Xisha Archipelago Research Articles

Microhabitat-differentiated distribution of culturable and antagonistic bacteria in marine ecosystem: Seawater as the original microbial provider

Abstract: Due to the typical tropical climatic characteristics, the Xisha archipelago in the South China Sea has derived typical biological treasures, particularly culturable bacteria. Microbial resources in nature will be valuable resources for industrial application only after being culturable microorganisms. Herein, the global distribution of culturable and antagonistic bacteria against aquatic pathogenic vibrios, including Vibrios parahaemolyticus, Vibrios harveyi, Vibrios alginolyticus, Vibrios campbellii, and Vibrios owensii, in thirty-nine stations of the Xisha archipelago across four main microhabitats of sediment, seawater, corals, and marine organisms were investigated by cultivation-based strategies. Vibrio was the dominant genus (68.98%) among the culturable bacteria, followed by Pseudoalteromonas (9.36%) and Alteromonas (4.91%). Only four targeted antagonistic strains with an antagonistic specificity of a certain Vibrio were obtained, namely the strains of SP52, C219 and W198, which provided a new insight of precise disease prevention/treatment by further revealing the targeted antagonistic mechanism. The correlations between the abundance of culturable bacteria and environmental factors in the whole Xisha archipelago and four microhabitats showed that salinity as the main factor significantly affected the distribution of 6 microbial genera, while total inorganic nitrogen (TIN), total carbon (TC), and PO43− significantly related to 3 genera, 2 genera, and 1 genus, respectively. Importantly, we found that microhabitat discrepancy rather than distance discrepancy of environmental factors made a greater contribution to the distribution of culturable bacteria and antagonistic bacteria and the capability of antagonistic bacteria, which was emphasized as an important factor in shaping bacterial communities. Additionally, seawater was the original microbial provider of other microhabitats in the marine ecosystem, which was the opposite of the terrestrial ecosystem in that the soil is the original microbial provider.

Quantitative detection of Vibrio alginolyticus strain XSBZ14 by a newly developed RT-PCR method

PurposeCoral degradation is a worldwide ecological problem. Bacterial diseases are a great danger to coral health. The pathogenic bacterium Vibrio alginolyticus XSBZ14 isolated from diseased coral had been identified as the pathogenic bacterium of Porites andrewsi White syndrome (PAWS) in Xisha Archipelago on transmission experiment. To date, the molecular mechanism by which this pathogen causes disease is unknown, and molecular diagnostics for diseases caused by this bacterium have not been developed. In an effort to restore damaged coral ecosystems in the South China Sea, efforts are underway to transplant flat-branch shore corals. There is therefore an urgent need to further develop specific and rapid detection methods for V. alginolyticus XSBZ14 in order to prevent this epidemic and ensure the successful implementation of compilation transplants.MethodsAt first, a low sequence identity single-copy sequence S2 was selected from the genome by in-house Perl script. Using the designed specific primers, four different types of standard curves were subsequently plotted for the accurate quantification of the strain XSBZ14 in four different samples (DNA, bacterial suspension, coral tissue, seawater). Then, use the strain to infect the Galaxea fascicularis and test the strain in the coral culture water during the week.ResultsThe rapid detection method of pathogenic bacteria by RT-PCR was established. The limit of detection (LOD) of the RT-PCR was 0.88 pg/reaction (0.44 pg/μL) in DNA, 2 CFU/reaction (1000 CFU/mL) in bacterial suspension, 2 CFU/reaction in coral tissue, and 20 CFU/reaction in seawater for the strain XSBZ14, respectively. In addition, according to the detection results of the RT-PCR, the strain XSBZ14 could survive in Galaxea fascicularis for a week, and the strain could also be detected from its reared seawater.ConclusionThese results indicated that the RT-PCR detection method of a coral pathogenic strain XSBZ14 was established. The method is a robust tool for the rapid detection and quantification of the coral pathogen, XSBZ14, and is very useful for PAWS epidemiological survey and specific pathogen-free coral transplantation in the South China Sea. And other coral species and their habitats might act as an important reservoir for the strain XSBZ14 and mediated its horizontal transmission in coral reefs.

