Reliance on 210Pb Chronology Can Compromise the Inference of Preindustrial Hg Flux to Lake Sediments

Lake sediments are frequently used to reconstruct the rate and magnitude of human impacts on the biogeochemical cycle of mercury (Hg). The vast majority of these studies rely on excess Pb-210 inventories in short cores to temporally constrain recent trends in Hg deposition, revealing an approximately 3-fold increase in Hg deposition since preindustrial times. However, the exhaustion of unsupported Pb-210 and the onset of widespread global Hg pollution converge temporally in the late 19th century, raising the possibility that preindustrial Hg fluxes are poorly constrained. Here, we combine Pb-210 and accelerator mass spectrometry (AMS) C-14 dated lake sediment records from arctic and Andean lakes to assess the reliability of Pb-210-derived chronologies in the estimation of preindustrial Hg fluxes. For all four studied lakes, relying on Pb-210 chronologies results in an overestimate of preindustrial Hg fluxes, because extrapolated basal Pb-210 sedimentation rates are systematically overestimated in comparison to accumulation models that include C-14 dates. In the Andes, the use of C-14 dates is critical toward assessing the full history of Hg pollution, which extends beyond the industrial era. In the Arctic, C-14 dating suggests that Hg deposition may have increased >10-fold since the Industrial Revolution, rather than the commonly quoted 3-fold increase. The incorporation of IT dates may therefore be necessary if accurate Hg flux histories are sought from oligotrophic lake sediments.