Northern South China Sea Research Articles

Analyzing the coastal sea level trends from SCMR-reprocessed altimeter data: A case study in the northern South China Sea

With improved altimeter data near the coast, it is possible to use them in analysis of coastal sea level trends. In this study, we assess the sea level trends over January 2002-April 2022 with the SCMR-reprocessed altimeter dataset in the coastal zone of the northern South China Sea (SCS). The reliability of this dataset is confirmed by the inter-comparison with the CMEMS (Copernicus Marine Environment Monitoring Service) Level-3 along-track data product and the ESA CCI (European Space Agency Climate Change Initiative) v2.2 virtual station product, along with the validation against tide gauge records. Compared to two altimeter products, the SCMR has a higher number of data points for the analysis of sea level trends in the study region (28439 vs 1408 for CMEMS and 868 for ESA CCI). The monthly SLA time series from the SCMR dataset are more consistent with those from tide gauge records in terms of higher correlation coefficients (0.37–0.83) and lower root mean square of the differences (RMSD, 33.98–92.19 mm) between altimeter and tide gauge. The along-track sea level trends within the 0–20 km coastal strip range between 2–5 mm yr−1, and the degradation of sea level trends occurs when the along-track trends show significant fluctuations at the spatial scales of ∼ 300 m. Therefore, a further data editing strategy is recommended in this regard. We also find that the ENSO-related signals mainly impact on sea levels in the west of Luzon Strait and of the Philippine coast, but are dampened along the Chinese coast over 2002–2022. The regional mean sea level trend is reduced from 3.54 ± 0.85 mm yr−1 to 3.00 ± 0.84 mm yr−1 after removing the ENSO-related low-frequency variability. Finally, the variation of sea level trends from offshore oceans to coasts is found to be nonlinear, indicating the complexity of sea level changes in the study area. The blending of tide gauge records and the SCMR dataset has been successfully conducted in four tide gauges over a longer period of January 1993-April 2022, which would contribute to the analysis of long-term sea level trends at the local scales.

Indo-Pacific humpback dolphins face extirpation in Shantou waters

The Indo-Pacific humpback dolphin is an inshore species occurring in productive estuarine and coastal areas of the Indo-Pacific. Since the end of the 20th century, coastal region of China has experienced tremendous economic expansion, exposing this dolphin species to a wide range of human activities. Despite frequent voices of concern in the past three decades calling for research and conservation attention, protracted debates were not followed by constructive conservation actions due to the lack of or inconsistent use of demographic data. The current study examined 13-year sighting and photo-identification data collected for a small and little known population of humpback dolphins off the coast of Shantou (ST) in the northern South China Sea. Compared to conspecific populations inhabiting large or medium-sized estuarine systems, the ST dolphins have much larger home range (mean = 181.3 km2, S.D. = 87.7) which, however, has been shrinking recently due to coastal reclamations and expanding aquaculture farming. Although at least two dolphin births were recorded between 2010 and 2022, there was no successful recruitment of a non-calf individual. Meanwhile, the population shrank by 29.4 %, with the most updated population size of only 12 individuals, estimated in 2022. Based on the current demographic trend, we predict that the ST humpback dolphins will be extirpated within one or two decades. Furthermore, as the dolphins are aging and becoming less reproductively viable, there is little chance to reverse the declining trend even without additional stressors. Moreover, we noticed prevailing vertebral column deformities amongst the ST dolphins, possibly a sign of inbreeding depression of a small population. The present study exemplifies a typical case of a small, isolated and locally endangered dolphin population that is approaching local extinction without efficient monitoring program and conservation action. We emphasize that without solution-based research and efficient conservation planning, other humpback dolphin populations in China might soon follow in the steps of ST humpback dolphins.

Dynamic accumulation of a high-grade gas hydrate system: insights from the trial production gas hydrate reservoir in the Shenhu area, northern South China Sea