Seismic architecture of Yongle isolated carbonate platform in Xisha Archipelago, South China Sea

This study presented recently reprocessed multi-channel seismic data and multi-beam bathymetric map to reveal the geomorphology and stratigraphic architecture of the Yongle isolated carbonate platform in the Xisha Archipelago, northwestern South China Sea. Our results show that the upper slope angles of Yongle carbonate platform exceed 10° and even reach to ∼32.5° whereas the lower slope angles vary from .5° to 5.3°. The variations of slope angles show that margins of Yongle Atoll belong to escarpment (bypass) margins to erosional (escarpment) margins. The interior of carbonate platform is characterized by sub-parallel to parallel, semi-continuous to continuous reflectors with medium-to high-amplitude and low-to medium-frequency. The platform shows a sub-flat to flat-topped shape in its geometry with aggradation and backstepping occurring on the platform margins. According to our seismic-well correlation, the isolated carbonate platform started forming in Early Miocene, grew during Early to Middle Miocene, and subsequently underwent drowning in Late Miocene, Pliocene and Quaternary. Large-scale submarine mass transport deposits are observed in the southeastern and southern slopes of Yongle Atoll to reshape the slopes since Late Miocene. The magmatism and hydrothermal fluid flow pipes around the Yongle Atoll have been active during 10.5–2.6 Ma. Their activity might intensify dolomitization of the Xisha isolated carbonate platforms during Late Miocene to Pliocene. Our results further suggest that the Yongle carbonate platform is situated upon a pre-existing fault-bounded block with a flat pre-Cenozoic basement rather than a large scale volcano as previously known and the depth of the basement likely reached to 1400 m, which is deeper than the well CK-2 suggested.

Cyclonic and Anticyclonic Asymmetry of Reef and Atoll Wakes in the Xisha Archipelago

A high-resolution (∼500 m) numerical model was used to study the reef and atoll wakes in the Xisha Archipelago (XA) during 2009. Statistical analyses of simulation data indicated strong cyclonic dominance in the mixing layer (above ∼35 m) and weak anticyclonic dominance in the subsurface layer (35∼160 m) for both eddies and filaments in the XA. The intrinsic dynamical properties of the flow, such as the vertical stratification and velocity magnitude, and the terrain of reefs and atolls had a significant effect on the asymmetry. Without considering the existence of reefs and atolls, the “background cyclonic dominance” generated under local planetary rotation (f≈4.1×10−5 s−1) and vertical stratification (with mean Brunt–Väisälä frequency N = 0.02 s−1 at 75 m) was stronger for filaments than eddies in the upper layer from 0∼200 m, and the larger vorticity amplitude in the cyclonic filaments could greatly enhance the cyclonic wake eddies. Furthermore, inertial–centrifugal instability induced selective destabilization of anticyclonic wake eddies in different water layers. As the Rossby number (Ro) and core vorticity (Burger number, Bu) decreased (increased) with the water depth, a more stable state was achieved for the anticyclonic wake eddies in the deeper layer. The stratification and slipping reefs and atolls also led to vertical decoupled shedding, which intensified the asymmetry.

Seismic Imaging and 3D Architecture of Yongle Atoll of the Xisha Archipelago, South China Sea

AbstractYongle atoll in the Xisha (Paracel) Archipelago is an isolated carbonate platform developed on Precambrian metamorphic and Mesozoic volcanic rocks since the early Miocene. To identify the 3D stratigraphic architecture and evolution of this platform, 13 high‐resolution seismic profiles and shallow‐to‐deep water multi‐beam data were processed and analyzed to reveal seismic facies, sequence boundary reflectors, seismic units, and platform architecture. Nine types of seismic facies were recognized based on their geometry, which included seismic amplitude, continuity, and termination patterns; additionally, six reflections, i.e., Tg, T60, T50, T40, T30, and T20, were identified in the Cenozoic strata. Five seismic units, SQ1 (lower Miocene), SQ2 (middle Miocene), SQ3 (upper Miocene), SQ4 (Pliocene), and SQ5 (Quaternary), were identified from bottom to top across the platform. The platform grew rapidly in the middle Miocene and backstepped in the late Miocene–Pliocene. Here, we discuss the developmental characteristics and evolution of the Yongle Atoll, in combination with drilling wells, which can be divided into four stages: the initiation stage in the early Miocene, the flourishing stage in the middle Miocene, the partial‐drowning stage in the late Miocene–Pliocene, and modern atoll in the Quaternary.

