The paleoceanography for the last 30,000 years in the southeastern Indian Ocean by means of calcareous nannofossils

Abstract

The paleoceanography during the latest Quaternary (last ∼30 kyr), based on calcareous nannofossils, was studied in the southeastern Indian Ocean through the analysis of 202 sediment samples taken from three deep-sea cores taken along a north–south transect between 12°S and 25°S offshore western Australia. A total of 24 taxonomic groups of calcareos nannofossil was identified by light microscope observation. Two biotic proxies were applied: Gephyrocapsa morphotypes as a sea-surface temperature (SST) indicator; and the ratio of small (<2.5 μm) placoliths of Reticulofenestra and Gephyrocapsa against the lower-photic species indicating the nutrient condition. The relative abundance of Gephyrocapsa types, defined by the angle between the bridge and the long axis of the coccolith, corresponds well with the modern SST. Small placoliths commonly occur in the northern and central cores, while they are rare in the southern core during the Holocene period. The Java upwelling system (JUS) operates above the northern core site and induces higher counts of small placoliths. In the central and southern core sites, located outside of the JUS, the nutrient levels and primary productivity are usually low. The difference in the small placolith abundances between the latter two core sites was explained by an appearance of a small scale local coastal upwelling which induces a common occurrence of the small forms at the central core site; the continuous presence through time of Gephyrocapsa low-angle type supports evidence for upwelling. An intensification of upwelling offshore southern Java during the last glacial period has been inferred from the foraminiferal record (e.g. Martı́nez et al., 1999. Palaeogeogr., Palaeoclimatol., Palaeoecol. 147, 73–99), and the high abundance of small placoliths, indicative of eutrophication as well as low SST values deduced from the nannoflora recognized here are concordant with the postulated upwelling. Low SSTs at the central core site prior to the middle of marine isotope stage 2 indicate a reduction of the Leeuwin Current (LC). Cooler temperatures at the southern site interpreted from the nannoflora indicate that the LC did not reach this site during stage 3 and early stage 2; instead, the South Indian Current dominated there during this period. Small placoliths commonly occurred not only at the central core site but also at the southern core site during stage 3, but disappeared from the southern one for the entire stages 2 and 1. A similar temporal change was also observed in the Tasman Sea (Hiramatsu and De Deckker, 1997. Palaeogeogr., Palaeoclimatol., Palaeoecol. 131, 391–412) and the southeast Indian Ocean (Okada and Wells, 1997. Palaeogeogr., Palaeoclimatol., Palaeoecol. 131, 413–432).