Heavy-metal enrichments, bacterial sulphate reduction and iron sulphide formation in shallow-water carbonate sediments offshore from a smelting complex at Port Pirie, South Australia

Abstract

The interactions of Fe and above normal concentrations of Pb, Zn and Cd with the products of bacterial sulphate reduction have been investigated using vibrocores from shallow subtidal seagrass-colonized carbonate environments near to and remote from a PbZn smelting complex on the shore of Spencer Gulf, South Australia. Sediments remote from the smelter are coarse grained, rich in seagrass, low in Pb, Zn and Cd, and contain only traces of sulphide. Near the smelter an additional 10–15-cm thick layer of relatively fine grained Pb-, Zn- and Cd-rich carbonate is present at the surface and inhibits exchange of the sediment pore waters with the overlying oxic seawater. As a result, bacterial sulphate reduction has become strongly established and concomitant diagenetic reactions have produced dissolved sulphide, iron monosulphides and trace quantities of pyrite. Sulphur-isotope data indicate that the present stage of sulphate reduction in which the amount of sulphide in the deeper coarse-grained sediments is equivalent to the amount of sulphate removed from the pore waters, may have been preceded by a period in which the first-formed sulphide was reoxidized within the sediments. The high ratio of iron monosulphides to pyrite results from a rapid rate of deposition of the surface layer and, in the deeper sediments, limitations on the oxygen supply. Pore waters associated with the metal-rich surface layer contain high concentrations of Zn (392 μg 1 −1) and low concentrations of Pb and Cd, but this has not resulted in significant spatial separation of these metals in the sediment column. In the surface sediments, Pb and Zn are probably not present as sulphides, although in the deeper sediments Zn concentrations are at a minimum in a zone of high dissolved sulphide concentration.