Internal solitons in the northern South China Sea from insitu observations

In this paper, we demonstrate the characteristics of strong internal solitons at the southern edge of Dongsha Islands in the northern South China Sea （SCS） during May-June 1998, using in situ time series data from the conductivity temperature depth （CTD）, acoustic Doppler current profiler （ADCP） and thermistor chain. Our measurements indicated that the strongest internal solitons were larger than 90 m in wave amplitude, and propagated approximately westward in traveling direction, with the maximum current speed of about 2 m/s and the period of 10-20 min. The strongest internal solitons occurred noticeably during May 14- 16, 1998, which is consistent with the occurrence period of the diurnal-dominated spring internal tides.     

Internal tide characteristics over northern South China Sea continental slope

21-day in-situ ADCP current and CTD data from an anchored observational station by R/V Shiyan 3 are used to investigate the characteristics of intemal tide in the northern South China Sea continental slope. The results show that tide and internal tide are both mixed type in the observed region and dominated by diurnals; the maximum internal tidal major axes of K1, O1 were largest and reached 11 cm/s, 10 cm/s, respectively, and their current vectors rotate anticlockwise. The 300 m-deep internal tide presents obvious quasi-diurnal oscillation and its average amplitude reaches 50 m. Furthermore, internal tide carried high energy; in the upper layers above 140 m, the kinetic energy of intemal tides accounts for 15%--37% of overall kinetic energy, most of internal tide energy concentrates around the thermocline.     

The barrier layer in the southern region of the South China Sea

By analysing the CTD data in the southernregion of the South China Sea gathered during six cruisesbetween 1989 and 1999, a barrier layer with seasonalvariation just like what exists in the equatorial oceans isfound in this region. It is the first discovery in such amarginal sea yet. It is strong in autumn and a little weak in summer and winter. The thicker the barrier layer, the higher the average temperature of the upper mixed layer. The region with the thicker barrier layer overlaps the region with thehigher average temperature of the upper mixed layer, andaccords with the thicker region of the warm pool in the South China Sea got from the Levitus data. The barrier layer in the southern region of the South China Sea has significantinfluence on the heat storage of the upper ocean there.     

南海表面海温异常对南海季风影响的数值模拟

采用P-σ混合坐标系区域气候模式模拟了4－7月南海季风的爆发、演变过程，并进行了3组敏感性数值试验，研究南海表面海温异常对南海季风的影响，得到以下结论：（1）南海4月份海温异常对南海季风的爆发日期影响不大，但对季风爆发后的强度有所影响，异常增温造成南海季风增强，异常降温则南海季风减弱。（2）南海季风爆发和强度的变化与南海本身的海温变化情况有密切的关系，尤其是5月份南海海温异常。5月份南海异常增温可以使南海季风提前爆发，季风增强，南海海温异常降低时，南海季风爆发的时间推迟，季风减弱。（3）南海海温持续异常可以影响南海及中国大陆的高低空环流变化，海温持续异常增温可以使南海季风提前爆发，显地加强南海季风，并有利于南海季风向北推进，但当海温在6月份进一步持续增温时，则有利于季风维持在较南地区，阻碍季风向北发展；当海温持续异常降低时，南海季风推迟爆发，且明显减弱。     

Background current affects the internal wave structure of the northern South China Sea

The internal wave modal equations are solved with the consideration of background currents. Analytical and numerical solutions of some specific examples, including observations in the northern South China Sea (SCS), are obtained to investigate the effect of background current on internal wave vertical structure. The effects of current shear and curvature on internal wave vertical structure are evaluated separately. It is found that the phase speed and wave structure are modified by background currents, the current shear has little effect on wave structure, whilst the current curvature could have strong impact on wave structure. The extent of the effect by the current curvature on the wave structure depends on the magnitudes of current curvature, relative wave speed, and buoyancy frequency, sometimes the effect by the current curvature may even cause the wave to attenuate severely with depth. A new method to obtain the real eigenfunction with depth in the case that the waves become evanescent is also put forward. It is shown that, the residual tidal current in the northern SCS is strong enough to cause the wave to attenuate severely at upper layer.     

Background current affects the internal wave structure of the northern South China Sea

The internal wave modal equations are solved with the consideration of background currents. Analytical and numerical solutions of some specific examples, including observations in the northern South China Sea （SCS）, are obtained to investigate the effect of back- ground current on internal wave vertical structure. The effects of current shear and curvature on internal wave vertical structure are eval- uated separately. It is found that the phase speed and wave structure are modified by background currents, the current shear has little effect on wave structure, whilst the current curvature could have a strong impact on the wave structure. The extent of the effect by the current curvature on the wave structure depends on the magnitudes of current curvature, relative wave speed, and buoyancy frequency, sometimes the effect by the current curvature may even cause the wave to attenuate severely with depth. A new method to obtain the real eigenfunction with depth in the case that the waves become evanescent is also put forward. It is shown that the residual tidal current in the northern SCS is strong enough to cause the wave to attenuate severely at the uooer laver.     

