Composition and behaviour of trace metals in post-oxic sediments of the Gulf of Papua, Papua New Guinea

Trace metal composition and behaviour were examined in post-oxic muds of the Gulf of Papua in January 1993. Porewater profiles and flux measurements indicate significant Fe and Mn reduction in these coastal muds. Vertical profiles, rates of sediment-water exchange, and correlation analyses imply that these silt-dominated muds mobilize Al and are a sink for Mo. Undetectable or very low porewater concentrations and flux rates of other metals (Ni, Cu, Cr, Co, Pb, Cd) indicate immobilization into solid-phase minerals. Solid-phase metal concentrations are consistent with those in average shale, suggesting pristine conditions in the gulf. Low (<1-2 mu M) free sulphides, low clay content, and bioturbation are partly responsible for trace metal composition and concentrations, but several lines of evidence imply non-steady-state diagenesis in these muds: (1) the lack of a clear zonational sequence of solutes with sediment depth; (2) lack of agreement between vertical profiles of dissolved and solid-phases of most metals; (3) significant, but weak, correlations between dissolved metals and dissolved organic carbon, and between solid-phase metals and total organic carbon; and (4) lack of correlation between conservative elements (e.g. dissolved Mo and Cl-; dissolved Al and SiOH). Intense, massive physical reworking is cited as the major factor fostering apparent non-steady-state behaviour of trace metals in the surface muds of the Gulf of Papua. This scenario is very similar to that occurring on the Amazon shelf, and may be typical of other wet tropical regions with high-energy coastlines receiving large amounts of highly weathered, river-borne material. (C) 1996 Academic Press Limited