Backstepping patterns of an isolated carbonate platform in the northern South China Sea and its implication for paleoceanography and paleoclimate

Backstepping of carbonate platform margins is an important process for understanding how carbonate platform responds to paleoenvironment changes. The products of backstepping, such as backstepped margin, backstepping surface and backstepping succession have been widely documented on both modern and ancient platforms around the world. However, the causes of platform backstepping have not been understood fully yet. This study presents a detailed description of variations in backstepping pattern of Xuande platform in the northern South China Sea using 2D seismic and bathymetric data. The results show two episodes of backstepping events occurred during the late Middle Miocene and the latest Miocene to the Early Pliocene. The former backstepping event occurred on the eastern margin of the platform and likely resulted from upwelling and turbulence related to the onset of contour currents in the Xisha Archipelago. The latter backstepping event shows a change of backstepping distances at different platform margins with more pronounced backstepping along the winter-monsoon windward side compared to the corresponding leeward side. It is proposed that this event was largely the result of vigorous currents and waves impinging on the platform margins related to an intensification of the East Asian monsoon and a relative sea-level rise. A difference in hydrodynamic energy was responsible for the variations in backstepping distances at platform margins. Platform margins exposed to higher-energy environment backstepped further than margins exposed to lower-energy envrionment. Results from this work indicate the morphology and stratigraphic architecture of carbonate platforms are strongly controlled by the hydrodynamic energy, here related to currents and waves driven by East Asian monsoon winds.

Soil water dynamics and water balance on a tropical coral island

AbstractUnderstanding soil water dynamics and the water balance of tropical coral islands is important for the utilization and management of their limited freshwater resources, which is only from rainfall. However, there is a significant knowledge gap in the influence of soil water on the water cycle of coral islands. Soil water dynamics and the water balance of Zhaoshu Island, Xisha Archipelago were thus investigated using soil moisture measurements and the Hydrus‐1D model from October 2018 to September 2019. Over the study period, vegetation transpiration, soil evaporation, groundwater recharge and storage in the vadose zone were approximately 196, 330, 365 and 20 mm, occupying 22%, 36%, 40% and 2% of annual rainfall total (911 mm), respectively. For the wet season (from May to October) these values became 75, 202, 455 and 40 mm, occupying 10%, 26% and 59% and 5% of the seasonal rainfall total (772 mm), respectively. During the dry season (from November to April), a dry soil layer between 40 and 120 cm depth of the soil profile was identified that prevented water exchange between the upper soil layers and the groundwater resulting in the development of deep roots so that vegetation could extract groundwater to supplement their water requirements. Vegetation not only consumes all dry season rainfall (140 mm) but extracts water deeply from groundwater (90 mm) as well as from the vadose layer (20 mm). As such, the vegetation appears to be groundwater‐dependent ecosystems. The research results aid us to better understand the process of water dynamics on coral islands and to protect coral island ecosystems.

High-resolution acoustic data revealing periplatform sedimentary characteristics in the Xisha Archipelago, South China Sea

Newly collected high-resolution acoustic data are used to describe the morphologies and sedimentary characteristics along the slopes of isolated carbonate platforms in the Xisha (Paracel) Archipelago. Based on multibeam bathymetric and seismic data, we have identified seven types of morphological and depositional features around the platforms, including erosive grooves, mass transport deposits, gullies, confined channels, canyons, mounded sediments, and marginal contourite depressions. The occurrence of erosive channelized features around the platforms indicates that gravity flow is a major sediment-export mechanism for the transportation of excess neritic carbonates towards the slope. Marginal contourite depressions and mounded sediments are interpreted as bottom-current reworking products, suggesting an important role of bottom current and deepwater circulation of the South China Sea in shaping the slope morphology of the carbonate platforms. Furthermore, the bottom-current products tend to develop along the slopes of the carbonate platforms at the Xisha Uplift margin; in particular, erosional features formed by bottom current frequently occur in the southern side of the carbonate platforms. By comprehensive analysis of morphological and depositional features, we establish a facies model around the carbonate platforms to interpret different sedimentary transport processes, such as off-platform and along-slope sediment transport processes. These findings highlight a notable and complex relay-style channelized transport system consisting of grooves/gullies-channels-canyons, which has implications for linking neritic carbonate platforms into deep-sea basins.

