South China Sea Continental Shelf Research Articles

Oceanic and ecological response to native Typhoons Cempaka and Lupit (2021) along the northern South China Sea continental shelf: comparison and evaluation of global and regional Operational Oceanography Forecasting Systems

The Global Operational Oceanography Forecasting System from the Mercator Ocean (MO) and the regional South China Sea Operational Oceanography Forecasting System (SCSOFSv2) were compared and evaluated using in situ and satellite observations, with a focus on the oceanic and ecological response to two consecutive native typhoons, Cempaka and Lupit, that occurred in July–August 2021. Results revealed a better simulation of the chlorophyll a (Chla) structure by SCSOFSv2 and a better simulation of the temperature profile by MO in the Pearl River Estuary. In addition, SCSOFSv2 sea surface temperature (SST) and MO Chla variations corresponded well with observations along the northern SCS shelf. Simulated maximum SST cooling was larger and 2–3 days earlier than those observations. Maximum Chla was stronger and led the climatological average by 2 days after the typhoon passage. Typhoon-induced vertical variations of Chla and NO3 indicated that different Chla bloom processes from coastal waters to the continental shelf. Discharge brought extra nutrients to stimulate Chla bloom in coastal waters, and model results revealed that its impact could extend to the continental shelf 50–150 km from the coastline. However, bottom nutrients were uplifted to contribute to Chla enhancement in the upper and middle layers of the shelf. Nutrients transported from the open sea along the continental slope with the bottom cold water could trigger Chla enhancement in the Taiwan Bank. This study suggests considering strong tides and waves as well as regional dynamics to improve model skills in the future.

Tempo-Spatial Distribution of an Endangered Fish Species, Threadfin Porgy Evynnis cardinalis (Lacepède, 1802), in the Northern South China Sea

Accurate information on the temporal and spatial distribution of fish stocks is necessary for informed management and conservation of fisheries resources. We report on the temporal and spatial variation in biomass and density of a hitherto little-studied species of commercial importance, the threadfin porgy Evynnis cardinalis, it was listed as endangered (EN) in a recent International Union for Conservation of Nature (IUCN) red list, on the northern South China Sea continental shelf, using data collected during fisheries surveys from 2014 to 2015. Seasonal variation in the frequency of occurrence of E. cardinalis in trawl samples on this shelf was highest during summer (68.57%) and lowest during winter (31.43%). Average biomass was highest in autumn (33.05 kg·km−2) and density was highest in spring (936 ind·km−2). The highest annual average biomass (41.6 kg·km−2) and density (1021 ind·km−2) occurred off Shantou, and the lowest occurred off Zhanjiang (7.2 kg·km−2 and 165 ind·km−2, respectively); seasonal patterns in density are more similar in waters off Yangjiang and Shantou than elsewhere. The highest annual average biomass occurred at 40 m depth (56.9 kg·km−2), and the highest annual average density (1084 ind·km−2) occurred at 30 m depth. Density of E. cardinalis varied with depth, with lower densities occurring in shallow and deeper waters, and higher densities at intermediate depths.

Potential competition between marine heterotrophic prokaryotes and autotrophic picoplankton for nitrogen substrates

AbstractHeterotrophic prokaryotes have the capacity to uptake inorganic nitrogen (N) substrates. However, it remains unclear what the potential competition is between heterotrophic prokaryotes and autotrophic plankton for N in the ocean, which would shunt the flow of N supporting primary production. To date, it has been difficult to distinguish heterotrophic prokaryotic N uptake from that of autotrophic picoplankton, especially in oligotrophic oceans dominated by cyanobacteria. We carried out field‐based DNA stable isotope probing incubation experiments in the South China Sea combining measurements of uptake rates of ammonium, nitrate, nitrite, and urea to estimate the taxon‐specific potential N assimilation. The results indicate that phylogenetically diverse heterotrophic prokaryotes significantly incorporated multiple N sources, contributing approximately 17–41% and 19–55% of total N uptake potential in the euphotic zone of the South China Sea continental shelf and open ocean, respectively, potentially competing with cyanobacteria (mainly Prochlorococcus). Notably, heterotrophic prokaryotes made a higher contribution to bulk uptake of nitrate in the incubation systems of the open ocean relative to regenerated N, and thus there was a tendency to overestimate the f‐ratio. Extrapolating our results to the oligotrophic, low‐latitude ocean via a global model suggests the f‐ratio would decrease ~ 18%. This suggests a more complicated biogeochemical role of heterotrophic prokaryotes in the biological carbon pump than hitherto assumed, with important implications for N and carbon cycling in the vast open ocean.

