THE SIGNIFICANCE OF MARGINOPORA-VERTEBRALIS (FORAMINIFERA) IN SURFICIAL SEDIMENTS AT ESPERANCE, WESTERN-AUSTRALIA, AND IN LAST INTERGLACIAL SEDIMENTS IN NORTHERN SPENCER GULF, SOUTH AUSTRALIA

Surficial sediments at Esperance, on the southern coast of Western Australia, are predominantly bioclastic carbonates in which tests of foraminifera are a major constituent. The megascopic species Marginopora vertebralis is prolific, occupying relatively protected areas of algal turf and seagrasses. The abundance of this normally tropical foraminifer at southern latitudes can be attributed to the Leeuwin Current, which brings warm waters from the tropics to southwestern Australia, and then eastwards, past Esperance into the Great Australian Bight. The Leeuwin Current is significantly warmer than the cool interfacing waters of the Southern Ocean that otherwise characterise the southern Australian continental shelf. Last interglacial sediments of the Glanville Formation, in northern Spencer Gulf of South Australia, host an assemblage of fossil foraminifera which is similar to that of the modern environment at Esperance, especially the presence of M. vertebralis. Two bivalve molluscs, Anadara trapezia and Pinctada carchariarium, both characteristic of warm waters, are also present in the Glanville Formation. Marginopora vertebralis and other faunal elements migrated to South Australia by means of the last interglacial equivalent of the Leeuwin Current, which apparently had greater definition at that time. It may therefore be inferred that, during other times of global warming and corresponding higher sea levels, the palaeo-Leeuwin current had greater ecological influence across the continental shelf of Southern Australia. Thus, warm-water marine fossils in the Glanville Formation do not necessarily signify last interglacial water temperatures for the open Southern Ocean, beyond the continental shelf. Similarly, it cannot be assumed that these fossils necessarily indicate a last interglacial climate for terrestrial southern Australian that was warmer than presently prevails.