Geological environment in the South China Sea

The South China Sea(SCS),situated in southern China,at the junction of the Pacific Plate,the Eurasian Plate,and the Indian Ocean Plate,is a northeast-southwest trending semi-enclosed sea.It spans an area of approximately 3.5 million square kilometers and has an average water depth of about 1200 m,its deepest point reaching 5559 m.In 2021,a scientific expedition(called as U1 voyage)in the South China Sea was organized by the Innovation Research Team of Guangdong Special Key Program from March to April,this marks the first comprehensive scientific research voyage to the southern Uboundary corridor.Consisting of a total of 30 papers,this special issue is to share a portion of the research findings from this scientific expedition U1 voyage,covering six aspects:1)characteristics of the marine ecosystem in the SCS and its response to marine dynamic processes;2)multi-scale marine dynamic processes,sea-air interactions,and forecasting techniques in the SCS;3)geomorphology and geological structure;4)sedimentary processes and resource potential in the SCS;5)geostrategy,rights and interests maintenance and strategic countermeasures in the SCS;6)marine scientific instruments.By integrating the scientific research with the study of history,jurisprudence and international strategies,this issue presents new insights into the formation history and scope evolution of the SCS,and it also seeks to establish a new scientific framework based on the marine governance and development of the SCS.

Mg/Ca,Ba/Ca,and S/Ca ratios as environmental and growth proxies for bivalve shells from the Haima cold seep,South China Sea

Bivalve shell fossils,cemented by authigenic carbonates,are widely spread in the Haima cold seep,Qiongdongnan Basin of the South China Sea(SCS).In this study,we examined an element profile of Gigantidas platifrons in the Haima cold seeps at a water depth of 1381 m.Based on the scanning electron microscope(SEM)analyses,the prismatic layer and nacreous layer were identified,which are characterized by prismatic structure and stratified structure,respectively.In addition,the profile can be subdivided into two parts:altered and unaltered zones.Laser inductively coupled plasma mass spectrometry(LA-ICP-MS)mapping shows that the element concentrations of the altered zones were influenced by the authigenic carbonate rocks,whereas the element concentrations of unaltered zones remain stable.In-situ X-ray diffraction(XRD)analyses show that the mineral constituent of the prismatic layer is mainly composed of aragonite.Along with the growth profile,Mg/Ca ratios of unaltered zones have minor variations,ranging 0.72-0.97 mmol/mol(mean=0.87 mmol/mol),with estimated temperatures of 3.8-4.1℃,indicating that the temperature of the surrounding seawater remains constant and agree with the measured data of 3.9℃which was conducted by a conductivity-temperature-depth system(CTD).The minor variations of Ba/Ca ratios(0.01-0.06 mmol/mol;mean=0.04 mmol/mol)indicate a relatively stabilized salinity of the surrounding seawater.S/Ca ratios show large variations of 0.04-4.15 mmol/mol(mean=1.37 mmol/mol).S/Ca ratios have regular variations which generally correspond to the variations of the Mg/Ca ratios,highlighting that the S/Ca ratios of bivalve shells show the potential to reflect the growth rate of the Gigantides.However,further studies should be carried out on the understanding of the links between the S/Ca ratios and seepage intensity of cold-seep fluids.

An Analysis of the Indian Modi Administration's Policy on the South China Sea Issue

Geographically, India is not a country in the region of the South China Sea, nor is it a country concerned in the issue on a geopolitical level or a US ally in the Asia-Pacific region. It has been very cautious to avoid taking sides in the South China Sea issue. However, since Modi took office in 2014, changes have taken place in the external environment that India faces, in maritime safety strategy, as well as in India's policies toward the US and China. India is becoming increasingly active in the issue of the South China Sea, voicing criticism and taking precautions against China. While India's policy on the South China Sea will not be completely the same as that of the United States and Japan, due to concerns for its own interests in the future, it is quite possible that India will take advantage of the issue of the South China Sea and will adopt more comprehensive ways to get involved in the issue.

