Radiolarian record to paleoecological environment change events over the past 1.2 MaBP in the southern South China Sea

This note studies the Radiolarian fossil groups since 1.2 MaBP in ODP leg 184 site 1143, the southern South China Sea (SCS). The result shows that radiolarian abundance experienced a significant variation: before 0.9 MaBP it remained at the extremely low level, but increased with low extent between 0.9–0.65 Ma, which corresponded to the Mid-Pleistocene Transition stage, and it increased rapidly after 0.65 MaBP. During the whole process, the average abundance became higher and higher in each stage, and showed regularly periodic fluctuations. The obvious increase after 0.65 MaBP is inferred to result from the enhanced upwelling in this region, which was induced by the intensified monsoon circulation after the “Mid-Pleistocene Transition”. An outstanding result of the spectral analyses is that a long oscillation of ∼ 0.2 Ma cycle was found in the records of radiolarian abundance and complex diversity, which corresponds well to the result of other paleoceanographic indexes. This probably indicated a special cycle characteristic of paleoecological environment evolution in this area. In addition, all of the radiolarian indexes show an obvious boundary in about 0.47 MaBP, indicating the abrupt variation of the community structure and radiolarian abundance level before and after 0.47 MaBP. So we suppose that there existed a distinct change event of oceanic ecology environment during that period.     

Siliceous microplankton fluxes and seasonal variations in the central South China Sea during 1993-1995:monsoon climate and El Ni(n)o responses

Seasonal variations of radiolarian and diatom fluxes in the central South China Sea during 1993-1995 were overwhelmingly controlled by monsoon climate. Radiolarian and diatom increased obviously during the Northeast (from November to February) and Southwest (from June to September) monsoons and decreased during the periods between the monsoons. The change of circulation driven by the monsoons improved water exchange in the different areas that brought rich nutrient materials for the surface microplankton, thereby enhancing radiolarian and diatom fluxes. Variation of radiolarian flux coincided with organic carbon flux, surface primary and export productivities. High radiolarian flux corresponded to high surface primary productivity. Radiolarian and diatom fluxes raised abnormally during 1994-1995 could be attributed to the El Ni(n)o event during the period.     

Siliceous microplankton fluxes and seasonal variations in the central South China Sea during 1993–1995: monsoon climate and El Niño responses

Seasonal variations of radiolarian and diatom fluxes in the central South China Sea during 1993–1995 were overwhelmingly controlled by monsoon climate. Radiolarian and diatom increased obviously during the Northeast (from November to February) and Southwest (from June to September) monsoons and decreased during the periods between the monsoons. The change of circulation driven by the monsoons improved water exchange in the different areas that brought rich nutrient materials for the surface microplankton, thereby enhancing radiolarian and diatom fluxes. Variation of radiolarian flux coincided with organic carbon flux, surface primary and export productivities. High radiolarian flux corresponded to high surface primary productivity. Radiolarian and diatom fluxes raised abnormally during 1994–1995 could be attributed to the El Nino event during the period.     

Quaternary high-resolution opal record and its paleoproductivity implication at ODP Site 1143,southern South China Sea

The correlation of opal content and MAR with oxygen isotopic records of benthonic foraminifera at Site 1143, southern South China Sea indicates that,since about 900 ka,the increasing opal content and MAR during the interglacial periods is inferred to reflect the bigher surface productivity ,for the intensified summer monsoon during the interglacial periods would result in the enhanced upwelling and nutrient supply,Time-sequence spectral analyses of oxygen isotopic record ,opal content and MAR at intervals of 0-900 ka reveal that the changes of surface productivity were dominantly forced by the variations of the earth orbital cycles.     