Observation of internal tides and near-inertial motions in the upper 450 m layer of the northern South China Sea

Mooring ADCP current observations from August to November are used to study the barotropic tides, baroclinic tides and near-inertial motions in the upper 450 m layer of the northern South China Sea. The barotropic and baroclinic tides at the mooring station are all dominated by M2, K1, O1, P1, with the barotropic amplitudes being 7.8 cm·s^-1, 7.0 cm·s^-1, 5.4 cm·s^-1 and 3.5 cm·s^-1 respectively. The amplitudes of M2, K1, O1, P1 internal tides vary greatly, which are 12--15 cm·s^-1 in the thermocline, and then decrease with increasing depth. The amplitude of the barootropic near-inertial motions is less than 1 cm·s^-1, contributing little to the barotropic currents. However, that of the baroclinic nearinertial motions can be as large as 5 cm·s^-1. The inclination of tidal ellipse tends to increase with increasing depth, implying upward propagation of energy, while that of the near-inertial ellipse tends to decrease with increasing depth, implying downward propagation of energy.     

Pleistocene precession forcing of the upper ocean structure variations of the southern South China Sea

The precession plays a dominant role in driving the tropical monsoon variations. Our high resolution, millennial scale marine isotope records from ODP Site 1143 in the southern South China Sea (SCS) present the detailed history of the upper ocean structure variations over the past 1.56 Ma on glacial/interglacial timescale. The cross spectral analyses between the Earth's orbital variations and the isotopic differences reveal a high coherency between the East-Asian-monsoon-related thermocline and nutricline variations of the SCS and the precession. The variations of monsoon-related isotopic difference between species also demonstrate periodicities of 11-, 12- and 14- thousand years near semi-precession which originates in the tropics, highlighting the importance of the precession in driving the east Asian monsoon changes.     

Lithospheric thermal-rheological structures of the continental margin in the northern South China Sea

Thermal structures of three deep seismic profiles in the continental margin in the northern South China Sea are calculated, their "thermal" lithospheric thicknesses are evaluated based on the basalt dry solidus, and their rheological structures are evaluated with linear frictional failure criterion and power-law creep equation. "Thermal" lithosphere is about 90 km in thickness in shelf area, and thins toward the slope, lowers to 60-65 km in the lower slope, ocean crust and Xisha Trough. In the mid-west of the studied area, the lithospheric rheological structure in shelf area and Xisha Islands is of four layers: brittle, ductile, brittle and ductile. Because of uprising of heat mantle and thinning of crust and lithosphere in Xisha Trough, the bottom of the upper brittle layer is only buried at 16 km. In the eastern area, the bottom of the upper brittle layer in the north is buried at 20 km or so, while in lower slope and ocean crust, the rheological structure is of two layers of brittle and ductile, and crust and uppermost mantle form one whole brittle layer whose bottom is buried at 30-32 km. Analyses show that the characteristics of rheological structure accord with the seismic result observed. The character of rheological stratification implies that before the extension of the continent margin, there likely was a ductile layer in mid-lower crust. The influence of the existence of ductile layer to the evolution of the continent margin and the different extensions of ductile layer and brittle layer should not be overlooked. Its thickness, depth and extent in influencing continent margin's extension and evolution should be well evaluated in building a dynamic model for the area.     

Benthic foraminiferal fauna turnover at 2.1 Ma in the northern South China Sea

Quantitative analysis of benthic foraminifera from ODP Site 1146 in the northern South China Sea (SCS) shows that abundance of Bulimina alazanensis, sometimes up to about 90%, decreased gradually since 3.2 Ma, especially at 2.1 Ma. Abundance of other benthic foraminiferal species, Globobulimina sub-globosa and Cibicidoides wuellerstorfi, increased after 2.1 Ma. Comparison with changes in oxygen and carbon isotopes of planktonic and benthic foraminifera shows that high abundance values of B. alazanensis corresponded with lower values of oxygen isotope, but for carbon isotope, high values of the species were consistent with heavier carbon isotope of benthic foraminifera and lighter carbon isotope of planktonic foraminifera, respectively, and vice versa. Considering factors such as uplift of Bashi Strait, expansion of the North Hemisphere Glaciation, strengthening of East Asian winter mon-soon and variations in oxygen and carbon isotope of foraminifera, changes of B. alazanensis in ODP Site 1146 suggest that the source of deep water masses of the northern South China Sea changed from the warm Pacific deep water with high oxygen content to Pacific Intermediate water with low oxygen content at 2.1 Ma. In addition, the strengthened East Asian winter monsoon resulted in increased pri-mary productivity, high nutrient and suboxic bottom water. Variations in species of B. alazanensis seemed to be unable to tolerate environmental stress induced by deep water masses and productivity changes.     

