Southern South China Sea Research Articles

Ancient hydrocarbon slicks recorded by a coral atoll in the South China Sea

The intensifying oil slicks exert deleterious effects on coral reefs, hence attracting many investigations on coral's tolerance and resilience to anthropogenic oil pollution. We hypothesize that intense seafloor hydrocarbon seepages in the past could also produce oil slicks and subsequently contaminate coral reefs nearby, which may provide clues on their responses to natural oil contamination. Here, a long coral reef core (NK1) drilled in the isolated Meiji Atoll in the southern South China Sea is analyzed for aliphatic hydrocarbons to test this hypothesis. Two depth intervals, centered at 45–65 m and 95–110 m, corresponding to ages of ca. 0.3–0.6 Ma and 1.3–1.5 Ma, respectively, are characterized by geological-configured hydrocarbons indicative of ancient natural oil pollution. Because the Meiji Atoll stands on a volcanic seamount and in isolation in the sea, we deem that natural oils were incorporated into contemporary coral reefs by surface oil slicks, which were derived from hydrocarbon seepages at the seafloor of the adjacent petroliferous basins. The natural hydrocarbon seepages should result from regional tectonic movements that occurred extensively in the middle and late Pleistocene, likely in association with the arc-continent collision in the Taiwan region since 2 Ma. Albeit the two significant contamination events, it appears that they exerted insignificant disturbances on the growth and development of coral reefs, perhaps protected by certain holobiotic oil-degrading bacteria as indicated by excess production of bacterial branched fatty acids during the events. Our study thus suggests a high resilience of coral reefs to oil pollution under natural conditions, which, of course, does not mean that they would survive readily from the abrupt and intense anthropogenic pollution.

Observation of Near-Inertial Internal Gravity Waves in the Southern South China Sea

Two sets of more than 850 days of mooring records and satellite altimeter data are used to explore the features and spatiotemporal evolution of near-inertial waves (NIWs) near Nansha Island in the southern South China Sea (SCS). The observed NIWs are dominated by clockwise (downward energy propagation) motions and show a clear blue shift with a distinct peak frequency of 1.09 f during two large NIW events. The near-inertial kinetic energy (NIKE) is primarily concentrated in the upper layer and radiated downward. The largest value of depth-integrated NIKE reaches 3.5 KJ/m2. Besides, the NIWs are dominated by the first three modes, which account for 80% of the total NIKE. Moreover, the depth-integrated NIKE exhibits an apparent seasonal variation, with the largest NIKE in winter, which is almost three times larger than that in other seasons. Every large NIKE event is attributed to the passage of storms and is dominated by mode-2 NIWs. The dominance of the mode-2 NIWs is likely caused by the interaction between NIWs and mesoscale eddies.

Coexistence of savanna and rainforest on the ice-age Sunda Shelf revealed by pollen records from southern South China Sea

Exposure of the presently submerged continental shelf of Southeast Asia during low sea-level stands of the Quaternary is thought to be deeply involved in the biogeographic evolution of this biodiversity hotspot. Inland proxy records are mounting to argue grassland expansions over Southeast Asia during glacial periods, lending supports to the existence of a transequatorial “savanna corridor” as hypothesized. However, pollen records retrieved from marine sediments have indicated widespread rainforests on the exposed northern Sunda Shelf. To reconcile these seemingly conflicting observations, here we present new, multisite marine pollen records from the southern South China Sea focusing on the Last Glacial Maximum (LGM) time interval. It is found that the sites in front of the Sunda Shelf palaeo-rivers contain substantially more Poaceae pollen, in particular a smaller size fraction, than the site offshore North Borneo. Among the sites, abundant lowland tree pollen with similar major taxa is also observed. The co-occurrence of grass and rainforest pollen suggests a longitudinal climate gradient with increasing aridity toward the interior of glacial Sundaland. Combining our results with various palynological evidence from marine sediments seems to indicate that the vegetation on the exposed Sunda Shelf might not be fully recognized by previous studies. This is likely due to an admixture of pollen from other source regions, particularly Borneo where the rainforest was retained during glacial periods. Therefore, the extant vegetation records, from both terrestrial and marine archives, support the hypothesis of a “savanna corridor” over the LGM sundaland.