Exploring cyclic changes of the ocean carbon reservoir

A 5-Ma record from ODP Site 1143 has re-vealed the long-term cycles of 400—500 ka in the carbon isotope variations. The periodicity is correlatable all over the global ocean and hence indicative of low-frequency changes in the ocean carbon reservoir. As the same periodicity is also found in carbonate and eolian dust records in the tropical ocean, it may have been caused by such low-latitude proc-esses like monsoon. According to the Quaternary records from Site 1143 and elsewhere, major ice-sheet expansion and major transition in glacial cyclicity (such as the Mid-Brunhes Event and the Mid-Pleistocene Revolution ) were all pre-ceded by reorganization in the ocean carbon reservoir ex-pressed as an episode of carbon isotope maximum (d 13Cmax), implying the role of carbon cycling in modulating the glacial periodicity. The Quaternary glacial cycles, therefore, should no more be ascribed to the physical response to insolation changes at the Northern Hemisphere high latitudes alone; rather, they have been driven by the 揹ouble forcing? a combination of processes at both high and low latitudes, and of processes in both physical (ice-sheet) and biogeochemical (carbon cycling) realms. As the Earth is now passing through a new carbon isotope maximum, it is of vital impor-tance to understand the cyclic variations in the ocean carbon reservoir and its climate impact. The Pre-Quaternary varia-tions in carbon and oxygen isotopes are characterized by their co-variations at the 400-ka eccentricity band, but the response of d 13C and d 18O to orbital forcing in the Quater-nary became diverged with the growth of the Arctic ice-sheet. The present paper is the second summary report of ODP Leg 184 to the South China Sea.     

High-resolution climate evolution derived from the sediment records of Erlongwan Maar Lake since 14 ka BP

Varve counts with AMS 14 C,137 Cs and 210 Pb dating of sediments(0-900 cm) from Erlongwan Maar Lake,NE China were used to establish a high-resolution chronology series for the late Quaternary.Dry density,total organic carbon(TOC) content,total nitrogen(TN) content,TOC/TN ratios and stable organic carbon isotope(13 C org) ratios were continuously analyzed on this sediment profile.On the basis of lithological characters,sporo-pollen assemblages and geochemical analyses,we identified 6 climate stages within the last 14 ka BP.The time before the Holocene(14-11.4 ka BP) represents a higher-order oscillation climatic transitional period(I).The entire Holocene climate development(from 11.4 ka BP to present) exhibited an increasing temperature trend,although there were cold and warm alternations(II-VI).The periods included were:II(11.4-9.05 ka BP) warm-wet stage,III(9.05-7.4 ka BP) cold and warm fluctuation stage,IV(7.4-4.2 ka BP) smoothly warming climate stage,V(4.2-1.67 ka BP) climate optimum stage,and VI(from 1.67 ka BP to present) cool and drier stage.Each climate stage began with a warming event and ended with an abrupt cooling event.This climate change cycle had unequal time spaces that were progressively shorter over time.Several abrupt climate shifts occurred at about 9.4-9.05,8.5-8.2,7.8-7.4,4.6-4.2,3.7-3.25,2-1.67 and 0.3-0.03 ka BP.Thus,it can be seen that the climate has been warming since 1920 AD,which indicates a new climate stage.     

Late Neogene radiolarian absence event in the southern South China Sea and its paleoceanographic implication

Down-core variations of radiolarians at ODP Site 1143 in the southern South China Sea （SCS） are presented for the last 12 Myr. The fluctuations of radiolarian abundances since the Late Miocene can be divided into three stages： a high abundance stage at 12-5.96 Myr, a radiolarian absent stage at 5.96-3.30 Myr and a gradual increasing stage after 3.30 Myr. The three stages correspond to the forming and vicissitudes of the Western Pacific Warm Pool （WPWP） and the Eastern Asian Summer Monsoon （EASM）. The radiolarian absence event （RAE） was also absent of diatom and occurred nearly simultaneously with the closures of the Panama Isthmus and the Indonesian seaway, which probably caused the reorganization of oceanic circulation systems. Accompanied this circulation reorganization was the weakening of the WPWP and the EASM, which probably led to a weakened upwelling in the southern SCS. In addition dissolved silica （Dsi） content in surface seawater might be very low during 5.96-3.30 Myr due to the ＂biogenic bloom＂ event, which consumed a large amount of Dsi in the surface seawater. All these factors together might lead to a great decrease of siliceous production in the southern SCS and consequently caused the RAE. Moreover, the dissolution of siliceous skeletons might also influence the abundance of radiolarians.     

Diatoms from the southwestern continental slope, South China Sea, and their paleoenvironmental significance since the last glacial times

A quantitative study was undertaken on diatoms from cores (SA08-34) obtained from the southwestern continental slope of the South China Sea (SCS). A total of 165 diatom species belonging to 45 genera were identified. We constructed a stratigraphic subdivision and correlation according to the characteristics of diatom assemblages together with ¹⁴C dating and carbonate analysis. We also discuss the sedimentary environment in the sea area since the last glacial times. The research shows that the diatom assemblages coincide with interglacial and glacial times, and changes in diatom abundance reflect the instability of the climate in the southern part of the SCS, such that short-term, temperature descending events correlate with the interglacial interval. The abundance of diatoms is relevant to interglacial and glacial times, since high abundance values were associated with an interglacial interval, and low abundance values with the last glacial maximum. We assume that strong upwelling developed in the interglacial interval, the development of which was influenced by variations of monsoons in the East Asian region.     