The ultimate enrichment level and quantity of gas hydrate resources are influenced by the dynamic process of accumulation and preservation. High-resolution 3-D seismic data, logging while drilling (LWD), pressured coring, and in situ testing were used to characterize the dynamic accumulation and preservation of the trial production high-grade gas hydrate reservoir (HGGHR) in the Shenhu area. Through seismic variance analysis and ant-tracking, we found that newly identified mud diapir-associated faults with three development stages controlled the migration and accumulation of gas hydrate and shifted the base of the gas hydrate stability zone (BGHSZ), resulting in dynamic accumulation and dissociation of gas hydrates. The recognized double bottom simulating reflectors (BSRs) were concluded to have been formed due to the shift of the BGHSZ caused by the variational equilibrium conditions. The interval between the double BSRs was inferred to be a disequilibrium zone where gas recycling occurred, contributing to the coexistence of gas hydrates and free gas and the dynamic formation of the HGGHR. Multiple gliding faults formed within the GHSZ in the late period have altered the HGGHR and control the present thickness and distribution of the gas hydrates and free gas in the hanging wall and footwall. Under the influence of geothermal fluids and the fault system associated with the mud diapir, the HGGHR experienced dynamic accumulation with three stages, including early accumulation, medium-term adjustment, and late alteration and preservation. We conclude that four factors affected the formation, distribution, and occurrence of the HGGHR: the geothermal fluids accompanying the deep mud diapir below the reservoir, the dual supply of thermogenic gas and biogenic gas, the recycling of hydrate gas beneath the BGHSZ, and the post-gas hydrate faults developed within the GHSZ. A geological model illustrating the dynamic formation of the trial production HGGHR was proposed, providing a reference for future exploration of HGGHRs with a great production potential in deepwater settings.

The latest activity of the slope fault zone (Pearl River Mouth) in northern South China Sea and implications for earthquake hazard assessments

In the northern region of the South China Sea, located at the intersection of the continental shelf and slope, there have been recorded four earthquakes with magnitudes M ≥ 6.0. This pattern of seismic activity suggests the presence of a previously unidentified active submarine fault. Notably, no active faults have been documented in this area prior to our investigation, with earlier studies predominantly focusing on the structural morphology and sedimentary basin evolution associated with the South China Sea's spreading phenomenon. Through the examination of seismic profiles and historical earthquake records, we have identified a newly discovered fault, termed the Slope Fault Zone (SFZ). The SFZ exhibits diverse structural components, including a listric normal fault in the western sector and a strike-slip fault zone in the eastern sector. Analysis of shallow seismic profiles and borehole data has revealed that the SFZ intersects with strata from the late Pleistocene epoch, confirming its classification as an active fault. The derived fault parameters indicate that the western segment of the SFZ has the potential to generate earthquakes of considerable magnitude, ranging from M 6.7 to 7.2. Furthermore, given the significant occurrence of submarine landslides along the southern boundary of the SFZ, there exists a risk that such seismic events could trigger slope failures and subsequent tsunamis. In summary, our research has unveiled the presence of an active fault capable of precipitating submarine earthquakes, landslides, and tsunamis.

Nitrogen fixation of different seasons and parts of seagrass Enhalus acoroides in Xincun Bay, northern South China Sea

In coastal ecosystems, the nitrogen fixation of seagrasses is crucial. However, the nitrogen fixation rates of seagrasses can vary spatially and temporally, especially in tropical regions. To investigate the nitrogen fixation rates of Enhalus acoroides, we used principal component analysis to examine the rates of nitrogen fixation in the roots, rhizome, and leaves of E. acoroides during the southwest monsoon (July) and the northeast season (October and January), respectively. The results showed that the nitrogen fixation rates in all three parts of E. acoroides were significantly higher during the southwest monsoon period compared to the northeast period. The rates of nitrogen fixation in the leaves of E. acoroides were found to be higher than in the roots and rhizomes. The difference in nitrogen fixation between the various parts of E. acoroides was highly significant (p<0.01). The nitrogen fixation activity of E. acoroides was significantly affected by temperature, as determined by principal component analysis. The spatial variations observed in different regions of E. acoroides were influenced by environmental factors in addition to the temperature-driven seasonal changes. The results suggest that temperature plays a vital role in the nitrogen fixation ability of E. acoroides.