Microbial diversity in cold seep sediments from the northern South China Sea

South China Sea （SCS） is the largest Western Pacific marginal sea. However, microbial studies have never been performed in the cold seep sediments in the SCS. In 2004, ＂SONNE＂ 177 cruise found two cold seep areas with different water depth in the northern SCS. Haiyang 4 area, where the water depth is around 3000 m, has already been confirmed for active seeping on the seafloor, such as microbial mats, authigenic carbonate crusts and bivalves. We investigated microbial abundance and diver- sity in a 5.55-m sediment core collected from this cold seep area. An integrated approach was employed including geochemistry and 16S rRNA gene phylogenetic analyses. Here, we show that microbial abun- dance and diversity along with geochemistry profiles of the sediment core revealed a coupled reaction between sulphate reduction and methane oxidation. Acridine orange direct count results showed that microbial abundance ranges from 105 to 106 cells/g sediment （wet weight）. The depth-related variation of the abundance showed the same trend as the methane concentration profile. Phylogenetic analysis indicated the presence of sulphate-reducing bacteria and anaerobic methane-oxidizing archaea. The diver- sity was much higher at the surface, but decreased sharply with depth in response to changes in the geochemical conditions of the sediments, such as methane, sulphate concentration and total organic carbon. Marine Benthic Group B, Chloroflexi and JS1 were predominant phylotypes of the archaeal and bacterial libraries, respectively.

Thermal Ionization Mass Spectrometry (TIMS)-U-Series Ages of Corals from the South China Sea and Holocene High Sea Level

Coral samples collected from the waters off the Leizhou Peninsula, the Hainan Island and the Yongxing Island of the Xisha Islands were analyzed by thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) for their U-Th systematics. Their uranium contents are mostly lower than 3μg/g, and the δ+{234}U(T) values are within the range of 150±5. X-ray diffraction analysis showed that the corals are all composed of aragonite. This demonstrates that the corals determined have maintained a close system for 7000 years and their skeletons bear much information about the primitive tropical marine environment. The highly precise age sequence of the corals determined has revealed that there occurred three phases of high sea level in the South China Sea waters, which are dated at 6799-6307 a B.P., 4472-4285 a B.P. and 1279-1012 a B.P., respectively. The three phases lasted 492 a, 187 a and 267 a, respectively. These three phases of high sea level are corresponding to the warm environments that had appeared in China during the Megathermal Period and the Medieval Warm Period in the past ten thousand years.

Geochemical and microbial characters of sediment from the gas hydrate area in the Taixinan Basin, South China Sea

The Taixinan Basin is one of the most potential gas hydrate bearing areas in the South China Sea and abundant gas hydrates have been discovered during expedition in 2013. In this study, geochemical and microbial methods are combinedly used to characterize the sediments from a shallow piston Core DH_CL_11（gas hydrate free） and a gas hydrate-bearing drilling Core GMGS2-16 in this basin. Geochemical analyses indicate that anaerobic oxidation of methane（AOM） which is speculated to be linked to the ongoing gas hydrate dissociation is taking place in Core DH_CL_11 at deep. For Core GMGS2-16, AOM related to past episodes of methane seepage are suggested to dominate during its diagenetic process; while the relatively enriched δ18O bulk-sediment values indicate that methane involved in AOM might be released from the ＂episodic dissociation＂ of gas hydrate.Microbial analyses indicate that the predominant phyla in the bacterial communities are Firmicutes and Proteobacteria（Gammaproteobacteria and Epsilonproteobacteria）, while the dominant taxa in the archaeal communities are Marine_Benthic_Group_B（MBGB）, Halobacteria, Thermoplasmata, Methanobacteria,Methanomicrobia, Group C3 and MCG. Under parallel experimental operations, comparable dominant members（Firmicutes and MBGB） are found in the piston Core DH_CL_11 and the near surface layer of the long drilling Core GMGS2-16. Moreover, these members have been found predominant in other known gas hydrate bearing cores, and the dominant of MBGB has even been found significantly related to gas hydrate occurrence. Therefore,a high possibility for the existing of gas hydrate underlying Core DH_CL_11 is inferred, which is consistent with the geochemical analyses. In all, combined geochemical and microbiological analyses are more informative in characterizing sediments from gas hydrate-associated areas in the South China Sea.