Characteristics of hydrocarbon accumulation and exploration potential of the northern South China Sea deepwater basins

Although the huge potential of the northern South China Sea deepwater basins has been proven by a series of discoveries that followed the exploration breakthrough of well LW 3-1-1, recent drilling and other studies have demonstrated the uniqueness and complicated nature of hydrocarbon accumulations of the deepwater basins there. Based on a review of previous work and the latest exploration activities and studies, the purpose of this paper is to discuss the critical controls for hydrocarbon accumulations in the deepwater basins of the northern South China Sea. A terrestrial-marine transitional coal-bearing source rock is proposed to be the primary source rock for the deepwater basins. A marine source rock, which was first identified as contributing to hydro-carbon generation in this region, probably plays a significant role in the deep-and ultra-deep water basins south to the Pearl River Mouth and Qingdongnan basins. The shelf margin delta depositional systems in the Baiyun Sag, sourced from the Pearl River, are currently primary exploration targets in the deepwater part of the Pearl River Mouth Basin, whereas the western Red River delta-ic-submarine fan depositional systems, initially proven by drilling, are the possible major exploration reservoirs in the Qing-dongnan deepwater areas. Current deepwater exploration targets at the large-sized structural traps and deep and ultra-deep areas in the south of the Pearl River Mouth and Qingdongnan basins will be the future exploration focus. Deepwater exploration activities and relevant fundamental studies, supporting and promoting each other, are of great importance to the national energy supply of China, the basic regional studies of the South China Sea, advancements in technology, and development of related deepwater industries, and will safeguard national sovereignty and territorial integrity.     

The deep thermal characteristic of continental margin of the northern South China Sea

Heat flow plays an important role in the study of thermal structure and thermal evolution of continental margin of the northern South China Sea. The analysis of heat flow value shows that margin heat flow in the northern South China Sea is relatively high setting, but the percentage of crustal heat flow is lower than 35% in terrestrial heat flow. The terrestrial heat flow exhibited a current of rise from the Northern Continental Margin to the Southern Central Basin. However, the proportion of crustal heat flow in terrestrial heat flow slowly dropped down in the same direction. It is suggested that the main factor causing high heat flow setting is the moving up of hot material from asthenosphere.     

南海深海塑料垃圾污染

渔业捕捞及商业航运活动的塑料排放是南海深海塑料污染的主要来源。南海深海微塑料污染始于20世纪80年代,具有明显陆源输入的特征,陆架近岸区域微塑料污染严重。陆坡深海峡谷是塑料/微塑料向深海盆地输运的主要通道,近底浊流在输运中发挥了重要作用。综述了南海海底塑料垃圾深潜研究的最新进展,首次提出了深海塑料垃圾生态系统的概念。     

南海浊流地貌

南海深海底部浊流地貌十分发育。陆坡区有大量的海底峡谷,部分峡谷的谷底、越岸区或出口部位分布有超临界流成因的大型沉积物波和周期阶坎底形。多数峡谷在陆坡脚的出口处并未形成像样的海底扇,而深海平原中却保有高丰度的浊流沉积。这可能暗示,南海峡谷浊流的能量较高,即使在经历陆坡脚的减速之后仍有足够的能量维持其沿平缓的深海平原作较长距离的搬运。浊流地貌的出现大多始于晚中新世,其成因与南海及周缘强烈的构造活动有关。     

Diagnostic analysis on the northern South China Sea winter counter-wind current

The winter counter-wind current （also named the South China Sea Warm Current （SCSWC）） in the northern South China Sea （SCS） has been known well for decades, but its mass and momentum origination have not be quantitatively evaluated before. In this paper, the high resolution three-dimensional ocean circulation model is adopted to reproduce the circulation in the northern SCS. The diagnostic analyses are performed to investigate the momentum budget in the northern SCS continental shelf/slope and the momentum propulsion of the SCSWC. It is indicated that the across-shelf pressure gradient and the across-shelf transport are responsible for the formation of the SCSWC, while the along-shelf pressure gradient is balanced by the surface stress, bottom stress, and Coriolis force. The magnitude of the terms in the along-shelf momentum equation is smaller than that in the across-shelf one. The analysis on the momentum budget in the northem SCS will benefit the marine environmental prediction in the future.     

