Oceanographic controls on sedimentological and geochemical variations in temperate carbonates off Western Tasmania, Australia

The unrimmed shallow (<200m) continental shelf off western Tasmania covers an area of about 16,000 km2 in latitudes 40° 30́ to 44° 30́S and longitudes 144° to 146° 30́ E. This is an ideal shelf environment for carbonate sedimentation due to availability of nutrients, preferred substrate for the growth of cool temperate biotic carbonates, a constant flow of different water masses, and limited terrigenous input. The sediment distribution on the shelf is characterised by a dominance of siliciclastic sediments close to the coast grading to carbonates towards the shelf edge and in deeper water. The dominant grain size is very coarse to medium sand, followed by fine sand, gravel fractions and minor fines (mud). Bryozoans dominate in all bulk samples, while foraminifera and molluscs occur in minor amounts. Skeletal fragments, debris and non-skeletal grains, are the other important components. Bryozoans occur mainly in gravel and very coarse to medium sand fractions, and foraminifera mainly in the fine sand. The higher percentage of bryozoans in the bulk sediments and in all different size fractions is attributed to low salinity, nutrient rich (6 μM NO3 - ) upwelling sub-Antarctic water, the absence of fines (mud) and low amounts of terrigenous material along middle to deep shelf off western Tasmania. The low abundance of foraminifera in the samples relative to tropical areas is due to lower seawater temperature. Bryozoans and foraminifera contents increase with increasing water depth and molluscs are mainly concentrated around 130 meters. Mineralogy of the bulk sediments is a mixture of high-Mg (mean 60%), to low-Mg (30%) calcite with some aragonite (10%) at all depths and latitudes. This is due to low seawater temperature and presence of bryozoans as the dominant components in these sediments. Sr values decrease with depth and latitude due to decrease in aragonite and high-Mg calcite. Na values increase with increasing water depth due to greater biochemical fractionation caused by faster growth rate of bryozoans and the presence of relatively saline Zeehan Current. Mn and Fe values decrease with increasing water depth, due to low terrigenous input to deeper parts of the shelf. The enrichment of δ18O values of bulk sediments in some areas of the shallow shelf is due to the intrusion of cool upwelling sub-Antarctic water. The higher δ13C values towards the deeper parts of the shelf reflect the influence of the warmer Zeehan Current.