The record of deposition and the migration of elements in salt marshes

Activities of two radionuclides, 137Cs and 210Pb, have been analysed throughout the cores and may be interpreted to determine average accretion rates for the marsh sediments. Distinct sub-surface activity peaks are observed for 137Cs, which may be correlated with the known fallout history of the isotope, whereas 210Pb profiles exhibit an approximately exponential decline with depth. The mean accumulation rates yielded by the two independent techniques are in good agreement for all cores studied and slightly lower than the estimated mean rate of sea-level rise across the region. Extrapolation of the sediment accretion rate allows a chronology to be established for the prism which may be used to attempt the reconstruction of pollution trends for trace metals. Inventories of these isotopes contained within the salt marsh sediments are compared to their predicted flux histories. This comparison is enhanced for 210Pb through the continuous collection, over three years, of local rain water samples for 210Pb activity measurements. The depositional flux of this isotope, determined here for the Southampton Water region, is in good agreement with previous measurements of the mean annual atmospheric 210Pb flux within the U.K.Differences arise in the behaviour of various trace elements to the changing conditions encountered upon burial. The resulting competition between the preservation of a depositional record and the removal of such by early diagenetic processes is considered. The historical introduction of anthropogenic copper to the marsh can be related to the operations of the Esso refinery at Fawley and the sediment record shows close correlation to the estimated discharge pattern. Other trace elements (As, P, Zn and Ni) exhibit considerable diagenetic lability and are often found in association with secondary phases (principally manganese oxides, iron oxyhydroxides and sulphides and organic substrates). Stable lead isotope analysis is effective in revealing the probable sources which have contributed to the accumulation of this element over time, although remobilisation of lead following burial is implied by its observed association with diagenetic features of the sediment core geochemistries. The mixing of natural lead in the study area with anthropogenic lead derived from a range of sources is demonstrated by isotope ratio plots, thereby enabling source apportionment. The influence of imported lead (such as that derived from Australian and Canadian Precambrian ores) is evident throughout the sediments analysed.