Evolution of Pacific circulation in the Miocene: Radiolarian evidence from DSDP Site 289
Karen Romine, Gail Lombari, 1985. "Evolution of Pacific circulation in the Miocene: Radiolarian evidence from DSDP Site 289", The Miocene Ocean: Paleoceanography and Biogeography, James P. Kennett
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A time-series of abundance data on Radiolaria in the Miocene at Site 289, western equatorial Pacific, shows variations that appear to be related to the evolution of oceanic circulation patterns and climate. In the early Miocene, unusually large abundances (up to 70 percent of the population) of two highly silicified radiolarian species, Stichocorys wolffii and Calocycletta robusta group, occurred during a period of relatively high silica accumulation rates, with highest abundance at the maximum in accumulation rate at about 17 Ma. This species dominance is difficult to explain, but may have been due to the development of a specific ecological niche that these species were able to dominate. Their rapid proliferation and decline suggests that the niche was a temporary development that probably occurred as certain threshold conditions were satisfied during progressive changes in circulation patterns in the equatorial Pacific. As both climate and circulation continued to evolve, the niche changed character or perhaps disappeared, resulting in the rapid decline and extinction of S. wolffii and the C. robusta group.
The extinction of Stichocorys wolffii and the evolution of Stichocorys peregrina in Site 289 occurred in conjunction with the rapid development of an assemblage whose species characterize the western portion of the transitional water mass of the late Miocene North Pacific. The timing of these faunal developments is contemporaneous with a large increase in accumulation of continental ice on East Antarctica during the middle Miocene (15 to 13 Ma). A major increase in the transitional assemblage, a decrease in abundances of Stichocorys spp., a decrease in silica accumulation rates in both the central and western equatorial Pacific, and an increase in mass accumulation rates of eolian dust in the central North Pacific are coincident with the closure of the Indo-Pacific passage to significant westward flow from the Pacific (about 12 to 10 Ma). The faunal changes indicate the development or intensification of a North Pacific transitional water mass at this time, a result of the intensification of the subtropical gyre due to the diversion of westward equatorial flow towards the poles and probably influenced by the intensification of the westerly winds.
An assemblage of species resistant to dissolution indicates three major and one minor interval of increased silica dissolution in the middle and late Miocene, 15–13 Ma, 12–11 Ma, 9–8 Ma (minor), and 7–6 Ma. The major intervals correlate with significant paleoceanographic events: a significant increase in δ18O interpreted as an increase in accumulation of ice on East Antarctica (15–13 Ma); closure of the Indo-Pacific passage (12–11 Ma); and the Messinian and/or δ13C shift (7–6 Ma). Two of these events occurred during a time of increased hiatus abundance in the western Pacific (11 Ma) and during a time of low silica accumulation (11 Ma and 7–6 Ma).