SEDIMENTARY AND GEOCHEMICAL VARIATIONS IN A SALT-MARSH MUD FLAT ENVIRONMENT FROM THE MESOTIDAL HAMBLE ESTUARY, SOUTHERN ENGLAND

The sediment record in a salt marsh contains valuable information on anthropogenic and natural inputs. The reliability of this record for a single core depends on how representative the sample is for the whole marsh and whether the various indicator elements are immobile. A detailed radiometric and geochemical study has been carried out on a series of salt marsh cores from the Hamble estuary, southern England, a temperate mesotidal estuary. Cores have been taken in two transects to assess cross-marsh variations in sediment accretion, trace element deposition and early diagenesis. From this, conclusions are drawn about variations in sedimentary processes and marsh stability, trace element focusing and the effect of early diagenetic movement on historical pollution records. Sediment accumulation rates across the salt marsh vary between 4 and 8 mm yr(-1) (Cs-137 and Pb-210 dating) and are apparently independent of elevation in the marsh. Pb-210, Cs-137 and anthropogenic Cu data show that the fronting mud flat is eroding, which may lead to increased wave attack and erosion at the marsh edge. The salt marsh itself, however, is accumulating at a rate significantly higher than the local rate of mean sea-level rise. The atmospheric deposition record of Pb-210(xs) is not well-preserved in the more organic-rich sediment at the rear of the salt marsh. Pb-210 and Pb are apparently mobilised in highly reduced sediments beneath the permanent water table and precipitate in overlying partially reduced sediment with hydrous Mn and Fe oxides. Such diagenetic movement of Pb-210 and Pb is localised and is not laterally continuous. At sites showing possible early diagenetic remobilisation of Pb-210 the accuracy of Pb-210 dating is reduced. Remobilisation of Pb-210 does not preclude Pb-210 dating, however, if peaks arising from redox mobility are identified and eliminated by comparison with other geochemical data (Fe, Pb, S, etc.). Of the trace elements examined, Cu shows a clear pollution spike. Anthropogenic Cu introduced into the Hamble estuary from the Esso refinery at Fawley, Southampton Water peaked around 1970 and has significantly reduced since 1971. Cu, Pb-210(xs) and Cs-137 are focused to some degree at the front of the marsh due to input of material labelled with these elements which has been eroded from surrounding mud flat areas.