Evolution of the Miocene ocean in the eastern North Pacific as inferred from oxygen and carbon isotopic ratios offoraminifera

Oxygen and carbon isotopic ratios of planktonic and benthonic foraminifera haveprovided information about the evolution of the oceans at low- and mid-latitude sites inthe Miocene eastern North Pacific Ocean.DSDP Site 495 (12° N; 91° W) provides a record of early and middle Mioceneoceanographic conditions in the eastern equatorial Pacific. Oxygen isotopic evidence indicatesthat G. sacculifer, D. altispira and G. siakensis were shallow-dwelling, tropicalplanktonic species. G. venezuelana deposited its test at greater depths, probably belowthe thermocline. Carbon isotopic evidence conflicts with that of the oxygen isotopes inthat it suggests that G. siakensis calcified under conditions similar to those ofG. venezuelana.Temperature variability at Site 495 during early and middle Miocene time wasrelatively small. However, while middle Miocene deep waters at this site cooled, simultaneouslywith a major phase of growth of the Antarctic ice sheet, surface and nearsurfacewaters 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 relativelystable, but were probably lower than modern surface temperatures.At Site 173 (40° N; 125° W) middle and late Miocene surface temperatures wereconsistently lower than those at the more southerly Site 470, and were also significantlymore variable. There is no indication that surface temperatures have changed significantlyat Site 173 since late Miocene time. The inferred greater variability of surfacetemperatures at Site 173 may reflect greater variability of the intensity of upwelling atthat site than at Site 470 during Miocene time.At Site 495 both the planktonic and benthonic foraminiferal carbon isotopic recordsvary sympathetically with published benthonic foraminiferal isotopic records fromthe Atlantic and Pacific Oceans, indicating that the carbon isotopic ratios at that sitelargely reflect global fluctuations in the isotopic composition of dissolved inorganiccarbon.At Site 470 the planktonic carbon isotopic record fluctuates sympathetically withpublished benthonic records, indicating that the middle and late Miocene 13C/12C ratiosof dissolved inorganic carbon in surface waters at this site reflected global fluctuations in¹³C/¹²C. The planktonic carbon isotopic record at Site 173 could not be correlated withglobal carbon isotopic fluctuations, indicating that, in part, local effects controlled the¹³C/¹²C ratio of dissolved inorganic carbon in surface waters at that site.