Geochemistry and particle size of surface sediments of Exmouth Gulf, northwest shelf, Australia

Exmouth Gulf is a shallow (mean depth 11.9 m, 2600 km(2) area) inverse estu arine embayment on the northwest shelf of Australia, with a catchment area of 6400 km(2) with low human and livestock populations in an arid climate. The Gulf is an extreme example of a tide and wave dominated estuary, with v ery 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 parti culate nitrogen, TP=total dissolved and particulate phosphorus). Holocene d eposits in the mangrove margins are less than Im thick, and are relict and partly aeolian in origin. Surficial sediments are largely quartz and calcit e 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 se diments. The thin ribbon of stunted mangroves along the eastern margin of t he Gulf has little influence on the composition of sedimentary organic matt er in the Gulf, and the mangrove sediments are not a trap for trace element s as has been found in other parts of the tropical world. Ba, Li, Pb, and C u vary in direct proportion to the abundance of the bulk sediment aluminosi licate fraction, boron is enriched by evaporation in the supratidal saltfla t regions, and cadmium has large variations perhaps related to the carbonat e fraction of the sediments. There is little evidence for coastal trapping of sediment and oceanic elements in Exmouth Gulf, and some regions of the i ntertidal mangrove and saltflat zones are being eroded into the Gulf and ad jacent shelf. (C) 2001 Elsevier Science Ltd. All rights reserved.