Search Results (7)

The Shenhu area, located on the northern continental slope of the South China Sea, is a confirmed gas hydrate-enriching region, but the sedimentary unit, causative mechanisms, and evolution processes of the strata that contain hydrate remain unclear. Furthermore, the recognition of bottom simulating reflectors (BSRs) rests on qualitative description; there is no quantitative method for the characterization or three-dimensional depiction of BSRs. This review examines the sedimentary features and key factors controlling gas hydrate distribution in the region, based on high-resolution sequence stratigraphy combined with drilling and logging data from hydrate drilling projects in the Shenhu area. The main findings of this study include (1) BSRs are mainly distributed in the ridges of the continental slope and in the slip blocks of the side slope, with hydrates developing along a thin layer (10–40 m) below the hydrate stability zone, as confirmed by drilling results; (2) The distribution of BSRs is strongly influenced by the presence of gas chimneys, the migration of deepwater channels, and the erosion and sedimentation processes of canyons, all of which are directly or indirectly related to the accumulation, distribution, and formation of hydrate reservoirs; (3) The sand factor is generally less than 10%, and BSRs accumulate in areas where the sand factor is higher (4–10%). Hydrate saturation shows a positive correlation with porosity. This research also identifies the early Pleistocene erosion/resedimentation event as a key factor that controls the non-homogeneous distribution of hydrates in the region. This research highlights the role of deepwater canyon erosion and slumping processes in controlling gas hydrate formation, offering new insights into the impact of dynamic geological processes on hydrate accumulation. This study provides valuable knowledge for future hydrate exploration and global resource assessments. Full article

The shelf-edge trajectory is comprehensively controlled by tectonics, sediment supply, sea level, and climate fluctuations; its migration and evolution have a strong influence on what happens in the deep-water depositional system during the Quaternary. The shelf-edge trajectory pattern, sediment-budget partitioning into deep-water areas, and reservoir evaluations are focused topics in international geosciences. In this paper, the Qiongdongnan Basin (QDNB) in the northern South China Sea is taken as an example to study how shelf-edge trajectory migration patterns can influence the types of deep-water gravity flow which are triggered there. Through quantitatively delineating the Quaternary shelf-edge trajectory in the QDNB, four types of shelf-edge trajectory are identified, including low angle slow rising type, medium angle rising type, high angle sharp rising type, and retrogradation-slump type. A new sequence stratigraphic framework based on the migration pattern of shelf-edge trajectory is established. There are four (third-order) sequences in the Quaternary, and several systems tracts named lowstand systems tract (LST), transgressive systems tract (TST), and highstand system tract (HST) are identified. This study indicates that the type of deep-water gravity flow can be dominated by the shelf-edge trajectory migration patterns. When the shelf-edge trajectory angle (α) ranged between 0° and 4°, the continental canyons were mostly small-scaled and shallowly incised, with multiple large-scale sandy submarine fan deposits with few MTDs found in the deep-water area. When the angle (α) ranged from 4° < α < 35°, the size and incision depth of the continental slope canyons increased, relating to frequently interbedded sandy submarine fan deposits and MTDs. When angle (α) ranged from 35° < α < 90°, only a few deeply-incised canyons were present in the continental slope; in this condition, large-scaled and long-distance MTDs frequently developed, with fewer submarine fans deposits. When angle (α) ranged from 90° < α < 150°, the valley in the slope area was virtually undeveloped, sediments in the deep-sea plain area consisted mainly of large mass transport deposits, and submarine fan development was minimal. Since the Quaternary, the temperature has been decreasing, the sea level has shown a downward trend, and the East Asian winter monsoon has significantly enhanced, resulting in an overall increase in sediment supply in the study area. However, due to the numerous rivers and rich provenance systems in the west of Hainan Island, a growing continental shelf-edge slope has developed. In the eastern part of Hainan Island, due to fewer rivers, weak provenance sources, strong tectonic activity, and the subsidence center, a type of destructive shelf-edge slope has developed. The above results have certain theoretical significance for the study of shelf-edge systems and the prediction of deep-water gravity flow deposition type. Full article

