NORTHEASTERN PACIFIC IRD EVENTS: EVIDENCE FOR MULTIPLE CORDILLERAN ICE SHEET RETREATS

Reconstructions of the Cordilleran Ice sheet have previously been limited to terrestrial sedimentary records. Here we present the first evidence based on glacial-marine sediments for multiple iceberg discharge from the Cordilleran Ice sheet. MD02-2496, Vancouver Island, British Columbia, (48^o 58' N; 127^o 02' W; 1190m water depth; 38.38m core length) is ideally located to capture glacial sediments delivered to the continental shelf of the Northwest Pacific. Grainsize, ice rafted detritus (IRD; > 250 µm grains gram^-1), magnetic susceptibility, and planktonic δ¹⁸O (G. bulloides) records are presented alongside a preliminary age model based 10 planktonic foraminiferal ¹⁴C dates.

Four IRD events are evident, defined by greater than 50 IRD grains gram^-1. The oldest IRD event preserved in this record (C4) occurred between 33.1-31.8 m (≈49 cal KyrBP/42 ¹⁴C KyrBP) and was marked by two phases (C4a and C4b). The more significant phase, C4a, displays a positive sawtooth pattern with IRD increasing from ≈50 to 517 grains gram^-1 during a period of high magnetic susceptibility. A smaller event (≈17.9 cal KyrBP; C3), occurred during an interval of high variability in <4 µm grains, high magnetic susceptibility, and high δ¹⁸O values. Another prominent two phase IRD event (≈16.2 cal KyrBP/14.4 14C KyrBP, 13.3–10.6 m), associated with a subtle decreasing trend in magnetic susceptibility (C2b), also exhibits a positive sawtooth pattern with IRD increasing from ≈50 to 467 grains gram^-1 (C2a). The youngest event (≈13.2 cal KyrBP/12 14C KyrBP; 6.9-6.8; C1) is coincident with peak deposition of <4 µm grains and high magnetic susceptibility. C1 is associated with high planktonic δ¹⁸O values during an interval of generally decreasing values associated with deglaciation.

Cordilleran Ice Sheet growth during the late stages of Oxygen Isotope Stage (OIS) 3, was associated with moderate iceberg discharge (C3). Following maximum extent onto continental shelf, intervals of rapid wastage are indicated by significant IRD deposition terminated by the highest number of IRD grains in the core. Preliminary data suggests the oldest event (C4) was associated with collapse after the penultimate Cordilleran Ice Sheet advance during OIS 4, while C2 was associated with the OIS 2 retreat. C1 suggests a brief return to glacial conditions during deglaciation.