Stable isotope stratigraphy of deep-sea cores from the Southwest Pacific region: aspects of late Quaternary palaeoceanography

Abstract

¹⁸O/¹⁶O and ¹³C/¹²C ratios of calcareous planktonic and benthic foraminifera from the Late Quaternary section of deep-sea sediment cores from the Tasman Sea and Southwest Pacific Ocean have been determined in this study. The core sites cover a latitudinal range from the equator to 49°S, and samples were provided by the Ocean Drilling Program and the New Zealand Oceanographic Institute. In most cases the cores were sampled at 10cm intervals. Generally, attention has been focused on the latest Quaternary (stages 1 to 5) glacial-interglacial oscillations of the δ¹⁸O and δ¹³C record, and aspects of their palaeoceanographic significance. Some longer sequences of core were also studied. Of the Southwest Pacific Ocean cores, 594 yielded the most detailed record for the past 130ky. Located just south of the Subtropical Convergence it has proved particularly sensitive to substage climatic fluctuations. In addition the high sedimentation rate (averaging ~13cm/ky) has reduced smoothing by bioturbation. Core 594 has yielded one of the most detailed isotopic records available world-wide for stages 1 to 6. Concurrent carbonate analysis by Cooke (1988) has revealed a similarly detailed carbonate record whose rises and falls are more or less synchronous with the isotopic record. To the west of New Zealand Core 593 was analysed to 44m sub-bottom depth and a clear record of isotopic fluctuations back to the 55/56 stage boundary was obtained. Sites 593 and 594 have thus provided useful isotopic reference curves for the Southwest Pacific region. Consistent changes in the planktonic δ¹³C record of some of the cores may reflect variations in CaCO₃ productivity linked to cyclical glacial-interglacial fluctuations. Glacial benthic δ¹³C results support a decreased volume of NADW production. Resumption of NADW flow is marked in many cases by a transient drop in δ¹³C values before rising to interglacial values as the relatively stagnant, isotopically lighter water is “flushed” from Deep Water flow paths. AABW appears to dominate during glacial periods. A northern shift of the Subtropical Convergence east of New Zealand is suggested (and supported by micropalaeontological data; Cooke, 1988), with a more restricted shift in the Tasman Sea. Interglacial Deep Waters appear to be dominated during interglacial times by AABW, whilst NADW is dominant in the Tasman Sea. Planktonic δ¹³C results suggest a more southerly position of the Subtropical Divergence in the Tasman Sea during interglacial periods. Lead-lag relationships in the δ¹³O signal between surface and bottom-dwelling foraminifera over Termination Iᴀ reveal a pattern of meltwater circulating first to the surface waters at higher latitudes, and to bottom waters at lower latitude sites. A modification of the meltwater “lid” is proposed, with the lid extending only partially across the ocean surface, and not into equatorial regions.