The Southward Extension of Cathaysia Block: Evidence from Zircon UPb Dates of Borehole Volcanics in the Northern South China Sea

Five Paleogene volcanics sampled from the northern South China Sea were analyzed via LA-ICP-MS zircon U-Pb dating, including basalt and andesite from Borehole SCSVI and volcanic agglomerate from Borehole SCSV2, respectively. A total of 162 zircon U-Pb dates for them cover an age range from Neoarchean to Eocene, in which the pre-Paleocene data dominate. The Paleogene dates of 62.5±2.2 Ma and 42.1±2.9 Ma are associated with two igneous episodes prior to opening of South China Sea basin. Those pre-Paleocene zircons are inherited zircons mostly with magmatogenic oscillatory zones, and have REE features of crustal zircon. Zircon U-Pb dates of 2518-2481 Ma, 1933- 1724 Ma, and 1094-1040 Ma from the SCSV1 volcanics, and 2810-2718 Ma, 2458-2421 Ma, and 1850-993.4 Ma from the SCSV2 volcanics reveal part of Precambrian evolution of the northern South China Sea, well comparable with age records dated from the Cathaysia block. The data of 927.0±6.9 Ma and 781±38 Ma dated from the SCSV2 coincide with amalgamation between Yangtze and Cathaysia blocks and breakup of the Rodinia, respectively. The age records of Caledonian orogeny from the Cathaysia block are widely found from our volcanic samples with concordant mean ages of 432.0±5.8 Ma from the SCSV1 and of 437±15 Ma from the SCSV2. The part of the northern South China Sea resembling the Cathaysia underwent Indosinian and Yanshannian tectonothermal events. Their age signatures from the SCSV1 cover 266.5±3.5 Ma, 241.1±6.0 Ma, 184.0±4.2 Ma, 160.9±4.2 Ma and 102.8±2.6 Ma, and from the SCSV2 are 244±15 Ma, 158.1±3.5 Ma, 141±13 Ma and 96.3±2.1 Ma. Our pre-Paleogene U-Pb age spectra of zircons from the borehole volcanics indicate that the northern South China Sea underwent an evolution from formation of Precambrian basement, Caledonian orogeny, and Indosinian orogeny to Yanshannian magmatism. This process can be well comparable with the tectonic evolution of South China, largely supporting the areas of the northern South China Sea as part of southward extension of the Cathaysia.

Classification and 3-D distribution of upper layer water masses in the northern South China Sea

Using the fuzzy cluster analysis and the temperature-salinity(T-S) similarity number analysis of cruise conductivity-temperature-depth(CTD) data in the upper layer(0–300 m) of the northern South China Sea(NSCS), we classify the upper layer water of the NSCS into six water masses: diluted water(D), surface water(SS),the SCS subsurface water mass(U_S), the Pacific Ocean subsurface water mass(U_P), surface-subsurface mixed water(SU) and subsurface-intermediate mixed water(UI). A new stacked stereogram is used to illustrate the water mass distribution, and to examine the source and the distribution of U_P, combining with the sea surface height data and geostrophic current field. The results show that water mass U_P exists in all four seasons with the maximum range in spring and the minimum range in summer. In spring and winter, the U_P intrudes into the Luzon Strait and the southwest of Taiwan Island via the northern Luzon Strait in the form of nonlinear Rossby eddies, and forms a high temperature and high salinity zone east of the Dongsha Islands. In summer, the U_P is sporadically distributed in the study area. In autumn, the U_P is located in the upper 200 m layer east of Hainan Island.

Crustal S-wave velocity structure across the northeastern South China Sea continental margin: implications for lithology and mantle exhumation