Variations of opal accumulation rates and paleoproductivity over the past 12 Ma at ODP Site 1143, southern South China Sea

The biogenic silica analysis was performed on 463 samples from ODP 184 Site 1143, the southern South China Sea. The results show that the opal content and MAR evidently increased between 12.3 and 5.7 Ma, which was contemporary with the high carbonate and total MARs, reflecting the high biogenic productivity in the late Miocene.This demonstrates the occurrence of the late Miocene "Biogenie Bloom Event" in the southern South China Sea, corresponding to the late Miocene-early Pliocene "Biogenic Bloom Event" in the Indian-Pacific Ocean. The increases of opal content and MAR after the middle Pleistocene (about 0.7 Ma)is inferred to result from the enhanced upwelling and nutrient supply, which was induced by the intensified monsoon circulation after the "Mid-Pleistocene Revolution".     

Deep-water Oligocene pollen record from South China Sea

Leg 184 of ODP recovered a record of deepwater sediments spanning the past 32.8 Ma from the South China Sea (SCS). The sediments of the Ofigocene (32.8-23.8 Ma) at Site 1148 contain relatively abundant fossil pollen. The pollen analysis at Site 1148 has established the first pollen assemblage sequence of deep-water Oligocene in the China Sea. The pollen assemblages of the Ofigocene are dominated by montane conifer tree pollen. The abundances of broad-leaved tree pollen are lower in the assemblages.Both of the montane conifer and broad-leaved tree pollen groups include mainly tropical-subtropical components. The pollen of cold and drought-enduring plants is present in lower content. The distinct change in pollen assemblage sequence of deep-water Oligocene of the SCS occurred at 32.0 Ma, indicative of an important shift of the Oligocene climate in the SCS region. The characteristics of the pollen flora of the deep-water Oligocene indicate the tropical montane ralnforest and lowland ralnforest developed on the areas around the SCS before 32.0 Ma, reflecting the warm and wet climatic condition. In the pollen flora of the Oligocene after 32.0 Ma, the temperate montane conifer and cool and drought-en- during deciduous tree taxa remarkably increased, indicating that the climate in the SCS region became comparatively cool and dry.     

过去1 300年南海东岛生态环境对气候变化的响应

东岛牛塘为目前在西沙群岛少见的淡水型湖泊,其中保存完好的粪土沉积物记录了距今1 300年来东岛生态环境演化历史.对DY4粪土沉积柱的研究结果表明,在1 400 AD前,东岛海鸟数量较多,牛塘湖岸周围植被生长较为繁盛,而在1 400~1 850 AD期间,东岛海鸟数量和植被生长都处于低谷.通过综合对比研究区、中国乃至北半球降雨量、温度变化记录,发现1 400 AD前岛屿生物量较高的时期对应于暖干的中世纪温暖期,而低生物量时期与凉湿的小冰期一致,证明南海热带孤立岛屿生态系统对区域乃至全球气候环境变化响应非常灵敏.     

Sea surface temperature records of core ZY2 from the central mud area in the South Yellow Sea during last 6200 years and related effect of the Yellow Sea Warm Current

Sea surface temperature (SST) records in the South Yellow Sea during the last 6200 years are reconstructed by the unsaturation index of long-chain alkenones (K 37 U ’) in sediment core ZY2 from the central mud area.The SST records varied between 14.1 and 16.5°C (15.6°C on average),with 3 phases:(1) A high SST phase at 6.2-5.9 cal ka BP;(2) A low and intensely fluctuating SST phase at 5.9-2.3 cal ka BP;and (3) A high and stable SST phase since 2.3 cal ka BP.Variation of the SST records is similar to intensity of the Kuroshio Current (KC),and corresponds well in time to global cold climate events.However,the amplitude of the SST response to cooling events was significantly different in different phases.The SST response to global cooling event was weak while the KC was strong;and the SST response was strong while the KC was weak.The difference in amplitude of the SST response is possibly caused by the modulation effect of the Yellow Sea Warm Current which acts as a shelf branch of the KC and a compensating current induced by the East Asia winter monsoon.The warm waters brought by the Yellow Sea Warm Current cushion the SST decrease induced by climate cooling,and both the Kuroshio and East Asian winter monsoon play important roles in the modulation mechanism.The SST records display a periodicity of 1482 years.The same period was found in the KC records,indicating that variation of the SST records in the central South Yellow Sea is strongly affected by KC intensity.The same period was also found in Greenland ice cores and North Atlantic and Arabian Sea sediment cores,showing a regional response of marine environmental variability in the East China Seas to that in the global oceans.     

