Southern Southern South China Sea Research Articles

Southern South China Sea boundary current transition from summer to winter

The characteristics and mechanism of the southern South China Sea (SCS) Western Boundary Current (WBC) summer-winter transition have been investigated. The transition typically starts in early October and lasts for about two weeks. Above the thermoclines (~100 m), the transition is simultaneous with depth, while below the thermoclines the transition in deeper layer significant lags that in the shallow layers. The geostrophic balance dominates the transition of WBC. Above the thermocline, the transition is determined by the barotropic pressure gradient component. Below the thermocline, the transition is determined by the competition between the barotropic and baroclinic pressures components. When the southern SCS WBC transition above the thermocline starts, the barotropic and baroclinic pressures components offset each other below the thermocline, resulting in the lag of the geostrophic balance. With the depth increases, more time is needed for the barotropic pressures component to enhance enough to dominate a geostrophic balance, which induces the transition lag with depth. Changes in the barotropic pressure gradient component are mainly due to the variations in SCS basin scale wind stress curl, while changes in the baroclinic pressure gradient component below the thermocline are associated with the warming of the deeper water column caused by the downwelling near the continental slope and the disappearing of upwelling off Vietnam.

The Impact of Changes in Sea Level and East Asian Monsoon on Sediment Transport on the Sunda Shelf Since the Last Deglaciation

AbstractThe Sunda Shelf plays a key role in the sedimentation of the southern South China Sea (SCS). However, the impact of past climate changes on sediment transfer to the Sunda Shelf and the southern SCS is still unresolved. Here, we present new data on grain size, clay mineralogy, and Sr‐Nd isotopes of three marine cores, river sediments, and surface sediments of the Sunda Shelf. The results indicate that clay fractions of the central Sunda Shelf and the Gulf of Thailand derived mainly from the Mekong River and the rivers of northern Thailand since the last deglaciation. Significant changes in sediment sources are observed at 7.5 cal ka BP. From 13.3 to 7.5 cal ka BP, sediments in the Gulf of Thailand mainly originated from the rivers of northern Thailand, with contributions from the Mekong River increasing after 7.5 cal ka BP. An opposite shift is observed in the central Sunda Shelf. Such variations in the spatial distribution result from a combination of sea level and East Asian monsoon effects. After 7.5 cal ka BP, less Mekong River sediments were transported southeastwards into the southern SCS, and more sediments were gradually trapped within the delta and transported in a southwesterly direction into the Gulf of Thailand due to intense East Asian winter monsoon, which forced a strong southwestward coastal current. Our results demonstrate the important impact of East Asian winter monsoon strengthening and sea level rise in the mid‐Holocene, which can change sediment sources and transport processes on the Sunda Shelf.

Glacial sea level low-stands regulated the upper ocean hydrology in the southern South China Sea over the past ∼ 280 kyr

The southern South China Sea (SCS) is an ideal region for investigating the low-latitude climatic and oceanic dynamics in the context of local and global effects. However, records of upper-water hydrography beyond the last two glacial cycles are still limited from the southern SCS. In this study, we present shell δ18O, Mg/Ca and Nd/Ca records of three planktonic foraminiferal species (Globigerinoides ruber, Globigerinoides sacculifer and Pulleniatina obliquiloculata) from Core TX05 to explore changes in upper-water hydrography in the southern SCS over the past ∼280 kyr. The results show that shell δ18O of the three species exhibit more positive values in glacials than interglacials. The δ18O residual (i.e. δ18Osw-ice) calculated by shell δ18O subtracting the signals of ice volume and temperature suggests fresher upper seawaters (lower salinity) during glacials than interglacials. This may be caused by terrestrial freshwater discharge from the paleo-Sunda continent that exposed during glacial sea level low-stands. The freshwater discharge may further be ascribed to relatively abundant precipitation over the paleo-Sunda continent during glacials, possibly pointing to the collective effects of the southward shift of the Intertropical Convergence Zone and the enhanced Pacific Walker circulation. The vertical thermal gradients (ΔTrub-obl and ΔTsac-obl) at Site TX05 show lower values during glacials than interglacials. The SST differences of the southern SCS from the northern SCS (ΔSSTS-N) and from the western tropical Pacific (ΔSSTSCS-WTP) correlate well with indicators from the Chinese Loess section, indicating relatively strong East Asian winter monsoon during glacials. The good correlation of ΔT, ΔSSTS-N and ΔSSTSCS-WTP to the relative sea-level suggests a remarkable effect of sea level low-stands and the strengthened East Asian winter monsoon during glacials on the upper ocean hydrography in the southern SCS.

