Centennial to Millennial Scale Variations of Amur River Discharge, Sea-ice Expansion, and Biogenic Carbonate in Marine Sediment of the Central Okhotsk Sea

The Okhotsk Sea is a seasonal sea-ice area that is the lowest latitude in the northern hemisphere. It is very sensitive for climate change such as global warming. Marine bottom sediment is sensitively recording environmental changes of climate and ocean changes in the past around the Okhotsk Sea. In order to understand centennial to millennial scale climate and ocean changes in the Okhotsk Sea, a series of marine sediment cores in the central Okhotsk Sea are obtained by several cruises (MR0604 by RV Mirai, YK07-12 by RV Yokosuka, XP07 by Prof. Khlomov). Age model of the cores was constructed by oxygen isotope stratigraphy, magnetostratigraphy, tephrochronology, and optically stimulated luminescence (OSL) dating since oxygen isotope stage 13. Correlation among the cores by magnetic susceptibility was also very useful for the stratigraphy in the Okhotsk Sea. Variation of sea-ice expansion was reconstructed by ice-rafted debris (IRD) that was obtained by percentage of >63 micrometer fraction, and volume magnetic susceptibility. Sea-ice has expanded during glacial, especially during stadial (cold) events in marine isotope stage 2 (Last glacial maximum) and 4. Sea-ice in the most part of the Okhotsk was seasonal during late Quaternary, but it was expected as perennial sea-ice condition only in the northern part of the sea. Enhanced arctic atmospheric circulation resulting strong wind field over the Okhotsk Sea would be the most important factor for sea-ice expansion in the Okhotsk Sea. Clay mineral assemblages of terrigenous particles show clear difference between warm (interglacial and interstadial events in glacial) and cold (glacial and stadial events in glacial). Illite relative content in fine silt fraction (2 to 32 micron meter in diameter) by X-ray diffraction analysis enriched during cold periods. The variation of illite also corresponded to K content of non- destructive measurement of XRF core scanner (TATSCAN-F2). The variation of illite and K content suggest increase of aeolian dust that is consistent of forcing of enhanced atmospheric circulation during cold periods. On the other hand, smectite relative content increased during warm period, especially interglacial representing the Amur River discharge. Biogenic carbonate enriched during interglacial and interstadials with centennial to millennial scale variations. Abrupt increasing biogenic productivity and/or preservation of carbonate would be related to changes of river runoff and thermohaline circulation. The environment around the Okhotsk Sea is controlled by sub-polar and polar atmospheric circulation, by fresh water input of the Amur river, and by intermediate and deep water formation that relates global and local thermohaline ocean circulation.