Asian monsoonal temperature and environmental changes since the penultimate deglaciation as revealed by alkenones and alkanes in sediments from the northern South China Sea

A new alkenone sea surface temperature (SST) record from the South China Sea (SCS) over the last 170 ka has been reported here. Our analysis of magnetic susceptibility and AMS 14C dating on the top portion of the core suggests that this record dates to the Marine Isotope Stage (MIS) 6. As indicated in Termination I and II, SST changes are correlated with clear glacial/interglacial cycles and likely dominated by atmospheric CO2 and Western Pacific Warm Pool temperature changes. But the two Terminations were different in magnitude. In Termination I, the minimum and maximum SSTs are 24.5 °C and 27.5 °C, respectively, showing a smaller rise (3 °C) than in Termination II (24–28.3 °C). The n-alkane indices, such as average chain length (ACL) and carbon preference index (CPI), exhibited high/low values during glacial/interglacial periods, and possible more arid climate during MIS 6 than MIS 2. During Termination II, the maximum surface warming in the SCS is accompanied by a progressive lag of approximately 7000 years in relation to maximum summer insolation until Termination I, when they are nearly synchronous. According to the study, SSTs are continuously changing and are influenced by solar insolation, global ice volume changes, and ocean–atmosphere interactions across hemispheres.

Dissolved Nitrous Oxide and Hydroxylamine in the Northern South China Sea in Summer

AbstractNitrous oxide (N2O) is a potent greenhouse gas and a strong ozone‐depleting substance. Ocean is an important natural source of atmospheric N2O, which has attracted increasing attention recently as to understand the biogeochemical cycles of nitrogen in marine systems. Nitrification is one of the major N2O production pathways during which N2O is produced as a side product and hydroxylamine (NH2OH) is produced as an obligate intermediate. However, few studies have reported the NH2OH variability in natural seawater and its relationship between N2O remains ambiguous. In summer 2022, spatial distribution and influencing factors of N2O and NH2OH in the northern South China Sea (SCS) were investigated. Dissolved N2O varied from 5.79 to 34.80 nmol L−1 in water column and generally increased with water depth above 800 m. N2O was correlated with dissolved oxygen and nitrate, inferring that N2O was mainly produced via nitrification. NH2OH varied from below detection limit to 3.91 nmol L−1. The adoption of multiple standard additions can largely reduce the bias in NH2OH measurement. Structural equation modeling showed that NH2OH has a potential effect favoring excessive N2O. Surface N2O saturation ranged between 102% and 278%, indicating that the northern SCS was a net source of atmospheric N2O in summer.

Sedimentary characteristics and geological significance of a mixed-process delta for petroleum exploration in the Zhuhai formation of the Baiyun Depression, Pearl River Mouth Basin, China

The Zhuhai Formation in the Baiyun Depression of the Pearl River Mouth Basin in the northern South China Sea is a marine delta influenced by multiple hydrodynamic forces, including rivers, waves, and tides, resulting in a mixed-process delta. As a result of the combined effects of these forces, the distribution patterns and reservoir structures in the sandstone formations of the Zhuhai Formation are extremely complex. This paper provides a detailed description of the coring section of the Zhuhai Formation drilled in the Baiyun Depression, focusing on the lithology, sedimentary structure, special containers, and depositional dynamics of the coring section. It also summarizes the characteristics of a hybrid power delta controlled by rivers, waves, and tides, and explores the geological significance of this mixed-process delta for oil and gas exploration. The study reveals that sedimentation controlled by rivers primarily consists of medium-to-fine sandstones with interbedded cross-bedding and parallel bedding. Scour-fill structures and a bell or box-shaped natural gamma ray logging curve (GR) are also observed. Seismic reflection exhibits strong amplitude and high continuity. Wave-controlled sedimentation, on the other hand, predominantly consists of silt and fine sandstones with scouring, wave-formed cross-bedding, abundant bioturbation, and a serrated and funnel-shaped GR curve. Seismic reflection shows medium amplitude and high continuity. Tidally-controlled deposits are characterized by thin interbeds of mudstone, siltstone, and fine sandstone, as well as vein laminations, undulating composite laminations, lenticular laminations, and biclastic layers. The GR curves for tidally-controlled deposits are sharp or jagged with funnel shapes, and seismic reflections exhibit medium amplitude and continuity. The hydrodynamic features controlling sedimentation during the depositional period of the Zhuhai Formation in the Baiyun Depression exhibit evident zonation on a planar scale, influenced by sedimentary paleogeography and sea-level changes. River control is predominant in the northern Zhuhai Formation, with local wave influence and minimal tidal influence. In the southern part, tidal action becomes increasingly important. In terms of reservoir properties, wave-controlled reservoirs display significantly better porosity and permeability compared to tidally-controlled and river-controlled reservoirs. Tidally-controlled reservoirs exhibit high heterogeneity with a wide range of porosity and permeability values. The varying influences of different hydrodynamic drivers in different locations have led to significant variations in the size, morphology, and physical properties of sand bodies within the marine delta sediments of the Zhuhai Formation in the Baiyun Depression. Thus, the type and intensity of sedimentary hydrodynamics become the primary controlling factors for the development of high-quality reservoirs.

