Quaternary surface-water stable isotope signal from calcareous nannofossils at DSDP Site 90-593, South Tasman Sea

Oxygen isotope values from calcareous nannofossils in four cores spanning the Quaternary from DSDP Site 593 in Tasman Sea are compared with the delta18O signal of planktonic and benthic foraminifers from the same samples. The classic mid-late Quaternary isotope stages are exhibited with stage 12 particularly well developed. When delta18O values of nannofossils are adjusted for coccolithophore vital effects they indicate larger (by 1-6°C) surface to bottom paleotemperature gradients and greater (by 1-3°C) changes in mean sea-surface temperature between full glacial and interglacial conditions than do delta18O values from planktonic foraminifers. Along with the foraminifers, the nannofossils record a bimodal distribution of delta18O between the early and mid-late Quaternary, indicating a significant change in global ice budget. The delta13C of nannofossils also shows a bimodal distribution, but is opposite to that for the foraminifers. Nannofossil delta18O values record a shift of c. -0.8‰ at isotope stage 8 corresponding to a major reduction in abundance of the previously dominant gephyrocapsids. A shift in delta13C of c. -1.5‰ also occurs at stage 8, and a shift in delta13C of c. +1.2‰ at around stage 14. The delta18O shift in nannofossils is at least a Pacific-wide phenomenon; the delta13C shifts are possibly global. The delta13C signal of nannofossils exhibits an antipathetic relationship to that of benthic foraminifers back to isotope stage 18 but no significant correlation beyond this level to the base of the Quaternary. This is interpreted as reflecting local productivity dominating global influences on delta13C since stage 18 at DSDP Site 593. The difference between nannofossil and benthic foraminifer delta13C signals (Delta13C) tends to be maximum during glacial stages and minimum during interglacials throughout the section, showing a strong correlation with the nannofossil delta180 signal. The increased partitioning of 13C between surface and bottom waters during the glacial periods may indicate heightened productivity in surface waters in the southern Tasman Sea at these times.

本次数据集包含塔斯曼海深海钻探计划（Deep Sea Drilling Project, DSDP）593站位4个第四纪沉积岩芯内钙质超微化石（calcareous nannofossils）的氧同位素数据，并将其与同一样品中浮游有孔虫、底栖有孔虫的δ¹⁸O信号开展对比分析。研究序列清晰呈现经典的第四纪中晚更新世同位素地层阶段，其中第12阶段的信号尤为清晰完整。当针对颗石藻（coccolithophore）的生命效应完成校正后，钙质超微化石的δ¹⁸O值显示，冰盛期与间冰期之间的表层-底层古温度梯度较浮游有孔虫的δ¹⁸O结果高出1~6℃，平均表层海水温度的变化幅度也比浮游有孔虫的测定结果高出1~3℃。与有孔虫的记录一致，钙质超微化石的δ¹⁸O值在第四纪早更新世与中晚更新世之间呈现双峰分布特征，指示全球冰量发生了显著改变。钙质超微化石的δ¹³C信号同样呈现双峰分布，但分布模式与有孔虫恰好相反。钙质超微化石的δ¹⁸O值在第8同位素阶段出现约-0.8‰的偏移，该偏移对应此前占绝对优势的桥石藻（gephyrocapsids）丰度大幅降低。第8同位素阶段同时伴随约-1.5‰的δ¹³C偏移，而在第14同位素阶段前后则出现约+1.2‰的δ¹³C偏移。钙质超微化石的δ¹⁸O偏移至少是太平洋全域范围内的现象；而δ¹³C偏移则可能具有全球性。可追溯至第18同位素阶段的钙质超微化石δ¹³C信号与底栖有孔虫的δ¹³C信号呈反相关关系，但在该阶段之前直至第四纪底部，二者并无显著相关性。这一现象被解释为：自深海钻探计划593站位的第18同位素阶段以来，当地初级生产力对δ¹³C的影响超越了全球尺度的调控作用。整个岩芯序列中，钙质超微化石与底栖有孔虫的δ¹³C差值（Δ¹³C）在冰期阶段达到峰值，间冰期阶段降至最低，且与钙质超微化石的δ¹⁸O信号呈现极强的相关性。冰期阶段表层与底层海水之间的¹³C分馏程度增强，或许指示当时塔斯曼海南部表层海水的初级生产力有所提升。