marine core from the continental slope off Vancouver Island, contains detailed late Quaternary Cordilleran Ice Sheet records. Multi-proxy data establish depositional patterns related to glacial phases while stable isotopes allow correlation between Cordilleran Ice Sheet activity and global climatic change. Magnetic susceptibility, color reflectance, and mass accumulation rates (MAR) of lithogenic sediment document the initiation of glaciomarine sedimentation (Fraser Glaciation) at &sim;30 Ka. Reduced magnetic susceptibility validates a brief ice retreat &sim;23 Ka (19.2 <super>14</super>C kyrs BP, Port Moody). MARS, grainsize, and cyclic sedimentation suggest the ice margin became proximal to the shelf edge at &sim;19 Ka. Spectral analysis of the interval 19 to 17 Ka suggests either lunar or solar forcing of ice sheet instability. Catastrophic ice sheet retreat is indicated by the gradual onset and abrupt cessation of ice rafted detritus deposition (IRD, >250mum grains g<super>-1</super>), between &sim;17 Ka and &sim;16.2 Ka. Another IRD pulse identifies a second ice sheet retreat at &sim;14.6 Ka. Glacial advance (the Sumas Events), between &sim;12.8 and 11.5 Ka, is indicated by increased abundance of medium-coarse silts, sands, with high magnetic susceptibility and poor sorting. Based on the sediment characteristics of this sequence of events, an older ice sheet advance with associated catastrophic retreat has been identified. Submillennial climate variability is revealed by the planktonic delta<super> 18</super>O record. Between stadial and interstadial conditions, sea surface temperatures (SSTs) ranged between 1.5 to 3&deg;C. Interstadial SSTs (7 to 9&deg;C) off southern British Columbia were &sim;2.5&deg;C cooler than those off Southern California, while stadial SSTs (&sim;5 to 6.5&deg;C) were similar between the regions. Increased western North American margin SST gradients during interstadials may have strengthened the North Pacific High and related northerly winds associated with upwelling. Environmental conditions during interstadials were similar to warm Pacific Decadal and negative North Pacific America Oscillations. The MD02-2496 planktonic delta<super>18</super>O record decoupled from other Pacific SST records between 28.5 and &sim;15 Ka. This anomalous negative regional delta<super>18</super>O_water indicates either freshening of local surface waters, or a precipitation shift from predominantly rain to snow. Evidence for freezing air temperatures could explain the absence of ice sheet response to submillennial climate change.