Nd isotopic variations and its implications in the recent sediments from the northern South China Sea

The chondrite-normalized REE distribution patterns in recent sediments from the northern South China Sea and surrounding areas are similar, with the HREE values being almost equal to or slightly higher, and the La_N/Lu_N ratio being lower, than PAAS standards. However, samples from the Xijiang River, a major branch of the Pearl River system, show opposite trends, i.e., with slightly lower HREE values and higher La_N/Lu_N ratio than PAAS. The distribution of Nd isotopes in sediments from the northern South China Sea was controlled by the Pearl River and the inshore area of South China, respectively. The volume of ε_Nd(0) from inshore areas of southern China is apparently higher than that from the Pearl River, and ε_Nd(0) values in offshore sediments and Taiwan Island are between these values. The results clearly show that ε_Nd(0) volume in the northeastern South China Sea is higher than in the southwest part of the northern South China Sea, indicating different source rock types. The main source rock of the Pearl River sediment is carbonate types and affects sediment deposition in the southwest of the northern South China Sea, while the source rock in the inshore area of southern China, mainly Mesozoic and Cenozoic granites, contributes to sediments in the northeastern area of the northern South China Sea. Due to different source rock types, the HREE values and the volume of ε_Nd(0) in recent sediments supplied by the inshore area of southern China are higher than those from the Pearl River. Over 80% of the rapidly accumulated sediments on the northern slope of the South China Sea in the southeast of Dongsha Islands area were derived from Taiwan Island and from inshore areas of South China, with only less than 20% from the Pearl River. Supported by National Natural Science Foundation of China (Grant Nos. 40276019, 40238060 and 40621063), and National Basic Research Program of China (Grant No. 2007CB819501)     

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0—3.5 km?s^-1 velocity are located in the sedimentary structure (2.0—6.0 km in depth and 2.0—4.6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5—6.0 km?s^-1 velocity generally existed in the middle crust (7.0—18.0 km in depth) with an about 2.5—6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.     

Distribution and identification of the low-velocity layer in the northern South China Sea

The low-velocity layer (LVL), closely related with tectonic activities and dynamic settings, has always been a hot topic in the deep crustal structure studies. The deep seismic (OBS/OBH) and onshore-offshore experiments have been extensively implemented in the northern South China Sea (SCS) since the 1990s. Six seismic profiles were finished on the northern margin of SCS by domestic and international cooperations. The features of crustal structures were revealed and five velocity-inversion layers were discovered. Among them three LVLs with 3.0—3. 5 km·s-1 velocity are located in the sedimentary structure (2.0—6.0 km in depth and 2.0—4. 6 km in thickness) of the Yinggehai Basin and Pearl River Mouth Basin. They were identified by the reflective and refractive phases for their shallow depth. The other two LVLs with 5.5—6.0 km·s-1 velocity generally existed in the middle crust (7.0—18.0 km in depth) with an about 2.5—6.0 km thickness in the transitional crustal structure of the northeastern and northwestern SCS. They were detected by the refractive phase from their overlain and underlying layers. We explored the possible tectonic formation mechanisms combining with previously reported results, which provided evidence for the formation and evolution of SCS.     

新生代早期南海北部水系演变

珠江在早渐新世仅是涉及华南沿海地区的小河;到晚渐新世,向西延伸到云贵高原前缘地带;到中新世,现代珠江流域格局初步形成。证据显示,南海北部还发育过一条源自南海西部隆起区的大型水系-昆莺琼古河,后淹没在南海之中,但在南海的沉积充填过程中扮演了重要角色。南海北部水系及沉积环境的重建,对于深刻认识南海新生代早期古地理特征以及该地区的油气勘探均具有重要意义。     

Satellite-derived surface water pCO2 and air-sea CO2 fluxes in the northern South China Sea in summer

An empirical approach is presented for the estimation of the partial pressure of carbon dioxide (pCO2) and air-sea CO2 fluxes in the northern South China Sea in summer using satellite-derived sea surface temperatures (SSTs), chlorophyll-a (Chl a) concentrations, and wind fields. Two algorithms were tested. The first used an SST-dependent equation, and the other involved the introduction of Chl a. Regression equations were developed for summer based on in situ data obtained in July, 2004. Using the monthly average SST and Chl a fields derived from the advanced very high resolution radiometer (AVHRR) and the SeaWiFS (sea-viewing wide field of view sen-sor), respectively, the monthly pCO2 fields were computed. The derived pCO2 was compared with the shipboard pCO2 observations con-ducted in July, 2000. This resulted in a root-mean-square error of 4.6 μatm, suggesting that the satellite-derived pCO2 was in general agreement with the in situ observations. The air-sea CO2 flux was further computed with the aid of the monthly mean QuikSCAT wind speed. We contend that more shipboard data are necessary for refining the empirical algorithms and reducing the uncertainty in the results.