Late Quaternary nannofossil indicators of climate change in two deep-sea cores associated with the Leeuwin Current off Western Australia

Abstract

The stratigraphic variations in nannoflora observed in the two deep-sea cores recovered from the offshore Western Australia revealed a sequence of paleoceanographic changes for the last 250 kyr. The dominance of small placolith taxa in the upper-photic flora indicates continuous mild upwelling for the isotope stages 7-5 in this region. The most important controlling factor for the abundance variation of Florisphaera profunda in this region is considered to be the lower-photic temperature rather than the stability of water column or the water turbidity. The significant reduction in the abundance of F. profunda combined with the stable (in the northern core RS96GC21) and reduced (in the southern core RS9-150) abundance of the small taxa suggest an intensified upwelling for the Penultimate Glaciation. The sharp increase in F. profunda abundance indicates a weakening of upwelling at the Last Interglacial Climax (LIC) offshore Western Australia. After isotope stage 5, the existence of upwelling is not clear except in stage 2, when it was not strong enough to produce blooms of Emiliania huxleyi. Among the subordinate taxa, Calcidiscus leptoporus, Calciosolenia murrayi, Neosphaera coccolithomorpha and Oolithotus fragilis provide meaningful paleoceanographic signals supporting the conclusions of previous studies: a weakening or cessation of Leeuwin Current for the glacial periods. The shift of dominance from small Gephyrocapsa (mostly G. ericsonii) to small Reticulofenestra (mostly R. parvula) can be an useful datum event offshore Western Australia. The reduced diversity of nannoflora during the interglacial periods can be explained by a lessened competition within nannoplankton community caused by particularly favorable conditions for the opportunists, such as E. huxleyi, F. profunda and small placoliths. The variations in nannofossil abundance suggests an increased aeolian flux at the stages 6 and 2 offshore North West Cape, as well as at stage 6 offshore southwestern Australia.