Quaternary calcareous nannofossil stable isotope signal in the southwest Pacific Ocean, DSDP Site 90-594

Oxygen and carbon isotope values from calcareous nannofossils in the mid-late Quaternary (Brunhes) interval of core from DSDP Site 594 off eastern South Island, New Zealand are compared with the delta18O and delta13C signal of planktic and benthic foraminifers from the same samples. Nannofossil delta18O values appear strongly affected by non-equilibrium fractionation, in agreement with previous in-vitro studies. The nannofossil delta18O and delta13C signals show a strong down-core cross-correlation and a significant positive linear correlation. This is interpreted as the result of kinetic isotope effects producing non-equilibrium fractionation of both 18O and 13C. A previously reported isotope stage 7/8 carbon-shift appears to be related mainly to a Pacific-wide reduction in the abundance of gephyrocapsids rather than to any significant change in the partitioning of carbon isotopes. The strong correlation between nannofossil delta18O and delta13C can also be interpreted as being influenced by increased productivity during glacial intervals and decreased productivity during interglacials in subantarctic waters.针对新西兰南岛东部外海深海钻探计划（Deep Sea Drilling Project, DSDP）594站位岩芯更新世中晚期（布容正向极性时，Brunhes）层位的钙质超微化石（calcareous nannofossils）氧、碳同位素值，本研究将其与同一样品中浮游有孔虫（planktic foraminifers）和底栖有孔虫（benthic foraminifers）的δ¹⁸O及δ¹³C信号进行了对比。结果显示，钙质超微化石的δ¹⁸O值显著受非平衡同位素分馏作用影响，这与此前体外实验研究的结论一致。钙质超微化石的δ¹⁸O与δ¹³C信号沿岩芯纵向呈现极强的互相关关系，且存在显著的正线性相关。该现象可通过动力学同位素效应（kinetic isotope effects）导致¹⁸O与¹³C均发生非平衡分馏来解释。此前报道的同位素阶段7/8碳同位素偏移现象，似乎主要与太平洋全域桥石球藻属（Gephyrocapsa）丰度降低相关，而非碳同位素分配比例发生显著变化。钙质超微化石δ¹⁸O与δ¹³C之间的强相关性，也可归因于亚南极水域冰川期生产力提升、间冰期生产力下降的影响。