Institutional Repository of South China Sea Institute of Oceanology, CAS
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The influence of earthquakes on Zhubi Reef in the Nansha Islands of the South China Sea
No full textIt is exceedingly important to estimate the stability of coral reefs. In recent years, growing construction projects have been carried out on the reef flat in the South China Sea. As a special marine geotechnical medium, it is made of the reef debris underwent overwhelmingly long geological age. Reefs grow thickly on the carbonate platform after the Late Oligocene and have five to six main sedimentary facies. It can be used as a recorder to measure the occurrence time of recent earthquake. A model of reef body is presented to study the influence of earthquakes according to the geological structure characteristic of reefs in the Nansha Islands. Furthermore, GeoStudio is used to simulate stress and deformation situations within it under various earthquake intensities. A safety factor is calculated by the limit equilibrium method, and the possible scenarios of earthquake-induced landslides and sliding scale are defined with a Newmark sliding block method, as well as stress distribution and deformation behaviors. Therefore, the numerical results suggest that the connections between the coral reef and the earthquake are as follows: (1) the reef body has a good stability under self-gravity state; (2) after the earthquake, it may cause slope's instability and bring out slumping when the safety factor is smaller than 1 (F (S)< 1); (3) the safety factor decreases with the increasing earthquake intensity, and fluctuates around a particular value after a while; and (4) as a new developed part of the reef, the smaller shallow landslide will be easily subject to collapse caused by the earthquake. It is concluded that it is feasible to provide a reference for evaluating the stability of coral reef using a geotechnical engineering simulation method. This can help the engineering constructions in the South China Sea
First discovery of a bone handaxe in China
No full textHandaxes, the characteristic tool of the Acheulean industrial complex, are predominantly made of stone. Handaxes made of bone are much less common. Only a few have been reported, from sites in Africa, Europe and western Asia. In this study we report a bone handaxe from Chongqing, southwest China, which represents the first bone handaxe ever discovered in China or any other part of East Asia. Typologically, it is somewhat inconsistent with the classic Acheulean handaxe morphology and is therefore classified as a proto-handaxe in this study. The bone from which the handaxe was made, which is now fossilized, has been dated to -170 ka based on a U-series technique. The handaxe was manufactured from the mandible of an individual of the stegodontid Stegodon orientalis, a typical member of the Middle Pleistocene Ailuropoda-Stegodon fauna (sensu stricto) of southern China. This artifact represents the earliest evidence for a tradition of bone handaxe manufacture in East Asia, and provides important evidence regarding the lifestyle, technology, and environmental surroundings of the humans that occupied the upstream region of the Yangtze River during the Middle Pleistocene. (C) 2014 Elsevier Ltd and INQUA. All rights reserved
Regional hard coral distribution within geomorphic and reef flat ecological zones determined by satellite imagery of the Xisha Islands, South China Sea
No full textCoral reefs in the Xisha Islands (also known as the Paracel Islands in English), South China Sea, have experienced dramatic declines in coral cover. However, the current regional scale hard coral distribution of geomorphic and ecological zones, essential for reefs management in the context of global warming and ocean acidification, is not well documented. We analyzed data from field surveys, Landsat-8 and GF-1 images to map the distribution of hard coral within geomorphic zones and reef flat ecological zones. In situ surveys conducted in June 2014 on nine reefs provided a complete picture of reef status with regard to live coral diversity, evenness of coral cover and reef health (live versus dead cover) for the Xisha Islands. Mean coral cover was 12.5% in 2014 and damaged reefs seemed to show signs of recovery. Coral cover in sheltered habitats such as lagoon patch reefs and biotic dense zones of reef flats was higher, but there were large regional differences and low diversity. In contrast, the more exposed reef slopes had high coral diversity, along with high and more equal distributions of coral cover. Mean hard coral cover of other zones was < 10%. The total Xisha reef system was estimated to cover 1 060 km(2), and the emergent reefs covered similar to 787 m(2). Hard corals of emergent reefs were considered to cover 97 km(2). The biotic dense zone of the reef flat was a very common zone on all simple atolls, especially the broader northern reef flats. The total cover of live and dead coral can reach above 70% in this zone, showing an equilibrium between live and dead coral as opposed to coral and algae. This information regarding the spatial distribution of hard coral can support and inform the management of Xisha reef ecosystems
Toward a better guard of coastal water safety-Microbial distribution in coastal water and their facile detection
