Abstract
This paper investigates the role of intervening transport processes on lake sediment records of the atmospherically deposited radionuclides 210Pb and 137Cs. Brotherswater is of particular interest to this issue in that its large catchment/lake area ratio and short water residence time are likely to amplify the influence of these processes, both from the catchment and through the water column. Brotherswater is also unique in being the site of two earlier multicore studies that, together with the present study, span a period of 4 decades. Measurements of fallout radionuclides were made on soil cores, suspended sediments and sediment cores, and the results combined with those from earlier studies to construct mass balances for 210Pb and 137Cs in Brotherswater. The results showed that catchment inputs accounted for 63% of 210Pb entering the lake. Further, just 47% of 210Pb entering the water column was delivered to the sediment record. For comparison, in an earlier study at nearby Blelham Tarn with a relatively smaller catchment but longer water residence time it was shown that 47% of 210Pb inputs were delivered via the catchment, 75% of which were delivered to the sediment record. Results from both sites suggest that 210Pb is predominantly transported on fine particulates with a mean particle size of 3–4 μm. Their relatively slow removal from the water column allows them to be transported relatively uniformly throughout the lake and may help account for the fact that simple 210Pb dating models are relatively reliable in spite of the complexities of the transport processes. Mass balance calculations for 137Cs are more complicated because of the variable fallout record. Measurements of 137Cs in the input stream and water column showed that catchment inputs are still significant 30 years after the last significant fallout (Chernobyl). Modelled results showed that catchment inputs delayed the date of peak inputs of weapons test fallout to the lake though by no more than 2 years. Although the results presented here are primarily concerned with fallout radionuclides and their reliability for dating, they also have implications for the use of sediment archives in reconstructing historical records of other atmospherically deposited substances such as trace metals or persistent organic pollutants.
Highlights
In spite of their relevance to paleolimnology, there have been relatively few detailed studies of the pathways by which atmosphere contaminants deposited on the landscape are delivered to the sediment records at the bed of a lake
The purpose of the present study was to investigate the transport processes controlling the fate of fallout radionuclides deposited on the catchment of Brotherswater in the English Lake District, and in particular the impact of those processes on the supply of 210Pb and 137Cs to the sediment record in the lake
Choosing 1986, the year of the Chernobyl accident, as a common reference date, results from the 1976/77 cores indicated that total fallout from this source at Brotherswater decay corrected to 1986 amounted to 7886 Bq m-2
Summary
In spite of their relevance to paleolimnology, there have been relatively few detailed studies of the pathways by which atmosphere contaminants deposited on the landscape are delivered to the sediment records at the bed of a lake. A more detailed understanding of transport pathways is of particular relevance to records of the fallout radionuclides 210Pb and 137Cs used to date sediments. Chronostratigraphic dating by 137Cs matches features in the 137Cs activity versus depth record with the known historical fallout record. Both assumptions are potentially questionable at sites where a significant fraction of the supply to the sediment record consists of older catchment derived material that may have been deposited years or even decades earlier. A further uncertainty is the extent to which inputs to the water column are lost via the outflow
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