THE INFLUENCE OF NONEQUILIBRIUM ISOTOPE FRACTIONATION ON THE QUATERNARY CALCAREOUS NANNOFOSSIL STABLE-ISOTOPE SIGNAL IN THE SOUTHWEST PACIFIC-OCEAN, DSDP SITE-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 deltaO-18 and deltaC-13 signal of planktic and benthic foraminifers from the same samples. Nannofossil deltaO-18 values appear strongly affected by non-equilibrium fractionation, in agreement with previous in-vitro studies. The nannofossil deltaO-18 and deltaC-13 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 O-18 and C-13. 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 deltaO-18 and deltaC-13 can also be interpreted as being influenced by increased productivity during glacial intervals and decreased productivity during interglacials in subantarctic waters.