Abstract

The Lincoln Shelf, located along the southern Australian continental margin, is a vast region covered by cool-water carbonate sediments. It is influenced by several regional water masses, including the warm, oligotrophic, eastward-flowing subtropical surface water transported by the South Australian Current; the cold, mesotrophic, westward-flowing slope water periodically upwelled from deeper layers; and the saline evaporated water seasonally exiting Spencer Gulf. During summer, upwelled Slope Water interacts with Shelf Waters, forming Cooled Evaporative Water and mixed slope water.

Surface sediments on the shelf consist of relict, stranded, and clean Quaternary carbonate particles. Holocene carbonate sediments are dominated by mollusks on the inner shelf, while bryozoans and benthic foraminifera prevail on the outer shelf. Stable oxygen isotopes were analyzed from sediments and overlying seawaters to identify the water masses driving carbonate sedimentation. Seawater isotope values were corrected to VPDB for calcite precipitation, and sediment values were adjusted for fractionation effects due to aragonite and Mg calcite.

The corrected oxygen isotope values reveal a mismatch between sediments and most shelf water masses, with seawater values being more negative than sediment values. Notably, Slope Waters exhibit isotope values nearly identical to sediments and are enriched in nutrients compared to other water masses. Sediments from the Kangaroo Island corridor, characterized by alternating upwelling and downwelling, show isotope values similar to adjacent facies despite the influence of saline outflow and nutrient-rich upwelling.

These findings suggest that carbonate sedimentation on the open shelf, dominated by biogenic, neritic, heterozoan carbonates, is not primarily driven by year-round water masses. Instead, it is influenced by short, seasonal pulses of nutrient-rich Slope Water, which promote active growth of benthic calcareous organisms. Similar patterns observed in Bahamian sediments highlight that while oxygen isotopes can elucidate the origins of neritic carbonates, they do not always reflect broader regional oceanographic conditions.