Formation and implication of cold-seep carbonates in the southern South China Sea

Southern South China Sea (SCS), rich in oil and gas resources, can provide a favorable condition for the formation and development of cold seeps. However, the geochemical effect of cold seep activities and the carbon cycle in the SCS are not well understood. Cold-seep carbonates as a window to exploration of gas hydrate are of great significance to investigate the material circulation in cold-seep system and to reconstruct the paleo-climate environment in the SCS. In this research, eight cold-seep carbonates collected from the Beikang Basin, southern SCS, are generally presented as yellow-black irregular column, crust and block, and mainly composed of high magnesium calcite (HMC), indicating they probably formed inside the deep fluid migration pathway. The middle rare earth element (MREE) enrichment with no or positive Ce anomaly in the carbonates from BH-CL37A suggests that they precipitated in anoxic pore water, while heavy rare earth element (HREE) enrichment in BH-CL32A carbonate shows great effect of seawater and they were formed in shallower depth. Different enrichments of redox sensitive trace elements and (Mo/U) EF indicate the strong anoxic environment. It is concluded that the difference of fluid environment could affect the enrichment degree and precipitation rate of trace elements by the relations between trace elements and Al. Extremely depleted 13C, slightly enriched 18O and the variation of Sr isotopes have archived the main biogenic methane source and the co-effect of seawater and seep fluid driven from a deep source (possibly containing the water from hydrate dissociation). The unique and complex geological conditions in the southern SCS will lead to the distinct regional specificity of the cold seep, which is different from the cold seep system in the northern SCS. This study plays an essential role in fulfilling the understanding of the activities of cold seeps in the SCS, revealing the element migration and transformation in cold seeps and their influencing factors.

Impact of Thermal Forcing over the Southeast of the Tibetan Plateau on Frequency of Tropical Cyclones Affecting Guangxi during Boreal Summer

Tropical cyclones entering coastal areas adversely affect southern China. However, changes in the frequency of tropical cyclones affecting the west of southern China remain unclear. Our study reveals the possible impact of the thermal forcing anomaly over the southeast Tibetan Plateau (TP) on the frequency of tropical cyclones affecting Guangxi formed within the west of 120° E during boreal summer. Further analysis indicates that the cooling over the southeast TP is accompanied by local descending motions over southeastern TP and compensating ascending motions over eastern Indochina Peninsula and results in a reduced 850–200 hPa vertical wind shear over the north of 15° N in South China Sea (SCS), which is conducive to the westward development of tropical cyclones and favorable conditions for the formation of TCs affecting Guangxi over the SCS. Finally, the results from a linear baroclinic model experiment also verify that the changes in the 850–200 hPa vertical wind shear over southern SCS and compensating vertical motions over eastern Indochina Peninsula are associated with the thermal forcing anomaly over the southeast TP. Our results imply that in summer the thermal forcing anomaly over TP should be emphasized when interpreting and predicting the frequency of tropical cyclones affecting local areas in southern China.

Mangrove sediment erosion in the Sunda Shelf during meltwater pulses: Insights from biomarker records

Mangroves have received increasing attention in recent years for their high carbon storage capacity. The effects of sea-level change during the last glacial period on terrestrial deposition and coastal mangrove ecosystems were investigated using various organic lipid proxies in marine sediment cores from the Sunda Shelf in the southern South China Sea (SCS). The gradual reduction in the content of long-chain n-alkanes and n-alkanols, BIT index, and increasing δ13Corg trend from the last glacial to the Holocene indicated that the depocenter moved landward from the studied location during the deglacial sea-level rise. Remarkable peaks in the mangrove proxy (Taraxerol/n-C28 alcohol ratio) occurred during meltwater pulse (MWP) events in the Sunda Shelf area, associated with the drowning and destruction of mangroves that could not withstand the rapid sea-level rise. The decomposition of carbon-rich mangrove deposits may have contributed to atmospheric CO2 concentration during two strong MWP events in Sunda Shelf. Our results suggest the vulnerability of mangrove systems upon rapid sea level change with positive feedback for global warming.

