Eastern South China Sea Research Articles

Depth Influencing Factors and Coping Techniques of Logging and Logging while Drilling in Deep Water

Aiming at the influence of tide, wave, ocean current and drilling engineering on logging and logging while drilling depth system and cuttings lag depth, the methods of equipment-related professional sensors, physical indicators, controlling drilling parameters and proofreading in drilling process are used to achieve the purpose of collecting all kinds of geological data in the harsh deep water environment. It is of guiding significance to ensure the accuracy of geological related data in deep-water exploration operation. At present, many deep-water exploration wells in the eastern South China Sea have achieved good results.

Sub-basin scale inhomogeneity of mantle in the South China Sea revealed by magnesium isotopes

The South China Sea (SCS) is the largest extensional basin in the western Pacific and was formed after rifting of the Euro–Asian continental margin. The nature of its underlying mantle remains enigmatic due to the lack of sampling of the seafloor’s igneous crust. The International Ocean Discovery Program Expedition 349 cored seafloor basalts of the southwestern (Site U1433) and eastern (Site U1431) SCS sub-basins. The recovered basalt samples exhibit different source lithologies and geochemistries. The Mg isotopic compositions of seafloor basalts from these sites were investigated to elucidate the origin of this large-scale mantle inhomogeneity. Results indicate that the Site U1431 basalts have a mantle-like average δ26Mg value of –0.27‰ ± 0.06‰ (2SD; n = 10). Together with inhomogeneous Sr–Nd–Pb–Hf isotopic compositions, the Site U1433 basalts have an average δ26Mg value (–0.20‰ ± 0.06‰; 2SD; n = 8) higher than those of the Site U1431 basalts and normal mantle. Their heavier Mg isotopic compositions and low 206Pb/204Pb ratios (~17.7) indicate that the Site U1433 basalts were affected by the re-melting of detached continental-arc lithosphere in the sub-ridge mantle. The coupling of Mg and Sr–Nd isotopes provides robust evidence that the mantle-like δ26Mg values of the Site U1431 basalts resulted from mixing between detached continental arc lithosphere and the nearby Hainan plume, with respective supra- and sub-normal δ26Mg values. From the perspective of Mg isotope, the mantles of the southwestern and eastern sub-basins are compositionally inhomogeneous, with their mantle evolutionary histories being distinct.

Continental Interior and Edge Breakup at Convergent Margins Induced by Subduction Direction Reversal: A Numerical Modeling Study Applied to the South China Sea Margin

AbstractThe dynamics of continental breakup at convergent margins has been described as the results of backarc opening caused by slab rollback or drag force induced by subduction direction reversal. Although the rollback hypothesis has been intensively studied, our understanding of the consequence of subduction direction reversal remains limited. Using thermo‐mechanical modeling based on constraints from the South China Sea (SCS) region, we investigate how subduction direction reversal controls the breakup of convergent margins. The numerical results show that two distinct breakup modes, namely, continental interior and edge breakup (“edge” refers to continent above the plate boundary interface), may develop depending on the “maturity” of the convergent margin and the age of the oceanic lithosphere. For a slab age of ~15 to ~45 Ma, increasing the duration of subduction promotes the continental interior breakup mode, where a large block of the continental material is separated from the overriding plate. In contrast, the continental edge breakup mode develops when the subduction is a short‐duration event, and in this mode, a wide zone of less continuous continental fragments and tearing of the subducted slab occur. These two modes are consistent with the interior (relic late Mesozoic arc) and edge (relic forearc) rifting characteristics in the western and eastern SCS margin, suggesting that variation in the northwest‐directed subduction duration of the Proto‐SCS might be a reason for the differential breakup locus along the strike of the SCS margin. Besides, a two‐segment trench associated with the northwest‐directed subduction is implied in the present‐day SCS region.

