Early diagenesis of lead in Laurentian Trough sediments

Abstract

The depth distributions of various solid-phase and porewater fractions of Pb, Fe and Cd in boxcores from three stations in the Laurentian Trough demonstrate that Pb is subject to early diagenetic change. High total-Pb concentrations, compared to background levels, indicate that much of the Pb in these cores is of anthropogenic origin. Patterns of mobilization and fixation, and of changes in the chemical reactivity towards extradants, are consistent from core to core. Pb is mobilized both near the sediment surface and at intermediate depth. In these zones dissolved Pb concentrations attain levels of 8–13.5 nM, which are two orders of magnitude greater than in the overlying bottom waters of the Laurentian Trough (0.1 nM). A fraction of Pb, soluble in a pH 5 sodium acetate-acetic acid solution, is produced diagenetically within the sediment. It is most abundant at intermediate depth, where it accounts for as much as 31% of the total Pb. Another fraction, soluble in hydroxylamine hydrochloride-acetic acid solution, decreases in abundance with depth. Flux calculations indicate that significant amounts of Pb are subject to postdepositional transformation. Parallels in the vertical distribution of the various fractions suggest that some Pb, like Cd, is mobilized near the sediment surface during the aerobic degradation of organic matter and that other forms are implicated in the redox-controlled cycles of Fe diagenesis.