The Research Of Paleoenvironment Evolution In The Okinawa Trough During The25ka BP

We selected core MD05-2908, which collected from southern Okinawa Trough andcore PC-1, which collected from northern Okinawa Trough, as the research materials.By anglicizing the AMS¹⁴C aging technology, fauna of planktonic foraminifera,oxygen and carbon isotopic, we set up the age model of the two cores, research thelocal environmental events, descript the characters of high-frequency-climate eventsin the Okinawa Trough, and reconstruct the paleoenvironment of Okinawa Troughduring the25ka BP.The results showed that there were five high-sediment rate events in the mid-lateHolocene; these events maybe be connected with the increase of input of landresource materials, also relative to the bloods in the Holocene. During the period of6790a BP to4000a BP, warm and wet climate environment,corresponded to theHolocene Climate Optimum, was benefit for the breaking out of blood. The period of4000a BP to1100a BP, sediment rate is relatively steady and the frequency of bloodwas rather lower. After1100a BP, with the enhancing of the human activity influenceon the natural environment, the frequency of blood was increasing largely, at the sametime largely affect the prime paleoproductivity in the southern Okinawa Trough.Q-mode analysis of planktonic foraminifera indicated that there were two primaryfactors to influence the marine environment in the southern Okinawa Trough:Kuroshio Currents and Flesh Water. During the period of6790a BP to4000a BP,Kuroshio Currents has the extend to enhance steadily; the period of4000a BP to3000a BP, the strength of Kuroshio Currents weakened or excursion eastward, theinfluence of which on FW enhanced; after3000a BP, the strength of KuroshioCurrents enhanced steadily, the period after1650a BP was the strongest period sincethe mid-late Holocene, and in the time of600a BP and140a BP, Kuroshio Currentsweakened provisionally. The time when Pullenitina Minimum Event (PME) occurred in the southern Okinawa Trough was exactly ensured from3900a BP to2800a BP,which lasted for1100years maybe connect to the enhance of East Asian WinterMonsoon(EAWM).Multiply indexes results showed that the leading factors to control the evolution ofmarine environment of northeastern Taiwan were two aspects——ocean currents(such as Fresh Water and Kuroshio Currents) and East Asian Monsoon system. Duringthe period of7000a BP to3800a BP, the change of marine environment was primarilycontrolled by fresh water and Kuroshio Currents. The EAM began to enhance since6000a BP, to the period of3800a BP to3500a BP, the steadily enhancing EAMreplaced FW and Kuroshio Currents and become the primary factor to control themarine environment. The period thereafter3500aBP to1100a BP, EAM was still theprimary factor to control the change of marine environment; during the period ofwhich in3500a BP to1650a BP, EAM weakened step by step, then began to enhancefrom1650a BP to1100a BP. During the period of1100a BP to350a BP, followingwith the enhancing of the Kuroshio Currents and the Fresh Water in turn, oceancurrents, main contents of which were Kuroshio Currents and Fresh Water acted as theprimary role to control the marine environment. After350a BP, with the enhancing ofEAM, it becomes the primary controlling factor of marine environment again.High-frequency climate events have been responded in the northern Okinawa Troughduring the last glacier, such as H2, H1and Younger Drays (YD) have imprinted incore PC-1, at the same time, these events have influenced on the oxygen and carbonisotopic, fauna of planktonic foraminifera and sea surface temperature, whichindicated that the high-frequency climate events were globally.There was a steadily cold eddy——upwelling in the northern Okinawa Trough all theyear around. About24.2ka B.P., the activity of upwelling in the northern OkinawaTrough was strong; after which to14.5ka B.P., with the activity of upwellingweakened gradually, changes in upwelling strength was primarily driven by the EastChina Sea shelf water. After14.5ka B.P., with global warming and sea level rising,the influence of Kuroshio Warm Currents on the Okinawa Trough increased gradually,and upwelling activity was primarily controlled by changes in the Kuroshio Currents.The rapid increase Kuroshio upwelling activities more easily lead to the development.We synthetically analyzed thirteen sedimentation cores selected from the OkinawaTrough. The results indicated that on the background of worldwide cooling during theLGM, the Northern Equator Currents (NEC) weakened, which induced the weakness and slowness of the Kuroshio, and the mainstream of the Kuroshio bended greatlywhen it moved northward. The existence of the Ryukyu-Taiwan land bridge resultedin the excursion eastward to the Pacific Ocean of the mainstream axis of the Kuroshioand bypassed outside of the Ryukyu Arc, then the mainstream axis of the Kuroshioentered the Okinawa Trough from middle of the Ryukyu Island Arc, and made stronginfluence on the middle and mid-north of the Okinawa Trough. When it bypassednorthward, the Kuroshio divided into northwest embranchment and northeastembranchment. Finally, the embranchments of the Kuroshio reconvened in thesoutheast of Japan and entered the northwest Pacific Ocean.