Patricia Wells

Response of nannoplankton to major changes in sea-surface temperature and movements of hydrological fronts over Site DSDP 594 (south Chatham Rise, southeastern New Zealand), during the last 130 kyr

Abstract A high-resolution history of paleoceanographic changes in the subpolar waters of the southern margin of the Subtropical Convergence Zone during the last 130 kyr, is present in foraminiferal assemblages of DSDP Site 594. The foraminifera indicate that sea-surface temperatures during the Last Interglacial Climax were warmer than today, and that between substage 5d through to the end of isotope stage 2, temperatures were mostly cooler than Holocene temperatures. The paleotemperatures suggest that (1) the Subtropical Convergence was located over the site during substage 5e, later moving further north, then moving southwards to near the site during the Holocene, and (2) the Polar Front was positioned over the Site during glacial stages 6, 4, 2 and possibly parts of stage 3. Several major events are indicated by the nannofloral assemblages during these large changes in sea-surface temperature and associated reorganization of ocean circulation. First, the time-progressive trends between E. huxleyi and medium to large Gephyrocapsa are unique to this site, with E. huxleyi dominating over medium Gephyrocapsa during stages 5c-a, middle part of stage 4 and after the middle point of stage 3. This unusual trend may (at least partly) be caused by the shift of the Polar Front across the site. Second, upwelling flora (E. huxleyi and small placoliths) increase in abundance during stages 1, 3 and 5, suggesting that upwelling or disturbance of water stratification took place during the interglacials. Thirdly, there are no significant differences between the distribution patterns of the various morphotypes of medium to large Gephyrocapsa, and the combined value of all medium Gephyrocapsa increases in abundance during glacials (stages 2 and 4 and the end of stage 6), similar to the abundance trends in benthic foraminifera. Finally, subordinate nannofossil taxa also show distinctive. climatic trends during the last glacial cycle: (1) Syracosphaera spp. are present in increased abundance during warmer extremes in climate (substages 5e, 5a, and stage 1); (2) Coccolithus pelagicus and Calcidiscus leptoporus dominate the subordinate nannofossil taxa, and their relative proportions seem to provide a useful paleoceanographic index, with C. pelagicus dominating when the Polar Front Zone is over the site (stages 6, 4 and 2), whilst C. leptoporus is relatively more abundant when the STC is positioned over the site (stages 1 and 5e). Increased abundance of C. pelagicus also can indicate intensified coastal upwelling.

Large-scale reorganization of ocean currents offshore Western Australia during the Late Quaternary

Abstract The history of coastal surface currents in the eastern Indian Ocean over the last ∼ 130 kyr has been studied through the application of the FI-2 transfer function to planktic foraminifera in deep-sea cores in a N-S transect offshore Western Australia. The concept of sea-surface temperature (SST) gradient residuals is the amount by which the sea-surface temperature gradient of a given time differs from that in the modern sea. The SST gradient residuals for several episodes of climate extremes in the Late Quaternary [the last glacial maximum (∼ 20 kyr B.P.), the last interglacial maximum ( ∼ 122-120 kyr B.P.), and at the end of the penultimate glaciation ( ∼ 130 kyr B.P.)] indicate that areas north of ∼ 18°S off Western Australia have changed very little during the extremes in climate over the last ∼ 130 kyr, while south of ∼ 18°S there have been significant changes in surface-water temperatures. Large areas of anomalously cool surface-water were established off North West Cape (22°S–24°S) during both glacial and interglacial extremes. These reflect the increased influence of the West Australian current on the surface-water circulation. During the last glacial maximum and at the end of the penultimate glaciation, cold sea-surface temperature anomalies were established near to the coast, indicating that the Leeuwin Current did not warm the coastal zone as it does today. During the last interglacial maximum the Leeuwin Current flowed as an intensified current in a narrow zone close to the coast, but did not significantly warm surface-waters offshore as far as 20°–25°S, where a zone of cold water was maintained.

Response of deep-sea benthic foraminifera to Late Quaternary climate changes, southeast Indian Ocean, offshore Western Australia

The Late Quaternary benthic foraminifera of four deep-sea cores off Western Australia (ODP 122–760A, ODP122–762B, BMR96GC21 and RC9–150) have been examined for evidence of increased surface productivity to explain the anomalously low sea-surface paleotemperatures inferred by planktic foraminifera for the last and penultimate glaciations. The δ13C trends ofCibicidoides wuellerstorfi, and differences between the δ13C trends of planktics (Globigerinoides sacculifer) and benthicsC. wuellerstorfi) in the four cores indicate that during stage 6 bottom waters were significantly depleted in δ13C, and strong δ13C gradients were established in the water column, while during stage 2 and the Last Glacial Maximum, δ13C trends did not differ greatly from that of the Holocene. Two main assemblages of benthic foraminifera were identified by principal component analyses: one dominated byUvigerina peregrina, another dominated byU. proboscidea. Abundance of these Uvigerinids, and of taxa preferring an infaunal microhabitat, and ofEpistominella exigua andBulimina aculeata indicate that episodes of high influx of particulate organic matter were established in most sites during glacial episodes, and particularly so during stage 6, while evidence for upwelling during the Last Glacial Maximum is less strong. The Penultimate Glaciation upwellings were established within the areas of low sea-surface paleotemperature indicated by planktic foraminifera. During the Last Interglacial Climax, upwelling appears to have been established in an isolated region offshore from a strengthened Leeuwin Current off North West Cape. Last Glacial Maximum δ13C values ofC. wuellerstorfi at waterdepths of less than 2000 m show smaller than global mean glacial-interglacial changes suggesting the development of a deep hydrological front. A similar vertical stratification/bathyal front was also established during the Penultimate Glaciation.