Sedimentary records of mercury stable isotopes in Lake Michigan

Mercury (Hg) concentrations and Hg isotopic composition were investigated in three sediment cores in Lake Michigan (LM). Two cores were collected from Green Bay, a region heavily impacted by Hg contamination and one core from an offshore region of LM absent of direct point source Hg. Historical trends of Hg influxes suggest increased Hg deposition began in the 1890s in Green Bay and in the early 1800's in offshore LM. Recently deposited sediment reflecting more anthropogenic influence shows similar delta Hg-202 values (-1.0 to -0.5 parts per thousand) for all three cores however, deep core sediments, reflecting pre-industrial eras, show much lower delta Hg-202 values (-1.7 to -1.2 parts per thousand). Using a binary mixing model based on delta Hg-202 signatures, the proportion of anthropogenic Hg was estimated. Model output confirms that Green Bay is more contaminated by local point source than the offshore LM. An increase in positive Delta Hg-199 values (-0.02 to +0.27 parts per thousand) was observed from inner Green Bay to the offshore of LM, which may indicate increased input of atmospheric Hg and decreased watershed inputs along this transect. Overall, this study suggests that sedimentary Hg isotopes maybe a useful tracer in understanding Hg sources and history of Hg contamination in large lakes.