LATE NEOGENE HISTORY OF THE KUROSHIO EXTENSION, ODP SITE 1208, SHATSKY RISE

The Kuroshio Current (KC) is one of the largest western boundary current systems in the world as part of the North Pacific Gyre, transporting 35 Sv of tropical waters northward. The KC is influential on atmospheric processes, and how it affected climate during key intervals in Earth history are crucial for understanding future climate change. This study is a preliminary assessment of the late Neogene paleoceanographic history of the Kuroshio Extension (KE) based on data from Ocean Drilling Program Site 1208 drilled on Shatsky Rise located under the present-day KE. Abundances of biogenic elements were calculated from splits of the >63 μm, along with the Planktic/Benthic Ratio (P:B; % planktics to total forams) and Planktic Fragmentation Index. Previously published alkenone SST estimates, along with species associations, were compared to the new data sets. Three major cooling steps are identified at 6.5 Ma, 2.3 Ma, and 1.1 Ma. Prior to 6.5 Ma in the middle to late Miocene, radiolarians dominated the assemblages, species richness was very low, P:B ratios were low, and planktic fragmentation was high due to intervals of intense dissolution. SSTs were relatively warm (>22°C) and foraminiferal assemblages were dominated by warm-water taxa. SSTs dropped ~2˚C after 6.5 Ma, and are associated with a decrease in warm water taxa and an increase in transitional and cool water taxa, decreased fragmentation and highly variable but markedly increased P:B. A major biotic change at 4.5 Ma shows increased P:B, decreased radiolarians, decreased fragmentation, and a marked increase in species richness. There were no significant changes in SST at this time. The cooling steps at 2.3 Ma and 1.1 Ma are associated with greatly decreased fragmentation, decreased cool-water species, and high P:B. The major change at 4.5 Ma may be a product of the intensification of the KE in the North Pacific due to tropical Pacific gateway closure. Alternatively, as Shatsky Rise moved northwest Site 1208 may have moved from warm, oligotrophic waters into cooler, more seasonally variable waters of the KE. The cooling steps are likely linked to increased productivity along the KE. Future analyses, including stable carbon and oxygen isotope analyses, will resolve water mass changes, ocean circulation patterns, and productivity gradients at Site 1208.