No full textProsperous development in marine-based tourism has raised increasing concerns over the sanitary quality of coastal waters with potential microbial contamination. The World Health Organization has set stringent standards over a list of pathogenic microorganisms posing potential threats to people with frequent coastal water exposure and has asked for efficient detection procedures for pathogen facile identification. Inspection of survey events regarding the occurrence of marine pathogens in recreational beaches in recent years has reinforced the need for the development of a rapid identification procedure. In this review, we examine the possibility of recruiting uniform molecular assays to identify different marine pathogens and the feasibility of appropriate biomarkers, including enterochelin biosynthetic genes, for general toxicity assays. The focus is not only on bacterial pathogens but also on other groups of infectious pathogens. The ultimate goal is the development of a handy method to more efficiently and rapidly detect marine pathogens. (C) 2017 Elsevier Ltd. All rights reserved
The role of rifting in the development of the continental margins of the southwest subbasin, South China Sea: Insights from an OBS experiment
No full textThe continental margins of the southwest subbasin in the South China Sea mark a unique transition from multi-stages magma-poor continental rifting to seafloor spreading. We used reflection and refraction profiles across the margins to investigate the rifting process of the crust. Combining with the other seismic profiles acquired earlier, we focused on the comparative geological interpretation from the result of multichannel seismic analysis and wide-angle seismic tomography. Our result provides the evidence of upper crustal layer with abundant fractures below the acoustic basement with a P-wave velocity from 4.0 to 5.5 km s(-1). It indicates extensive deformation of the brittle crust during the continental rifting and can make a good explanation for the observed extension discrepancy in the rift margins of the South China Sea. The seismic chronostratigraphic result shows the possibility of the intra-continental extension center stayed focused for quite a long time in Eocene. Additionally, our evidence suggested that continental margin of the southwest subbasin had experienced at least three rifting stages and the existence of the rigid blocks is an appropriate explanation to the asymmetric rifting of the South China Sea
Morphology of Shatsky Rise oceanic plateau from high resolution bathymetry
No full textNewly collected, high resolution multi-beam sonar data are combined with previous bathymetry data to produce an improved bathymetric map of Shatsky Rise oceanic plateau. Bathymetry data show that two massifs within Shatsky Rise are immense central volcanoes with gentle flank slopes declining from a central summit. Tamu Massif is a slightly elongated, dome-like volcanic edifice; Ori Massif is square shaped and smaller in area. Several down-to-basin normal faults are observed on the western flank of the massifs but they do not parallel the magnetic lineations, indicating that these faults are probably not related to spreading ridge faulting. Moreover, the faults are observed only on one side of the massifs, which is contrary to expectations from a mechanism of differential subsidence around the massif center. Multi-beam data show many small secondary cones with different shapes and sizes that are widely-distributed on Shatsky Rise massifs, which imply small late-stage magma sources scattered across the surface of the volcanoes in the form of lava flows or explosive volcanism. Erosional channels occur on the flanks of Shatsky Rise volcanoes due to mass wasting and display evidence of down-slope sediment movement. These channels are likely formed by sediments spalling off the edges of summit sediment cap
Models of the rapid post-rift subsidence in the eastern Qiongdongnan Basin, South China Sea: implications for the development of the deep thermal anomaly
No full textThe Qiongdongnan Basin is one of the largest Cenozoic rifted basins on the northern passive margin of the South China Sea. It is well known that since the Late Miocene, approximately 10Ma after the end of the syn-rift phase, this basin has exhibited rapid thermal subsidence. However, detailed analysis reveals a two-stage anomalous subsidence feature of the syn-rift subsidence deficit and the well-known rapid post-rift subsidence after 10.5Ma. Heat-flow data show that heat flow in the central depression zone is 70-105mWm(-2), considerably higher than the heat flow (<70mWm(-2)) on the northern shelf. In particular, there is a NE-trending high heat-flow zone of >85mWm(-2) in the eastern basin. We used a numerical model of coupled geothermal processes, lithosphere thinning and depositional processes to analyse the origin of the anomalous subsidence pattern. Numerical analysis of different cases shows that the stretching factor (s) based on syn-rift sequences is less than the observed crustal stretching factor (c), and if the lithosphere is thinned with (c) during the syn-rift phase (before 21Ma), the present basement depth can be predicted fairly accurately. Further analysis does not support crustal thinning after 21Ma, which indicates that the syn-rift subsidence is in deficit compared with the predicted subsidence with the crustal stretching factor (c). The observed high heat flow in the central depression zone is caused by the heating of magmatic injection equivalently at approximately 3-5Ma, which affected the eastern basin more than the western basin, and the Neogene magmatism might be fed by the deep thermal anomaly. Our results suggest that the causes of the syn-rift subsidence deficit and rapid post-rift subsidence might be related. The syn-rift subsidence deficit might be caused by the dynamic support of the influx of warmer asthenosphere material and a small-scale thermal upwelling beneath the study area, which might have been persisting for about 10Ma during the early post-rift phase, and the post-rift rapid subsidence might be the result of losing the dynamic support with the decaying or moving away of the deep thermal source, and the rapid cooling of the asthenosphere. We concluded that the excess post-rift subsidence occurs to compensate for the syn-rift subsidence deficit, and the deep thermal anomaly might have affected the eastern Qiongdongnan Basin since the Late Oligocene