Application of DINCAE to Reconstruct the Gaps in Chlorophyll-a Satellite Observations in the South China Sea and West Philippine Sea

The Data Interpolating Empirical Orthogonal Functions (DINEOF) method has demonstrated usability and accuracy for filling spatial gaps in remote sensing datasets. In this study, we conducted the reconstruction of the chlorophyll-a concentration (Chl-a) data using a convolutional neural networks model called Data-Interpolating Convolutional Auto-Encoder (DINCAE), and we compared its performance with that of DINEOF. Furthermore, the cloud-free sea surface temperature (SST) was used as a phytoplankton dynamics predictor for the Chl-a reconstruction. Finally, four reconstruction schemes were implemented: DINCAE (Chl-a only), DINCAE (Chl-a and SST), DINEOF (Chl-a only), and DINEOF (Chl-a and SST), denoted rec1, rec2, rec3, and rec4 respectively. To quantitatively evaluate the accuracy of these reconstruction schemes, both the cross-validation and in situ data were used. The study domain was chosen to be the Northern South China Sea (SCS) and West Philippine Sea (WPS), bounded by 115–125°E and 16–24°N to test the model performance for the reconstruction of Chl-a under different Chl-a controlling mechanisms. The in situ validation showed that rec1 performs best among the four reconstruction schemes, and that adding SST into the Chl-a reconstruction cannot improve the reconstruction results. However, for cross validation, adding SST can slightly improve spatial distributions of the root mean square error (RMSE) between the reconstructed data and the original data, especially over the SCS continental shelf. Furthermore, the potential of DINCAE prediction is confirmed in this paper; thus, the trained DINCAE model can be re-applied to reconstruct other missing data, and more importantly, it can also be re-trained using the reconstructed data, thereby further improving reconstruction results. Another consideration is efficiency; with similar reconstruction conditions, DINCAE is 5–10 times faster than DINEOF.

Physical characteristics and distribution of bottom sediments from the Kelantan River Delta towards the South China Sea continental shelf, Malaysia

Physical characteristics and distribution of bottom sediments from the Kelantan River Delta towards the South China Sea continental shelf, Malaysia

Anti-phase relationship between the East Asian winter monsoon and summer monsoon during the Holocene?

The relationship between the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM) during the Holocene is complicated and remains controversial. In this study, analysis of grain size and benthic foraminiferal oxygen isotope, as well as accelerator mass spectrometry 14C dating was performed on a sediment core retrieved from the newly revealed muddy deposit on the northern South China Sea continental shelf. The history of the EAWM and EASM were reconstructed for the last 8200 a BP. Further analysis in conjunction with previously published paleo-climate proxies revealed that the relationship between the EAWM and EASM during the Holocene is more complex than a simple and strict anti-phase one-both negative and positive correlations were identified. The EAWM and EASM are negatively correlated around 7500, 4800, 4200, 3200, and 300 a BP (cooling periods), while positively correlated around 7100, 3700, and 2100 a BP (warm periods). In particular, both the EAWM and EASM intensified during the three positive correlation periods. However, we also found that the relationship between these two sub-monsoons is anti-phase during the final phase of particularly hot periods like Holocene Optimum and Medieval warm period. The possible impact from variations of solar irradiance on the relationship between the EAWM and EASM was also discussed.

