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.