Pollen morphology of selected tropical plants in Peninsular Malaysia and its implication in the paleoecological reconstruction of Southeast Asia

Marine pollen records from the southern South China Sea (SCS) have yielded critical information regarding epicontinental vegetation succession since the last glacial period, which involves understanding the role of tropical rainforests in driving climate change. However, the rich biodiversity and pollen taxa in Southeast Asia are challenging for palynological work, such as detecting pollen signals from various vegetation types and tracing the source areas of pollen grains deposited in the ocean; therefore, improving the taxonomic level of fossil pollen identification is significantly crucial for pollen analysis in tropical regions. To enrich modern pollen references of Southeast Asia, we observed the pollen morphology of 124 identified species (111 genera and 42 families) from tropical Malaysia using a light microscope and scanning electron microscope (SEM). The results show 19 pollen morphotypes, of which the tricolporate type occurs in half of the samples, reflecting the complexity of fossil pollen classification and identification. Pollen sculpture and aperture details of the selected taxa are clearly shown under SEM, which significantly improves the morphological understanding of tropical pollen (e.g., fabaceae, euphorbiaceae, and rubiaceae). This result presents the potential of extracting unique pollen signals to distinguish different vegetation types that contain many common species in Southeast Asia. It highlights the significance of improving pollen identification levels to clarify whether there was a widespread rainforest on the exposed continental shelf during the last glacial period in the southern SCS.

Geochemical characteristics of sediment and pore water affected by cold seeps in southern South China Sea

Cold seep, characterized by active material circulation and methane seepage, is of great importance to reconstruct the paleo-marine environment and trace the origin of life and the occurrence of minerals. Southern South China Sea (SCS) with ample oil and gas resources is an ideal platform for cold seep study, but information and researches on cold seeps here are rather deficient. We studied the geochemical characteristics of sediment cores and pore water combined with seismic profile information in the Beikang Basin, aiming to further understand the nature of cold seeps in this area. Results show the extremely low δ 13CDIC of pore water in each core and we also found sulfate content decreases with depth, the high content of inorganic carbon (DIC) and the corresponding minimumδ 13CDIC in pore water, the kink-type depth profiles of Sr, Ba and Ni enrichment factors and Sr/Mg in the sediments of BH-CL37, the reducing sedimentary environment constrained by the features of Mo, U and REE. The extremely negative δ 13CDIC in the pore water of the sediment cores indicates strong AOM effect in cold seeps and the main biogenic origin of methane. The SO42- depth variation trends of pore water, the high DIC content and the lowest value of δ 13CDIC can define the approximate SMTZ depth of each sediment core effectively. We thought that fluid fluxes and tectonic settings greatly influence the sedimentary environment and geochemistry, leading to the manifest regional differences in the properties and activities of cold seeps. This study of cold seep in southern SCS proves the variation of cold seep in time and space, stressing the necessity of further region-specific analysis towards different cold seeps; and the complex conditions of SCS do function well in offering multiple types of cold seeps.

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.

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.

Provenance of Oligocene–Miocene sedimentary rocks in the Cuu Long and Nam Con Son basins, Vietnam and early history of the Mekong River

The offshore Cuu Long (CLB) and Nam Con Son (NCSB) basins of SE Vietnam are two important Cenozoic hydrocarbon-bearing basins in the southern South China Sea (SCS), which can contribute to understanding the evolution of major SE Asian river systems, in particular the Mekong River. The Oligocene to Early Miocene basin fill of the Cuu Long Basin is dominated by sediment sourced locally from the Da Lat Zone basement on land. Sandstones have abundant Cretaceous detrital zircons and heavy mineral assemblages dominated by apatite and epidote. The Bach Ho Unconformity at c. 16 to 20.5 Ma marks a major provenance change and the overlying Middle to Late Miocene Con Son and Dong Nai formations were sourced by a large river system, which drained Indochina or even the Himalaya, resembling the present-day Mekong River. These formations have heterogeneous detrital zircon populations dominated by Triassic ages and zircon–rutile–tourmaline-dominated heavy mineral assemblages. The Oligocene Cau and Early Miocene Dua formations of the Nam Con Son Basin have a similar provenance to the CLB Con Son and Dong Nai formations, indicating a comparable drainage history of a large proto-Chao Phraya. At the Dua Unconformity at c. 15.4 to 17 Ma the Indochina provenance signature changes to a predominant Sibumasu signature.The early Mekong River evolution is suggested to have involved two river systems. A proto-Mekong 1 initially filled the CLB, while at the same time a proto-Chao Phraya River with a capture area into the Himalaya filled the Malay Basin and/or the NCSB. At the end of the Early Miocene drainage was completely reorganised, and the proto-Mekong 1 captured the headwaters of the proto-Chao Phraya and became the proto-Mekong 2, while the NCSB was filled by sediment from the Malay–Thai Peninsula. This major change marks a very important drainage reorganisation in the southern SCS region.

