Sedimentary organic matter record of recent environmental changes in the St. Marys River ecosystem, Michigan–Ontario border

Lake George, located in the St. Marys River, has been heavily impacted by human-induced environmental changes over the past century. The effects of human impacts starting in the late nineteenth century and of natural, gradual diagenesis can be distinguished in the bulk organic matter and molecular contents of the sedimentary record. Organic carbon concentrations increase from 0.5% in sediments deposited 200 years ago to 4% in recent sediments. A fourfold increase in organic carbon mass accumulation rates accompanies the change in concentrations. Elevated C/N ratios in near-modern sediments indicate that increased delivery of land-derived organic matter has been responsible for much of the recent increases in sedimentary organic carbon. Organic δ13C and δ15N values change significantly and coincidentally with the environmental changes, reflecting depressed algal productivity since the introduction of industrial effluents to the aquatic system, increased delivery of land-derived organic matter and some impacts of acid rain. Increases in microbial and petroleum hydrocarbon contributions occur in sediments deposited since 1900. Fatty acid distributions provide evidence of substantial microbial reworking of organic matter throughout the sedimentary record.