Comprehensive identification of CO2 nonhydrocarbon gas layers by relative CO2 content and apparent porosity calculated from well logs: A case study in the Baiyun deep-water area in the eastern South China Sea

The accurate identification of [Formula: see text] nonhydrocarbon gas layers plays an important role in the exploration and development of natural gas reservoirs. The logging response characteristics of layers containing nonhydrocarbon gas and hydrocarbon gas are very similar because there are gas fluids in both kinds of layers, which leads to the uncertainty in the interpretation results of gas types. We have developed a comprehensive method for the identification of [Formula: see text] nonhydrocarbon gas layers by using [Formula: see text]-relative content and apparent porosity. First, based on the formation component bulk volume model of the coexisting reservoir of the [Formula: see text] nonhydrocarbon gas layers and the hydrocarbon gas layers, we develop an optimization algorithm to quantitatively calculate the [Formula: see text] relative content in the reservoir. Second, by analyzing and determining the logging response value of the [Formula: see text] nonhydrocarbon gas under the formation temperature and pressure conditions, we establish a qualitative identification chart using the difference between the calculated value of the apparent porosity of [Formula: see text] and the hydrocarbon gas. Finally, we were able to accurately identify the [Formula: see text] nonhydrocarbon gas layers by comprehensively analyzing the calculation results of the [Formula: see text] relative content in the reservoir and the identification results of the [Formula: see text] apparent porosity identification chart. We apply the above workflow in the Baiyun deep-water area in the eastern South China Sea, and it shows high efficiency in the identification of [Formula: see text] nonhydrocarbon gas layers.

E&P Notes (August 2020)

E&P Notes (August 2020)

Impact of ENSO on the South China Sea during ENSO decaying winter–spring modeled by a regional coupled model (a new mesoscale perspective)

Studies have suggested that the ocean dynamics in the South China Sea (SCS) play a crucial role in local air–sea processes and the regional weather or climate system. We used a high-resolution regional coupled model to investigate the impact of the El Niño Southern Oscillation (ENSO) on the atmospheric and oceanic processes in the SCS during ENSO decaying winter–spring. The coupled model demonstrates the important roles of mesoscale features in the regional air–sea interaction induced by the anomalous ENSO conditions. During El Niño, the Kuroshio intrusion into the SCS is enhanced which further modifies the upper and middle layer circulation of the SCS. The reduced planetary vorticity transport during El Niño results in a weakened cyclonic circulation and thinner mixed layer in the southern SCS. The modulations of the cyclonic circulation in the southern SCS, Vietnam offshore jet, and the coastal current during ENSO event enhance oceanic advection that favors an ocean-forcing energy exchange to the atmosphere across the air–sea interface. The relative importance of atmospheric and oceanic forcing on the SST change during the ENSO evolution is verified using a heat budget analysis for different regions of SCS. In the El Niño following spring, the advective term due to anomalous Ekman drift dominates the cooling process in the northern and eastern SCS. These thermal structure changes in the SCS are contributed by the wind–current–topography interactions.

Spatiotemporal variations of benthic macrofaunal community in the Xiamen Amphioxus Nature Reserve, eastern South China Sea

In order to realize the spatiotemporal variations of benthic macrofaunal communities at the “Amphioxus Sand” habitat, six surveys including four seasons and three consecutive summers (i.e., 2014, 2015 and 2016) were conducted in two core sites, i.e., Huangcuo (HC) and Nanxian-Shibaxian (NX), in the Xiamen Amphioxus Nature Reserve in China. A total of 155 species of macrofauna were recorded, therein, polychaetes were dominant in terms of species number and density. Significant spatiotemporal variations of macrofaunal communities were observed. The density of polychaetes and the biomass of molluscs in the HC site were higher than those in the NX site. Macrofauna were more diverse and abundant in the cold seasons (winter and spring) than that in the warm seasons (summer and autumn). The annual variations of macrofaunal communities may be attributed to the changes in sediment texture among the three years of the survey. The variations in macrofaunal communities were mainly related to the proportion of polychaetes within the community. In addition, the density of amphioxus (include Branchiostoma japonicum and B. belcheri) was negatively correlated to that of polychaetes, bivalves, and crustaceans. Amphioxus was less likely to be found in the sediments with higher silt and clay content. Five biotic indices including Margaref’s richness index (d), Peilou’s evenness index (J’), Shannon-Wiener diversity index (H’), AMBI and M-AMBI were calculated in the present study. AMBI seems suitable in assessing benthic health at the “Amphioxus Sand” habitat, and a potential risk of ecological health in Xiamen Amphioxus Nature Reserve should be aware.