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. Full article

This study describes the development of a long-range hybrid autonomous underwater vehicle (AUV) for ocean turbulence measurement. It is a unique instrument, combining the characteristics of the conventional AUV and the buoyancy-driven glider, with a variety of flexible motion modes, such as cruise mode, glider mode, drift mode, and combination of multiple motion modes. The hybrid AUV was used for continuous turbulence measurement in the continental slope of the northern South China Sea in 2020. A total of ten continuous profiles were completed covering a horizontal span of 25 Km and a depth of 200 m. The hybrid AUV was operated in the combined glider and cruise mode. The hybrid AUV’s flight performance was stable and satisfied the requirement for turbulence observation. The measured velocity shears from both probes were in good agreement, and the noise-reduced shear spectra were in excellent agreement with the Nasmyth spectrum. The water column in the study area was highly stratified, with a thick thermocline. The dissipation rate (ε) varied from 1.41 × 10⁻¹⁰ to 4.18 × 10⁻⁷ W·kg⁻¹. In the surface mixed layer, high values of ε (10⁻⁹∼10⁻⁸ W·kg⁻¹) were observed toward the water surface. In the thermocline, ε was 10^−9.5∼10⁻⁹ W·kg⁻¹, which was smaller than the level of the surface mixed layer. This result was mainly because of the strong “barrier”-like thermocline, which damped the transmission of wind and heat energy from the surface mixed layer to the deep layer. Overall, this study demonstrates the utility of hybrid AUVs for collecting oceanic turbulence measurements. They are a powerful addition to traditional turbulence instruments, as they make it possible to survey large areas to obtain high-quality and high-resolution data in both vertical and horizontal directions over long durations. Full article

Investigating the deposition evolution and stratal stacking patterns in continental rift basins is critical not only to better understand the mechanism of basin fills but also to reveal the enrichment regularity of hydrocarbon reservoirs. The Pearl River Mouth Basin (PRMB) is a petroliferous continental rift basin located in the northern continental shelf of the South China Sea. In this study, the depositional evolution process and stacking pattern of the Zhu III Depression, western PRMB were studied through the integration of 3D seismic data, core data, and well logs. Five types of depositional systems formed from the Eocene to the Miocene, including the fan delta, meandering river delta, tidal flat, lacustrine system, and neritic shelf system. The representative depositional systems changed from the proximal fan delta and lacustrine system in the Eocene–early Oligocene, to the tidal flat and fan delta in the late Oligocene, and then the neritic shelf system in the Miocene. The statal stacking pattern varied in time and space with a total of six types of slope break belts developed. The diversity of sequence architecture results from the comprehensive effect of tectonic activities, sediment supply, sea/lake level changes, and geomorphic conditions. In addition, our results suggest that the types of traps are closely associated with stratal stacking patterns. Structural traps were developed in the regions of tectonic slope breaks, whereas lithological traps occurred within sedimentary slope breaks. This study highlights the diversity and complexity of sequence architecture in the continental rift basin, and the proposed hydrocarbon distribution patterns are applicable to reservoir prediction in the PRMB and the other continental rift basins. Full article

Understanding the marine hydro-thermohaline environment is essential for terrestrial meteorology and the coastal ecosystem. Here, we provide insight into the hydro-thermohaline environment at the Qiongdongnan continental slope of the northern South China Sea and the mechanism controlling it, with focus on its short-term characteristics. We employ a well-validated three-dimensional unstructured-grid-based Finite Volume Coastal Ocean Model (FVCOM) to analyze the spatial-temporal behavior of its hydro-thermohaline structures and to quantify the transport fluxes over a full tidal period. The analysis reveals a two-layer flow structure with directionally oppositely moving layers in the along-isobaths direction. Furthermore, transport patterns undergo periodic changes. During the spring tide, the downslope (along-isobaths) transport of water/heat/salt is approximately 119%/70%/120% higher (62%/62%/62% lower) than during the neap tide. From analyzing the different terms in the thermohaline balance equation, we find that the main dynamic factors controlling heat transport over a tidal period are the gravitational convention and the mean flow, while the salt transport is only dominated by the mean flow. The data of the short-term thermohaline evolution of the QDNS provided in this study may be of use for future studies of the northern SCS, including its marine ecology and marine fisheries. Full article

A total of 388 surface sediment samples taken from the northern South China Sea (SCS) continental shelf were analyzed to characterize the signature of their rare earth elements (REEs). The average REEs concentration was 192.94 μg/g, with a maximum of 349.07 μg/g, and a minimum of 32.97 μg/g. The chondrite-normalized REEs pattern exhibits a remarkably light REEs accumulation, a relatively flat heavy REEs pattern, and a negative Eu anomaly. We subdivided the study area into three zones using the characteristics of REEs and statistical characteristics. Zone I: continental shelf off western Guangdong Province. Here, the sediment provenance is mainly river-derived from the Pearl River, Taiwanese rivers, and those in the adjacent area. Zone II: Qiongzhou Strait and Leizhou Peninsula. Here, the sediment provenance consists of the Qiongzhou Strait and the Hainan Island. Zone III: Hainan Island and SCS slope sediments are dominated. The REEs compositions are mainly controlled by source rock properties, hydrodynamic conditions, and an intensity of chemical weathering. We reconstructed the sediment dispersal and transport route using the REEs compositions, grain size, and other geochemical characteristics throughout the study area. Full article