Use of lead isotopes for developing chronologies in recent salt-marsh sediments

Dating of recent salt-marsh sediments is hindered by the radiocarbon plateau and the moving ∼100 year window of 210Pb accumulation histories. Introduction of anthropogenic Pb to the environment is a means to date salt-marsh sediment deposited over the last 200 years by correlating downcore changes in concentration and isotopic ratios to historical production and consumption. We investigated use of Pb as a chronometer in a core of salt-marsh sediment from New Jersey, USA. Changes in Pb concentration identified horizons at AD 1875, 1925, 1935 and 1974 that correspond to features of historic U.S Pb production and consumption. Stable leadisotopes (206Pb:207Pb) constrained ages at AD 1827, 1857 and 1880, reflecting Pb production in the Upper Mississippi Valley with its unusual isotopic signature and at AD 1965 and 1980 from leaded gasoline. Confirmatory evidence for the gasoline horizons came from increased Sb concentrations caused by vehicle emissions and industrial activity. These nine chronostratigraphic markers of fixed dates provide precise constraints on sediment age in the northeastern USA and enable salt-marsh records of coastal evolution and wetland development to extend beyond the period of instrumental measurements