Seasonal oceanographic changes and their implications for the abundance of small pelagic fishes in the southern South China Sea

The seasonal variations in shelf circulation and their implications for the distribution of light fishing vessels positions (VBD) that capture small pelagic fishes in the southern South China Sea (SSCS) are investigated by using multi-datasets between 2008–2014. It is found that the SSCS shelf circulation is indicated by the strong seasonal reversal western boundary current (WBC) and prominent cyclonic circulation (eddy). The southeastward WBC brings homogeneous salty, colder, and high primary production of nanophytoplankton (PPN) water during winter-time from the northern and the cyclonic eddy region into the southern SSCS. In contrast, less salty, warmer, and less PPN are advected northwestward from Karimata into the northern region. These factors significantly influence the distribution of VBD and in turn the production of captured small pelagic fishes (CPUE), revealing two maxima (peaks) of both VBD and CPUE during the monsoon breaks (MB) with a time-lag of 2–3 month from the monsoon peaks. Relationship between oceanographic parameters and CPUE is examined statistically revealing that ocean current and PPN are significantly affect the CPUE. A relatively high PPN and strong current during the peaks of monsoon are correlated with lower CPUE. In contrast, during the MBs period CPUE maxima are associated with a relatively weak ocean current and consumed PPN that favor the abundance of small pelagic fishes.

Potential geochemical evidence of Porites corals responding to coral bleaching in the 20th century in the Nansha Islands, southern South China Sea

To better understand the long-term survival of modern coral reefs under global warming, the history of past coral bleaching events prior to modern marine ecological surveys must be investigated. Here, 11 dead Porites and 2 Porites with growth hiatuses growing in the 20th century were collected from the Nansha Islands, southern South China Sea (SCS) for monthly Sr/Ca, δ18O and δ13C analyses to explore the climatic conditions and metabolic information corresponding to these growth structures. Among the 13 episodes, the results showed that 8 mortalities occurred in the summers, coral MJ8 died in spring, and the other 2 mortalities and 2 growth hiatuses occurred in the cooling season. In addition to the dead surface of the coral mortalities, stress bands with significantly reduced growth rates could also be observed in the corals. Negative shifts in δ13C occurred prior to 5 coral mortalities and most stress bands were caused in summer, suggesting a sharp reduction in photosynthesis by decreased symbiotic zooxanthellae or photosynthetic pigments. Overall, the mortalities can be concluded to have occurred in summer and stress bands with abrupt δ13C decreases in the last century were mainly caused by thermal coral bleaching in the southern SCS. This in turn suggests that the repeated coral bleaching and mortalities occurred much earlier before the 1980s, which is a longer-term response to modern global warming during the last century.

Origin of large-scale variegated reef limestones in the southern South China Sea: Implications for Miocene regional and global geological evolution

