ABSTRACI': In eastern Tasmania temperate shelf carbonates occur in latitudes between 40030' and 44°8 in water depths from approximately 14 to 250 m. Increasing water depths correspond to decreasing water temperatures and salinities. Bryozoans (total, not species) increase with increasing water depth, bivalves are high in shallow-depths, foraminifera are high in mid-depth and gastropods are mostly located around 130 m. The amount of calcite relative to aragonite increases with increasing water depth due to decreasing water temperatures. The Mg, Sr andNa values increase with increasing water depth due to changes in carbonate mineralogy, the type ofbiota and the temperature. Mn and Fe values in bulk carbonates decrease with increasing water depth, due to the decreasing of terrigenous contentThe 5 1 '(} values of bulk sediments, bryozoans, benthic foraminifera and brachiopods increase with increasing water depth, due to decreasing seawater temperatures and salinity, and the changes in carbonate mineralogy. The 5 13C values of most of these carbonates increase with increasing water depth, mainly due to mixing of water masses and decreasing seawater temperatures. Latitudinal variations in sedimentology, carbonate elemental and isotopic compositions and mineralogy caused by seawater temperatures and salinities are small when compared to changes caused by increasing water depth. Combining present oceanographic features with those deduced from sedimentological and geochemical properties enables better understanding of the paleoceanography off Tasmania since the Last Glacial Maximum, related to seawater temperatures, salinity, mixing of water masses, sea-level changes, sedimentation and diagenesis. Figure 1. The occurrence of temperate carbonates and siliciclastics (quartz sandand mud) on the Tasmanian shelf from the coast to200m water depth. Notetheoccurrence of siliciclastics in coastal environments and carbonates from shallow to offshore shelfenvironments.
The unrimmed shallow «200m) continental shelf off western Tasmania covers an area of about 16,000 km 2 in latitudes 40 0 30 to 44 0 30S and longitudes 144 0 to 146 0 30 E. This is an ideal shelf environment for carbonate sedimentation due to availability ofnutrients, 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 tiM N0 3 -) upwelling sub-Antarctic water, the absence offines (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 ofthe bulk sediments is a mixture ofhigh-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 ofthe shelf. The enrichment of 8180 values of bulk sediments in some areas of the shallow shelf is due to the intrusion of cool upwelling sub-Antarctic water. The higher 8 13C values towards the deeper parts of the shelf reflect the influence of the warmer Zeehan Current.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.