Sedimentary Organic Nitrogen Isotopic Constraints on the Mid‐Holocene Transition in the Nitrogen Dynamics of the Northern South China Sea

AbstractMillennial‐scale nitrogen (N) cycling processes in marginal seas and their response to climate change have not been well understood. Here, we present high‐resolution (ca. 110 years) organic nitrogen isotope (δ15Norg) data since the last deglaciation (16.1 ka) derived from a highly resolved sediment core in the northern South China Sea, aiming to explore millennial‐scale N cycling processes in this area. Unlike most bulk nitrogen isotope (δ15Nbulk) records from the South China Sea, the δ15Norg records show a clear response to well‐defined climatic episodes during the last deglaciation and early Holocene (EH, ∼11.7 to 9 ka), but exhibit a gradually decreasing trend in mid‐to‐late Holocene (since ca. 9 ka). During the last deglaciation and EH, the upper water column N dynamics are controlled by the lateral transport of surface nitrate from eastern tropical Pacific (ETP) presumably via the North Pacific Intermediate Water (NPIW) and to some extent, influenced by the altered input of terrigenous matter driven by sea level change. The significant decrease in δ15Norg since the mid‐Holocene (at ca. 9 ka) can be best explained by the increase in local N2 fixation forced by enhanced El Niño. This mechanism is consistent with modern observations. Overall, our results may reflect the main controlling factors of surface ocean N dynamics have shifted from zonal transport of nitrate from the ETP to El Niño since the mid‐Holocene.

Controlling Effect of Particle Size on Gas Hydrate Enrichment in Fine‐Grained Sediments

AbstractThe particle size of sediments below the seabed is a crucial factor affecting the formation and enrichment of gas hydrates. Apart from the formation and enrichment law of gas hydrate in coarse‐grained sediments (dominated by a sandy‐sized fraction), in the fine‐grained sediments (&lt;62.5 μm) which accounts for more than 90% of offshore gas hydrate resources globally, the control effect of sediment particle size on gas hydrate is still unclear. Therefore, understanding the relationship between the fine‐grained sediment particle size and gas hydrate enrichment is essential for revealing the global distribution and dynamic evolution of gas hydrates. Here, we analyzed the vertical gas hydrate saturation, particle size parameters of sediments, whole‐rock minerals, and clay mineral components based on drilling data and sediment samples from fine‐grained gas hydrate reservoirs (GHRs) in the Shenhu area of the northern South China Sea. The results show that in fine‐grained sediments, the coarse particles cannot improve the reservoir quality or enrich the gas hydrate because many fine particles fill the intergranular pores formed by the coarse particles. Meanwhile, the fine particles were dominated by clay minerals, especially in the illite/smectite mixed layer, which significantly reduced the permeability of the sediment layer and was not conducive to the enrichment of gas hydrates. Moreover, sedimentary processes directly control the sediment particle size and mineral composition, which play an essential role in controlling GHRs at the macroscale. In the fine‐grained sediments, very fine sediments (&lt;8 μm) have a more significant negative impact on gas hydrate enrichment.

Provenance history of the eastern Pearl River Mouth Basin: Implications for the evolution of the South China margin

The provenance history of the Pearl River Mouth Basin is of great significance for understanding how the South China margin evolved from a Mesozoic Andean-type subduction margin into the present-day rifted passive margin. However, the long-term provenance history of continental margin basins in the northern South China Sea is poorly understood. Newly identified Mesozoic−Cenozoic successions from the Lufeng Sag in the eastern Pearl River Mouth Basin provide valuable records of the provenance history and related evolution of the South China margin. Here, we trace the provenance of the eastern Pearl River Mouth Basin using U-Pb ages of zircon and ZTR values (proportions of zircon, tourmaline, and rutile in all transparent heavy minerals) of heavy minerals, and determine the tectonic setting and crustal thickness using trace element ratios and Eu anomalies in zircon. The results show that from the middle Eocene strata—which have a U-Pb age spectrum similar to strata of the Upper Cretaceous—to the Miocene strata, the proportion of Mesozoic age populations gradually increases, while the proportion of Paleozoic age populations decreases. In addition, following the middle Eocene high values, the ZTR values decreased during the late Eocene and then increased during the early Oligocene. Based on the variations in zircon age populations and ZTR values, the middle Eocene sediment is interpreted to have been recycled from Upper Cretaceous to middle Eocene successions, followed by an early Oligocene provenance change from intrabasinal to extrabasinal sources. Moreover, the zircon trace element ratios and Eu anomalies indicate that a widespread Mesozoic continental magmatic arc formed before ca. 90 Ma along the South China margin, and the crust simultaneously thickened during this time. In accordance with the provenance history, the evolution of the South China margin is divided into three tectonic phases: the Paleo-Pacific, Proto-South China Sea, and South China Sea. During the transition from the Paleo-Pacific to the Proto-South China Sea tectonic phase, sediment was recycled from Upper Cretaceous to middle Eocene successions in the eastern Pearl River Mouth Basin. During the transition from the Proto-South China Sea to the South China Sea tectonic phases, the provenance changed from intrabasinal to extrabasinal sources.

