GLACIAL LAKE MISSOULA OUTBURST FLOODS AND THE SOUTHWESTERN CORDILLERAN ICE SHEET

A detailed record of glaciomarine sedimentation is preserved in the well-dated ~50-kyr jumbo piston core MD02-2496 (48.97˚ N, 127.04˚ W, water depth of 1243 m) collected from the continental slope ~75 km off Vancouver Island. The in-situ bulk elemental composition of the 35 m core was determined at 1 mm intervals with a 5-second count time from a Mo X-ray tube using an ITRAX X-ray Fluorescence (XRF) Core Scanner (Cox Analytical Instruments) at the Sediment Geochemistry Lab of the College of Earth, Ocean, and Atmospheric Sciences at Oregon State University. The core is well-dated with 40 mixed planktonic foraminifera and bulk organic carbon ¹⁴C ages.

A notable feature of this core is a series of >81 layers of fine-grained sediments with ancient (K/Ar ages of~300 Ma and e_Nd of ~-8) shale-like (high Rb counts) compositions. These layers are interspersed by coarser grained, young (K/Ar ages of~100 Ma and e_Nd of ~-3) sediments containing ice-rafted debris (IRD). The composition and age of the layers indicates the sediments originated in Glacial Lake Missoula and were transported by ocean currents ~250 miles north along the west coast of North America. The flood layers begin at ~19.5 ka with five thin (<5 cm thick) layers before thick flood layers (>5 cm thick) appear after ~19.3 ka. Thick flood layers are often associated with more felsic, coarse sediment at the base of the layer and an average time of 38 years between floods layers. Flood frequency decreases between ~17.7-17.2 ka and between ~15.7-15 ka. At ~17.1 ka, IRD concentrations increase from <1 grain g^-1 to ~20 grains g^-1, and remain >50 grains g^-1 from ~16.5-16.35 ka, except in flood layers, as the Juan de Fuca Strait deglaciates. During this ~150-year interval of increased IRD deposition, flood sediments are thin and discontinuous. Another 16 flood layers occur from ~16.3-15.65 ka; however, the base and top of these layers are diffuse rather than abrupt like earlier flood layers. The final flood layers from ~14.9-14.5 ka are thin (<2 cm thick). This well-dated sequence displays how Glacial Lake Missoula flood sedimentation changed during the retreat of the Cordilleran Ice Sheet.