Checklist of the coral fish fauna of Xisha Islands, China.

BackgroundThe Xisha Islands are composed of the Yongle Islands and the Xuande Islands in Hainan Province, China. It has one of the highest species diversity in the world and is also a typical oceanic distribution area of coral reefs globally. The ichthyofauna of the Xisha Islands were recorded by underwater visual census in May 2019 and July 2020. The survey data were combined with previous records of species into the checklist of the Xisha Islands presented herein. A total of 691 species, belonging to 24 orders and 97 families, was recorded. The major families were Labridae, Pomacentridae, Serranidae, Chaetodontidae, Hexanchidae, Lutjanidae, Scaridae, Gobiidae, Scorpaenidae and Carangidae. In this study, the Coral Fish iversity Index (CFDI) of six families (Chaetodontidae, Pomacanthidae, Pomacentridae, Labridae, Scaridae and Acanthuridae) was 229, indicating 756 coral fishes. In terms of the IUCN Red List, one species is Critically Endangered (Glyphis gangeticus), six species are Endangered (Stegostoma fasciatum, Aetomylaeus maculatus, Aetomylaeus vespertilio, Epinephelus akaara, Cheilinusundulatus sp. and Xiphias gladius), 16 species are Vulnerable, and 13 species are Near Threatened in the Xisha Archipelago, so conservation should be strengthened in this area in the future.New informationOne species is a new record for China (Dischistodus pseudochrysopoecilus) and 23 species are newly found in the Xisha Islands.

A pioneering survey of deep-diving and off-shore cetaceans in the northern South China Sea.

A pioneering boat-based survey was conducted in 2019, to gather baseline information regarding the presence, composition, relative abundance, and spatial distribution of deep-diving and off-shore cetaceans in the northern South China Sea (SCS). A total of 27 sightings comprising at least 8 cetacean species were recorded during the 13-day survey, including 5 deep-diving species (i.e. Risso's dolphin [Grampus griseus], short-finned pilot whale [Globicephala macrorhynchus], sperm whale [Physeter macrocephalus], Cuvier's beaked whale [Ziphius cavirostris], and an unidentified beaked whale [either the ginkgo-toothed beaked whale, Mesoplodon ginkgodens, or Deraniyagala's beaked whale, Mesoplodon hotaula]), as well as 3 off-shore dolphins (i.e. pantropical spotted dolphin [Stenella attenuate], striped dolphin [Stenella coeruleoalba], and Fraser's dolphin [Lagenodelphis hosei]). With the exception of pantropical spotted dolphins, all other species were sighted and recorded at sea in the northern SCS for the first time. The pantropical spotted dolphin was the most frequently sighted species, comprising 30% of the total sightings. Deep-diving cetaceans were mainly sighted in the northern Xisha Archipelago, whereas off-shore dolphins were distributed across the survey area. The pantropical spotted dolphin was observed in aggregations of more than 100 individuals and nearly all encountered species included calves; these findings suggested that the survey area functions as an important feeding and calving ground for various cetacean species. This pioneering survey provides fundamental information regarding cetacean fauna in the northern SCS and highlights the need to strengthen research and conservation efforts concerning these species.

Multi-scale Spectral-Spatial Remote Sensing Classification of Coral Reef Habitats Using CNN-SVM