Shoaling of the internal solitary waves over the continental shelf of the northern South China Sea

The activities of internal solitary waves (ISWs) over the continental shelf of the northern South China Sea (SCS) are of high complexity. In this study, we investigated the spatial-temporal characteristics of the shoaling ISWs over the northern SCS continental shelf using the satellite images and the results of numerical simulation. The examination of the ISW signals in the satellite optical images revealed the existence of three types of ISWs in the region north to the Dongsha Island, namely, mode-1 depression ISW, mode-1 elevation ISW, and mode-2 convex ISW. The geographical distributions of these ISWs were derived from the satellite images. Numerical results exhibited the process of polarity conversion of ISWs, by which mode-1 elevation waves were transformed from the shoaling mode-1 depression waves. The mode-2 convex ISWs generally followed the mode-1 depression ISWs. The numerical results suggested that the interaction of the mode-1 depression ISWs with the up-slope topography locally generated mode-2 ISWs, and such waves of high vertical mode dissipated rapidly during the inshore propagation.

Evolution status of the distal mud deposit associated with the Pearl River, northern South China Sea continental shelf

The sedimentary characteristics of distal mud deposits, a product of along shore transport of river-discharged suspended sediment, contain information on their different evolutionary stages. In the present study, the distal mud associated with the Pearl River, southern China, is investigated using the data sets obtained from seabed sediment sample analysis, shallow geophysical survey and 210Pb dating. The results indicate that although the mud deposit in consideration occupies an area of more than 8000km2, the thickness is small with a young age (i.e., <102yrs). Shallow seismic survey reveals that the Holocene strata have a thickness of around 10m or less, with the lower layers being characterized by reworked deposits, rather than the distal mud deposits. The internal sedimentary structure shows that clinoforms are poorly developed. Compared with the distal muds of the Yangtze River (on the inner shelf of the East China Sea) and Yellow River (in the northern Yellow Sea), the distal mud here is still at its young stage. In contrast, those associated with the Yangtze and Yellow Rivers have already reached their growing and mature stages, respectively. This difference in the evolution stage is caused by the estuarine processes that control the timing and duration of the distal mud formation. Furthermore, since both river mouth deltas and distal mud deposits, at their mature stage, would be recognized as deltaic deposits in the geological record, it is necessary to establish appropriate criteria that can be used to distinguish between the two types of deposits. They contain different signals of sea level positions; hence, caution should be taken in interpreting the formation of “shelf edge deltas”, which have been found on the outer shelf regions of the South China Sea and in many places elsewhere.

Cenozoic tectonic jumping and implications for hydrocarbon accumulation in basins in the East Asia Continental Margin

Tectonic migration is a common geological process of basin formation and evolution. However, little is known about tectonic migration in the western Pacific margins. This paper focuses on the representative Cenozoic basins of East China and its surrounding seas in the western Pacific domain to discuss the phenomenon of tectonic jumping in Cenozoic basins, based on structural data from the Bohai Bay Basin, the South Yellow Sea Basin, the East China Sea Shelf Basin, and the South China Sea Continental Shelf Basin. The western Pacific active continental margin is the eastern margin of a global convergent system involving the Eurasian Plate, the Pacific Plate, and the Indian Plate. Under the combined effects of the India-Eurasia collision and retrogressive or roll-back subduction of the Pacific Plate, the western Pacific active continental margin had a wide basin-arc-trench system which migrated or ‘jumped’ eastward and further oceanward. This migration and jumping is characterized by progressive eastward younging of faulting, sedimentation, and subsidence within the basins. Owing to the tectonic migration, the geological conditions associated with hydrocarbon and gashydrate accumulation in the Cenozoic basins of East China and its adjacent seas also become progressively younger from west to east, showing eastward younging in the generation time of reservoirs, seals, traps, accumulations and preservation of hydrocarbon and gashydrate. Such a spatio-temporal distribution of Cenozoic hydrocarbon and gashydrate is significant for the oil, gas and gashydrate exploration in the East Asian Continental Margin. Finally, this study discusses the mechanism of Cenozoic intrabasinal and interbasinal tectonic migration in terms of interplate, intraplate and underplating processes. The migration or jumping regimes of three separate or interrelated events: (1) tectonism-magmatism, (2) basin formation, and (3) hydrocarbon-gashydrate accumulation are the combined effects of the Late Mesozoic extrusion tectonics, the Cenozoic NW-directed crustal extension, and the regional far-field eastward flow of the western asthenosphere due to the India-Eurasia plate collision, accompanied by eastward jumping and roll-back of subduction zones of the Pacific Plate.