Research progress on deep geophysics and continental dynamics in Eastern Tibetan Plateau
No full textThe study of deep geophysics and continental dynamics in the Tibetan Plateau has been taking vigorous progress in the last decades, which results in substantial achievements. Published in this special issue of the Chinese Journal of Geophysics, No. 6 of 2017, are 36 articles on the recent achievements made in the studies of deep geophysics and continental dynamics of the Eastern Tibetan Plateau. The studies cover three research fields, including seismic characteristics and continental dynamics, seismic anisotropy and crust-mantle structure, and deep electrical structure and the interpretation of gravity-magnetic data. In this review, the studies are briefly outlined and the key achievements of the works are highlighted to share with the readers
Authigenic carbonates from newly discovered active cold seeps on the northwestern slope of the South China Sea: Constraints on fluid sources, formation environments, and seepage dynamics
No full textAuthigenic carbonates recovered from two newly discovered active cold seeps on the northwestern slope of the South China Sea have been studied using petrography, mineralogy, stable carbon and oxygen isotopic, as well as trace element compositions, together with AMS C-14 ages of shells of seep-dwelling bivalves to unravel fluid sources, formation conditions, and seepage dynamics. The two seeps (ROV1 and ROV2), referred to as 'Haim seeps' herein, are approximately 7 kilometers apart, and are typified by abundant carbonate rocks represented bycrusts and nodules. Aragonite and high-Mg calcite are the main carbonate minerals. Based on low delta C-13(carbonate) values ranging from -43.0 parts per thousand to -27.5 parts per thousand (V-PDB) methane is apparently the predominant carbon source of seep carbonates. The corresponding 8180 values, varying from 2.5 parts per thousand to 5.8 parts per thousand (V-PDB), mostly are higher than calculated values representing precipitation in equilibrium with seawater (2.5%o to 3.8 parts per thousand), which probably reflects past destabilization of locally abundant gas hydrates. In addition, we found that carbonates with bivalve shells are generally aragonite-dominated, and bear no barium enrichment but uranium enrichments, reflecting shallow formation depths close to the seafloor. In contrast, carbonate crusts without bivalve shells and nodules contain more calcite, and are characterized by major molybdenum enrichment and different degrees of barium enrichment, agreeing with precipitation at greater depth under strictly anoxic conditions. AMS 14C ages suggest that a major episode of carbonate precipitation occurred between 6.1 ka and 5.1 ka BP at the Haima seeps, followed by a possibly subordinate episode from approximately 3.9 ka to 2.9 ka BP. The common occurrence of dead bivalves at both sites indicates that chemosynthesis-based communities flourished to a greater extent in the past, probably reflecting a decline of seepage activity in recent times. Overall, these results confirm that authigenic carbonates from gas hydrate-bearing areas can provide insight into long-term seepage dynamics and the genesis and fate of marine gas hydrate reservoirs
Bathymetry of the Coral Reefs of Weizhou Island Based on Multispectral Satellite Images
No full textShallow water depth measurements using multispectral images are crucial for marine surveying and mapping. At present, relevant studies either depend on the use of other auxiliary data (such as field water depths or water column data) or contain too many unknown variables, thus making these studies suitable only for images that contain enough visible wavebands. To solve this problem, a Quasi-Analytical Algorithm (QAA) approach is proposed in this paper for estimating the water depths around Weizhou Island by developing a QAA to estimate the diffuse attenuation coefficients and simplifying the parameterization of the bathymetric model. The approach contains an initialization sub-approach and a novel global adjustment sub-approach. It is not only independent of other auxiliary data but also greatly reduces the number of unknowns. Experimental results finally demonstrated that the Root Mean Square Errors (RMSEs) were 1.01 m and 0.77 m for the ZY-3 image and the WorldView-3 (WV-3) image, respectively, so the approach is competitive to other QAA bathymetric methods. Besides, the global adjustment sub-approach was also seen to be superior to common smoothing filters: if the Signal-to-Noise Ratio (SNR) is as low as 42, i.e., ZY-3, it can smooth the water depths and improve the accuracies, otherwise can avoid the over-smoothing of water depths