Reliability of salt marshes as ''geochemical recorders'' of pollution input: A case study from contrasting estuaries in southern England

Temperate, mesotidal salt marshes are usually good ''geochemical recorders'' of pollutant input. Dated salt marsh cores from the Hamble, Itchen (Southampton Water), and Beaulieu estuaries (southern U.K.) are assessed. Sediments show clear labeling from effluents, which varies depending on their proximity to major urban or industrial areas. For elements where input is dominantly from a single source and periods of peak discharge are known (i.e., Cu, Cs-137, and (CO)-C-60), historical records of pollutant input can be reconstructed, provided redistribution through sediment mixing or early diagenetic processes is minimal. Where the pollutant has a range of sources (i.e., Pb) or where physical mixing in-estuary produces a time-integrated signal, it can prove extremely difficult to relate concentration depth profiles to discharge histories. Using concentration and stable Pb isotope data, the observed temporal input of Pb to these marshes is shown to reflects complex, mixed marine/atmospheric input from regional (automobile emission) and local (urban/industrial) sources. While general trends in pollutant loading may still be observed, it is extremely difficult to reconstruct accurately temporal trends in Pb input and sources of Ph in these estuaries using salt marsh records due to the importance of local, poorly-defined Pb sources and in-estuary mixing processes.