Assessment of mercury contamination and food composition in commercially important marine fishes in the southern South China Sea

This study examined mercury contamination in common commercial marine fish caught from the South China Sea. Specifically, the muscle tissue mercury levels of four species (n = 10), one each from the Scombridae, Serranidae, Carangidae, and Nemipteridae families, was examined using a direct mercury analyzer (MA-3000); their trophic level was assessed using TrophLab tm software. Provisional tolerable weekly intake levels (PTWI) and the relationships between length–mercury concentration and weight–mercury concentration of species using Pearson correlation coefficient were identified. Samples from the Serranidae family had the highest muscle mercury concentrations, followed by Nemipteridae, Carangidae, and Scombridae in descending order. The results show that the highest mercury concentrations were found in Epinephelus heniochus (2.34 ±0.22μg/g) from the Serranidae family, while the lowest mercury concentrations were found in Scomberomorus guttatus (0.05 ± 0.00 μg/g) from the Scombridae family. This study suggests that factors such as size, composition of food in the fish gut, and habitat had the greatest influence on the mercury levels found in these fishes. The PTWI data showed that only 31% of the species in the current study had not exceeded the recommended safety level of 1.6 μg/kg BW/week methylmercury intake. A consideration of PTWI can help to reduce fish consumption risks to consumers by providing a recommended consumption level of each fish.

Modeling dissolved organic carbon exchange across major straits and shelf breaks in the South China Sea

The South China Sea (SCS) is the largest semienclosed marginal sea in the western Pacific. The advective supply of dissolved organic carbon (DOC) from the open ocean to the SCS influences its carbon cycle and biogeochemical processes. However, quantitative studies of DOC exchange across different straits and the shelf to the SCS are particularly limited. In this study, we investigated seasonal DOC transport across four major straits and cross-shelf breaks using a coupled physical–biogeochemical model. The modeling results show that the seasonal variations in DOC transport through the major straits are largely controlled by the monsoon. In winter, associated with the northeast monsoon, the magnitudes of DOC transport from the four straits are generally larger than those in summer with the southwest monsoon. DOC transport is eastward at the Luzon Strait and northward at the Taiwan and Karimata straits during the southwest monsoon and changes direction during the northeast monsoon. The annual mean of depth-integrated DOC transport is 86.7 Tg C yr−1 westward at the Luzon Strait, 19.1 Tg C yr−1 northward at the Taiwan Strait, 25.9 Tg C yr−1 southward at the Karimata Strait, and 46.0 Tg C yr−1 southward at the Mindoro Strait. DOC transport across the shelf break is a result of geostrophic or quasigeostrophic balance. In the north and west, the annual net cross-shelf break transports are onshore with magnitudes of 8.8 and 31.1 Tg C yr−1, respectively. In the southern SCS, the annual net transport is offshore with a magnitude of 18.3 Tg C yr−1. The net export of total labile and semilabile DOC from the SCS has a flux of 10.7 Tg C yr−1. This study provides seasonal budgets of DOC exchange between the SCS and its adjacent oceans, which contribute to a better evaluation of the carbon cycle in the SCS.

Zooplankton Fecal Pellet Characteristics and Contribution to the Deep‐Sea Carbon Export in the Southern South China Sea

AbstractZooplankton fecal pellets play a significant role in particulate organic carbon (POC) export, which has been widely reported in different ocean regions. However, zooplankton fecal pellet in the tropical marginal sea and its contribution to the total POC flux has not been sufficiently investigated yet. Here, for the first time we report zooplankton fecal pellet characteristics and fluxes in the South China Sea from June 2020 to May 2021 via one mooring with two sediment traps deployed at 500 and 2,190 m depths. Average fecal pellet numerical flux was 2.30 × 104 and 4.22 × 104 pellets m−2 d−1 at 500 and 2,190 m, respectively, corresponding to average fecal pellet carbon (FPC) flux from 0.31 to 0.71 mg C m−2 d−1. Small ellipsoidal and spherical pellets accounted for more than 90% of the total numerical flux, while large cylindrical pellets, although relatively rare (6%), accounted for almost 15% of the FPC flux. Both fecal pellet numerical and carbon fluxes were higher in winter, when the strong East Asian winter monsoon winds and heavy rainfall promoted the marine primary production. Higher fecal pellet fluxes combining with the presence of extra‐large pellets at 2,190 m compared to those at 500 m suggested the in‐situ repackaging of deep‐dwelling zooplankton communities in the mesopelagic and bathypelagic zones. Contribution of fecal pellets to the total POC flux ranged from 0.4% to 30.0% (averaging 9.0%), with higher values occurring during winter monsoon, at which time fecal pellets became a critical conveyor of carbon export to the deep sea.

