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    The Pliocene to Recent History of the Kuroshio and Tsushima Currents: a multi-proxy approach

    Gallagher S J, Kitamura A, Iryu Y, Itaki T, Koizumi I, Hoiles P

    Kuroshio Current, Tsushima Current, Pliocene, Pleistocene, Holocene, Paleoceanographic proxies, Biogeography, Microfossils, Macrofossils, Geochemistry

    Figure 1: The southern path of the Kuroshio Current showing the inferred extent of the Ryukyu Reefs in glacial and interglacial periods (adapted from Iryu et al. [2006])

    Figure 2: (i) The distribution of G. ruber and Tsushima Current molluscs in interglacials of the Omma Formation (adapted from Kitamura and Kimoto [2006]) correlated to the LR04 benthic stack of Lisiecki and Raymo ([2005]). The red dots (1.6 to 1 Ma) are interglacials where migratory Indo-Pacific Warm Pool benthic foraminifera are present (Hoiles et al. [2012]). Red dots at 0 and ~3 Ma are Warm Pool benthic foram ingressions described in Gallagher et al. ([2009]). Reconstructions of the paleogeography of the Tsushima Strait after 1.6 Ma (ii) and prior to 1.7 Ma (iii) adapted from Kitamura and Kimoto ([2006])

    The Kuroshio Current is a major western boundary current controlled by the North Pacific Gyre. It brings warm subtropical waters from the Indo-Pacific Warm Pool to Japan exerting a major control on Asian climate. The Tsushima Current is a Kuroshio offshoot transporting warm water into the Japan Sea. Various proxies are used to determine the paleohistory of these currents. Sedimentological proxies such as reefs, bedforms, sediment source and sorting reveal paleocurrent strength and latitude. Proxies such as coral and mollusc assemblages reveal past shelfal current activity. Microfossil assemblages and organic/inorganic geochemical analyses determine paleo- sea surface temperature and salinity histories. Transportation of tropical palynomorphs and migrations of Indo-Pacific species to Japanese waters also reveal paleocurrent activity. The stratigraphic distribution of these proxies suggests the Kuroshio Current reached its present latitude (35 °N) by ~3 Ma when temperatures were 1 to 2 °C lower than present. At this time a weak Tsushima Current broke through Tsushima Strait entering the Japan Sea. Similar oceanic conditions persisted until ~2 Ma when crustal stretching deepened the Tsushima Strait allowing inflow during every interglacial. The onset of stronger interglacial/glacial cycles ~1 Ma was associated with increased North Pacific Gyre and Kuroshio Current intensity. This triggered Ryukyu Reef expansion when reefs reached their present latitude (~31 °N), thereafter the reef front advanced (~31 °N) and retreated (~25 °N) with each cycle. Foraminiferal proxy data suggests eastward deflection of the Kuroshio Current from its present path at 24 °N into the Pacific Ocean due to East Taiwan Channel restriction during the Last Glacial Maximum. Subsequently Kuroshio flow resumed its present trajectory during the Holocene. Ocean modeling and geochemical proxies show that the Kuroshio Current path may have been similar during glacials and interglacials, however the glacial mode of this current remains controversial. Paleohistorical studies form important analogues for current behavior with future climate change, however, there are insufficient studies at present in the region that may be used for this purpose. Modeling of the response of the Kuroshio Current to future global warming reveals that current velocity may increase by up to 0.3 m/sec associated with a northward migration of the Kuroshio Extension.