Oxygen and carbon isotopic ratios of planktonic and benthonic foraminifera have provided information about the evolution of the oceans at low- and mid-latitude sites in the Miocene eastern North Pacific Ocean.

DSDP Site 495 (12° N; 91° W) provides a record of early and middle Miocene oceanographic conditions in the eastern equatorial Pacific. Oxygen isotopic evidence indicates that G. sacculifer, D. altispira and G. siakensis were shallow-dwelling, tropical planktonic species. G. venezuelana deposited its test at greater depths, probably below the thermocline. Carbon isotopic evidence conflicts with that of the oxygen isotopes in that it suggests that G. siakensis calcified under conditions similar to those of G. venezuelana.

Temperature variability at Site 495 during early and middle Miocene time was relatively small. However, while middle Miocene deep waters at this site cooled, simultaneously with a major phase of growth of the Antarctic ice sheet, surface and near-surface waters warmed.

The oxygen isotopic record at Site 470 in the eastern North Pacific (29° N; 117° W) indicates that middle and late Miocene surface temperatures at this site were relatively stable, but were probably lower than modern surface temperatures.

At Site 173 (40° N; 125° W) middle and late Miocene surface temperatures were consistently lower than those at the more southerly Site 470, and were also significantly more variable. There is no indication that surface temperatures have changed significantly at Site 173 since late Miocene time. The inferred greater variability of surface temperatures at Site 173 may reflect greater variability of the intensity of upwelling at that site than at Site 470 during Miocene time.

At Site 495 both the planktonic and benthonic foraminiferal carbon isotopic records vary sympathetically with published benthonic foraminiferal isotopic records from the Atlantic and Pacific Oceans, indicating that the carbon isotopic ratios at that site largely reflect global fluctuations in the isotopic composition of dissolved inorganic carbon.

At Site 470 the planktonic carbon isotopic record fluctuates sympathetically with published benthonic records, indicating that the middle and late Miocene ¹³C/¹²C ratios of dissolved inorganic carbon in surface waters at this site reflected global fluctuations in ¹³C/¹²C. The planktonic carbon isotopic record at Site 173 could not be correlated with global carbon isotopic fluctuations, indicating that, in part, local effects controlled the ¹³C/¹²C ratio of dissolved inorganic carbon in surface waters at that site.   PDF