Paleoenvironment Evolution In The Okinawa Trough During The Late Pleistocene And Holocene

Cores E017, DOC082 and DOC024, retrieved from the Okinawa Trough in the East ChinaSea, respectively, are used to reconstruct productivity, climate and marine environmentalconditions changes in the middle-southern and the northern part of the Okinawa Trough basedon a multi-proxy approach including geochemical and micropaleontological proxies,combined with detailed AMS 14C dates, stable isotope and other data acquired by previousstudies, during the late Pleistocene (~18-0ka BP) and Holocene. In addition, the mechanismsof productivity, climate and marine environmental conditions fluctuations are discussed tostudy their significations. In core E017 from the middle-southern part of the Okinawa Trough, the geochemicalpaleoproductivity proxies such as organic carbon, calcium carbonate, opal and biogenicbarium, also the accumulation rates of planktonic and benthic foraminifera, radiolaria andsilicoflagellate show the productivity decreased gradually from a late Pleistocene (~18ka BP)maximum to a late Holocene minimum, so that four stages including 18-16ka BP, 16-9ka BP,9-3ka BP and 3-0ka BP, can be recognized. In general, the paleoproductivity was higherduring the glacial and deglacial than the postglacial and Holocene. Also, the variation patternof paleoproductivity is evidenced by changes of planktonic foraminiferal assemblages andbenthic foraminiferal species that are correlated with the productivity. About the mechanismof paleoproductivity evolution, the planktonic foraminiferal δ13C value, organic matter C/Nratio and organic carbon δ13C (Wahyudi, 1997) suggest that it was controlled by the terrestrialorganic matter input induced by the regional and global climate changes and the fluctuationsof sea level. In the middle-northern part of the Okinawa Trough, the geochemical andmicropaleontological paleoproductivity proxies from core DOC082 indicate that theproductivity was low during the early and middle Holocene (11-3ka BP); In contrast, duringthe late Holocene (3-0ka BP) it increased abruptly to a strikingly high value. This may reflectthe occurrence of stronger and more frequent El Ni?o events. The productivity was highermarkedly about 7ka BP of the Holocene than the period of 6-0ka BP in the northern part ofthe Okinawa Trough, suggested by the data from core DOC024; but the peak fluxes ofsilicious microfossils (radiolaria and silicoflagellate) occurred at 3.0ka BP, it showedincreased trend before 3.0ka BP and declined from 3.0 to 0ka BP. The variation pattern ofproductivity and the differences in different paleoproductivity proxies may reflect thevariability of the Kuroshio Current, the cold eddy and upwelling associated with this warmcurrent during the Holocene. iii常凤鸣 冲绳海槽晚更新世——全新世的古环境演化 Mean annual sea surface temperature fluctuations in cores DOC082 and DOC024 fromthe middle and northern part of the Okinawa Trough have been reconstructed through the last11ka using the transfer functions of microfossils. The results show, during the Holocene, threeperiods of the regional climate changes can be identified. During the early Holocene(11-8.4ka BP) it shifted gradually from cold to warm; the middle Holocene Climatic Optimum,between 8.4ka BP to 3.1ka BP, was warm; and the optimum climate was replaced by a cooland fluctuant one after 3.1ka BP of the late Holocene. Also, six cold events (at~0.2-0.6, 1.2,3.0, 4.6, 5.5 and 8.1ka BP) occurred through the last 11ky. The Holocene climate changes andthe cold events in the Okinawa Trough appear to correlate well with similar climatic changesrecorded in different regions of the word, suggesting comparability of Holocene globalclimatic change trend and ubiquity of Holocene climatic instability. The ratio of mixed-layer/thermocline-dwelling planktonic foraminiferal species and theresult estimated using transfer function suggest that the depth of thermocline grad...