Meridional response of Western North Pacific paleocyclone activity to tropical atmospheric circulation variability over the past millennium

Sedimentary records of past tropical cyclone (TC) activity indicate large variability and spatial heterogeneity on the interannual-century scales. However, the meridional variation patterns and climatic forcing mechanisms for regional TC activity are not well understood because of the short and incomplete observational record. Here, we present a near-annually resolved record of intense TC activity derived from a back-reef lagoon at Anle Atoll in the southern South China Sea since the Little Ice Age (LIA). Our reconstruction indicates that TC activity was higher during the early LIA than what has been observed throughout the instrumental record. Basin-wide compiled records of the western North Pacific (WNP) cyclogenesis revealed strong evidence of a meridional seesaw pattern for storm activity between relatively low and high latitudes over the past millennium. This pattern suggests a centennial-scale link with Pacific hydroclimate atmospheric circulation shifts. A combination of southward displacement of the Intertropical Convergence Zone and westward migration of the Pacific Walker Circulation, as well as damped El Niño Southern Oscillation variability, likely contributed to enhanced TC activity during the LIA in the low latitudes of the WNP. External forcing of solar irradiance budgets profoundly affects the hydroclimate and TC climatology in the WNP.

Marine Heatwaves in the South China Sea: Tempo-Spatial Pattern and Its Association with Large-Scale Circulation

A marine heatwave (MHW) can significantly harm marine ecosystems and fisheries. Based on a remotely sensed sea surface temperature (SST) product, this study investigated MHWs behaviors in the South China Sea (SCS) throughout the warm season (May to September) from 1982 to 2020. The distributions of the three MHW indices used in this study showed significant latitudinal variations: more frequent, longer, and more intense MHWs appear in the northern SCS, and less frequent, shorter, and weaker MHWs appear in the southern SCS. Using the empirical orthogonal function (EOF) method, we found that the first leading modes of the three MHW indices account for more than half of the total variance. The first leading modes reveal uniform anomalies throughout the SCS, with the maximum in the deep central portion and its surroundings. Their corresponding time series showed significant interdecadal variations, with a turning point around 2009. Since 2010, the SCS has seen an increase in the frequency, length, and severity of MHWs. The incidence of MHWs has been linked to the presence of stable near-surface anticyclonic anomalies, which reduced cloud cover and increased solar radiation. This abnormal pattern was usually accompanied by the intensification and westward shift of the western North Pacific subtropical high (WNPSH). The findings imply that MHWs in the SCS may be predictable on interannual and decadal scales.

Geomorphic development of an unvegetated shingle cay on the Ximen Reef in the southern South China Sea

Unvegetated shingle cay (USC) is a type of coral island typically located in the southern South China Sea (SCS). Currently, there is a lack of detailed understanding of its development process and dynamic mechanism. The geomorphic development of a representative USC at the Ximen Reef in the southern SCS was examined through a field survey and high-resolution imagery analysis. The USC consisted of shingle sediments composed of coral skeletal clastics. It exhibited a composite geomorphic structure with shingle ridges and a central depression. From 2000 to 2020, the USC showed highly dynamic changes such as an increase in fully emerged area, morphological variations, and southeastward migrations. The USC development model has four main stages: 1) a shingle bank representing the embryonic foundation of the USC, 2) a mound-shaped shingle cay representing the early growth of the USC, 3) a single shingle ridge-depression representing the formation of the composite structure, and 4) a multiple shingle ridges-depression representing the adjustment of the geomorphic structure. The USC has undergone all development stages multiple times over the past two decades. The centripetal refracted wave convergence, which is controlled by the elliptical reef shape, is the key dynamic factor influencing the USC development. Monsoons and tropic cyclones play a joint role in the morphological variations in the USC during the past two decades. The short-term development of the USC at the Ximen Reef helps us understand the long-term evolution of the USCs in the southern SCS under past and future climate changes.