Acid-Extracted Hydrocarbon Anomalies and Significance in the Chaoshan Depression of the Northern South China Sea

To predict the favorable zones and the types of reservoirs, acid extraction has been used in the Chaoshan depression to detect trace amounts of light hydrocarbons, heavy hydrocarbons, and the δ 13C (‰) of methane. As a result, two integration anomalous zones for exploration activity were blocked out in the northeastern and southwestern parts of the Chaoshan Depression, respectively. By analyzing the differentiation law and structural characteristics of hydrocarbon gases, as well as the stable carbon isotope ratio of methane, the underlying reservoirs were predicted to be gas reservoirs, and the seismically interpreted Dongsha-A (DS-A) structure was predicted to be a gas-rich structure. By correlating the seismic profile and geochemical anomalies, it was determined that fault planes and micro-fractures are the main controlling factors for the occurrence of the seabed’s geochemical anomalies. A composite formation mechanism of “lower generation, upper accumulation and micro fractures leaking” is proposed for the control of the underlying petroleum reservoirs, as well as for the micro-fracture control of permeability and surface adsorption control. Acid-extracted hydrocarbon anomalies have favorable indicating significance for exploration activity.

Changes in diazotrophic community structure associated with Kuroshio succession in the northern South China Sea

Abstract. Kuroshio intrusion (KI) is a key process that transports water from the western Pacific Ocean to the northern South China Sea (nSCS), where KI-induced surface water mixing often causes variations in microbial assemblages. Yet, how interannual KIs affect the biogeography of diazotrophs and associated environmental factors remains poorly characterized. Here, by quantifying the degree of KIs in 2 consecutive years, coupled with monitoring the diversity and distribution of nitrogenase-encoding nifH phylotypes with quantitative PCR and high-throughput sequencing, we show that changes in the diazotrophic community structure in the nSCS are highly correlated with KI-induced variations in a range of physicochemical parameters. Specifically, the filamentous cyanobacteria in the genus Trichodesmium were more abundant at stations strongly affected by KI and thereby with a deeper mixed layer and higher surface salinity and temperature; the unicellular N2-fixing cyanobacteria in group B (UCYN-B) were more abundant at stations least affected by KI and correlated with nutrient availability, whereas UCYN-C and the γ-proteobacteria were prevalent at stations moderately affected by KI. The neutral community model further demonstrated that dominant diazotrophic subcommunities were significantly affected by environmental factors in 2017 when KI was stronger compared to 2018 when KI retreated. Our analyses provide insightful evidence for the role of KI in shaping the diazotrophic community structure primarily as a stochastic process, implying a potential region-scale redistribution of diazotrophs and nitrogen budget, given that KIs are projected to intensify in a future warming ocean.

Migration and controls of Shenhu submarine canyons in the upper continental slope of northern South China Sea: Insights from three‐dimensional seismic data mapping

