Geochemistry and particle size of surface sediments of Exmouth Gulf, Northwest Shelf, Australia

Exmouth Gulf is a shallow (mean depth 11.9 m, 2600 km2 area) inverse estuarine embayment on the northwest shelf of Australia, with a catchment area of 6400 km2 with low human and livestock populations in an arid climate. The Gulf is an extreme example of a tide and wave dominated estuary, with very small river inputs. Freshwater supply from the land is very small, and terrestrial run-off nutrient supply is estimated to be 0.8 mmol TN and 0.4 mmol TP per square metre Gulf area per year (TN=total dissolved and particulate nitrogen, TP=total dissolved and particulate phosphorus). Holocene deposits in the mangrove margins are less than 1 m thick, and are relict and partly aeolian in origin. Surficial sediments are largely quartz and calcite sand, and fine-grained recent sediments are probably removed by energetic Indian Ocean tide and wave mixing and circulation. Geochemical variations in sediment composition across the Gulf are small, and appear to be little affected by redox cycles. Marine organic matter is efficiently decomposed, leaving behind excess phosphorus associated with carbonate and iron-rich sediments. The thin ribbon of stunted mangroves along the eastern margin of the Gulf has little influence on the composition of sedimentary organic matter in the Gulf, and the mangrove sediments are not a trap for trace elements as has been found in other parts of the tropical world. Ba, Li, Pb, and Cu vary in direct proportion to the abundance of the bulk sediment aluminosilicate fraction, boron is enriched by evaporation in the supratidal saltflat regions, and cadmium has large variations perhaps related to the carbonate fraction of the sediments. There is little evidence for coastal trapping of sediment and oceanic elements in Exmouth Gulf, and some regions of the intertidal mangrove and saltflat zones are being eroded into the Gulf and adjacent shelf.