SEDIMENTARY ENVIRONMENT AND ACID POTENTIAL OF THE PYRITIC SEDIMENTS IN A SAND BARRIER ESTUARINE SYSTEM, EASTERN AUSTRALIA

Three cores of Holocene sediments were investigated from an infilled s and barrier estuary, eastern Australia. The estuary formed as a result of drowning of the previous river valley following the postglacial ma rine transgression. The sedimentary environments evolved over time at the investigated site from a saline prodelta to a less saline delta fr ont to fresh flood overbank sedimentation. Both prodelta and delta fro nt sediments contained large amounts of pyrite, but pyrite accumulatio n was limited in the riverine flood overbank sediments. The accumulati on of pyrite in the estuarine sediments appears to have been controlle d by the combined effect of sedimentation rate and the supply of organ ic matter, while palaeowater salinity relating to the supply of sulfat e ion did not limit pyrite accumulation except for the top 1 metre of the freshwater sediments. The degree of pyritization (DOP) of the inve stigated estuarine sediments is comparable to that of aerobic marine s ediments (Raiswell er al., 1988), although the concentrations of organ ic C, reactive Fe and pyrite-S were much greater. This may be attribut ed to higher availability of organic matter and reactive Fe compounds, and consequently pyrite-S enrichment in an embayed estuary than in th e open sea. Owing to the presence of large amounts of pyrite, the inve stigated estuarine sediments posed great acid potential upon the oxida tion of pyrite. In general, increasing pyrite concentration in the sed iments resulted in increasing acid potential, but this was also accomp anied by an increase in self-neutralizing and buffering capacity (NBC) of the sediments although the finely-grained prodelta sediments tende d to have greater NBC values than did the coarser delta front sediment s on an equivalent pyrite concentration.