AbstractSubmarine canyons, prominent features on continental margins, hold significant scientific and engineering implications. Despite extensive research on various types of submarine canyons, current understanding of those canyons developed on the upper continental slope, recognized as the most common type globally, remains limited. The Shenhu Canyons on the upper continental slope of the northern South China Sea, comprising over 19 subparallel features persistently developed from the Late Miocene to Quaternary, offer an excellent opportunity for in‐depth study. Previous assumptions suggested a unidirectional eastward migration of these canyons parallel to the host margin, primarily based on observations from two‐dimensional transverse seismic profiles. However, this study's analysis using three‐dimensional seismic data mapping reveals that not all canyons have migrated eastward; 28% display an opposite or a zigzag patterned migration, challenging previous assumptions. The eastward migration of the Shenhu Canyons on the eastern slope of the palaeo‐Pearl River shelf‐edge delta can be linked to the south‐east–southward progradation of the host margin and the palaeo‐shelf‐edge delta. However, the overall eastward migration pattern is complicated by submarine landslides and turbidity current processes. Landslides can influence the direction and magnitude of canyon migration through four mechanisms: turbidity current diversion; retrogressive development of canyon‐wall landslides; retrogressive landslides on surrounding open slopes; and emplacement of landslides in canyon thalwegs. These mechanisms may result in widening, bifurcation, confluence or obstruction of canyons, contributing to canyon migration. Erosion by turbidity currents can trigger canyon‐wall failures, which in turn promote the lateral migration of the canyons. If overspilled turbidity currents lead to the growth of levées in the lower reaches, canyon confinement increases, suppressing lateral migration. The inherited high‐gradient Palaeogene syn‐rift tectonic slope, combined with the depositional delta‐front slope of the palaeo‐shelf‐edge delta, creates favourable conditions for the initiation and persistent development of the straight Shenhu Canyons. This comprehensive analysis provides valuable insights into the complex dynamics and factors influencing the migration of upper continental slope submarine canyons.

Identification of Mass Transport Deposits and Insights into Gas Hydrate Accumulation in the Qiongdongnan Sea Area, Northern South China Sea

Identification of Mass Transport Deposits and Insights into Gas Hydrate Accumulation in the Qiongdongnan Sea Area, Northern South China Sea

Quantitative characterization of 3D pore structures and percolation characteristics in bioturbated reservoir media based on X-ray micro-CT: a case study of the Neogene Sanya Formation in the Qiongdongnan Basin, Northern South China Sea

Organisms alter the primary texture of sediments, leading to alteration in petrophysical properties mediated by a textural and mineralogical contrast between burrow-fill and sediment host, which affects reservoir properties and fluid flow characteristics. In this article, in order to understand the microscopic pore structure and flow state of fluids in microscopic pores of bioturbated reservoirs, Ophiomorpha-bearing bioturbated reservoirs in the Neogene Sanya Formation of the Qiongdongnan Basin in the northern South China Sea were selected to research. Micro-CT was applied to scan selected core plugs, and a 3D pore structure model was established. The geometric characteristics of the microscopic pore structure were quantitatively and visually characterized by a modified maximal ball algorithm, and connectivity analysis of the pore structure was carried out. Numerical simulations of the percolation characteristics of the analyzed bioturbated reservoir samples were performed using digital core software (Avizo) and multi-physics field simulation software (Comsol). The results show that: (1) 3D pore structures reveal that pore volume, pore area, pore equivalent radius, throat area, throat equivalent radius and throat length have a large distribution range; among them, the pore volume has the largest distribution range, which can vary by six orders of magnitude, indicating that the pore size distribution of the bioturbated reservoir is uneven and has strong heterogeneous characteristics; (2) the connected pore structure is very complex, and as the equivalent radius of connected pore increases, the coordination number also gradually increases, the better the connectivity. Numerical percolation simulation results also suggested that larger connected pore space plays a key role in the effective permeability of the reservoir. This study has important implications for analyzing the modification effect of bioturbation on oil and gas reservoirs, and enhancing production and recovery in the study area.

Representative Dynamic Accumulation of Hydrate-Bearing Sediments in Gas Chimney System since 30 Kyr BP in the QiongDongNan Area, Northern South China Sea

A stratigraphic complex composed of mass transport deposits (MTDs), where the gas occurrence allows for the formation of a gas chimney and pipe structure, is identified based on seismic interpretation in the QiongDongNan area of the northern South China Sea. During the Fifth Gas Hydrate Drilling Expedition of the Guangzhou Marine Geological Survey, this type of complex morphology that has close interaction with local gas hydrate (GH) distribution was eventually confirmed. A flow-reaction model is built to explore the spatial–temporal matching evolution process of massive GH reservoirs since 30 kyr before the present (BP). Five time snapshots, including 30, 20, 10, and 5 kyr BP, as well as the present, have been selected to exhibit key strata-evolving information. The results of in situ tensile estimation imply fracturing emergence occurs mostly at 5 kyr BP. Six other environmental scenarios and three cases of paleo-hydrate existence have been compared. The results almost coincide with field GH distribution below the bottom MTD from drilling reports, and state layer fracturing behaviors always feed and probably propagate in shallow sediments. It can be concluded that this complex system with 10% pre-existing hydrates results in the exact distribution and occurrence in local fine-grained silty clay layers adjacent to upper MTDs.

