Doctor of Philosophy
Dr. Fred Longstaffe
Late Quaternary histories are investigated here for sediment cores from Lakes Huron and Michigan, using the oxygen- and carbon-isotope compositions of biogenic carbonates and the oxygen- and hydrogen-isotope compositions of porewater. Age models for these cores are based on sedimentological information, and where possible, radiocarbon and pollen dates. The Michigan Basin cores provide a thick record of the late Pleistocene, whereas the Huron Basin cores primarily record Holocene deposition. Taken together, the histories recorded in biogenic carbonates from these sediments provide a clear account of lakewater isotopic changes – and their significance – over much of the history of the Great Lakes Basin.
Biogenic carbonates, and ostracodes in particular, serve as excellent recorders of lakewater δ18O values. Modern specimens from Lake Huron have been used to assess non-equilibrium oxygen- and carbon-isotope fractionation effects during shell formation within this setting. This information provides improved ability to calculate the oxygen-isotopic compositions of paleolakewater using fossil biogenic carbonates from the cores. The fossil shell compositions indicate intervals characterized by low δ18O meltwater originating from the retreating Laurentide Ice Sheet and/or associated proglacial lakes, and periods dominated by higher δ18O water that are more reflective of regional precipitation, runoff and climatic conditions. Differences in the oxygen- and carbon-isotope compositions of deep- versus shallower-water ostracode species in the Michigan Basin cores suggest isotopic stratification of its lakewater during the late Pleistocene. Results for previously unstudied parts of the Huron Basin suggest that sub-basins acquired different oxygen-isotope compositions during periods of very low lake levels, reflecting different water sources and/or different conditions at such times. Early Holocene influxes of glacial meltwater into the Huron Basin, and to a lesser extent the Michigan Basin, correlate with major global climate perturbations that were likely triggered by release of large volumes of glacial meltwater into the Arctic and/or North Atlantic Oceans.
Porewater did not preserve original lakewater δ18O and δD values. These compositions are largely controlled by downward diffusion of modern lakewater.
Macdonald, Rebecca A., "Late Quaternary Histories of Lakes Huron and Michigan: A Stable Isotope Investigation of Sediment Cores and Modern Biogenic Carbonates" (2012). Electronic Thesis and Dissertation Repository. 457.