马蹄参属的起源和地理分布

根据现代理分布及近缘化石的分布状况,讲座了马蹄参属的起源和地理分布,划定了该属植物的分布区类型,确认马蹄参属曾广泛分布于欧亚大陆和北美洲,可能于晚白垩纪起源于中、西欧,散布途径主要有两条：从起源地经西亚沿提特斯海北岸传播进入东南亚和中国南部;从起源地经格陵兰群岛进入北美洲。马蹄参属的现代分布格局主要是由以下三方面因素形成的：气候变迁、地史变迁、植物自身生物特性及对不良环境的适应能力。     

Thirty million year deep-sea records in the South China Sea

In the spring of 1999 the Ocean Drilling Program (ODP) Leg 184 Shipboard Party cored 17 holes at 6 deep water sites in the northern and southern parts of the South China Sea (SCS). Chinese scientists actively participated in the entire process of this first deep-sea drilling leg off China, from proposal to post-cruise studies. More than 30 categories of analyses have been conducted post-cruise in various Chinese laboratories on a large number of core samples, and the total number of analyses exceeded 60 thousand.The major scientific achievements of the Leg 184 studies are briefly reported in three successive papers, with the first one presented here dealing with deep-sea stratigraphy and evolution of climate cycles. This ODP leg has established the best deep-sea stratigraphic sequences in the Western Pacific: the 23-Ma isotope sequence from the Dong-Sha area is unique worldwide because of its continuity; the last 5-Ma sequence from the Nansha area represents one of the best 4 ODP sites worldwide with the highest time-resolution for that time interval, and the sequences of physical properties enable a decadal-scale time resolution. All these together have provided for the first time high-quality marine records for paleoenvi-ronmental studies in the Asian-Pacific region. This new set of stratigraphic records has revealed changes in climate cyclicity over the last 20 Ma with the fluctuating power of the 100 ka, 400 ka, 2000 ka eccentricity cycles, indicating the evolving response of the climate system to orbital forcing along with the growth of the Antarctic and Northern Hemisphere ice sheets.     

Carbonate dissolution and deep-water paleoceanography of the South China Sea since the Middle Pleistocene

Based on the data of oxygen isotope, micropaleontology and paleomagnetism, we set up the detailed chronology of Core 17957-2 from the southern South China Sea and discuss the change of carbonate dissolution over the last 800 ka. Down-core variation of carbonate content records the “Pacific-type” cycle of higher glacial values and lower interglacial values, though the core lies above the modern lysocline. Carbonate dissolution indices indicate that several severe dissolution of CaCO₃happened during the transition from interglacial to glacial stages. Spectral analyses of these indices show that the carbonate dissolution periodicities are mainly made up of 500 ka and 100 ka. Compared with the cycles of carbonate dissolution of the Indian Ocean, the long-term (500-ka) periodicity reflects the characteristic of the deep-water circulation of the oceans.     

Radiolarian transfer function for paleo-primary productivity in the South China Sea

Basic data of radiolarian group and primary productivity were from 46 investigation stations of the South China Sea. The factor analysis of principal component and the least squares estimation in both linear regression and nonlinear estimation were applied to establishing the radiolarian transfer function for paleo-primary productivity in this sea area. The results show that the transfer function of nonlinear regression analysis for the radiolarian assemblage and primary productivity has much better parameters. Factors analysis has accumulating variance of 83.6% and communalities of more than 0.940347. Nonlinear regression analysis gives multiple correlation coefficient as 0.84888 and explained variance as 72.059%. The equation of transfer function is with an average estimated error of about 53.778 mg · C/m² · d, which is 14.69% in average observed values. Calculation accuracy is 85.31% for the system of transfer function.     