Wan, J.X. and Ma, Y., 2020. Multi-scale spectral-spatial remote sensing classification of coral reef habitats using CNN-SVM. In: Jung, H.-S.; Lee, S.; Ryu, J.-H., and Cui, T. (eds.), Advances in Geospatial Research of Coastal Environments. Journal of Coastal Research, Special Issue No. 102, pp. 11-20. Coconut Creek (Florida), ISSN 0749-0208.In recent years, coral reefs have undergone serious degradation globally, prompting the use of remote sensing as an effective means to monitor these reefs on a large scale. Deep learning, which is a state-of-the-art image processing technique suitable for remote-sensing applications, can be used to learn nonlinear characteristics of images and is therefore applicable to the classification of small-scale coral reefs. This paper proposes a multi-scale method based on a convolutional neural network and support vector machine (CNN-SVM) to classify the coral reef habitats of Zhaoshu Island and Zhong Island in the Xisha Archipelago, China. This method combines spectrum, texture, and bidimensional empirical mode decomposition (BEMD) based scale separation algorithm to fully learn multi-scale information of coral reefs. Remote-sensing images captured by the WorldView-2 and Gaofen-2 (GF-2) satellites are used to evaluate the performance of the proposed CNN-SVM framework. The results indicate that the proposed method performs accurately and efficiently. Compared with SVM, random forest (RF), CNN, and CNN-RF, the overall accuracy is improved by 10.57 %, and the accuracy of classifying reef-clumping areas is improved by 17.44 %.

Facies model on the modern isolated carbonate platform in the Xisha Archipelago, South China Sea

The Xuande Atoll, with a discontinuous reef rim caused by platform drowning, is a typical ellipse isolated platform on the northern slope of the South China Sea. Based on the data of drilling holes, the analysis of surface sediments and high resolution acoustic data, six sedimentary facies are identified in the Xuande Atoll, including reef outer slope, reef front, reef flat, aeolianite sand island, back-reef slope and lagoon. Facies map shows that the elevated reef-rimmed margins with islands are more noticeable on the windward of the platform. With an elevated and discontinuous reef-rimmed margin as well as coarse-grained sediment in the interior platform, Xuande can serve as a case of a leaky bucket atoll which is characterized by initially drowning scenarios. The facies architecture of the Xuande Atoll is evidently influenced by complex and interacting parameters including antecedent topography, sea level, wind-driven currents, water depth and tectonic subsidence. Different controlling factors exist in the drowning of the windward and leeward sides of the platform margin. The study of modern depositional settings in the Xuande Atoll will provide insight on predictions for ancient carbonate provinces.

A numerical study of island wakes in the Xisha Archipelago associated with mesoscale eddies in the spring

Eddies caused by island wakes often appear downstream of an island where upwelling currents are coupled with nutrient transport. Several island wake eddies downstream of the Yongle atoll (YA), Huaguang Reef (HR) and Yuzhuo Reef (YR) in the Xisha Archipelago (XA) are found based on SAR and GF-1 satellite images taken on April 14, 2009 and May 30, 2014, respectively. The present study aims to explore the features and mechanisms of spring island wakes based on a high-resolution numerical model as well as satellite data. Our results are: 1) The cyclonic wake eddies (CWEs) are surface-intensified, and introduce a 20 – 40 m uplift of the isopycnal for the most intense eddies; While the anticyclonic wake eddies (AWEs) reach a subsurface maximum at a depth of 25 – 50 m, which leads to the weak isopycnal downwelling. The CWEs are generally more abundant than the AWEs in different radius classes under different incoming flows, which is due to the inertial-centrifugal instability (ICI) of the AWEs. 2) In the XA, lower SST features observed in the downstream side of the reefs demonstrate the presence of current-induced island wakes. In the near wakes of YA, HR and YR, the periodic vorticity generation is mainly caused by shear flow instabilities arising from the side boundaries of the reefs. Anticyclonic vorticity generation is slightly larger than that of the cyclonic for these three reefs. 3) For the submesoscale wake dynamics, the barotropic instability is more significant than the baroclinic in the XA. Strong toroidal vortices of alternating sign, which strongly perturb the density, were observed as the existence of the ICI in the transects of an unstable intense AWE. But these could hardly be found in a weak AWE. The unstable AWEs may have a larger decay of the velocity at their eddy edges and large persistent vorticities at their cores, which lead to an increase of the ratio between the core vorticity and the Rossby number Γ. While stable AWEs have longer lifetimes with nearly constant Γ values that are similar to those of the stable CWEs.

