Sedimentary Characteristics Of Submarine Canyon System And Their Environmental Response Since The Late Pleistocene

This study focuses on the paleoenvironmental evolution and characteristic ofsubmarine canyon system in the west slope of the Okinawa Trough since50ka, aimingto resolve various scientific problems with regard to current and monsoon system; Thebrode current system of East China Sea and the unique geographic and geomorphicfeatures of submarine canyon systems in the west slope of the Okinawa Trough providehigh resolution, representive records for paleoenviromental reconstruction.We chose four sediment cores A27,Oki02,A7,Oki01from axis of south submarinecanyon, deep-sea fan and basin respectively. We have first time unravelled independentKuroshio Current (F3and EM1) and East Asian winter monsoon (GSI and EM2)proxies by using V-PCA and EMMA on grain size matrix of detrital components ofsediments in the Okinawa Trough, so as to discuss the transport mechanisms. Combinedwith clay mineral and geochemistry analysis, we have further differentiated theprovenance of research area. Based on clear discurssions on provenance and transportmechanisms, we have described the terrigenous sediment influx using XRF scannnerelemental ratios log(Ti/Ca) and log(Fe/Ca), and thus reconstruct the history of KuroshioCurrent and monsoon system. Further comparison with other monsoon and rainfallproxies located on the zone of ITCZ and ENSO proxies have been conducted for betterelucidate the potential forcing and thus resolve various scientific problems.Our results indicates that core A27in the south Okinawa Trough has experiencedaccumulation-erosion-accumulation processes in response to the evolution of regionaltectonic activity, current system. From50.0-48.0ka, the sediments in submarine canyonare mainly composed of hemipelagic silty mud with accumulation rate of65cm/ka,when the Kuroshio Current was outside the Okinawa Trough and clay mineral derviedmainly from Yellow and Yangtze River. The intensified tectonic activity and thesubsequent volcano and earthquake activity not only cause the erosion and sedimentmissing in submarine canyon but also open the bridge bwtween Taiwan and Ryukyuislands gradually. From35.0-34.0ka, tectonic activity has weakened gradually and thetriggered-turbidity sediment has began accumulating on the submarine canyon system, which thus has enhanced the sedimentation rate of core A27also；From34.0-12.0ka，the bridge between Taiwan and Ryukyu islands has collapsed completely and theKuroshio Current has entered the Okinawa Trough, which not only has broughtsediments of Taiwan Island to core A27, but also has resisted much sediments fromYangtze and Yellow River. The clay minerals of core A27have shifted from Yellow andYangtze River to Taiwan island and East China Sea continental shelf, when theaccumulation rate has descended correspondingly. Since the Holocene, the KuroshioCurrent has intensified greatly in response to precession-induced enhancement of EastAsian summer monsoon and decrease of “ENSO power”. The intensified KuroshioCurrent and the subsequent formation of Kuroshio-induced “water barrier” not onlymasks the fluvial signal driven by summer monsoon but also blocks the clay mineralssourced from Yellow River, one of the major contributors prior to12ka BP. This isevident by the shift of clay mineral provenance from Yellow and Yangtze River tomixture of East China Sea continental shelf and Yangtze River and the decrease oflog(Ti/Ca) and log(Fe/Ca). On the contrary, the strong East Asian winter monsoon andweak Kuroshio “water barrier” may favor the export of abundant terrigenous materialfrom East China to the Okinawa Trough via monsoon forcing Ekman transport andstrong mixing. Meanwhile, the relatively steady sea level and Kuroshio Current in theHolocene minimizes the sea level effect on the circulation patter, when the “storage insummer and export in winter” model has been constructed. The newly proposed indexGSI, EM2and log(Ti/Ca), which well represents winter monsoon, are consistent withother winter monsoon proxies. The consistency further validate the credibility of ourmonsoon proxies. The newly proposed index GSI, EM2and log(Ti/Ca) exhibits anegative correlation with the18O record in Dongge Cave in China. Such anti-correlation suggests the southward migration of the Intertropical Convergence Zone(ITCZ) in Holocene. The consistency among our records and rainfall records in Peru,monsoon records in Oman and averaged summer insolation pattern at30°Nfurthersupport the global-wide impact of ITCZ on monsoon systems.For quantitative description of the relationship of monsoon system and unravelingthe internal ENSO forcing and external insolation forcing from monsoon proxy, we also have conducted cross-correlation analysis, wavelet analysis and global spectra analysisto access the East Asain monsoon proxies. Cross-Correlation Analyses for GSI andlog(Ti/Ca) against18O record in Dongge Cave further unravel a decoupling betweenthe East Asian winter and summer monsoon during5500-2500cal yr BP, and a greatercomplexity in recent2500yr. Such decoupling is attributable to exacerbated ENSOmode alternation activity and even human interference that superimposed on insolationand ITCZ forcing. Global spectra and wavelet analysis indicate that the East Asianwinter and summer monsoon show similar1500,500and260year cycle and the cyclevary with the change of time.The newly proposed grain size index F3and EM2, indicators of Kuroshio Current,show consitent variation with other Kuroshio Current proxies includingP.obliquiloculata in Core A7and MD0124-04, sortable silt(10-63μm) of CoreODP1202and δT of two cores near the coastal area of Japan Sea. The comparisonbetween Kuroshio proxies and ENSO, monsoon indicators, coupling with modernobservation results, indicate that the enhancement of Kuroshio Current since theHolocene is a response of precession forcing decrease of “ENSO power” andintensification of East Asian summer monsoon. The centennial scale variation since5.0ka is ascribed to higher ENSO variation superimposed on orbital scale variation.Global spectra analysis, wavelet analysis on Kuroshio Current proxies indicate that theKuroshio Current shows2600,1500and260year cycle, which further illustrate thecomplex and norlinear relationship between middle latitude Kuroshio Current and lowlatitude ENSO and monsoon.