Carbon Loading to Lake Michigan Sediments

This thesis represents a comprehensive assessment of carbon concentrations in Lake Michigan sediment. By integrating radiocarbon dating with elemental analysis, grain size and other co factors, the concentrations and loadings of total carbon, (TC), black carbon (BC), organic (OC) and inorganic (IC) carbon were quantified both spatially and temporally. As a part of the Great Lakes Sediment Surveillance Program (GLSSP) the aim was to characterize Lake Michigan sediment for carbon in different forms. The physical-chemical characteristics of sediment and the variations in OM, BC, TC, and OC are fundamental to our understanding of the potential for contaminant uptake and also the changes in the overlaying water that may reflect greater changes in the whole region. This project represents the most in-depth analysis of deep water sediment (>30m) to date in Lake Michigan. An accurate and reproducible method for the determination of OC was developed and validated. It was found that some previously published methods for Lake Michigan were not idealized for the type of sediment present and thus a valid process was developed. The ratio of OC to OM has increased for the open lake region and in southern basin. Two hypotheses have been put forward to explain this cultural eutrophication (CE) or labile OM degradation (LD). Through analysis of the surface area (SA) and stable C13 isotope to source appoint OM, it was found that cultural eutrophication is the most likely cause of the observed trend. A trend was observed with OM loading increasing from North of the Lake to the South and decreasing temporally with core depth. The C/N ratios indicate that lacustrine algae is the predominant source of OM to the lake this further substantiated the CE hypothesis. BC represents a small fraction of the total carbon, with low variance across Lake Michigan. BC concentrations follow a trend of decreasing distance from Chicago and North Indiana’s more urbanized areas. Historically BC concentrations peaked around the 1930s-40s this is in keeping with expected industrial and urban output for the region. Comparing the OC, BC and C/N ratios we can conclude that anthropogenic OC input, in the form of BC, is significant in the Southern basin. Heavy metals also followed a similar trend to that of BC, though the bioavailability of these metals is hard to estimate and determinant on a range of factors. The findings of this study offer a comprehensive understanding of the source and the deposition of carbon in its main forms to Lake Michigan sediment. The results of which can be used to further develop the framework information of the GLSSP project. By first validating the methodology and using multiple detection strategies the findings here have been rigorously validated and tested. It is hoped that this work can be used alongside the radiocarbon dating and the other factors in the wider GLSSP framework to understand the effects of natural and anthropogenic changes to Lake Michigan over the last 150 years.