Carbonate-rich sequences in the deep sea of the eastern South China Sea subbasin (IODP Hole U1431D) during the last 600 kyr

Here, we examined changes in carbonate preservation in a deep-sea record (Hole U1431D, 4240 m water depth) retrieved from the South China Sea (SCS) during the International Ocean Discovery Program (IODP) Expedition 349. To evaluate these changes, we applied a micropaleontological and geochemical multiproxy approach including, planktonic foraminifera (PF) total abundance, fragmentation (FI%) and dissolution (FDX) indexes, and total organic carbon (TOC). The results of this study showed that during the last 600 kyr, Hole U1431D site has been under the effect of carbonate dissolution associated with the deepening of the lysocline and the Calcite Compensation Depth. Nonetheless, during MIS 8 and MIS 7, there was a significant increase in PF total abundance and TOC content. The low PF total abundance found in some intervals resulted in an underestimation of the FI% and FDX indexes. Although high PF total abundance events occur at Hole U1431D turbidite sequences, these are also interpreted in the context of higher primary productivity (PP) in the central SCS. High TOC contents from MIS 10 to MIS 6 were associated with the effects of enhanced PP during the Mid-Brunhes Dissolution Interval as well as the dynamics of the East Asian Monsoon (EAM). During MIS 8 and 7, increased abundances of PF and TOC contents may have been related to enhanced PP promoted by the seasonal EAM. Our data suggest that enhanced PP associated with the occurrence of turbidite currents at the East SCS subbasin allowed the preservation of PF at Hole U1431D.

Relationships between chemical elements of PM2.5 and O3 in Shanghai atmosphere based on the 1-year monitoring observation

Mass level of fine particles (PM2.5) in main cities in China has decreased significantly in recent years due to implementation of Chinese Clean Air Action Plan since 2013, however, O3 pollution is getting worse than before, especially in megacities such as in Shanghai. In this work, O3 and PM2.5 were continuously monitored from May 27, 2018 to March 31, 2019. Our data showed that the annual average concentration of PM2.5 and O3 (O3-8 hr, maximum 8-hour moving average of ozone days) was 39.35 ± 35.74 and 86.49 ± 41.65 µg/m3, respectively. The concentrations of PM2.5 showed clear seasonal trends, with higher concentrations in winter (83.36 ± 18.66 µg/m3) and lower concentrations in summer (19.85 ± 7.23 µg/m3), however, the seasonal trends of O3 were different with 103.75 ± 41.77 µg/m3 in summer and 58.59 ± 21.40 µg/m3 in winter. Air mass backward trajectory, analyzing results of potential source contribution function model and concentration weighted trajectory model implied that pollutants from northwestern China contributed significantly to the mass concentration of Shanghai PM2.5, while pollutants from areas of eastern coastal provinces and South China Sea contributed significantly to the mass level of ozone in Shanghai atmosphere. Mass concentration of twenty-one elements in the PM2.5 were investigated, and their relationships with O3 were analyzed. Mass level of ozone had good correlation with that of Ba (r = 0.64, p < 0.05) and V (r = 0.30, p > 0.05), suggesting vehicle emission pollutants contribute to the increasing concentration of ozone in Shanghai atmosphere.

The response of surface chlorophyll to mesoscale eddies generated in the eastern South China Sea