The northeastern margin of the South China Sea (SCS), developed from continental rifting and breakup, is usually thought of as a non-volcanic margin. However, post-spreading volcanism is massive and lower crustal high-velocity anomalies are widespread, which complicate the nature of the margin here. To better understand crustal seismic velocities, lithology, and geophysical properties, we present an S-wave velocity (VS) model and a VP/VS model for the northeastern margin by using an existing P-wave velocity (VP) model as the starting model for 2-D kinematic S-wave forward ray tracing. The Mesozoic sedimentary sequence has lower VP/VS ratios than the Cenozoic sequence;in between is a main interface of P-S conversion. Two isolated high-velocity zones (HVZ) are found in the lower crust of the continental slope, showing S-wave velocities of 4.0–4.2 km/s and VP/VS ratios of 1.73–1.78. These values indicate a mafic composition, most likely of amphibolite facies. Also, a VP/VS versus VP plot indicates a magnesium-rich gabbro facies from post-spreading mantle melting at temperatures higher than normal. A third high-velocity zone (VP : 7.0–7.8 km/s;VP/VS: 1.85–1.96), 70-km wide and 4-km thick in the continent-ocean transition zone, is most likely to be a consequence of serpentinization of upwelled upper mantle. Seismic velocity structures and also gravity anomalies indicate that mantle upwelling/ serpentinization could be the most severe in the northeasternmost continent-ocean boundary of the SCS. Empirical relationships between seismic velocity and degree of serpentinization suggest that serpentinite content decreases with depth, from 43% in the lower crust to 37% into the mantle.

Evidence of China's sea boundary in the South China Sea

According to a series of important historical maps,i.e.,the Location Map of the South China Sea Islands,the Nansha Islands,Zhongsha Islands,Xisha Islands,Yongxing Island and Shidao Island,and Taiping Island（archived by the Territorial Administration Division of the Ministry of Interior of Republic of China in 1946）,and the Administration District Map of the Republic of China published in 1948,the dashed line surrounding the South China Sea Islands represents China＇s sea boundary in the South China Sea at that time.It was both connected with,and an extension of,the land boundary of China.At that time the dashed line was used to represent the waters boundaries while the solid line was used to represent the land boundary—a universal method used in maps that was then recognized internationally.The above observation provides historical and scientific evidence of China＇s sea boundary in the South China Sea that is useful for the international maritime delimitation over the South China Sea area.

Geochemical characteristics of Oligocene-Miocene sediments from the deepwater area of the northern South China Sea and their provenance implications

Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values（110–115 Ma for U1435 and L21; 150 Ma for H1）, indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River（Pearl River）, as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.

Identification of Microplankton in South China Sea with Image-Matching Individual PCR

In this study, marine microplankton were identified by combining standard light microscopy with Sanger 18S rRNA gene sequencing. The image-matching individual PCR technique was applied to identify the image collectable unicellular microplankton to genera. Instead of pure strain culture and morphological identification, microplankton individual cells were isolated and fixed with glutaraldehyde, frozen and stored for months. Finally, they were imaged under a microscope and molecularly identified via phylogenetic analysis of their 18S ribosomal RNA gene(18S rDNA). Microplankton cells were collected at 30 locations in South China Sea, and were assigned to 21 known and 4 unidentified genera(2 uncultured fungi and 2 uncultured stramenopiles) with phylogenetic analysis in parallel to the morphological identification.

The Turn of the Tide:Explaining China’s Growing Assertiveness in the South China Sea

Before the late 2000s,China employed a diplomatic“next generation”approach to its territorial disputes in the South China Sea(SCS).Beginning in the late 2000s,however,China has assumed threatening attitudes,and taken a strong stand against other disputants.Why has China recently shown its assertiveness when dealing with the South China Sea issue?This study argues that two factors-China’s growing naval capabilities and the growing presence of China’s strategic rivals around the South China Sea-led China to adopt a more assertive policy regarding the South China Sea issue.

南海地区新局势、新挑战与新思路——2023年度“南海论坛”综述

2023年南海地区局势复杂多变,危机频发,南海问题依然是全球最为复杂的海洋争端和影响中国-东盟关系的重要因素。如何在中美战略博弈大背景下应对亚太海洋安全新形势,相关问题亟须深入的探讨与总结。在南京大学中国南海研究协同创新中心主办的2023年度“南海论坛”上,与会专家围绕南海维权维稳斗争的新形势与新问题、美国与小多边机制干涉南海的挑战与应对、南海岛礁主权与海洋权益的历史视角和法律斗争新态势与新挑战,以及增进南海合作的新思路新办法等议题进行了深入讨论。面对法律、认知、话语斗争等诸多形势变化,中国需要进一步强化证据链建设和话语权构建,结合具体条件重塑相关实践的规范性,增进南海合作和共同开发。

