Sedimentary Evolution In Northern Okinawa Trough And Their Environmental Response Since The Last 400 Ka

Based on the sediment grain-size,Sedimentary mineralogy including detrital minerals,monomineral and clay mineralogy,as well as sedimentary geochemistry including isotopic and major and trace element geochemistry,sediments from IODP Site U1428 and U1429 in the northern Okinawa Trough were used to study the history of sedimentary evolution and its control mechanism since the middle Pleistocene.According to the sedimentary proxy records,the eastern Asian marginal seas evolution during the middle Pleistocene has been reconstructed.Besides,proxy of the Kuroshio Current intensity evolution since the middle Pleistocene has also been established.Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu since the last glacial.Clay-sized sediments transported from the Yellow River to the study site were strongly influenced by sea-level fluctuation.During low sea-level stage,the paleo-Yellow River mouth was positioned closer to the northern Okinawa Trough,favoring large fluvial discharge or even direct input of detrital sediments to the study area.During high sea-level stage,retreated river mouth combined with Kuroshio Current blocking effect,reduced the sediment input in a large extent.The input of Kyushu-derived clay-sized sediments to the study site was mainly controlled by the Kuroshio Current with increased input in phase with weakened Kuroshio Current.Proxy of Kuroshio Current intensity evolution has been constructed based on the variation of Kyushu Island sediment input since the last glacial,which suggested good coherency between Kuroshio Current intensity and repid climate change events during the last glaciations on the millennial-scale.Further the role of Kuroshio Current in the global climate change has been discussed: During El Ni?o conditions,a weakened North Pacific subtropical gyre led to reduced heat transport by the western boundary currents,such as Kuroshio Current,from the Tropical Pacific to the northern high latitude,which resulted in increased Pacific sea surface temperatures.In turn this would have led to a larger latitudinal temperature gradient.As a result,a stronger Hadley cell transported more moisture to high latitudes and influenced the North Atlantic via the westerlies.Ultimately this drove ice melting,which turned off the North Atlantic deep water formation,and led to hemispheric(and possibly global)climate change.Silicate weathering and erosion history of sediment provenance have been reconstructed.On glacial-interglacial scale,silicate weathering proxies indicate that the core sediment was more weathered during the last glacial and deglacial than that in the modern Yellow River.We attribute this to the increased upper reach sediment input,which supplied more weak weathered sediment to the study site during the Holocene.Besides,composite effect of weathering during glacial and deglacial sediment production and additional weathering upon subaerial exposure of shelf deposits during low sea-level stage,as well as older weathered sediments reworking during sea-level rise could also account for this weathering regime.On multi-millennial scale,during sea-level relative stable stage,silicate weathering was mainly controlled by the East Asian summer monsoon,with a cooling and drying climate associated with weakened summer monsoon activity.During rapid sea-level rise stage,strong reworking of older weathered sediments overwhelmed the East Asian summer monsoon in controlling silicate weathering,which induced a continuous increase in chemical alteration of the sediments during this period.Provenance analysis indicates that the sediments were mainly derived from the Yellow River and the island of Kyushu since the last 424 ka.The Kyushu clay-sized sediment input was controlled by the Kuroshio Current with increased input in phase with weakened Kuroshio Current.Proxy of Kuroshio Current intensity evolution has been constructed based on the variation of Kyushu Island sediment input since the last 424 ka.On the long-term scale,Kuroshio Current exhibits the cycles of 62 ka and 21 ka,which are the same with the cycles of ENSO activity and East Asian summer monsoon.The proxy indicates the Kuroshio Current intensity was cohereancy with the East Asian summer monsoon during 424-370 ka?260-160 ka?60-0 ka,which suggests a strengthened Kuroshio Current pacing with the enhanced East Asian summer monsoon.During 370-260 ka and 160-60 ka,the Kuroshio Current intensity was cohereancy with the ENSO activity.During El Ni?o-like period,there was a weakened Kuroshio Current,and the reversed situation occurs during La Ni?a-like period.On the long time scales,Yellow River clay-sized sediment input to the study site mainly controlled by the East Asian winter monsoon.During 424-100 ka,strengthened winter monsoon increased the capacity of Yellow River sediments breaking through the Kuroshio Current water barrier,resulted an increased Yellow River sediment input.After 100 ka,weakened winter monsoon reduced the materials input from the Yellow River.On glacial-interglacial scale,Yellow River clay-sized sediment input was regulated by sea-level change,with increased/reduced sediment input during low/high sea-level.Provenance analysis indicates that sediments from both sand and imbedded fine-grained calcareous-rich clay layers at the IODP Site U1428 were mainly derived from the Yellow River,which firstly indicates the Yellow River sediments could influence the northern Okinawa Trough since at least the middle Pleistocene.Significant transformation of lithology at IODP Site U1428 at ~424 ka was attributed to the large-scale subsidence of the Zhe-Min Uplift.Before ~424 ka,the Zhe-Min Uplift was emersed in the East China Sea.During the low sea-level stage,the paleo-Yellow River influenced low relief region of the eastern side of the Zhe-Min Uplift near the Cheju strait,and favored large fluvial discharge of coarse-grained sediments,which eroded from the river floodplain and lacustrine deposits in the northern East China Sea and Yellow Sea shelves input to the northern Okinawa Trough.Strong sediment reworking in the study area induced by the narrow transgressive channel during sea-level rise could also supply coarse-grained sediments to the study area.After the large-scale subsidence of the Zhe-Min Uplift at ~424 ka,large-scale transgression in northern East China Sea and Yellow Sea along with the distant Yellow River mouth during high sea-level stages resulted fine-grained sediments transported to the northern Okinawa Trough.During low sea-level stages of the last 424 ka,sediments eroded from the marine facies deposit on the exposed East China Sea and Yellow Sea shelves could still provide fine-grained particles to the study site.