COLLAPSE OF THE JUAN DE FUCA LOBE OF THE CORDILLERAN ICE SHEET

Juan de Fuca lobe (JdFL) of the Cordilleran Ice Sheet extended west along the Strait of Juan de Fuca, between British Columbia and Washington, to a terminus on the continental shelf west of Neah Bay. New lidar topography for the area east of Lake Ozette, WA dictates reinterpretation of published ¹⁴C data on the timing of ice-lobe advance and retreat: 4 analyses (13,010 – 13,380 ¹⁴C yr BP) of wood samples previously thought to be from ablation drift (Heusser, 1973) are actually from lodgment till and thus predate the maximum extent of JdFL. Post-glacial wood ages from east of Bellingham, WA (Kovanen and Easterbrook, 2001) show JdFL was gone well before 12,380 ¹⁴C yr BP.

With these constraints we reinterpret core MD02-2496 (Hendy and Cosma, 2008) from the continental slope northwest of JdFL. The ~1000 yr duration of ice-rafted debris (IRD) event C2 is too long to coincide with retreat of JdFL as previously proposed. We suggest initiation of C2 (1–10 grains >250µm/g) corresponds to ice reaching the shelf-slope break and the onset of calving into deep water. Increased IRD deposition (20–>100 grains >250µm/g) in the latter part of C2 reflects increased calving as JdFL retreated. Retreat of the ice margin from the shelf-slope break to the Coupeville moraine near the San Juan Islands, some 200 km, took ~210 yr if sedimentation rate was constant during C2. If sedimentation was more rapid during retreat, retreat was faster. Mg/Ca geochemistry of fossil surface-dwelling foraminifera indicate the sea surface warmed 1–3°C coincident with increased IRD deposition.

For much of its existence JdFL was grounded (Herzer and Bornhold, 1982). We infer that onset of ice retreat corresponds to floating of the marine-terminating JdFL. Floating of grounded ice requires ice thinning or sea level rise. Thinning of JdFL was not likely the result of regional atmospheric warming (or reduced snowfall) as the Puget lobe of the ice sheet did not thin at the same time. We suggest that sea level rise initiated JdFL collapse, aided by warming seawater. The below-sea-level floor of JdFL largely sloped east, towards the ice source, thus floating of its terminus likely initiated a positive feedback loop that hurried collapse.