Winter–Summer Transition in the Southern South China Sea Western Boundary Current

Abstract The winter–summer transition in the southern South China Sea (SCS) western boundary current (WBC) is studied. Two categories have been identified. In case 1, the southern SCS WBC transition in the lower layer (below the thermocline) lags that in the upper layer (above the thermocline). In case 2, there is no transition lag at full depth. In both categories, the geostrophic balance dominates the transition. In case 1, the upper layer geostrophic balance is dominated by the sea surface height pressure gradient (SSHPG) and Coriolis forcing during southern SCS WBC transition. Therefore, there is no transition lag with depth in the upper layer. Below the thermocline layer, the competition between the SSHPG and the density pressure gradient (DPG) determines the transition. During the transition, the amplitudes of the SSHPG and DPG are basically equivalent. The SSHPG needs time to develop sufficiently larger than the DPG. Therefore, the transition in the deeper layer significantly lags that in the shallower layer. The reversal of the SSHPG is mainly attributed to the change in the basin-scale wind stress curl over the southern SCS. The change in the DPG is mainly associated with the cooling of the water along the western continental slope, which is induced by upwelling. In case 2, there is no cooling along the western continental slope, and then the amplitude of the DPG is always far smaller than that of the SSHPG. Responding to the change in the SSHPG, the southern SCS WBC transition behaves consistently at full depth. Significance Statement We have a comprehensive understanding of the South China Sea (SCS) circulation patterns in winter and summer. However, their seasonal transitions remain unclear, and a better understanding of them is potentially helpful for improving ocean circulation modeling and prediction. This paper focuses on the winter–summer transition in the SCS western boundary current (WBC). Above the thermocline (∼100 m), the transition behaves consistently in the vertical direction and is controlled by the conversion of the sea surface height–induced pressure gradient. Below the thermocline, the transition in the deeper layer of the WBC significantly lags that in the shallower layer of the WBC, which is associated with the competition between the SSH-induced pressure gradient and the density-induced pressure gradient at the sea surface.

Spatial and seasonal variations of near-inertial kinetic energy in the upper South China Sea: Role of synoptic atmospheric systems

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper ocean of the South China Sea (SCS) are examined using the Global 1/12° Analysis Hybrid Coordinate Ocean Model and the Navy Coupled Ocean Data Assimilation reanalysis product. The annual mean NIKE in the upper SCS is characterized by a rapid decay with depth and a southwestward decrease from the west of Luzon Island to the southern SCS, reflecting the pattern of near-inertial energy input by the atmospheric wind field. Owing to the changes of near-inertial wind forcing as well as the mixed layer depth, NIKE in the upper SCS exhibits a pronounced seasonal cycle. The magnitude of mixed layer NIKE averaged in the SCS in November-January is found to be approximately twice of that in April-June. Further analysis shows that the variation of near-inertial wind forcing in the SCS can, to a large extent, be explained by northerly cold surges during the winter SCS monsoon and strong tropical cyclones from the Tropical Pacific. In addition, the wind reversal during the monsoon transitional period and the low-level convergence formed by the elevated terrain of islands also contribute to oceanic near-inertial energy in the SCS.

Spatio-temporal distribution and mechanism of Cenozoic magmatism in the South China Sea and adjacent areas: Insight from seismic, geochemical and geochronological data

