The palaeoceanographical significance of diatoms in Late Quaternary sediments from the south-west Pacific.

摘要

Diatoms in late Quaternary sediments from an area of the SW Pacific Basin east of New Zealand have been used to determine aspects of palaeoceanography in this region for essentially the last 130ka (the last climatic cycle). One hundred and fifty-eight diatom taxa are documented from 44 core-top samples and 11 cores which collectively cover the last 210ka, (to late oxygen isotope stage 7) Calender year ages are used throughout this report. For the last 5 oxygen isotope stages, abundance and accumulation rate data are used to infer changes in the intensity of deep-and surface-water circulation, and to provide qualitative information on sea-surface temperatures: - Allochthonous diatoms, endemic to antarctic and subantarctic waters, plus types reworked from sea-floor sediments are used in conjunction with sedimentary textural and stable oxygen isotope data to infer a reduction in the formation of Antarctic Bottom Water in Antarctica during the Last Glacial Maximum and stadial substage 5d. The last and present interglacials are inferred as times of increased bottom water formation. For the area north and east of the Chatham Rise, upwelling and high productivity diatom indicators are used in conjunction with organic carbon and delta13C data to infer an increase in upwelling intensity (by virtue of increased wind intensity) commencing in oxygen isotope stage 4 and reaching maximum intensity during the Last Glacial Maximum. Upwelling declined in a series of distinct stages commencing at -17.7ka (18ka) calendar years, during the deglaciation phase of Termination 1. For the last climatic cycle, relative abundance data of warm water diatoms are used in conjunction with delta18O signal from planktonic foraminifera to infer the warmest sea-surface temperatures during the last and present interglacial periods, for surface water mass, (Cool) Subtropical Water, north of the Subtropical Convergence. Cooler, but still relatively warm surface water is inferred for interstadial oxygen isotope stage 3, and the coolest surface waters are inferred for the Last Glacial Maximum. Post-glacial surface-water warming commenced at -17.7ka (18ka) calendar years. On non-climatic issues, differences in the distribution of two distinct morphologies of the extant planktonic diatom Azpeitia tabularis (Grunow) Fryxell and Sims are reported: - A slightly larger, more heavily silicified and more coarsely areolated type, with a broader hyaline ring between valvar and mantle areolae, is restricted in its distribution to south of the Subtropical Convergence. A slightly smaller, more weakly silicified and more finely areolated type does not appear to be restricted in its distribution by the Subtropical Convergence, but is more common north of this front. Finally, the effects of silicic volcanic ash airfall (tephra) on diatom abundance/preservation are assessed, and an age-model based on diatom abundance patterns is suggested for a core with no carbonate and therefore no delta18O stratigraphy).