Trace metals in lacustrine and marine sediments: A case study from the Gulf of Carpentaria, northern Australia

Sediment cores taken from the shallow (<70-m depth), epicontinental Gulf of Carpentaria record a late Quaternary marine transgression over a well-established lacustrine sequence. Bulk sediment samples taken along the length of a single core have been analysed for their Ca, Al, Fe, Ti, Mn, Mg, Na, K, Sr, Ba, V, Ni, Co, Cr, Cu, Sc, Zr, Y, La and Ce contents to examine compositional changes across a continuous sedimentary sequence in which the depositional environment evolved from freshwater lacustrine to saline lacustrine to open marine over the past 40 ka. The data form remarkably linear covariation trends suggesting physical mixing of two sedimentary components as the predominant process controlling the compositions of both lacustrine and marine sediments. One component has high Ca, Sr and Mn, suggestive of biogenic carbonate. The other component contributes virtually all of the sedimentary budget of the other elements, and is probably terrigenous clay, either water-borne or aeolian. Minor contributions from heavy minerals such as zircon may account for the higher values of Zr and Y for some horizons in the core. Anomalously high concentrations of zircon may have been brought into the Gulf as dust particles during episodes of increased aeolian activity. Carbonate is most abundant in the marine sediments, resulting in the higher observed Ca and Sr concentrations. V and Mn seem to be the metallic elements most sensitive to depositional environment and water chemistry. Environmental conditions had little or no effect on sedimentation of the other metals. V and Mn were delivered to the lacustrine sediments more efficiently than to the marine sediments, probably reflecting changes in solubility as slightly acid lacustrine waters gave way to more neutral oceanic conditions. Metal-organic complexing was not an important process in these oxidised sediments.