This study investigates the response of surface chlorophyll (CHL) to mesoscale eddies generated in the eastern South China Sea (ESCS) during their lifecycles based on global eddy dataset and chlorophyll data from January 2003 to December 2016. The results show that the responses of CHL anomalies to cyclones are stronger than anticyclones generated in the Luzon Strait (LS) and in the area off northwest Luzon Island (NWLI). CHL anomalies are higher when cyclones disappear than when they are generated in the LS and in the NWLI. Furthermore, impacts of different eddy dynamic mechanisms (eddy advection, eddy trapping, eddy pumping and eddy-induced Ekman pumping) on CHL anomalies are quantified during eddy lifecycles based on empirical orthogonal function (EOF) method. Our results reveal that CHL anomalies in cyclones generated in the NWLI are affected by eddy pumping and its strengthening effect enhanced until the middle stage of their lifecycles. CHL anomalies in cyclones generated in the LS are modulated by eddy advection and linear Ekman pumping. CHL anomalies in anticyclones generated in the LS are jointly induced by eddy trapping, nonlinear Ekman pumping and eddy pumping. This study helps us attribute surface CHL to specific mechanisms and understand the differences in the response of surface CHL to mesoscale eddies generated in the ESCS during their lifecycles, and thus gains a deeper understanding of the role of mesoscale eddies in regulating the marine primary production.

Ventilation in the South China Sea: Subduction versus obduction

We investigate ventilation in the South China Sea (SCS) based on a recently developed framework for annual subduction/obduction in monsoon-dominated oceans. It is found that subduction occur in winter, and it was greatly weakened by ~58% due to the summer monsoon. Furthermore, there is a second obduction window opened up during the summer monsoon period, besides the winter obduction window.The winter subduction is concentrated in two narrow regions: one is located in the northern SCS and the other in the eastern SCS from 10°N to 15°N. The winter obduction distributes widely in two areas: the northeastern SCS and the area south of 10°N, while the summer obduction occurs primarily near the Vietnam. The key factors regulating subduction/obduction and their variability at different areas are different: the lateral induction term is the key contributor for subduction in the northern SCS; however, for the subduction in the eastern SCS and the obduction process, the vertical pumping term is dominant. Furthermore, the subducted (obducted) water in the SCS would penetrate into (come from) the subsurface ocean at the depth ~ 100m after (before) one year.

Analysis of surface chlorophyll a associated with sea surface temperature and surface wind in the South China Sea

In this study, the spatial and temporal variability in surface chlorophyll a (Chl-a) in the whole South China Sea (SCS) was investigated in detail by using 8-day, 4-km, gap-free MODIS-A data (2003–2016). Monthly climatology and empirical-orthogonal-functions analysis of Chl-a were performed in association with sea surface temperature and surface wind to aid in better understanding the physical mechanisms responsible for the Chl-a variability. The results are as follows: (1) Chl-a has out-of-phase variability between the coastal and open-sea regions due to different major factors controlling phytoplankton growth in each region; (2) in particular, Chl-a increases in the northern SCS during winter and in the western and southwestern SCS during summer mainly due to the effects of monsoons and orography; and (3) wind-driven coastal upwelling is stronger in the western SCS than in the eastern SCS. A wind-induced coastal upwelling not reported in the literature was detected along Palawan Island (8–12∘N, 117–120∘E) during winter. In the SCS, the Chl-a variability is influenced by the El Nino–Southern Oscillation with a time lag of 4–9 months, depending on the variability scales.

Similarity in benthic habitat and fish assemblages in the upper mesophotic and shallow water reefs in the West Philippine Sea

AbstractThe South China Sea (SCS) is a biodiversity hotspot, however, most biodiversity surveys in the region are confined to shallow water reefs. Here, we studied the benthic habitat and fish assemblages in the upper mesophotic coral ecosystems (MCEs; 30–40 m) and SWRs (8–22 m) at three geographic locations (Luzon Strait; Palawan; and the Kalayaan Group of Islands) in the eastern SCS (also called the West Philippine Sea) using diver-based survey methods. Mean coral genera and fish species richness ranged from 17–25 (per 25 m2) and 11–17 (per 250 m2) in MCEs, respectively; although none of these were novel genera/species. Coral and fish assemblages were structured more strongly by location than by depth. Location differences were associated with the variability in benthic composition, wherein locations with higher hard coral cover had higher coral genera richness and abundance. Locations with higher algae and sand cover had higher diversity and density of fish herbivores and benthic invertivores. Fishing efforts may also have contributed to among-location differences as the highly exploited location had the lowest fish biomass. The low variation between depths may be attributed to the similar benthic composition at each location, the interconnectivity between depths due to hydrological conditions, fish motility, and the common fishing gears used in the Philippines that can likely extend beyond SWRs. Results imply that local-scale factors and anthropogenic disturbances probably dampen across-depth structuring in coral genera and fish species assemblages.