Tsunami hazard assessment for atoll islands inside the South China Sea: A case study of the Xisha Archipelago

The potential tsunami hazard for atoll islands inside the South China Sea (SCS) has not been investigated previously. In this study, we assess the tsunami hazard for the Xisha Archipelago using a deterministic approach. The tsunami source considered in this study is hypothesized earthquakes in the Manila subduction zone. We employ COMCOT (COrnell Multi-grid Coupled Tsunami model) for tsunami simulations. Different with previous deterministic assessments, our study investigates the uncertainties caused by bathymetric resolution and rupture complexities (here refers to rupture speed and direction variations related to fault geometry complexity). Comparison between results of different resolution suggests that coarser grids may over/underestimate tsunami hazards around mid-ocean atoll islands and this depends on the surrounding bathymetry. But results of different datasets show that GEBCO may lead to a lower result than the multibeam data. We also investigate the uncertainties caused by different rupture speed and direction, which are highly affected by subducting seamounts/ridge. Different rupture direction will shift the hardest hit locations to areas, which are possibly underestimated in cases of instantaneous ruptures. Also, the distribution of slip and location of the rupture zone can affect the local tsunami wave height significantly. We suggest that near-island high-resolution bathymetric data and dynamic rupture process should be considered in future tsunami hazard assessment in locations with similar geological structures.

Nan'ao, an archaeological site of Song dynasty destroyed by tsunami

It has remained unknown whether the coastline of mainland China was ever hit by tsunamis due to the lack of geological investigations and reliable historical records. In 2013, we found sedimentary and biological evidences on Dongdao Island of Xisha Archipelago, indicating that a tsunami occurred in the South China Sea and struck the island about a thousand years ago. Results of tsunami modelling suggest that the possible trigger of the tsunami is an earthquake in the Manila Trench, and that the coastlines of Guangdong, Hainan Provinces of China and those of Thailand are under the impact of the tsunami. In this study, a geological investigation was conducted on Nan’ao Island of Guangdong Province to study the influence of the South China Sea tsunami. Tsunami deposits were identified in the coastal sedimentary profiles, and the radiocarbon ages of the samples are about 1000 a BP, identical to the age of the tsunami hitting Dongdao Island. The deposit layer contains small marine shells. The sedimentary samples have the same grain size distributions with the coastal sands, suggesting that the deposits have a coastal or nearshore sand origin. The sea level of the study area for the past 1500 years was 0.5 m lower than the present level; therefore the deposit layers was not sandy deposits formed by high sea level. There are no large rivers on the island, so the deposits were not formed by river, either. The absence of planar stratification and numerous individual laminations, typical textures of sandy storm deposits, excludes the influence of storm deposits. The single-layer, homogeneous and structureless textures presented by the deposit layers are indicative of rapid deposition. Giant boulders, typically requiring strong waves to transport, were distributed near the sampling sites. All these evidences strongly support that the southeastern coast of the island was struck by the paleotsunami. The calendar age of a bone sample collected from the event layer is 1090±20 AD ( 14 C age: 860±20 a BP). The dating results of the tsunami deposits of Xisha Archipelago ranges from 1017 to 1034 AD. Besides, the coastal regions of Chaozhou were struck by destructive sea wave in 1076 AD as recorded by historical archive. Taking all the chronological data into account, we conclude that the time of South China Sea tsunami is 1076 AD. The deposit layer also contains shards of ceramics, which were identified as Song dynasty ceramics. Ceramics remains produced before and after the Song dynasty were absent in the deposits. We analyzed the temporal distribution of the cultural relics of Nan’ao Island and found that the amount of the relics significantly decreased after the tsunami and remained low until the late Ming dynasty. Beside, more than 15000 ancient coins were retrieved in Nan’ao One, a shipwreck 2 km south off Nan’ao Island, and the characters on the coins indicated that they belonged to the 69-a period before the tsunami. These cultural evidences indicate a drastic cultural decline caused by the tsunami. This study confirms the risk of tsunamis in the South China Sea. Such risk should be considered in future planning and construction of nuclear power plant, harbor and petroleum reserve structure in the coastlines of China. This study also shows the cultural consequence of the tsunami and suggests archaeological excavation of the Song-dynasty relics of Nan’ao.

