During the last glacial maximum, the Cordilleran Ice Sheet advanced to cover significant portions of the northwestern United States. One component of the ice sheet, the Purcell Trench Lobe, surged southward, moving into the panhandle of Idaho and damming the outflow of the Clark Fork River. Behind this ice dam, Lake Missoula grew to maximum volumes of 2200-2600 cubic kilometers (about twice the size of Lake Erie) (Waitt, 1985). When the lake level reached a critical threshold, the ice dam began to float and tunnels formed, generating a glacial outburst flood. The flow rate was on the order of 10,000,000 cubic meters per second, greater than all of the modern world's rivers combined.
Figure 1: Map of Glacial Lake Missoula, flood path, pertinent geological provinces, and sediment core location.
However, there are many open questions about the floods: How many floods were there? When exactly did the floods occur? What were the sizes of the floods? The answers have implications for the pacing and patterns of deglacial climate change and fresh-water fluxes into the northern Pacific Ocean.
We have been investigating the Missoula Floods through a deep-sea sediment core located off the coast of Vancouver Island. This core, MD-02-2496, contains a sequence of laminated cyclic fine-grained sediments that, based on radiocarbon dating of foraminifera within the core, were deposited between 18.6 and 15.9 thousand years ago. Some of these layers, based on K/Ar, Rb/Sr, and carbon isotope analysis, appear to be derived from an old, continental source. Our research tries to match this geochemical fingerprint with the most likely source, establishing sediment provenance. The most plausible source for these geochemically unique layers is a sediment plume from the Missoula Floods.