Problems with the dating of sediment core using excess 210Pb in a freshwater system impacted by large scale watershed changes

Abstract

Pb-210 dating of freshwater and coastal sediments have been extensively conducted over the past 40 years for historical pollution reconstruction studies, sediment focusing, sediment accumulation and mixing rate determination. In areas where there is large scale disturbance of sediments and the watershed, the vertical profiles of excess 210Pb (210Pbxs) could provide erroneous or less reliable information on sediment accumulation rates. We analyzed one sediment core from Hendrix Lake in southwestern Arkansas for excess 210Pb and 137Cs. There is no decrease in excess 210Pb activity with depth while the 137Cs profile indicates sharp peak corresponding to 1963 and the 137Cs penetration depth of 137Cs corresponds to 1952. The historical data on the accelerated mercury mining during 1931–1944 resulted in large-scale Hg input to this watershed. Using the peak Hg activity as a time marker, the obtained sediment accumulation rates agree well with the 137Cs-based rates. Four independent evidences (two-marker events based on 137Cs and two marker events based on Hg mining activity) result in about the same sedimentation rates and thus, we endorse earlier suggestion that 210Pb profile always needs to be validated with at least one another independent method. We also present a concise discussion on what important factors that can affect the vertical profiles of 210Pbxs in relatively smaller lakes.