Marine sediments featuring special colors have always played an important role in disclosing regional or global environmental transitions and geological evolution. A deep well (Nanke-1), drilled on Meiji Atoll in 2017, penetrated Cenozoic reef strata in the central Nansha Block (also referred to as Dangerous Grounds) in the southern South China Sea (SCS) and revealed two pronounced variegated (containing colors of white, reddish and red brown) reef limestone layers in the Miocene. Similar large-scale variegated reef limestones have not been found in other reef drilling cores to date, so its coloration origin, formation mechanism and geological implications are unclear. Here, synthesized mineralogical, petrographic, and geochemical analyses demonstrated that the presence of trace amounts of amorphous microsized hematite with varying crystallinity imparted reddish or red brown colors to reef limestones. Hematite formed during meteoric water-induced karstification after the carbonate platform was exposed subaerially. Long-term exposures were mainly controlled by regional tectonic uplifts closely associated with the subduction of the proto-SCS, the seafloor spreading of the Southwest Sub-basin and the collision between blocks (i.e., the Sabah collision and the Palawan collision). The different tectonic settings have led to the different developments of carbonate platforms in the northern and southern SCS during most of the Early and Middle Miocene, but the major expansion of the high-latitude Antarctic ice sheets and the resultant significant drop in global sea level may have contributed to wide exposure of carbonate platforms both in the northern and southern SCS in the late Middle Miocene. Our studies demonstrated that the occurrences of variegated reef limestones in isolated oceanic reef carbonate platforms have great potential in indicating subaerial exposure, regional tectonic uplift or sea level fall.

Mesoscale Eddy Effects on Nitrogen Cycles in the Northern South China Sea Since the Last Glacial

Archaeal ammonia oxidation is the most important intermediate pathway in regulating the oceanic nitrogen cycle; however, the study of its specific role on a geological time scale is restricted to a specific part of marginal seas; thus far, only in the southern South China Sea (SCS). To explore the spatial pattern of the role of archaeal ammonia oxidation in the SCS, the GDGT-[2]/[3] ratio (Glycerol Dialkyl Glycerol Tetraether), an indicator of the archaeal ammonia oxidation rate, was analyzed and examined from the collected data profiles since the last glacial period in the northern SCS. The results showed that the GDGT-[2]/[3] ratio in the northern SCS was opposite to that in the southern SCS, with higher GDGT-[2]/[3] values during the Holocene compared to the last glacial period. Based on existing published depths of thermocline (DOT) data in the northern SCS since 30 ka, we believe that hydrological structural variations induced by mesoscale eddies caused this difference. Therefore, physical processes are very important factors that control the nitrogen cycle over a long-time scale. This study may provide new insights into the understanding of the role of archaeal ammonia oxidation within the marine nitrogen cycle over geological time scale.

Sediment distribution and dispersal in the southern South China Sea: Evidence from clay minerals and magnetic properties

Modern source-to-sink processes in the southern South China Sea (SCS) remain poorly understood. In this study, clay minerals and magnetic properties are used to identify the sources and transport pathways of surface sediments in the southern SCS. The distributions of four clay mineral species in the deep southern SCS are mainly controlled by sediment sources, while clay minerals on the outer Sunda Shelf mostly reflect reworking/recycling of older weathered sediments. Clay mineral analyses indicate that the Mekong River is the predominant contributor of clay minerals to the southern SCS, with minor contributions from Luzon and/or Palawan and northern Borneo. The contribution from the Luzon and/or Palawan arc to the southern SCS is semi-quantitatively evaluated as 20% and is mainly smectite. Most sediments in the southern SCS are magnetically dominated by mixtures of magnetite, hematite and goethite in variable proportions. Concentration-dependent magnetic parameters decrease gradually seaward and then sharply towards the outer deep sea. The grain size of magnetite particles also shows a significant fining trend with distance from the southwestern sources. In addition, the magnetic minerals change from hematite- and goethite-dominated to magnetite-dominated type as going further offshore. These magnetic characteristics suggest that the magnetic minerals in the southern SCS are mainly sourced from the southwest through summer monsoon-driven surface currents. Finally, combining the provenance signature indicated by clay minerals and the transport information revealed by magnetic properties, this study highlights that the Mekong River plays a major role in modern source-to-sink processes in the southern SCS. In brief, our study sets a good example for tracing sediment provenance and transport in other marginal seas worldwide.

Significant Changes in Microbial Communities Associated With Reef Corals in the Southern South China Sea During the 2015/2016 Global‐Scale Coral Bleaching Event

