Quaternary Planktonic Foraminiferal Assemblages In The Southern South China Sea And Paleoclimatic Variations

Planktonic foraminifera (PF) in a total of 997 samples from ODP Site 1143 (9°21.72'N, 113°17.11'E, water depth 2,772 m) were studied to reveal the past -2,100 kyr paleoceanographical changes in the southern South China Sea (SCS), with a resolution averaging -2 kyr. Glacial-interglacial fluctuations and long-term changes in carbonate dissolution and preservation, sea surface temperature (SST), depth of thermocline (DOT), paleoproductivity and predominant PF species as responding to the East Asian paleomonsoon and tropical climatic evolution were investigated. In addition, predominant PF species, DOT and paleoproductivity between Site 1143 and 1146 were compared to imply the differentiation of upper ocean water environments between the southern and northern SCS due to glacial-interglacial reversal of the East Asian winter and summer monsoons and sea level change.PF fragmentation and absolute abundance, percentage of resistant species (RSP%), carbonate content and coarse fraction (＞63μm) were used to indicate carbonate dissolution and preservation. Peaks of carbonate dissolution occurred during interglacial to glacial transitions, while preservation spikes were mainly observed during glacial to interglacial transitions. Carbonate cycle at ODP Site 1143 was influenced by both dissolution and terrigenous dilution. All carbonate dissolution and preservation indices are strongly coherent with -δ¹⁸O over the eccentricity and obliquity bands. In general, maximal carbonate preservation leads and intensified dissolution lags minimal ice volume on these two orbital bands.FP-12E, SIMMAX-28 and ANN (Artificial Neural Network) techniques were employed to estimate paleo-SSTs based on PF assemblages and the results were evaluated. These estimates generally show higher SST values in glacials than interglacials, likely a bias by predominant species. Before other quantitative methods independent of faunal assemblages are developed for ODP Site 1143, the ratio of Globigerinoides sacculifer to Globigerinoides ruber was used in this study to provide the solution to qualitatively estimating SST variations. Cross-spectral and phase analyses of G. sacculifer/G, ruber and benthic foraminiferalδ¹⁸O indicated that maximal SSTs led ice volume minima (lowestδ¹⁸O) on the eccentricity, obliquity and 23-and 19-ka precession bands. Moreover, G. sacculifer/G. ruber ratio displayed significant half-precessional powers. This implies that tropical processes, rather than ice volume, have been playing an important role in temperature variations in the southern SCS.DOT and productivity were related to changes in the wind stress of the East Asian monsoon. DOT was indicated by mix-layer and thermocline dwelling species and calculated by transfer function based on faunal assemblages. The ratio of asyrnbiotic to symbiotic species was proved to be a good indicator of paleoproductivity. The southern SCS was characterized by deeper thermocline and lower productivity in glacials than interglacials. This indicates that intensified East Asian winter monsoon occurred during glacials and summer monsoon enhanced during interglacials. On a longer timescale, the thermocline depth and paleoproductivity underwent profound shifts at 1,650 ka and 850 ka, likely as a direct response to the stepwise evolution of the East Asian monsoon. Besides the dominance of the 41-ka obliquity cyclicity, half-precession periodicities were remarkably significant in variabilities of the thermodine depth and paleoproductivity, indicating that tropical climate factors including the East Asian monsoon played a key role in fluctuations of upper ocean structure in the southern SCS.G. ruber, G. sacculifer, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata and Globorotalia menardii dominated the faunal assemblage at ODP Site 1143. The relative abundance of G. ruber, G. sacculifer and N. dutertrei exhibited vague glacial-interglacial fluctuations, probably in response to a combination of carbonate dissolution, DOT, SST and productivity. On a long time scale, G. ruber mirrored N. dutertrei in displaying profound changes at 1,600 ka and 850 ka, but G. sacculifer changed little. The opposite change in G. ruber and N. dutertrei likely reflects a long-term change in thermocline and paleoproductivity. The long-term behavior of G. sacculifer may be due to its adapting flexibility to thermocline change.Two tropical carbonate-dissolution species, G. menardii and P. obliquiloculata, both favor shallow thermocline but displayed different responses to glacial-interglacial cycles in the southern SCS. More abundant G. menardii in interglacials than glacials was due to the shallow interglacial thermocline in the southern SCS. However, P. obliquilculata is reversely correlated withδ¹⁸O) profile with high abundance in glacials since the Mid-Pleistocene Revolution (MPR). Investigation of its downcore variations at sites in western Pacific marginal seas indicated that the reverse correlation is unique to the southern SCS. Before the MPR, P. obliquiloculata was abundant in interglacials. High abundance of P. obliquiloculata during glacials after the MPR can be ascribed to increased regional seawater salinity, a connection of southern SCS waters to the western tropical Pacific or water conditions similar to the modern western tropical Pacific formed in the southern SCS.The pink morphotype of G. ruber has distinctive abundance in glacials and cool interglacial sub-stages, in contrast to records elsewhere. A deeper thermocline or the proximity of ODP Site 1143 to land during glacials is suggested to be responsible for the occurrence of large numbers of pink G. ruber, if the summer SST in the southern SCS provided an optimum temperature condition throughout the time interval.Orbitally tuned oxygen isotope strafigraphies from both Site 1143 and 1146 provided an accurate chronologic framework to compare paleoceanographic differences between the southern and northern SCS. The differences in dominant species, DOT and paleoproductivity indicated a profound differentiation of surface water environments between the two regions at～1,200 ka. This transition was considered to be a response in the long-term interaction between winter and summer monsoons to the progressive dominance of the eccentricity astronomical cycle in the late Quaternary. On the glacial-interglacial timescale, a see-saw like pattern was revealed in changes in DOT and associated paleoproductivity between the southern and northern SCS. During glacials, thermocline was deeper and productivity was lower in the south than in the north, and vice versa in interglacials. This is interpreted to be mainly due to differential glacial-interglacial impacts of winter and summer monsoons in the northern and southern SCS, as well as influence of glacial sea level lower-stands.