A Census of the 1993–2016 Complex Mesoscale Eddy Processes in the South China Sea

Mesoscale eddy process with at least one splitting and/or merging event can be defined as either a complex process or a simple process. Investigation of the difference between these two categories could provide new insights into how different factors, such as the seabed topography, Kuroshio intrusion, and winds, affect the origin, migration, and decay of the mesoscale eddies. This study compared the characteristics of the complex against the simple eddy processes in the South China Sea (SCS) from 1993 to 2016. We comprehensively analyzed the eddy processes with regards to their characteristic points, trajectories, and networks. The simple and complex processes share many similarities but do show significantly different behaviors. Both the simple and complex processes mainly start from the eastern SCS. However, the complex processes mainly vanish in the western SCS whereas the simple processes disappear almost everywhere across the SCS. The complex processes last longer and migrate more than the simple processes. Lastly, the complex processes mainly move westward within the community. The complex processes can be further categorized into complex anticyclonic and cyclonic eddy processes. Spatially, the splitting and merging events mainly occur in the southwest of Taiwan, northwest of the Luzon Island, and the southeast of Vietnam. Temporally, the merging and splitting events mainly occur in the fall. The interaction among the communities reveals the different migration patterns of the complex anticyclonic and cyclonic eddy processes in the SCS.

Eddy generation mechanism in the eastern South China Sea

Mesoscale eddy generation mechanisms in the eastern South China Sea (ESCS) are investigated using altimetry observations and solutions of a nonlinear, 1½-layer reduced-gravity model. We estimate the relative roles of the wind forcing in the interior South China Sea (SCS) and the remote forcing from the western tropical Pacific (WTP) in eddy generation in the ESCS. Model solutions show that the high-frequency wind in the interior SCS is the primary forcing for eddies, which explains about 54% of the mesoscale eddies generated in the ESCS. Signals from the WTP also play an important role. Wind-driven equatorial signals reach the west coast of Luzon Island through the Sibutu Passage and Mindoro Strait. The reflected Rossby waves from the west coast of Luzon Island propagate westward, become unstable, and turn into eddies. The signals driven by high-frequency wind from the WTP explain about 40% of the mesoscale eddies generated in the ESCS. The high-frequency wind forcing in both the SCS and the WTP is important for eddy generation in the ESCS.

The role of synoptic‐scale waves in the onset of the South China Sea summer monsoon

The role of premonsoon synoptic‐scale waves (2‐ to 10‐day oscillation) in the onset of the South China Sea summer monsoon (SCSSM) is investigated in this study. The results indicated that the onset date of the SCSSM from 1979–2016 was related to the intensity of premonsoon synoptic‐scale waves. The more active the premonsoon synoptic‐scale waves, the earlier the SCSSM tended to be established. This study focused on the accumulative effects of premonsoon synoptic‐scale waves on the SCSSM onset because active years often included more than one process. According to the regression analysis results, three premonsoon synoptic‐scale waves influenced the onset of the SCSSM. First, waves over the southern Bay of Bengal (BOB) were related to Kelvin waves, which accelerated the intrusion of southwesterly winds over the BOB into the South China Sea (SCS). Second, waves over the eastern SCS were related to tropical depression (TD)‐type waves. Westerly anomalies south of this type of synoptic‐scale wave were favorable for the onset of the SCSSM. Third, waves originating from the north were hypothesized to be related to the mid‐latitude fronts. The mid‐latitude vortex moved northeast of the SCS and induced the intrusion of southwesterlies over the SCS. All three waves continuously enabled the westerly anomalies and affected the SCSSM. In addition, the combined effects of these synoptic‐scale waves were investigated. The results showed that active waves led to significant anomalous westerlies along the boreal tropical region and cyclonic circulation over the Philippine Sea. Anomalous convection was enhanced over the BOB and SCS, and the western Pacific subtropical high was suppressed. The combined intensities of these premonsoon synoptic‐scale waves indicated their potential for use as a new predictor of the onset of the SCSSM.

