Historical rates of atmospheric Pb deposition using super(2) super(1) super(0) Pb dated peat cores: corroboration, computation, and interpretation

Lead-210 dating of peat cores is one approach that has been used to arrive at historical rates of heavy metal deposition. Despite concerns regarding the validity of super(2) super(1) super(0) Pb dating due to Pb mobility, super(2) super(1) super(0) Pb dating can be used if the dates are corroborated with some other independent dating technique. In this study, based on analyses of super(2) super(1) super(0) Pb dated, pollen corroborated peat cores from two sites in the Czech Republic (Jezerni slat and Bozi Dar Bog), we illustrate a previously unexplored problem concerning the computation of metal deposition, using Pb as an example. When peat cores are collected, sectioned into depth intervals, super(2) super(1) super(0) Pb dated and analyzed for metal contents, the super(2) super(1) super(0) Pb dates most appropriately correspond to the midpoint depth for each interval whereas the metal contents correspond to the interval between the top and bottom of each section. Thus the super(2) super(1) super(0) Pb dates and metal content values throughout the core are offset by half the distance of each depth interval. In calculating historical rates of heavy metal deposition two approaches are available for correcting for the depth interval offsets, the traditional approach of date interpolation and our newly proposed metal content interpolation. We see no a priori reason for choosing one approach over the other, and suggest simultaneous use of both date and metal content interpolation. Additionally, acid-insoluble ash (AIA), which has been proposed as a dating technique in and of itself, may be more useful as an interpretive tool which may provide insights into the nature or sources of atmospherically deposited Pb. For example, plots of Pb content per core section versus AIA content per core section for Jezerni slat, located in a relatively pristine area, reveal increased Pb content without increased AIA contents in depths shallower than 6 cm, indicating deposition of gasoline-derived Pb after its introduction in 1922. Similar plots for Bozi Dar Bog, located in a polluted industrialized region, indicate greater inputs of Pb than would be predicted from AIA, based on the Jezerni slat analyses. We interpret the apparent excess Pb deposition at Bozi Dar Bog as being contributed by soil-derived dust from local metal mining. Elevated rates in Pb deposition at Bozi Dar Bog are consistent with the history of local mining known to have occurred in the vicinity. Finally, mag