VARIATIONS IN THE UPPER WATER STRUCTURE OF NORTHERN SOUTH CHINA SEA DURING THE MID-PLEISTOCENE CLIMATE TRANSITION PERIOD: PLANKTONIC FORAMINIFERA OXYGEN ISOTOPE RECORDS OF ODP SITE 1146
The mid-Pleistocene Transition(MPT),during 1.2~0.7 million years ago(Ma),marks a critical time when the Glacial-cycles changed from a 41-ka-cycle dominated low-amplitude mode to a 100-ka-cycle dominated highamplitude mode. Climate changes during MPT are believed to be associated with processes including glacial intensification,carbon-cycle reorganization, ocean-circulation variation,as well as tropical sea-air feedbacks. However, studies about the changes in the behavior and mechanisms of tropical ocean thermocline are relatively short. To reconstruct and explore the mid- to Late-Pleistocene changes in the upper-water structure of northern South China Sea (SCS), here we analyzed the oxygen and carbon isotopes of Pulleniatina obliquiloculata, a sub-surface dwelling planktonic foraminifer, from the sediment records of site 1146(19.46°N, 116.27°E; 2092 m depth) recovered during the Ocean Drilling Program (ODP) Leg 184. In this work,a total of 223 samples of sediments between 9.80 ~ 140.17 m, deposited during about 4 ~ 1200 ka from ODP Site 1146 were analyzed. The overall trend of P. obliquiloculata delta~(18)O of ODP 1146 shows step-wise variations: (1)1.2~0.9 Ma, the amplitude of glacial-interglacial was relatively small (ca. 0.75) with a dominant cycle of 41 ka; (2) 0.9 ~0.6 Ma, the amplitude gradually increased with strengthening 100 ka eccentricity cycle; (3)0.6~0 Ma, the glacial cycle further increased to an amplitude of ca. 1.25%o with a dominant 100 ka periodicity. Compared with Globigerinoides ruber delta~(18)O and benthic foraminifera delta~(18)O, we have found that the amplitude of surface water delta~(18)O only increased during interglacial,but the subsurface water delta~(18)O amplified during both glacial and interglacial. The subsurface water delta~(18)O increased from approximately - 0.79%o to - 1.04%o in the interglacial and -0.06 to 0.2%o in the glacial during MPT. These phenomena may be associated with the strengthening of the East Asian winter monsoon since 0.9 Ma. The eccentricity cycle started to show at the same time of ca. 0.6 Ma in all planktonic and benthic foraminifera delta~(18)O while it was weaker in those of G. ruber and benthic foraminifer, and the obliquity and eccentricity cycles are significant in the delta~(18)O of P. obliquiloculata during 0.9 ~0.6 Ma. Both evidences of periodicity transition and amplitude intensification of glacial cycles indicates the special role played by changes in the thermocline structure during the Mid-Pleistocene climate transition in the northern SCS. In addition, the delta~(13)C of P. obliquiloculata shows obvious maxima in 0.02 Ma, 0.49 Ma and 0.99 Ma, consistent with the delta~(13)C maximum events observed in other records. The difference between surface and subsurface foraminiferal delta~(13)C reduced to near 0 during the delta~(13)C maximum events, probably suggesting weakened productivity in response to the delta~(13)C maximum events.