Relations between sedimentary sequence and paleoclimatic changes during last 200 ka in the southern South China Sea

Core NS-93-5 was taken from the mild slope terrace of the southern South China Sea (SCS), which has preserved the steadiness depositional record of the normal marine environment since late Quaternary. Sedimentary sequence and oxygen isotopic stratigraphy of high resolution in the near 200 ka of the southern SCS has been established. By the comparative analysis with GISP2's ice core, the depositional record of D/O's events 1-21 and Heinrich's events H1-H6 in the southern SCS that reflected the quick climate change in short time scale since the last interglacial stage is revealed, which indicates that in the last 200 ka in the southern SCS and the Arctic area there was tele-connection of paleoclimate and the unstability of the Western Pacific Warm Pool. This note shows that the sedimentary layers order of this sea area classified by color identification and by oxygen isotope stratum has the very good corresponding relation. The color feature of sediment changes along with the climate and the former arranges a stagnant time of 1-1.5 ka generally. It was suggested that the paleoclimatic changes resulted in the rise and fall of sea level, the open and close of strait thoroughfare and the upwelling activity, which causes the changes of the oxidation-reduction condition of deep water along with the dynamic environment in this sea area, thus producing the different sediment features of sea bed. In addition, a volcanic ash layer, about 17 cm thick, has been found in the transition of oxygen isotope 4/5 stage, which is related with the Toba's volcanic eruption.     

Deep sea pollen record during 12－1.6 Ma from the southern South China Sea and its response to envi-ronmental change

Based on deep-sea pollen results (512-76 m) from ODP Site 1143 in the southern South China Sea (SCS),the climate and vegetation evolution sequence on the surrounding islands and the exposed continental shelf are discussed. The pollen records show that the pollen influx was quite low before 8.15 Ma and increased dramatically afterwards. The influx changes can be ascribed,on one side,to tectonics deformations around the southern SCS resulting in rapid uplift of islands and subsequent increase of the sediment rates and pollen influx and on the other side to climate cooling and monsoon enhancement. Around 2.63 Ma was another obvious boundary,the increasing of pollen and spores influx since this time was mainly related to global climate cooling. Spectrum analysis of pollen influx values shows that 2 Ma,0.67 Ma,and 0.19-0.17 Ma cycles existed during 12-3.0 Ma,while 0.1 Ma and 46.9 ka cycles existed during 3.0-2.0 Ma.     

Changes in sea surface temperature in western South China Sea over the past 450 ka

Sea surface temperature over the past 450 ka was obtained by the unsaturation of molecular fossil-long chain alkenone with a resolution of about 1 ka from the western South China Sea. This is the longest temperature profile in the South China Sea at such high resolution. The U37^κ-SST results revealed similar glacial-interglacial cycles as the δ^18O profile of planktonic foraminifera, with SST variability of 23-25.5℃ for glacial and 25-28℃ for interglacial periods. The highest SST （28.4℃） was recorded at MIS5.5 and lowest SST （22.6℃） during MIS2. The SST record preceded the planktonic foraminiferal δ^18O on five glacial-interglacial transitions. Comparison of temperature records from the Southern and Northern Hemispheres indicated a more Southern Hemisphere-like pattern for the temperature variation in the SCS. Strong precession and semiprecession signals in the spectra of our SST record manifest the tropical phenomena.     

Millennial climate changes since MIS 3 revealed by element records in deep-sea sediments from northern South China Sea

High resolution elemental MD05-2903 from the northern South records in core China Sea were obtained from XRF scanning and grain size and mineral component analyses. The K/Ti curve reveals climate changes since MIS 3 on a millennial scale, representing one of the best such records so far from deep-sea sediments of the South China Sea. When compared our K/Ti record with other climate records from high latitude ice cores and from East Asia stalagmites, variations in the K/Ti record show some typical saw-shaped features of rapid climate changes in the Northern Hemisphere. Grain size analysis reveals a close relationship between variations in K/Ti and in the fine end-member grain size. XRD analysis confirms that high K/Ti layers were dominated by K-rich weathering minerals, especially illite, while low K/Ti layers contain low illite but high abundance of Ti-rich heavy minerals. Therefore, the K/Ti record reflects changes in the input and deposition of fine weathering minerals and heavy minerals in the northern South China Sea region during stadials and interstadials. These elemental changes were largely con- trolled by variations in rainfall, erosion, and fluvial trans- portation induced by East Asia Monsoon.