Biogeographic impact of the Leeuwin Current in southern Australia since the late middle Eocene

Abstract

The Leeuwin Current can be tracked from the western margin of the Australian continent to the southern margin by the record of fossilized organisms typical of warm-water marine environments. This transport smudges the normal latitudinal asymmetry in the biotas of opposing oceanic and continental margins, in which the eastern margins of oceans are cooler than the western margins and warmer biotas are restricted to lower latitudes in the east. The most comprehensive record is in the large benthic foraminifera, although fossils of benthic invertebrates, nektonic nautiloids and planktonic protists are also informative. In addition, organic biomarker hydrocarbons in stranded bitumens and resins demonstrate that they have travelled the same route from their ultimate sources in Cenozoic sedimentary basins and modern tropical rainforests of the Indonesian Archipelago. The earliest spoor of the current is in the later middle Eocene, at which time the part-deflection of counter-gyral circulation in the Indian Ocean to the southeast was stimulated by the accelerated opening of the oceanic gap between Australia and Antarctica. Thus the origin of the Leeuwin Current is twice the age of the previously suggested origin in the Miocene. The current turns on and turns off in the Great Australian Bight during the late Quaternary in concert with the interglacials and the glacials at scales of 105 yr. The switch can be seen in the faunal succession of planktonic foraminifera which are consistent with the neritic benthic faunas of the central gulfs: both communities show that, at the warm peak of the last interglacial, the current transported biota across the Bight more strongly than it has during the Holocene. The Cenozoic record of the past 40 Ma is in the same mode at 106 yr scales: the relevant fossils are found concurrently at major marine transgressions and warming reversals of the overall fall in global temperature. However, the fossil pattern is due to transport on the activated Leeuwin Current, not merely to general warming and the spread of friendly environments to higher latitudes.