ISOTOPE GEOCHEMISTRY OF SCABLAND FLOOD DEPOSITS

Argon and neodymium isotope geochemistry provide a means of tracking Scabland Flood sediment from source to sink. Analysis of sediment from locations along the flood path and from two marine sediment accumulation sites elucidate patterns of sediment mixing on land, subsequent unmixing at sea, and marine sediment transport pathways. Slackwater deposits in the Tucannon and Willamette valleys are characterized by old K/Ar ages of fine-grained sediment (240 to 360 Ma), younger ⁴⁰Ar/³⁹Ar ages of individual micas (mean ages from 70 to 150 Ma), and moderately negative ε_Nd values (-8.5 to -10.5). Similarly old K/Ar ages (210 to 390 Ma) and negative ε_Nd values (-8.5) link glacial outburst flood layers from sediment core MD02-2496 offshore Vancouver Island to the Scabland Floods. This isotopic link shows that Scabland Floods sent plumes of fine-grained sediment >400 km to the north in the Pacific Ocean. A high-resolution geochemical record from MD02-2496 identifies 44 Scabland Flood layers in the core. Sixteen radiocarbon dates spanning the glacial flood interval provide an absolute chronology, showing cyclical deposition with ~50- to 80-year periodicity between ~19.3 and ~14.9 ka. Additionally, ⁴⁰Ar/³⁹Ar age spectra of individual mica grains from a late Pleistocene Astoria Fan turbidite bed are similar to age spectra from Scabland Flood slackwater deposits, reinforcing the connection between this turbidite sequence and glacial outburst floods.