Semidiurnal Internal Tide Interference in the Northern South China Sea

Multiwave interference plays a crucial role in shaping the spatial variations of internal tides. Based on a combination of in situ mooring and altimeter data, interference of semidiurnal internal tides was investigated in the northern South China Sea. Mooring observations indicate the observed kinetic-to-potential energy ratio and group speed are both relatively lower than the theoretical values of mode-1 semidiurnal internal tides, indicating the presence of partly-standing waves. This is consistent with the altimeter result that the mooring was located at the antinode within the interference pattern formed by the superposition of the westward and southward semidiurnal internal tides from the Luzon Strait and the continental slope of the southern Taiwan Strait. However, the kinetic-to-potential energy ratio and group velocity were notably changed when an anticyclonic eddy passed by the mooring. By employing the ray-tracing method, we identified that mesoscale processes may induce a phase difference in the semidiurnal internal tides between the Luzon Strait and the continental slope of the southern Taiwan Strait. This alteration further leads to changes in the positions of nodes and antinodes within the interference pattern of the semidiurnal internal tides.

Analysis of Uncertainties and Associated Convective Processes in Simulations of Extreme Precipitation Over Cities With a Regional Earth System Model: A Case Study

AbstractThis study utilizes the Weather Research and Forecasting model coupled with an atmospheric chemistry model, a multi‐layer urban canopy model (UCM), and a building energy model to simulate the extreme rainfall event influencing the Guangzhou city in South China on 7 May 2017. By employing small variations in the longwave emissivity of buildings within the UCM, 11 convective‐permitting experiments are conducted. The maximal 18‐hr and hourly rainfall accumulation vary from a 2‐year return period to as high as a 20 or 40‐year return period with notable spatial differences. Comparisons between the more accurate group (GOOD) and less accurate group (POOR) simulations highlight that some minor differences in the near‐surface air thermodynamic conditions in urban area could lead to substantial differences in local convection and its impacts on subsequent convective systems. With persistent transportation of warm, moist airflows from the northern South China Sea, formation of a slow‐moving mesoscale outflow boundary to the north of the urban agglomeration leads to the development of a quasi‐stationary, compactly structured meso‐γ‐scale rainstorm in the GOOD simulations. The stronger low‐level to near‐surface convergence and mid‐level cyclonic shear within this system substantially enhance low‐level updrafts, leading to increased microphysical production and stronger horizontal advection of rainwater within the system. These findings offer some process‐based understanding about the uncertainties in simulating urban extreme rainfall and underscore the need to develop ensemble forecasting methods for convection‐permitting numerical models that incorporate increasingly complicated representations of anthropogenic influences.

Millennial calcification trends in Porites corals: Resilient skeletal density and vulnerable linear extension in response to environmental change

Global coral reefs are currently facing widespread declines in calcification rates, a trend that underscores the need to understand variations between calcification in contemporary and ancient corals. This study investigates the changes in coral calcification over the last millennium by analyzing 21 subfossil and 11 modern coral samples from a marginal reef in the northern South China Sea. Our results reveal that although coral skeletal density has remained relatively stable, significant differences are evident in linear extension rates between the Medieval Climate Anomaly (MCA, 900–1300 CE) and the Little Ice Age (LIA, 1550–1850 CE), as well as between the pre- and post-industrial periods. This finding suggests that inshore Porites corals have greater resilience in their skeletal density compared to linear extension rates when responding to environmental changes. Our analysis indicates that sea surface temperature (SST) is the primary environmental driver influencing coral calcification, with a positive correlation observed between SST and both modern coral linear extension and calcification rates. In contrast, factors such as seawater pH, salinity, and total solar irradiance appear to have minimal impact on coral skeletal growth. Therefore, the lower linear extension and calcification rates observed in the LIA corals, as compared to the MCA corals, are likely due to the cooler temperatures during the LIA. We also hypothesize that the extended period of cold SST from the LIA to the early 20th century may have led to the reduced linear extension and calcification rates in modern corals, with recent warming insufficient to facilitate a return to the optimal skeletal growth observed during the MCA.