Reconstructing vegetation cover during the retreat of the Puget Lobe of the Cordilleran Ice Sheet using organic matter from…

The dynamics of the Cordilleran Ice Sheet (CIS) over the last glacial period are complex, with several instances of ice retreat and readvance before the final retreat. This study demonstrates that in western Washington, marine sediment cores can be used to reconstruct continental ice cover. Here, I present a reconstruction of terrestrial landscape evolution within the Puget Sound during the last deglaciation, using geochemical analyses of organic matter from marine sediment cores. I measured the carbon isotope composition in bulk organic matter (d13Corg) and concentrations of leaf wax biomarkers (n-alkanes and fatty acid methyl esters) from three marine sediment cores off the coast of Washington State. Sedimentary organic matter is predominantly terrestrial in origin in all three cores and constitutes 75-95% of the total organic matter. The end of glacial sedimentation is marked by a positive carbon isotope excursion (2.3 - 6.9‰ in magnitude) and by an increase in concentration of leaf wax biomarkers, that most likely represents a period of rapid ice sheet retreat in the Puget Lowlands. Using biomarker abundances to produce an estimate for terrestrial organic matter (%Terr) and an isotope mass balance mixing model, I determined the isotopic excursion represents a change in source of vegetation cover among C3plant varieties. My calculations suggest that shrubs and forbs were more abundant than trees in the Puget Lowlands during the last glacial period. Furthermore, I propose the positive isotopic excursion marks the drainage of Glacial Lake Russell into the Strait of Juan de Fuca and the reorganization of continental drainage pathways as the Puget Lobe of the CIS retreated north.