Heavy metal distribution of the South China Sea continental shelf sediments off Sabah and Sarawak coastlines

Fifty-one surface sediment samples from offshore Sabah and Sarawak were collected from the two cruises of the Marine Vessel Southeast Asian Fisheries Development Centre (MV SEAFDEC) before and after the northeast monsoon. The sediments were analyzed for heavy metal contents to determine the effects of monsoon seasons and spatial patterns of distribution. The contents of heavy metals were measured from the 63 μm fraction of the dried sediments and analyzed using an Atomic Absorption Spectrophotometer. The heavy metals studied were Al, Fe, Cr, Cu, Zn, Pb and Mn. Results showed that some metals were in concentrations lower than the average Earth!s crust. The lower or higher-than-normal values of some of the heavy metals studied can be attributed to the intensive weathering in the study area, mineralogy, effect of the monsoon and the Rajang River. Furthermore, normalization was done using Al as normalizer to determine whether the high levels of Pb were due to anthropogenic input or not. It was found that the values were higher but not necessarily an indication of inputs from human activities as high levels were also found far from the coast. This may be attributed to input of particulates from the Rajang River and possibly movement of bottom sediments.

Postglacial sea-level changes in the northern South China Sea continental shelf: Evidence for a post-8200 calendar yr BP meltwater pulse

A postglacial sea-level record reconstructed through the study of 25 cores and 10 grab samples obtained from the siliciclastic-dominated northern South China Sea continental shelf between Hong Kong and Dongsha is presented. The seven sedimentary facies recognized are: (1) Pleistocene overconsolidated mud facies affected by palaeosol development due to sub-aerial exposure during the Last Glacial Maximum (LGM), (2) bioclastic sandy gravel facies occurring at a present-day seabed depth exceeding 120 m, (3) LGM bioclastic beach-dune sand facies at a present-day seabed depth between 110 and 120 m, (4) postglacial siliciclastic beach-dune sand facies formed during at least two episodes of sea-level stillstands at present-day seabed depths between 80 and 90 m, and 30 and 50 m, respectively, (5) postglacial transgressive sand facies of Late Pleistocene to Early Holocene age: radiocarbon dating shows that this facies was deposited in two stages before and after the 8200 calendar yr BP cold event, (6) postglacial sand and gravel-dominated facies with acoustic turbidity at a present-day seabed depth of about 50 m, (7) postglacial mud facies. Based on the radiocarbon ages obtained from samples located above and below the Holocene–Pleistocene hiatus in the two innermost shelf cores, and large variations of atmospheric radiocarbon concentration found during the last glacial period in a submerged stalagmite from the Bahamas (Beck et al., 2001. Science 292, 2453–2458), ages exceeding 8200 calendar yr BP are likely to represent minimum values. Ages postdating this limit, which are less affected by variations of atmospheric radiocarbon concentration, indicate the presence of a post-8200 calendar yr BP cold event meltwater pulse caused by eustatic sea-level rise from approximately 40 m below present to the present level by about 6000 calendar yr BP. Inaccuracies of the pre-8200 calendar yr BP radiocarbon ages may be attributed to large variations of atmospheric Δ 14C during the postglacial period with greenhouse gases released from the sub-aerially exposed continental shelf probably accounting for a significant proportion.

Satellite observation of internal solitary waves converting polarity

On a SPOT‐3 visible image over the South China Sea continental shelf, two contrasting signatures of internal solitary waves (ISWs) are observed: darker stripes leading brighter stripes in the lower part, and brighter stripes leading darker stripes in the upper part. The authors suggest that the contrasting patterns are caused by ISWs of different polarities. A packet of ISWs is observed in the process of converting polarity due to bottom shoaling, both in the propagation direction and in the transect direction. The evolution process can be obtained by transferring information from the spatial domain to the temporal domain. The conversion process starts with the original depression ISWs passing the 160 m isobath. The evolution process can be divided into two phases: the first is the broadening of the original depression waves and the second is the appearance of the new‐born elevation waves. These two phases are verified using numerical simulation.