Maturation of Neogene dolomite from Xuande Atoll of Xisha archipelago, the South China Sea

Abstract Dolomitization is a remarkable diagenetic process occurring in the subsurface carbonate beds at Xuande Atoll of Xisha islands in the South China Sea. Dolomite crystals initially appear at the depth of ∼180 m in Holocene-Pleistocene carbonate with relatively small concentrations, whereas extensive dolomitization has developed at the minimum burial depth of 290 m in Pliocene strata of the ZK-1 well. Petrographically, ZK-1 dolomites have undergone, from shallow to deep depth, a progressive maturation process indicated by fabric evolvement of dolomitized rocks, rock lithification induced by textural coarsening and continued growth of cements, increase in stoichiometry and cation ordering of dolomite downcore, and alteration of metastable dolomite to stable one, involving preferential dissolution and subsequent reprecipitation process. Under cathodoluminescence, dolomite cements are mostly dull red to nonluminescent or zoned, whereas the replacive matrix dolomite exhibits a homogeneous greyish-red cathodoluminescence. Whole-rock elemental analyses show that dolomite rock is characterized by low concentrations of Mn (

Post-rifting magmatism and the drowned reefs in the Xisha Archipelago domain

Fourteen isolated drowned reefs have been identified around the Xisha Uplift by multibeam and seismic data. The drowning processes of these reefs can be divided into three different stages, which correspond to three different accelerated tectonic subsidence periods. The drowning of the Xisha reefs is the result of the combined action of tectonic subsidence and sea level fluctuations, and the tectonic subsidence rate had to remain above 0.2 mm yr−1 for a long time. Three abrupt accelerated tectonic subsidence events that occurred in the late Miocene, Pliocene and early Quaternary in the Xisha Uplift were closely related to the thermal subsidence processes after three stages of post-rifting magmatism. The magmatism of the middle Miocene and the following thermal subsidence resulted in the drowning of reefs in the northwestern Xisha uplift (Zone A). During the early Pliocene, massive magmatic intrusions and volcanic eruptions occurred in the Xisha Uplift. Then, the subsequent thermal subsidence started the drowning process of reefs in the northeastern and western regions of the Xisha Uplift (Zone B and C). During the early Quaternary, large-scale magmatism also occurred in the Xisha Uplift. The subsequent thermal subsidence resulted in a new rapid tectonic subsidence, which caused the reefs in the southern and southeastern regions of the Xisha Uplift to drown (Zone D and E).

Change of Organicδ13C in Ornithogenic Sediments of the Xisha Archipelago, South China Sea and its Implication for Ecological Development

Change of Organic<i>δ</i>¹³C in Ornithogenic Sediments of the Xisha Archipelago, South China Sea and its Implication for Ecological Development

Palaeotsunami in the East China Sea for the past two millennia: A perspective from the sedimentary characteristics of mud deposit on the continental shelf

Palaeotsunami studies have revealed that a great tsunami occurred in the South China Sea (SCS) and hit Xisha Archipelago a thousand years ago, suggesting the risk of tsunami hazard in this region. However, knowledge about palaeotsunamis in the East China Sea (ECS) is still limited. In this study, the grain size sequence of a marine sediment core was analyzed and the history of tsunamis in the East China Sea for the past two millennia is presented. We first conducted a numerical simulation of the SCS tsunami to see its impact on the ECS. The results suggest that the impact of an Mw 8.0 earthquake-induced tsunami is small. For the sediment core, according to its mean grain size distribution, the sediments were formed by two components, the <57.8 μm fraction and the >57.8 μm fraction, consisting about 90% and 10% of the core. The most significant change in the grain size sequence of the <57.8 μm fraction is consistent with the SCS tsunami temporally and this might be the result of the westward transportation of coarse grains by the tsunami. For other abrupt changes in the grain size sequence, they have not exhibited greater amplitudes than the tsunami-affected layer, indicating that the impact of the SCS tsunami was the most significant during this period. Thus, the East China Sea has not been struck by great tsunamis in the past two millennia. For palaeotsunamis occurring before the late-Holocene, more studies based on various proxies are required to reveal their history.

Rare earth element and Yttrium (REY) geochemistry of Cenozoic carbonates on the Xuande Atoll of the Xisha Archipelago, the South China Sea: Implications of the impact of dolomitization on REY

Rare earth element and Yttrium (REY) geochemistry of Cenozoic carbonates on the Xuande Atoll of the Xisha Archipelago, the South China Sea: Implications of the impact of dolomitization on REY