AbstractMicrobial communities play important roles as coral symbionts, but their changes among coral species in response to thermal bleaching events are not well understood. Therefore, we focused on the microbial communities associated with coral species in the southern South China Sea (SCS) during the 2015/2016 global‐scale coral bleaching event (GCBE). Samples of eight typical coral species were collected before and during the GCBE, and the microbial communities, potential gene functions, and zooxanthellae densities (ZDs) were analyzed. We found that ZDs significantly decreased among all coral species during the GCBE. Alpha diversities of the microbial communities also significantly decreased at different taxonomic levels among these coral species. Principal coordinates analysis revealed significant differences in beta diversity among coral specimens, which were divided into two groups before and during the GCBE. Microbial gene functional prediction showed that microbial community physiology significantly changed during the GCBE, with decreased coverage in metabolism, membrane transport, replication and repair, and increased coverage in cell motility and signal transduction. Moreover, the abundance of potential pathogens such as the genus Vibrio greatly increased (from ~0.28% to ~52.92%) during the GCBE, whereas the abundance of several beneficial microbes such as Endozoicomonas significantly decreased (from ~26.10% to ~0.91%), resulting in an obvious decline in the coral‐holobiont physiological functions. Thus, the GCBE greatly affected the health of coral species in the southern SCS, by reducing the biodiversity of associated microbial communities and increasing the abundance of potential pathogens.

A high-resolution δ18O record of modern Tridacna gigas bivalve and its paleoenvironmental implications

The lack of monthly-resolved paleoenvironmental records beyond the instrumental era hinders our full understanding of global climate dynamics. Tridacna gigas shells have a large potential to quantitatively document the variability of the ocean surface environment on time scales from daily to interannual in the past, while the applicability of the geochemical proxies of Tridacna gigas needs to be quantitively investigated. Here we establish a chronology for a Tridacna gigas shell from the southern South China Sea (SCS) by identifying daily growth laminae using a laser scanning confocal microscope (LSCM). Our high-resolution Tridacna gigas shell δ18Oshell record throughout 1989–2013 shows a high correlation with changes in local precipitation and the index of El Niño-Southern Oscillations (ENSO) on the interannual time scale. Together with observed sea surface temperature (SST), we calculated seawater δ18O (δ18Osw) and derived sea surface salinity (SSS) change in the southern SCS based on the Tridacna gigas shell δ18O record. Our salinity-budget calculation results suggest that the SSS variability in the southern SCS is predominantly affected by the vertical mixing and sea surface freshwater flux. Our study reveals that the Tridacna gigas shell is an ideal archive for high-resolution and quantitative paleoenvironmental reconstructions in tropical ocean regions, thus providing a new window to explore paleoenvironmental changes on annual and interannual time scales.

Unique Authigenic Mineral Assemblages and Planktonic Foraminifera Reveal Dynamic Cold Seepage in the Southern South China Sea

Many cold seeps and gas hydrate areas have not been discovered beside the Beikan basin in the southern South China Sea (SCS), and their characteristics and histories also remain poorly known. Here we describe authigenic minerals and the carbon and oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber from sediment core 2PC, recovered from the gas hydrate zone of the Nansha Trough, southern SCS, to elucidate its history of dynamic cold seepage. We infer that the occurrence of authigenic gypsum crystals and pyrite concretions, and anomalously negative δ13C values of Globigerinoides ruber, reflect paleo-methane seepage. Two major methane release events were identified, based on remarkable excursions in foraminifera δ13C at depths of 150–250 cm and 350–370 cm. Euhedral gypsum crystals and tubular pyrite concretions co-occur with extremely negative planktonic foraminifera δ13C values, indicating a shift in the sulfate methane transition zone and a change in the methane flux. Our data suggest that authigenic mineral assemblages and δ13C values of planktonic foraminifera provide a valuable tool in elucidating the characteristics of dynamic methane seepage in a marine environment.

Vegetation evolution-based hydrological climate history since LGM in southern South China Sea

Abstract The reconstruction of vegetation evolution and climate changes at low latitudes since the Last Glacial Maximum (LGM) remains incomplete. In this study, we reconstructed the palynological history of vegetation changes since the LGM using a sediment core collected from the slope in the southern South China Sea (SCS). The chronology of the core was based on eight AMS14C dates, and 85 stratigraphic samples were palynologically analyzed. Our results denote that the vegetation reconstructed during the LGM in the southern SCS was characterized by a relatively high percentage of herb pollen and a modest decrease in the number of fern spores, suggesting a temperature decline and humidity change in this region. Further, the preservation of a rich diversity of pollen from the tropical lowland rainforest indicated that the rainforests might have covered large areas of the exposed Sunda Shelf during the LGM. The dynamic concentration of the Pinus pollen in the core, which is a proxy for the East Asian winter monsoon (EAWM), satisfactorily matched the changes in the Atlantic meridional overturning circulation and Australian-Indonesian summer monsoon, especially during the Heinrich Stadial 1. This suggests that the climate at the high latitudes of the Northern Hemisphere can drive the Intertropical Convergence Zone (ITCZ) southwards based on the enhanced EAWM. The southward-shifting ITCZ may change the hydrological climate of the equator and the Southern Hemisphere. The pteridophyte spores, which are good regional precipitation indicators, were positively correlated with the warm pool convection and inversely related with the EAWM, possibly indicating the considerable impact of the high-latitude climate in the Northern Hemisphere on precipitation in the tropical regions. Our results indicate that during the last deglaciation, the tropical hydrological changes were relatively moderate when compared with the drastic change observed at high latitudes and were attributed to the regulation of the El Nino-Southern Oscillation-like state. Our sequence of pollen and spores from the southern SCS can be used to observe the detailed dynamics of climatic evolution in tropical regions. The results of this study are essential to understand the complicated climate processes and the relation between high and low latitudes.

