Paleoenvironment Evolution In The Margin Region Of Tropical Western Pacific During The Late Quaternary

Cores Ph05-5, WP3, MD2908, Z14-6 and WP7, retrieved from the tropical Western Pacific margin region, respectively, are used to reconstruct CaCO3 cycle, oxygen and carbon stable isotopic character of calcareous nannofossils, paleoproductivity and upper water structure changes of the tropical Western Pacific margin region during the late Quaternary (~250-0 kyr BP)based on a multi-proxy approach including micropaleontological and geochemical proxies, combined with detailed AMS14C dates, stable isotope and other data acquired by previous studies.Two deep sea cores Ph05-5 and WP3 retrieved from the western Philippine Sea and Southeast of Taiwan respectively were selected to carry out the CaCO3 and calcareous nannofossil faunas study during the last 190 kyr BP. The carbonate contents showed higher values in the glacial periods and lower during the interglacial and Holocene periods, which characteristics was similar to the tendency of"Pacific Type"carbonate cycle. However, there were high carbonate contents in the warm period and low values during the cold interval, which displayed the same tendency with the"Atlantic Type"carbonate cycle during the last glacial period (MIS4-2) in the east of Phillipines. The variations of primary productivity and carbonate dissolution index indicated that the carbonate dissolution was a major factor controlling the carbonate content in the east of Philippines, and the variations in carbonate contents were mainly affected by the productivity of calcareous organism in the Southeast of Taiwan. The"Atlantic Type"carbonate cycle in the east of Phillipines during the last glacial period (MIS4-2) was an effect of the process of dissolution combined with the change of primary productivity.Theδ18O andδ13C values of calcareous nannofossils have been analyzed for the core Ph05-5. Theδ18O values of calcareous nannofossils were obviously lower in Holocene and the last interglaciation than those in the last and the penultimate glaciation. The nannofossilδ18O values during the last 190 kyr BP were positively correlated with those of planktonic and benthic foraminifera from the same core. But the averageδ18O value of nannofossils was higher than that of planktonic foraminifera Globigeriniodes ruber and was lower than that planktonic foraminifera Neogloboquadrina dutertrei, all which were much lower than that of benthic foraminifera Cibicides wullerstorfi. The similar trends forδ13C records and absolute abundance of calcareous nannofossils in core Ph05-5 corporately reflected the primary productivity variations in the western Philippine Sea. The primary productivity kept a stable but relatively low station at MIS 6 and MIS 5d. There was an abrupt rise at the beginning of MIS 5d for the primary productivity which retains high until the end of MIS 3. The prinary productivity showed a slight decline during MIS 2 and MIS 1, but still higher than that of MIS 6 and MIS 5.The deep sea core Ph05-5 was selected to carry out calcareous nannofossil and foraminifera study. Based on nannofossil and foraminifera indexes, the variations of upper water structure and paleoproductivity were reconstructed and its control factors and paleoceangraphy implications were discussed in this region during the last 190 kyr BP. The down core changes of Florisphaera profunda percentage content and DOT (depth of thermocline) estimated using planktonic foraminifera transfer function inferred that the nutricline and thermocline were shallower in glacial periods (MIS 6 and 5d~2) and deeper in interglacial periods (MIS 5e and Holocene), and the nutricline and thermocline shown the deepest depth in the MIS 5e during the last two glacial cycles. On a glacial-interglacial time scale, the primary productivity variations calculated from F. profunda conversion equation showed that the primary productivity was higher in glacial periods and lower in interglacial periods. Similarly, the lowest primary productivity appeared at MIS5e, which could be confirmed by the variations of absolute abundance of calcareous nannofossils. Theδ13C difference between benthic foraminifera Cibicides wullerstorfi and calcareous nannofissils (Δδ13CC. wullerstorf-coccolith), and theδ13C difference between benthic foraminifera C. wullerstorfi and planktonic foraminifera Neogloboquadrina dutertrei (Δδ13CC. wullerstorf-N. dutertrei) both clearly showed that the lower export productivity in interglacial periods and the higher in glacial periods. Also, the export productivity is remarkable low during MIS 5e. The paleoproductivity and upper water structure characters during MIS 5e in core Ph05-5 were consistent with modern La Niňa activity. So, during MIS 5e La Niňa might happen with a high frequency in the northern margin region of WPWP. Furthermore, we presume that the main current of Kuroshio should be intensified during MIS5e based on the trade wind fluctuation theory (Wyrtki, 1975).The productivity indexes based on calcareous nannofossils transfer function and benthic foraminifer's accumulation rate calculation function showed that the productivity was higher in glacial periods than interglacial periods in the WPWP during MIS 7 which was confirmed by the abundance and accumulation rate of benthic foraminifera, and the productivity was relatively stable during interglacial periods (MIS7, 5, 3 and the Holocene) than during glacial periods (MIS 6, 4 and 2). The decreases of productivity from glacial to interglacial were relatively slower than the increases from interglacial to glacial. Cross-spectral analyses of the two productivity proxies (U+B and N. dutertreiδ13C) showed obvious 100kyr, 41kyr and 23 (19) kyr periods, which indicated that the ice volume changes and the precession drive of the low latitude were both the controlling factors of productivity of WPWP. The paleoproductivity variations reflected by U+B and N. dutertreiδ13C were highly negatively correlated with CO2 concentration recorded in Vostok ice core on 30 kyr period bond during the last 250 kyr BP, which indicated that the primary productivity was the important"absorber"in global carbon cycle, and the 30 kyr period was the exhibition of the ENSO-like oscillation of marine productivity in the Equator Pacific. And, the productivity indexes of U+B and N. dutertreiδ13C led the ice volume variations about 2.6-7kyr (0.5kyr,2.6 kyr,4.2 kyr, 7kyr) on precession bond, which suggested that the sun irradiation was the directed deriver of productivity changes in low latitude region on millennial time scales. Cross-spectral analyses between productivity proxies (U+B andδ13C) and planktonic foraminiferaδ18O indicated highly significant coherence (all>90%) at 51ka period, and we had a hypothesis that the 51ka period was synthesized by 41 and 23ka period (1/23-1/41=1/52), which should attribute to a shift of energy from the 23 period to 51ka modulated by the 41ka period.Based on cross contrasting analysis of relative abundance of F. profunda in core WP7, Ph05-5, WP3 and Z14-6, we had made such conclusion: 1 the relative abundance of F. profunda was controlled by variations of upper water structure; 2 there was similarity between the relative abundance of F. profunda variation and foraminifer'sδ18O curve, and higher abundance corresponding to lighterδ18O; 3 the relative abundance variations of F. profunda in the middle of Okinawa Trough were still mainly controlled by the oscillation of Kuroshio Current based on contrast with core RN89-PC3, RN87-PC4 and KT84-14P1 in Ryukyu Island Arc; 4 the F. profunda relative abundance in core WP7 and Ph05-5 was correlated with WPWP variation. The relative abundance of F. profunda of core MD2908 was negatively correlated with the number of sun macula in northeast of Taiwan during the last 6800 yr BP which was obviously during the last 3700 yr BP. We hypothesized that EASM was mainly controlled by sun activity across the latitude variation of ITCZ based on the correlation between the relative abundance of F. profunda and the number of sun macula.