ABSTRACT

Three cores of Holocene sediments were investigated from an infilled sand barrier estuary, eastern Australia. The estuary formed as a result of drowning of the previous river valley following the postglacial marine transgression. The sedimentary environments evolved over time at the investigated site from a saline prodelta to a less saline delta front to fresh flood overbank sedimentation. Both prodelta and delta front sediments contained large amounts of pyrite, but pyrite accumulation was limited in the riverine flood overbank sediments. The accumulation of pyrite in the estuarine sediments appears to have been controlled by the combined effect of sedimentation rate and the supply of organic matter, while palaeowater salinity relating to the supply of sulfate ion did not limit pyrite accumulation except for the top 1 metre of the freshwater sediments. The degree of pyritization (DOP) of the investigated estuarine sediments is comparable to that of aerobic marine sediments (Raiswell et al., 1988), although the concentrations of organic C, reactive Fe and pyrite-S were much greater. This may be attributed to higher availability of organic matter and reactive Fe compounds, and consequently pyrite-S enrichment in an embayed estuary than in the open sea. Owing to the presence of large amounts of pyrite, the investigated estuarine sediments posed great acid potential upon the oxidation of pyrite. In general, increasing pyrite concentration in the sediments resulted in increasing acid potential, but this was also accompanied by an increase in self-neutralizing and buffering capacity (NBC) of the sediments although the finely-grained prodelta sediments tended to have greater NBC values than did the coarser delta front sediments on an equivalent pyrite concentration.