Radionuclide and geochemical studies of recent sediments from the Solent estuarine system
This study examines the geochemistry of salt marshes and intertidal sediments from the Solent estuarine system, southern England. Micro-, meso- and macro-scale studies have been carried out to assess the geochemical processes operating in these intertidal sediments and the use of radionuclides as tracers of sedimentary processes in the Solent area. Measurement of 210Pbxs, 137Cs and 60Co provides valuable information on sedimentary processes occurring in the Solent estuarine system. Subsurface maxima in activity are observed for 137Cs and 60Co (after normalisation to Al) which can be related to historical input, providing dated sediment horizons. This has enabled determination of Recent sediment accumulation rates and reconstruction of historical pollution records. Sediment accumulation rates obtained using 210PbXS/ l37Cs and 60Co in salt marsh environments provide an indication of the rate of Recent sea level rise, estimated as 4 mm y-1. This is in broad agreement with tide gauge data and when compared with longer term archaeological and radiochronological data may indicate an increase in the rate of mean sea level rise over the last 1000 years. Salt marsh areas with accumulation rates lower than 4 mm y 1 (from 137Cs and 60Co dating) are degenerating (undergoing die-back) indicating that they are failing to keep pace with the rising sea level. While local (intra-estuary) effects influence rates of sediment accumulation on a particular salt marsh the main control of sediment accretion over the whole Solent area is a rise in mean sea level. Dating of salt marsh sediment cores has allowed assessment of the historical record of anthropogenic pollutant input to the Solent (in particular for Pb and Cu). Pb isotope studies of salt marsh sediments from Southampton Water indicate increasing importance of anthropogenic Pb (from Precambrian ores) since the beginning of this century. A very prominent transient spike due to an anthropogenic Pb input is found in salt marsh sediments from the relatively unpolluted Beaulieu estuary. This may be related to military marshalling in the Beaulieu river before the Allied D-Day landings in 1944. Anthropogenic Cu is present in the Solent region mainly due to discharge from the Esso refinery at Fawley, Southampton Water. Analysis of Cu fluxes has shown that discharges peaked in ca.1970 and have significantly reduced since 1971 (due to effluent clean-up by the refinery). The reliable use of radionuclides as indicators of sediment accumulation depends on their chemical immobility (lack of redox migration). 60Co shows evidence for early diagenetic remobilisation at two salt marsh sites, and 2iopb m a v a i s o be mobile in some salt marsh areas. Remobilisation of 210Pb is apparently localised and may be aided by flooding/drainage cycles in the marsh. The use of these radionuclides as tracers of sediment accumulation in areas where significant early diagenetic remobilisation occurs is problematic. Where possible a number of chemically different radionuclides should be used to eliminate bias in calculated sediment accumulation rates due to redox mobilisation or diffusive processes. Detailed geochemical studies can assist in determining whether redox movement of radionuclides is occurring. A critical examination of the geochemical behaviour of both the stable counterparts of radionuclides and redox-sensitive elements is required if realistic sediment accumulation rates are to be obtained.