ABSTRACT: Spatial and temporal variation of the last ice age mega-floods in the Pacific Northwest: Sediment provenance using single-aliquot K/Ar dating.

CGRG Bibliography of Canadian Geomorphology
Search Results

Author :

Date :

Title :

Publication :

Issue :

Page(s) :

Abstract

Glacial outburst floods (jokulhaups) are important, but poorly understood, drivers of climate change during large-scale deglaciations. Meltwater fluxes are one of the climate forcings with the most uncertain histories included in climate modeling of the last 21 kyr. Outburst flood events are not included in modeled meltwater fluxes due to their small volume, though they are one of the fastest delivery mechanisms for freshwater to the ocean. The most significant outburst floods known to have occurred during the last deglaciation came from glacial Lake Missoula between 18 and 15 kyr BP. Although the volume of meltwater delivered to the North Pacific Ocean was small, the speed of delivery and sediment content of these floods was large. Here we present data that constrain the location and timing of outburst flooding to the N Pacific. Single aliquot bulk K/Ar dates of fine silts and clays from a deep sea core collected off Vancouver Island, British Columbia [MD-02-2496 (48°58’47”N: 127°02’14”W; 2043m water depth)] provide a geochemical fingerprint of the provenance of glacial marine sediments. K/Ar ages, combined with mapped continental geology, allow for the identification of potential source regions for individual flood layers in the core. K/Ar ages in this study represent an integrated age of the entire sediment package delivered to the core site. This includes the Proterozoic Belt-Purcell metasediments (which glacial Lake Missoula covered), the Miocene/Pliocene Columbia Plateau Basalts (which were heavily eroded by the Missoula Floods), and the largely Cenozoic rocks of the coastal and Cascade mountain ranges. Initial results indicate a significant variation in K/Ar ages between background sedimentation and flood deposition events. Missoula Flood outburst sediments appear to have K/Ar ages of around 300 million years while background sediments appear to have K/Ar ages of less than 100 million years. Timing of outburst flood deposits has been determined by 49 14C dates, of which 36 were derived from planktonic forminifera. Sedimentation rates during the interval of outburst flooding are high (~200 cm kyr-1). Cyclic outburst flooding with a return period of 80 years begins abruptly at 19.5 kyr BP and ends around 17.5 kyr BP when iceberg discharge from the nearby Juan de Fuca Lobe and Barklay Sound begins. Our data constrain the sedimentary source terranes of flood layers to an end member associated with cratonic sediments, suggesting that glacial Lake Missoula outburst flood sediments were transported to the site. The transport mechanism was likely similar to the modern Columbia River plume, which flows northward due to the coriolis effect during periods of high river flow. Schmidt (2010, Journal of Quaternary Science) cited "ice sheet responses on sub-millenial timescales" as a "specific target" where current climate models struggle and more paleo-climate study is needed. Thus, in addition to the study's inherent value for scientific curiosity, it may also provide valuable information to scientific modelers.

Bibliography of Canadian Geomorphology