ABSTRACT Cenozoic magmatism was highly active in the South China Sea (SCS), and mostly dominated by basalts. The magmatic rocks are mainly distributed in the lower slope and subbasins of the SCS, in addition to sporadic and small-scale Cenozoic volcanism on the SCS shelf. This paper presents a complete map of the Cenozoic magmatic rocks of the SCS with magmatic rock boundaries. This paper also presents an analysis of geological survey and published data in terms of seismic profiles, ages, geochemistry and isotopic systematics of the Cenozoic magmatic complexes of the SCS and adjacent areas. Many of wells or dredges are located in magmatic rocks identified by seismic profiles. Three periods of magmatic activity are distinguished: (1) a pre-spreading period (>33 Ma); (2) a syn-spreading period (33–15 Ma) and (3) a post-spreading period (<15 Ma). The thickest volcanic rocks encountered more than 1000 m (possibly Oligocene to Miocene) in the Boayan 1/1A well in the Palawan Trough of the southern SCS during Cenozoic. The oldest magmatic rocks are andesites (62.5 ± 2.2 Ma) in the northern SCS (offshore) and andesites (65 ± 3 Ma) in Penghu (on land). The latest modern magmatic activity was detected in 1923 offshore of a submarine volcanic eruption (10°09´29″N, 109°00′50″E, 20 m below sea level) and along the shore of a volcanic eruption in the Philippines. Cenozoic magmatic rocks of the SCS include at least 1008 rock bodies: the smallest is 0.00012 km2, the largest is 1528 km2 and the total area covers 46,309 km2. The Yangjiang-Yitong fault and Zhongnan-Reedbank fault divide the magmatic rocks into two different sections in the eastern and western SCS. The ages of the magmatic rocks in the eastern SCS are mostly older than those in the western SCS. The magmatic rocks in the northern, southern, eastern, western SCS and adjacent areas, and SCS basin are related to extension, subduction+collision, subduction, extrusion, extension+cooling+subsidence, respectively.

DISTRIBUTION OF BENTHIC FORAMINIFERA IN THE CORAL REEFS ECOSYSTEM OF PULAU BIDONG, TERENGGANU, SOUTHERN SOUTH CHINA SEA

A study on the distribution of benthic foraminiferal species in the surface sediments of coral reefs was carried out in Pulau Bidong in the southern South China Sea (SCS). Samples were collected at 12 sites around Pulau Bidong at water depths ranging from 5 m to 30 m. Live (rose bengal stained) and optimally preserved empty tests of benthic foraminifera were analysed based on their chamber and apertural characteristics observed under binocular microscope (Leica Zoom 2000). The results revealed that 32 genera of benthic foraminifera were found in the coral reefs sediments. Amphistegina was the most abundant genus with relative abundance of 68% and frequently recorded at depths of between 10-15 m and 25-30 m. Other common genera that were found were Calcarina, Elphidium, Heterolepa, Heterostegina, Quinqueloculina, Peneroplis and Triloculina. These genera showed depth distribution preference in the shallow water zone (5 m depth). The highest diversity index (2.3) and number of taxa (26 genera) were recorded at station W2, located at the western part of Pulau Bidong. The lowest diversity index (0.2) and number of taxa (seven genera) were recorded at station S3, located at the southern part of Pulau Bidong. The frequency distribution and diversity of benthic foraminifera in the study area were related to water depth.

A history of the latest and Neogene unconformities, offshore Palawan and the southern South China Sea

The age and character of the main unconformities offshore Palawan is resolved, and their history reviewed. Some very deep and very shallow unconformities have insufficient well data, but from the Oligocene to Late Miocene times a series of unconformities can be dated, characterised and correlated regionally to help develop a new tectono-stratigraphic framework. This framework does not support the widely cited idea of Early Miocene subduction of a Proto-South China Sea plate followed by collision over a plate suture.The absence or very weak expression of a Base Miocene Unconformity (c. 24 Ma), so strongly expressed across northern Borneo, also suggests a major revision of tectonic models is required. In spite of many cited reports, there is no evidence for an unconformity in the later Early Miocene (c. 17 to 15 Ma), which was a period of gradually increasing compression and uplift. The main mid-Neogene seismic unconformity is the Red Unconformity dated at about 12–13 Ma (equivalent to the DRU of Sabah). This was when the uplift of a foreland over-thrust system paused, and a locally erosional surface was rapidly transgressed.This review describes the neglect of analytical sciences that remain a crucial part of basic geological studies.

