Spatiotemporal characterization of mercury isotope baselines and anthropogenic influences in lake sediment cores

Abstract

Increasing mercury isotope ratios from pre-industrial (1510-1850) to present-day (1990-2014) in lake sediment cores have been suggested to be a global phenomenon. To assess factors leading to spatiotemporal changes, we compiled mercury concentration (THg) and mercury isotope ratios in 22 lake sediment cores located at various regions of the world. We find that the positive delta Hg-202 shifts together with THg increases from pre-industrial to present-day are a widespread phenomenon. This is caused by increased contribution of mercury from local to regional anthropogenic mercury emission sources, which lead to higher sediment delta Hg-202 (-1.07 +/- 0.69 parts per thousand, 1 SD) than pre-industrial sediments (-1.55 +/- 0.96 parts per thousand, 1 SD). The positive Delta Hg-199 shifts were observed in 15 lake sediment cores, which have low pre-industrial Delta Hg-199 (-0.20 +/- 0.32 parts per thousand) compared to the sediment cores with near-zero to positive pre-industrial Delta Hg-199 (0.08 +/- 0.07 parts per thousand). The magnitudes of delta Hg-202 (r(2) = 0.09) and Delta Hg-199 (r(2) = 0.20, both p > 0.05) changes from pre-industrial to present-day did not correlate with the magnitude of THg changes. Instead, the magnitudes of delta Hg-202 and Delta Hg-199 changes decreased with increasing pre-industrial delta Hg-202 and Delta Hg-199 values, suggesting that the baseline mercury isotope ratios play a more important role in determining the magnitude of mercury isotope changes compared to the degree of THg input. We suggest that the spatiotemporal assessments of delta Hg-202 in lake sediment cores can be used as an important proxy for monitoring changes in anthropogenic mercury sources for the Minamata Convention on Mercury.