Occurrence and impacts of tropical cyclones over the southern South China Sea

AbstractThis study investigates the seasonal variation of tropical cyclone (TC) occurrences over the southern South China Sea (SCS) based on the Joint Typhoon Warning Center dataset during 1979–2016. In this region, TCs are most active from October to December (OND), rather than in the active TC season from July to October (JASO) in other western North Pacific (WNP). According to the location of their genesis, the southern SCS TCs are divided into two groups. One consists of TCs generating in the WNP, which are induced by a westward steering flow and move into the southern SCS in OND. The other group contains TCs generated locally in the SCS that move into the southern SCS, which peak in OND due to the positive relative vorticity and high genesis potential index (GPI). In this region, TC genesis is influenced by the El Niño‐Southern Oscillation (ENSO). On an interannual scale, TCs tend to occur in La Niña years due to high GPI. In addition, the mid‐level relative humidity and the low level vorticity play important roles in the contribution of the environmental factors of GPI during ENSO. Both composite analysis and a case study show the impact of TCs on precipitation. TC occurrence is accompanied by heavy rainfall, which is triple the amount of climatological rainfall in OND during 1979–2016, over 15 mm⋅day−1. Nevertheless, the influence of locally generated TCs, rather than TCs generated in the WNP, contributes to more heavy rainfall in the southern SCS.

Diversity of Symbiodiniaceae in 15 Coral Species From the Southern South China Sea: Potential Relationship With Coral Thermal Adaptability.

It is well-known that the adaptability of coral-Symbiodiniaceae symbiosis to thermal stress varies among coral species, but the cause and/or mechanism behind it are not well-understood. In this study, we aimed to explore this issue based on zooxanthellae density (ZD) and Symbiodiniaceae genus/subclade. Hemocytometry and next-generation sequencing of the internal transcribed spacer region 2 (ITS2) marker gene were used to observe ZDs and Symbiodiniaceae genera/subclades associated with 15 typical coral species in the southern South China Sea (SCS). Average ZDs of all corals were in low levels, ranging from 0.84 to 1.22 × 106 cells cm−2, with a total of five Symbiodiniaceae genera, Symbiodinium, Cladocopium, Durusdinium, Fugacium, and Gerakladium, as well as 24 dominant subclades, were detected and varied among these coral species. Pocillopora verrucosa was dominated by Durusdinium (subclade D1/D1a), and other colonial corals were dominated by Cladocopium, but the subclades were varied among these species. Porites lutea and Montipora efflorescens were dominated by C15, and Echinopora lamellosa, Hydnophora exesa, and Coscinaraea exesa were dominated by C40. Acropora corymbosa, Merulina ampliata, and five species of Faviidae were mainly associated with Cladocopium types of C3u and Cspc. In contrast to other colonial corals, the dominant subclade of solitary Fungia fungites was C27, with high host specificity. Our study indicates that coral thermal stress adaptability is mainly affected by dominant Symbiodiniaceae type instead of ZD in the southern SCS. Some heat-sensitive corals, such as P. verrucosa corals, have acquired a high abundance of heat-tolerant Durusdinium to adapt to thermal stress. This could be the main reason for these corals becoming the dominant corals in this reef region. Background subclades analyses showed significant differences among coral species in subclade quantity and diversity. These suggest that numbers of coral species may have adapted to high environmental temperature by adopting various symbionts and/or associating with heat-tolerant Symbiodiniaceae.