The Formation of Atolls: New Insights From Numerical Simulations

AbstractSeveral theories have been proposed to explain atoll formation. While karst dissolution during glacial periods and preferential coral reef accretion along raised bank margins during deglaciations and interglacials have been invoked to explain atoll formation, the respective roles of karst dissolution and reef margin construction in atoll formation have not been adequately evaluated by simulations. In this study, we conducted three‐dimensional numerical simulations of the Quaternary development of Meiji Atoll in the southern South China Sea based on interpreted data from a 2020‐m‐deep borehole drilled on its northeast rim in 2018. Our results suggest that the origin of atolls is more likely due to spatially differential dissolution across margin and interior areas (i.e., minimal along margins and maximal in bank centers) rather than preferential reef accretion along margins of flat‐topped banks. Preferential reef accretion along margins of flat‐topped banks that can result in central lagoons and atoll morphology can hardly result in the formation of lagoonal patch reefs that reach mean sea level. Preferential reef accretion along margins is mainly predicated on the karst‐induced morphology that has a central depression surrounded by raised rims, that is, using antecedent karst morphologies. If topographic highs in the lagoon are similar in elevation to the margins, reef accretion on these topographic highs can be similar to that observed on the margins, resulting in lagoonal patch reefs that reach mean sea level. Our simulation shows that spatially differential dissolution across margin and interior areas is a critical driver of worldwide central lagoons and atoll formation.

TEMPERATURE AND SALINITY CHARACTERISTICS IN THE SOUTHERN SOUTH CHINA SEA AND STRAIT OF MALACCA

Temperature and salinity characteristics in the southern South China Sea and Strait of Malacca are investigated using CTD data and the global ocean model HYCOM. CTD data were obtained from cruises taken in March 2015, March 2019, and July-August 2019. The results showed that the average reading of temperature and salinity in the Strait of Malacca during March 2015 was 28.60±0.708 °C and 32.61±1.01 psu, respectively. The average readings during March 2019 were 29.31±1.593 °C and 32.38±0.884 psu, respectively. The average temperature and salinity in the Strait of Malacca during the August 2019 cruise were 29.63±0.701 °C and 32.52±0.553 psu, respectively. Meanwhile, in the southern South China Sea, the average readings in July 2019 were 28.98±0.310 °C and 33.07±0.126 psu and in August 2019 were 27.94±1.01 °C and 33.36±0.363 psu. Seasonal variations presented by the 15-years (1997-2012) HYCOM climatological model show that the sea surface temperature is warmer by approximately 1 - 2 °C during the southwest monsoon months (June-July-August) than during the northeast monsoon (December-January-February) in the southern South China Sea. The seasonal temperature variations in the Strait of Malacca are not significant except for the southern part. The sea surface salinity exhibits no clear seasonal cycle except for the coastal areas.

Coral community of Holocene coral reef in the southern South China Sea and its significance for reef growth

Ancient coral reefs are rich archives of historical process and vital baselines for future development. Coring gives a window into the growth of ancient coral reefs and the fossil coral community's response to paleoenvironment. Well NK-1 drilled from Meiji Reef is the longest scientific core with the highest recovery in the South China Sea. This study focused on the upper section of the Well NK-1 accumulated during the Holocene. From 8200 yr BP, Holocene reef initiated on the substrates of Pleistocene limestone and stopped accreting vertically at 4800 yr BP. It consisted of an initiation and fast reef growth (5.6 mm yr−1) from 8200 yr BP to 7300 yr BP, a shift to slow reef growth (2.0 mm yr−1) between 7300 yr BP and 5400 yr BP, a return to a rapid reef growth (8.8 mm yr−1) from 5400 yr BP to 4800 yr BP, and finally a cessation of upward accretion and a reinforced lateral progradation since 4800 yr BP. A total of 11 coral genera and 16 Acropora species were identified from the Holocene segment of the core. The diversity of coral communities had an impact on reef growth. Coral communities with less genera and absolute predominance of branching Acropora were corresponded with a rapid vertical accumulation of reef framework with higher accretion rate. Acropora corals with rapid growth and dispersion by fragmentation were principal reef-builders during the Holocene, with A.valida being the most prevalent coral species. The highest Acropora species richness occurred at the geological era of 5400–5300 yr BP when reef accumulated with the fastest upward accretion rate of 23.1 mm yr−1. This intricate and detailed pattern presented a new and novel model for the development of Holocene reef, indicating that the coral community and key reef builders were crucial to reef growth. These findings are extremely helpful for understanding the current ecological state of modern coral reefs as well as their potential response to global climate change.