SEDIMENT MANGANESE AND BIOGENIC SILICA AS GEOCHEMICAL INDICATORS IN ESTUARINE SALT MARSHES OF COASTAL GEORGIA, USA

Salt-marsh estuarine sediments are not homogeneous. It is obvious that a steady state cannot be assumed if the depositional environments und er which salt marshes accumulate change from subtidal through non-vege tated intertidal to vegetated intertidal state during their formation. In addition to these, the supply of sedimentary material depends on t he tidal prism which changes as salt marshes are formed. Based on the study of cores collected in the estuarine region of Georgia, USA, it w as found that the Mn/Al ratios in sediments of marsh cores change from subtidal through non-vegetated intertidal to vegetated intertidal. Th e relative variation of biogenic silica preserved in sediments of mars h core reflects the relative supply rate of organic carbon (phytoplank ton) produced in the water column to the sediment surface at the time of deposition. In this paper it is proposed that sediment manganese an d biogenic silica may be applied as geochemical indicators of changing depositional environments and organic carbon originating from phytopl ankton in the estuarine salt marsh sediments.