Overpressuring mechanisms in the Yinggehai Basin, South China Sea

Yinggehai Basin is an elongate Cenozoic rift basin on the northwestern margin of the South China Sea continental shelf. Its thick (17 km) basin fill is characterized by high geothermal gradient and high overpressure. Overpressure associated with nonequilibrium compaction mainly occurs at depths more than 2800 m at the basin center and more than 4000 m at the basin margin because the shallow-buried Neogene and Quaternary strata lack effective seals. This regional overpressure distribution, however, is disrupted at basin center where high overpressure occurs in permeable formations at a depth as shallow as 1400 m on top of a series of deep-seated faults and fractures. We studied the processes and mechanisms of overpressuring via numerical modeling that couples basin filling, sediment compaction, and thermal and pressure fields to approach the origin of the shallow high overpressure. Model results indicated that an increase of fluid volume due to natural-gas generation by organic cracking is not large enough to generate the overpressure because of the limited amount of organic matter. The shallow overpressure has probably been generated allogenically. Deep open faults have served as vertical hydraulic conduits and channeled the deep high pressure into shallow permeable formations.

Vertical Variations of Core Sound Velocity: Evidence of Paleooceanographic History Since the Pleistocene Epoch

The vertical variations in the borehole core sound velocity (Cp) of the submarine sediments are related to the events of marine transgression and regression in the geologic history. This article establishes the vertical variation of Cp curve for the borehole sound velocity in the northern South China Sea continental shelf and the East China Sea continental shelf, thus providing evidence for revealing at least (3) sedimentary cycle events of marine transgression and regression occurring in the west Pacific marginal sea since the Pleistocene Epoch. The sound velocity of marine sediments brought in the course of marine transgression is lower (1450-1510 m/s) while that of continental sediments formed in the course of marine regression is higher (1650-1720 m/s).

Nonlinear internal waves in the South China Sea: Observation of the conversion of depression internal waves to elevation internal waves

The conversion of depression nonlinear internal solitons to elevation internal waves has been observed twice at the South China Sea continental shelf break. Shipboard X‐band radar, tow‐yo CTD, ADCP and high frequency acoustic flow visualization of the process are presented. The data focuses on up slope propagation of depression internal solitons from a water depth of ∼264 m to a water depth of ∼110 m. Dissipation by large amplitude shear instabilities was observed. The kinetic energy density was found to be a decreasing monotonic function of range with an energy dissipation rate coefficient of 0.063 km−1. The rate of energy dissipation over the ∼16.5 km soliton propagation path was 0.17 W/m/m. Simple numerical models of the waveform evolution are qualitatively compared to the observations. The observations, extracted scale parameters and dissipation rates should be useful for testing a variety of evolution equation representations of internal soliton propagation.

Cenozoic plate tectonics and basin evolution in Indonesia

SE Asia comprises a complex array of Cenozoic basins. The chronostratigraphic evolution of these basins may be understood within the plate tectonic evolution of SE Asia. The relative motions of India, Australia, the Pacific and Eurasia provide the boundary conditions for this evolution. Indian collision and indentation destroyed a subducting northern Tethyan margin and led to major clockwise rotation of SE Asia. The South China Sea continental shelf developed after collapse of the West Pacific subducting margin. Crustal extension led to sea floor spreading and the formation of the Reed Bank Terrane and, at its trailing edge, the South China Sea. Sumatran basins opened due to back arc extension in the Eocene. Closure of a marginal ocean basin resulted in a major contractional event in the Late Oligocene. The Gulf of Thailand basins and Andaman Sea opened in response to rotation of Indochina and oblique convergence at the precursor to the Sunda trench. Inversion of the southern end of these basins and uplift in Borneo coincided with the collision of the Reed Bank Terrane with Borneo. Opening of the Makassar Straits, Kutei, Tarakan and Barito basins occurred during the Eocene. Inversion of these basins was a result of the collision of Australia and Australian derived microplates in the Late Miocene/Pliocene. Pliocene fold and thrust belt and foreland basin formation in New Guinea was a result of oblique arc collision. Basin evolution of SE Asia is not a result of major lateral extrusion in front of the Indian indenter. The major effect of this collision is in terms of the clockwise rotation of Indochina and extension along the Sumatran active margin.