CONTROL OF SOIL ACIDIFICATION BY FLUVIAL SEDIMENTATION IN AN ESTUARINE FLOODPLAIN, EASTERN AUSTRALIA

A shallow stratigraphic sequence with associated pyrite-induced soil a cidification was investigated along a transect from the levee to the b ackswamp in an estuarine floodplain of eastern Australia. Three sedime ntary layers were identified and interpreted to correspond with three depositional stages. Firstly, a layer of humic, pyrite-rich, silty mud was deposited under a saline, mangrove-inhabited, intertidal environm ent during the present high sea level episode. This pyritic layer is b uried by the second sedimentary layer of grey brown mud with limited p yrite content, that was deposited in a brackish lagoonal environment. This material now represents much of the contemporary backswamp surfac e. The third sedimentary layer is a sandy mud without pyrite, that has been deposited by freshwater overbank floods. It is concluded that fl uvial sedimentation has been increasingly important in the development of the stratigraphic sequence, controlling the pyrite content, thickn ess and occurrence depth of the pyritic layer. The present drainage co nditions have allowed oxidation of pyrite in the soils of the backswam p and the resulting acidification has caused elevated concentrations o f toxic aluminium that threaten the local environment. However, in the levee, the pyritic layer is covered by thick non-pyritic freshwater s ediments and low-pyritic lagoonal sediments, and the soil profiles are unlikely to contribute to any acidification hazard.