Analysis of ocean wave characteristic in Western Indonesian Seas using wave spectrum model

Understanding the characteristics of the ocean wave in Indonesian Seas particularly in western Indonesian Seas is crucial to establish secured marine activities in addition to construct well-built marine infrastructures. Three-years-data (July 1996 - 1999) simulated from Simulating Waves Nearshore (SWAN) model were used to analyze the ocean wave characteristics and variabilities in eastern Indian Ocean, Java Sea, and South China Sea. The interannual or seasonal variability of the significant wave height is affected by the alteration of wind speed and direction. Interactions between Indian Ocean Dipole Mode (IODM), El Niño Southern Oscillation (ENSO) and monsoon result in interannual ocean wave variability in the study areas. Empirical Orthogonal Functions (EOF) analysis produces 6 modes represents 95% of total variance that influence the wave height variability in the entire model domain. Mode 1 was dominated by annual monsoon and has spatial dominant contribution in South China Sea effected by ENSO and Indian Ocean influenced by IODM. Java Sea was influenced by Mode 2 which is controlled by semi-annual monsoon and IODM. A positive (negative) IODM strengthens (weakens) the winds speed in Java Sea during the East (West) season and hence contributes to Mode 2 in increasing (decreasing) the significant wave in Java Sea.

Shotgun metagenomic analysis of microbial communities in the surface waters of the Eastern South China Sea

Shotgun metagenomic analysis of microbial communities in the surface waters of the Eastern South China Sea

Model-simulated coastal trapped waves stimulated by typhoon in northwestern South China Sea

In this paper, we apply an unstructured grid coastal ocean model to simulate variations in the sea level and currents forced by two typhoons in the northwestern South China Sea (SCS). The model simulations show distinct differences for the two cases in which the typhoon paths were north and south of the Qiongzhou (QZ) Strait. In both cases, coastal trapped waves (CTWs) are stimulated but their propagation behaviors differ. Model sensitivity simulations suggest the dominant role played by alongshore wind in the eastern SCS (near Shanwei) and southeast of Hainan Island. We also examine the influence of the Leizhou Peninsula by changing the coastline in simulation experiments. Based on our results, we can draw the following conclusions: 1) The CTWs stimulated by the northern typhoon are stronger than the southern CTW. 2) In the two cases, the directions of the current structures of the QZ cross-transect are reversed. The strongest flow cores are both located in the middle-upper area of the strait and the results of our empirical orthogonal function analysis show that the vertical structure is highly barotropic. 3) The simulated CTWs divide into two branches in the QZ Strait for the northern typhoon, and an island trapped wave (ITW) around Hainan Island for the southern typhoon. 4) The Leizhou Peninsula plays a significant role in the distribution of the kinetic energy flux between the two CTW branches. In the presence of the Leizhou Peninsula, the QZ branch has only 39.7 percent of the total energy, whereas that ratio increases to 72.2 percent in its absence.

Interannual and long-term sea level variability in the eastern Indian Ocean and South China Sea

Sea level anomalies (SLAs) derived from satellite observations (over a period of 22 years) and tide gauge data compiled from 66 stations from the eastern Indian Ocean and South China Sea (SCS) and western Pacific Ocean have been analyzed to study the interannual to long term variation of SLAs in the eastern Indian Ocean and SCS. Spatial patterns of sea level variability on the eastern boundary of the Indian Ocean exhibit non-coherent variability with SCS but show coherent variability with western Pacific Ocean. We find coherent variability in interannual SLAs in the Bay of Bengal, Andaman Sea, Malacca Strait and Java Strait. A large fraction of interannual SLA variations in the eastern SCS and interior SCS is linked to El Nino-Southern Oscillation (ENSO) and rest of the region is characterized by small scale interannual variations. The interannual SLAs in the SCS show seasonality with pronounced variation during winter and fall seasons. Interannual surface wind anomalies associated with Indian Ocean Dipole (IOD) and ENSO explain sea level interannual variability in the eastern Indian Ocean and SCS. The decadal sea level variability associated with Pacific Decadal Oscillation (PDO) and decadal ENSO are observed in the SCS.