Distributions, inventories and isotopic composition of lead in 210Pb-dated peat cores from contrasting biogeochemical environments: Implications for lead mobility

Abstract

The use of concentration profiles of lead and other contaminant metals in 210Pb-dated ombrotrophic peat cores for reconstruction of historical trends in atmospheric deposition has become relatively well established. However, uncertainty remains over the validity of the assumption of post-depositional immobility of lead in peats. In particular, a number of studies have suggested that in saturated peat systems, lead is subject to diagenetic remobilisation and redistribution, with the result that 210Pb and lead profiles do not provide a historical record of deposition. Results are presented here for lead concentrations, inventories and stable isotope ratios and for 210Pb activities in two peat cores from locations close to the Glasgow industrial area in west-central Scotland. Contrasting biogeochemical conditions prevailed at the two sampling sites, with one being unsaturated and ombrotrophic while the other was saturated and minerotrophic. The results for the ombrotrophic peat core were compatible with information from other peat and lake sediment core studies, data for archived herbage samples and with known historical trends both in industrial activity and in the use of lead as an additive to petrol, giving a high degree of confidence in the 210Pb chronology and implied record of deposition. This provides strong support for the suggestion that lead is immobile in such systems. In contrast, the lead concentration profile and stable isotope data for the minerotrophic peat provided apparent temporal variations that were inconsistent with other studies and known historical trends, confirming that the saturated peat core did not provide a record of atmospheric deposition. This observation is consistent with the suggestion that lead is subject to diagenetic remobilisation and redistribution in saturated peats.