BIOGEOGRAPHIC IMPACT OF THE LEEUWIN CURRENT IN SOUTHERN AUSTRALIA SINCE THE LATE MIDDLE EOCENE

The Leeuwin Current can be tracked from the western margin of the Aust ralian continent to the southern margin by the record of fossilized or ganisms typical of warm-water marine environments. This transport smud ges the normal latitudinal asymmetry in the biotas of opposing oceanic and continental margins, in which the eastern margins of oceans are c ooler than the western margins and warmer biotas are restricted to low er latitudes in the east. The most comprehensive record is in the larg e benthic foraminifera, although fossils of benthic invertebrates, nek tonic nautiloids and planktonic protists are also informative. In addi tion, organic biomarker hydrocarbons in stranded bitumens and resins d emonstrate that they have travelled the same route from their ultimate sources in Cenozoic sedimentary basins and modern tropical rainforest s of the Indonesian Archipelago. The earliest spoor of the current is in the later middle Eocene, at which time the part-deflection of count er-gyral circulation in the Indian Ocean to the southeast was stimulat ed by the accelerated opening of the oceanic gap between Australia and Antarctica. Thus the origin of the Leeuwin Current is twice the age o f the previously suggested origin in the Miocene. The current turns on and turns off in the Great Australian Eight during the late Quaternar y in concert with the interglacials and the glacials at scales of 10(5 ) yr. The switch can be seen in the faunal succession of planktonic fo raminifera which are consistent with the neritic benthic faunas of the central gulfs: both communities show that, at the warm peak of the la st interglacial, the current transported biota across the Eight more s trongly than it has during the Holocene. The Cenozoic record of the pa st 40 Ma is in the same mode at 10(6) yr scales: the relevant fossils are found concurrently at major marine transgressions and warming reve rsals of the overall fall in global temperature. However, the fossil p attern is due to transport on the activated Leeuwin Current, not merel y to general warming and the spread of friendly environments to higher latitudes. (C) 1997 Elsevier Science B.V.