Stable carbon isotope ratios of planktonic foraminifera from ODP Hole 130-806B on the Ontong Java Plateau (Appendix)

We present delta13C records spanning the last 2 m.y. of two planktonic foraminiferal taxa (Globigerinoides sacculifer and Pulleniatina) from Ocean Drilling Program Site 806 on the Ontong Java Plateau, western equatorial Pacific. The sampling interval is between 4 and 5 k.y. The two taxa are thought to reflect conditions within the mixed layer and within the bottom of the mixed layer (or in the uppermost thermocline), respectively. The records were analyzed for trends and cycles on time scales greater than 100 k.y. and for periodicity in the orbital portions of the Fourier spectrum. The G. sacculifer record shows a long-term decrease in delta13C, superimposed on long-period fluctuations. The obliquity-related signal is distinct throughout the Quaternary. The Pulleniatina record shows subdued amplitudes, compared with G. sacculifer.
There is evidence, from comparison of the differences in delta13C with those of delta18O, that a shallowing of the thermocline in the late Quaternary (increase in delta delta18O) was not accompanied by increased contrast in delta13C between the two species. One possible explanation is that the oxygen minimum was weakened at the same time as the thermocline shallowed and the mixed layer thickness decreased. In addition, changes in symbiont activity in G. sacculifer may be involved.