20世纪上半叶日本在南沙群岛扩张的历史考察

20世纪上半叶,日本在对外扩张过程中密切关注中国南沙群岛,并将实现对其的控制和占领作为主要目的。日本政府对南沙群岛进行了多次非法调查,编造依据,企图“合法”侵占中国南沙群岛。日本不仅掠夺南沙群岛资源,而且在太平岛建设军事设施,为进军东南亚做准备。日本谋划、侵占中国南沙群岛是其推南进政策的重要一环。在此过程中,日本关注的重点是南沙群岛的战略地位,而非经济资源。日本的侵占导致南沙群岛成为二战后国际社会需要处理的问题之一。

Environmental Redox Changes of the Ancient Sea in the Yangtze Area during the Ordo-Silurian Transition

Extensive organic-matter （OM） rich facies （black shales） occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections （Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province） straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon （TOC）, pyrite sulphur, and different species of Fe, including dithionite-extractable Fe （FED）, pyrite Fe （FeP）, HCl-extractable Fe （FeH）, and total Fe （FeT）, in black shales, as well as other redox proxies, such as the SIC ratio, the ratio between highly reactive Fe （FeHR = FeD ＋ FeP） and FeT, and the FeP/（FeP ＋ FeH） ratio, known as the degree of pyritization （DOP）. In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios （0.20-0.77; avg. 0.45）, high DOP values （0.21-0.72; avg. 0.54）, and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low FeHR/FeT ratios （0.10-0.35; avg. 0.21）, low DOP values （0.11- 0.40; avg. 0.28）, and SIC values are clustering on the normal marine value （SIC = 0.36）. The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios （0.32-0.49; avg. 0.41）, high DOP values （0.46-0.68; avg. 0.53） and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post-glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios （ 0.00-0.08, avg. 0.02）, low FeHR/FeT ratios （0.10-0.37, avg. 0.27）, low DOP values （0.01-0.27, avg. 0.10）, and TOC contents （0.72%-4.27%, avg. 2.55%）. Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high （0.94- 9.32%, avg. 4.44%）, but low （0.35%-2.12%, avg. 1.29%） in the mid-early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.

A Corpus-Based Critical Discourse Analysis of Trump and Biden Administrations’China Policies

The theory of proximization is an effective discourse strategy to study the speaker’s ability to achieve his own legitimacy or reinforce the other’s illegitimacy,and its superiority can be maximized by means of quantitative and comparative analysis.In this study,we collected reports on Trump’s and Biden’s policies on China to build two small corpora,with a total of 11,030 words in the Trump corpus and 17,566 words in the Biden corpus.The critical discourse analysis is combined with proximization theory.With the help of BFSU Qualitative Coder 1.2,Antconc 3.5.7,and Log-Likelihood and Chi-Square Calculator 1.0,a critical cognitive score of the relevant discourse was conducted from the perspective of proximization theory.It has been found that:(1)Both Trump and Biden administrations resort to a large number of spatial proximization strategies to build ODCs converging to IDCs with China as the ODC,posing a threat to internal physical IDCs;(2)in the use of temporal proximization strategy,both administrations use primarily modal verbs and various entities to construct ODCs that extend indefinitely into the present and future,emphasizing the urgency and the threat of the effect and reinforcing the legitimacy of their actions;(3)in terms of axiological proximization strategy,the two administrations differ greatly from each other,indicating that there are still discursive biases.

Characteristics and Trends of the Biden Administration’s Latin American Policy

Since taking office in 2021, US President Joe Biden has rectified former President Trump’s domestic and foreign policies. However, Biden has continued the Trump administration’s idea of “excluding China and protecting the courtyard,” which permeates the entire process of policy adjustment on Latin America. The purpose is to damage and hinder China–Latin America relations and cooperation. After the outbreak of the Russia–Ukraine conflict, the Biden administration intensified efforts to exclude extraterritorial forces represented by China and Russia that seek to regain control of the backyard to a greater extent. The Biden administration has made progress in easing the antagonism between the US and Latin America and repairing ally system in the western hemisphere, however, its efforts to reshape U.S.–Latin America relations have encountered many challenges.