Upper Mississippi Pb as a mid-1800s chronostratigraphic marker in sediments from seasonally anoxic lakes in Eastern Canada

Abstract

Sediment cores from eight headwater lakes located in Southern Québec, Eastern Canada, were analyzed for Pb, stable Pb isotopes, and the radioelements 210Pb, 137Cs, 241Am and 226Ra. The depth profiles of stable Pb isotope ratios show, for the post-19th century period, the influence of several isotopically distinct anthropogenic lead sources, mainly including emissions from two Canadian smelters and from leaded gasoline combustion in Canada and in the United States. A most interesting feature of the profiles, however, is the presence of sharp stable Pb isotope ratio peaks near the depth horizon, where excess 210Pb becomes undetectable. Using a binary mixing model and assuming that natural Pb concentrations and isotopic compositions from the catchment are given by the pre-industrial sediments at the bottom of the cores, we find that a significant part of the anthropogenic Pb supplied to the sediments at this horizon originated from smelting activities in the Upper Mississippi Valley. We assess that the Pb isotope ratio peaks, also observed in the laminated sediments of the Pettaquamscutt Estuary, Rhode Island, USA, are an accurate chronostratigraphic marker for the validation of mid-19th century 210Pb-derived dates. Given that the study lakes are located up to 2000km from the Mississippi Valley, we conclude that this isotopic Pb signal provides a widely distributed time-marker that is key to validate 210Pb chronologies in environmental archives from Eastern North America.