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

摘要

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 ~(210)Pb (~(210)Pb_(xs)) could provide erroneous or less reliable information on sediment accumulation rates. We analyzed one sediment core from Hendrix Lake in southwestern Arkansas for excess ~(210)Pb and ~(137)Cs. There is no decrease in excess ~(210)Pb activity with depth while the ~(137)Cs profile indicates sharp peak corresponding to 1963 and the ~(137)Cs penetration depth of ~(137)Cs 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 ~(137)Cs-based rates. Four independent evidences (two-marker events based on ~(137)Cs and two marker events based on Hg mining activity) result in about the same sedimentation rates and thus, we endorse earlier suggestion that ~(210)Pb 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 ~(2100Pb_(xs) in relatively smaller lakes.