Particulate fluxes investigated in the central South China Sea (SCS) during 1993―1996 indicate that opal flux can be used to show primary productivity change, which provides a foundation for tracing the evolutionary relationship between the surface productivity and East Asian monsoon in the SCS during the late Quaternary glacial and interglacial periods. Based on the studies of opal % and their mass accumulation rates (MAR) at the six sites recovered from the SCS during the “Resolution” ODP Leg 184 and “Sonne” 95 cruise of the Sino-Germany cooperation, opal % and their MARs increased evidently in the northern sites since 470―900 ka, and they enhanced and reduced, respectively, during the glacial and interglacial periods. Whereas they increased obviously in the southern sites since 420―450 ka, and they augmented and declined, respectively, during the interglacial and glacial periods. The vari- ability in opal % and their MARs in the late Quaternary glacial cyclicity indicate the “seesaw” pattern of surface productivity in the SCS. The winter monsoon intensified during the glacial periods, surface productivity increased and decreased, respectively, in the northern and southern SCS. The summer monsoon strengthened during the interglacial periods, surface productivity increased and decreased, respectively, in the southern and northern SCS. The cross spectral analyses between the opal % in the northern and southern SCS during the Quaternary and global ice volume (δ 18O) and orbital forcing (ETP) indicate that the East Asian winter and summer monsoons could be ascribed to the different drive mechanisms. On the orbital time scale, the global ice volume change could be a dominant factor for the winter monsoon intension and temporal variations. As compared with the winter monsoon, the correlative summer solar radiation with the obliquity and precession in the Northern Hemisphere could be a mostly controlling factor for the summer monsoon intension and temporal variations.
WANG RuJian JIAN ZhiMin XIAO WenShen TIAN Jun LI JianRu CHEN RongHua ZHENG YuLong CHEN JianFang
The deep sea records from the ODP Sites 1143 and 1144 in the northern and southern South China Sea (SCS), including foraminiferal δ 18O and δ 13C, Opal% and pollen percentage, reveal that the variations of the east Asian monsoon have been closely correlated with the variations of the Earth’s orbital parameters (eccentricity, obliquity and precession) and the global ice volume on orbital scale. All the monsoonal proxies show strong 100 ka, 41 ka and 23 ka cycles. Although G. ruber δ 13C of Site 1143 is coherent with the ETP (ETP= normalized (eccentricity + obliquity-precession) at eccentricity, obliquity and precession bands, most of the coherent relationship focuses on the precession band, and the other monsoonal proxies are coherent with the ETP only at the precession band, which indicate that precession dominates the Pleistocene tropical climate changes. The phase relationship of the monsoonal proxies with the foraminiferal δ 18O implies that the global ice volume changes have played a significant role in modulating the east Asian monsoon climate, at least dominating the winter monsoon. This forcing mechanism of the east Asian monsoon is apparently different from that of the Indian ocean mon-soon. The variations of the east Asian monsoon at the precession band, at least that of the winter monsoon, have been controlled not only by the sensible heating but also by the latent heating of the surface water in the South China Sea.
TIAN Jun1,2, WANG Pinxian1, CHENG Xinrong1, WANG Rujian1 & SUN Xiangjun1,3 1. State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Neogloboquadrina pachyderma is the most abundant planktonic foraminifera species found in the modern polar oceans.The δ18O and δ13C of N.pachyderma from the Western Arctic Ocean sediments were analyzed to reveal the implications of the proxies to environmental changes.The δ18O from N.pachyderma in the Chukchi Sea reflect the water mass distribution in this area.Heavier δ18O values were found along the Anadyr Current(AC) and lighter values in the central and eastern Chukchi Sea.These may reflect the freshwater signal from the Alaska Coastal Current(ACC) and Bering Sea Shelf Water(BSSW) .The light δ18O signature in the high Arctic basin comes from the freshwater stored in the Arctic surface layer.The δ13C distribution pattern in the Chukchi Sea is also influenced by the current system.High primary productivity along the AC results in heavy δ13C.The relatively low primary productivity and the freshwater component from the BSSW and ACC may be the reason for this light δ13C signal in the central and eastern Chukchi Sea.Our data reveal the importance of well ventilated Pacific Water through the Chukchi Sea into the Arctic Ocean.
Terrigenous components in sediment core B84A from the Alpha Ridge,Western Arctic Ocean,have been investigated to reconstruct Mid to Late Quaternary variations in sedimentation,provenance,and related climate changes. The core stratigraphy,evaluated by a combination of variations in Mn content,color cycles,foraminiferal abundance,and lithological correlation,extends back to estimated Marine Isotope Stage 12. Twelve Ice Rafted Detritus(IRD,>250 μm) events were identified and interpreted to mostly occur during deglaciation.The Canadian Arctic,which was covered by ice sheets during glacial periods,is suggested to be the major source region. The IRD events likely indicate the collapses of ice sheets,possibly in response to abrupt climate changes.Grain size analysis of B84A indicates sedimentologically sensitive components in core B84A in the 4-9 μm and 19-53 μm silt subfractions,which are inferred to be mainly transported by currents and sea ice,respectively. Down core variability of these two fractions may indicate changes in ice drift and current strength. In accordance with previous studies in the central Arctic Ocean,the average sedimentation rate in core B84A is about 0.4 cm·ka-1.Compared with the relatively high sedimentation rates on the margins,sedimentation in the central Arctic Ocean is limited by sea ice cover and the correspondingly low bioproductivity,as well as the long distance from source regions of terrigenous sediment.
LIU WeinanWANG RujianCHEN JianfangCHENG ZhenboCHEN ZhihuSUN Yechen
