Vertical profiles and sediment-water solute exchange of iron and manganese were measured in reef- (central Great Barrier Reef Lagoon) and river-dominated (Fly Delta-Gulf of Papua) shelf regions of the Coral Sea. Solid-phase iron and manganese concentrations were greater in the Papuan deposits (Fe range: 42.8-98.3 mg g-1 sediment DW; Mn range: 423-2500 mug g-1 DW) than in the central GBR lagoon (Fe range: 4.0-40.0 mg g-1 DW; Mn range: 99-496 mug g-1 DW), reflecting generally smaller grain size and higher rates of terrigenous runoff from the Fly River and associated river systems onto the adjacent Papuan shelf as well as dilution by calcium carbonate and a decline in terrestrial input across the GBR shelf. Dissolved metal concentrations in the porewaters were usually very low (Fe range: 0.1-28.0 muM; Mn range: 0.1-97.0 muM) and not significantly different between shelves. Vertical profiles of dissolved and solid-phase Fe and Mn either did not vary with sediment depth or were irregular at most stations. The dissolved Mn/Fe (molar) ratio varied greatly among stations, but was generally higher in the Papuan sediments (range: 1.2-46.4) than in the GBR deposits (range: 0.3-18.1). The solid-phase Mn/Fe (molar) ratio varied little among stations, ranging from 0.01 to 0.03. Solid-phase metal concentrations correlated negatively with calcium carbonate content. When this relationship was held constant (partial correlation analysis), solid-phase Fe and Mn correlated negatively with grain size and positively with total N (Fe only) and P. Metal solutes in the porewater correlated negatively with redox potential and grain size (Mn only) and positively with porewater NH4+ (Mn only). Rates of Fe and Mn solute flux across the sediment-water interface were, at most stations, undetectable. Measurable fluxes ranged from -609 to +125 mumol m-2 day-1 for iron and from -218 to +400 mumol m-2 day-1 for manganese. The distribution and diagenesis of Fe and Mn in these tropical shelf sediments are more similar to their behavior in hemipelagic and pelagic deep-sea clays than in sediments of comparable grain size on temperate shelves. The lack of vertical structure, low porewater concentrations, and undetectable or slow rates of sediment-water solute exchange at most stations may be attributed to several factors, including moderate redox conditions, low reactivity of advected detritus, moderate to high production rates of hydrolytic and fermentative bacteria, and frequent physical disturbances.
