South China Sea Continental Slope Research Articles

The control of turbidite deposition on pyrite sulfur isotopic composition in sediments on the continental slope of the South China Sea

Sulfate reduction is a commonly reported process in a variety of continental margin settings that affects the carbon (C), sulfur (S), and iron (Fe) marine sedimentary cycles. However, sulfate reduction in unstable environment (turbidite deposition) is easily ignored, which results in a lack of understanding of the C, S, and Fe biogeochemical cycle. Therefore, to elucidate the role of turbidite sedimentation in the global C, S, and Fe biogeochemical cycle, total organic carbon (TOC), total sulfur (TS), iron speciation, pyrite contents, and sulfur isotopic composition in sediments (CL94) on the South China Sea continental slope were analyzed, along with sediment grain sizes, calcium carbonate (CaCO3) content, and K/Al and Ti/Al ratios. Based on petrographic characteristics, grain size parameters, and element geochemistry (K/Al and Ti/Al ratio), it was concluded that the mid-section (322–350 cm) of Site CL94 is a turbidite layer. Compared with normal sedimentary environment, δ34SCRS (−39.7 ‰ on average) exhibits a high value in turbidite layer, which could be related to the high deposition rate and relatively closed environment caused by the turbidity current deposition. Furthermore, in situ deposition is virtually entirely unaffected by external influences, and this is the primary source of sulfide and reactive iron in the relatively closed environment. Thus, pyrite concentration in the turbidity layer fluctuates in conjunction with variations in sulfide and active iron. Moreover, the identification and data extraction of methane seeps were further complicated by the fact that the TS/TOC ratio and δ34SCRS features induced by the turbidity current were extremely similar to those of sediments around methane seeps. The examination of pyrite sulfur isotopic composition and TS/TOC ratios for detecting methane seepage requires further investigation.

Observations of shoaling internal wave transformation over a gentle slope in the South China Sea

Abstract. Four oceanographic moorings were deployed across the South China Sea continental slope near 21.85∘ N, 117.71∘ E, from 30 May to 18 July 2014​​​​​​​ for the purpose of observing high-frequency nonlinear internal waves (NLIWs) as they shoaled across a rough, gently sloping bottom. Individual waves required just 2 h to traverse the array and could thus easily be tracked from mooring to mooring. In general, the amplitude of the incoming NLIWs tracked the fortnightly tidal envelope in the Luzon Strait; they lagged by 48.5 h and were smaller than the waves previously observed to the southwest near the Dongsha Plateau. Two types of waves, a waves and b waves,​​​​​​​ were observed, with the b waves always leading the a waves by 6–8 h. Most of the NLIWs were remotely generated, but a few of the b waves formed locally via convergence and breaking at the leading edge of the upslope-propagating internal tide. Waves incident upon the moored array with amplitude less than 50 m and energy less than 100 MJ m−1 propagated adiabatically upslope with little change of form. Larger waves formed packets via wave dispersion. For the larger waves, the kinetic energy flux decreased sharply upslope between 342 and 266 m, while the potential energy flux increased slightly, causing an increasing ratio of potential-to-kinetic energy as the waves shoaled. None of the waves met the criteria for convective breaking. The results are in rough agreement with recent theory and numerical simulations of shoaling waves.

Spatiotemporal patterns of sediment deposition on the northern slope of the South China Sea in the last 150,000\u2009years

The source-to-sink system of the northern South China Sea (SCS) has been widely discussed during the past few decades. Sr–Nd isotope, clay minerals and trace elements were extensively used as the proxies of sediment provenance, however, still little is known about the transport processes and controlling mechanisms on detailed spatiotemporal scales due to the limitations of these methods. Here we put forward the new provenance proxies RAK and RKCN based on major element compositions to study the spatiotemporal changes in sediment provenance since 150 ka mainly from four sites, DLW3101, MD12-3429, ZHS-176 and MD12-3432, which are located on the northern SCS continental slope. Our results show that, spatially, the pathways and intensities of contour currents and gravity flows play important roles in sediment transport. For alongslope processes, the South China Sea Branch of Kuroshio Current (SCSBKC) and the Deep Water Current (DWC) transport sediments from southwestern Taiwan, while the Intermediate Water Current (IWC) can carry sediments from Hainan, the Red River or the Indochina Peninsula. For downslope processes, gravity flows transport materials from the Pearl River delta and shelf to the slope. Moreover, seafloor bathymetry influences sediment transport by altering the pathways of ocean currents. Temporally, the impacts of sea level and monsoon rainfall fluctuations are always superimposed over the last 150 ka. Sea level fluctuations could significantly change the distance from the Pearl River estuary to the slope, while variations in the East Asian summer monsoon (EASM) rainfall would affect continental erosion rates in the source regions.

The South China sea continental slope morphology and its relation to the anomalous gravitational field

The bottom topography of the studied area of the South China Sea is characterized by an uneven distribution of depths, their sharp variations and the presence of a large number of seamounts. As a result of the research conducted, bathymetric profiles, a bathymetric map and a scheme of Faye anomalies were compiled. Geomorphological zoning was conducted. Sections of bottom distinguished by morphology were identified: 1) weakly dissected slope, steep (angles up to 15) in the upper part and more gentle (57) to the bottom of the deep-water step, 2) slope eroded by fluvial processes, 3) stepped slope, 4) chains of mountains and flat-topped hills, 5) accumulative bottom of the deep-water plain. It was established that the gravitational field as a whole reflects the complex structure of the bottom, and the local forms of the underwater relief are expressed in Faye anomalies. For elevations, zones of positive anomalies of Faye are marked, and sections of canyon-like valleys were mapped by an intense negative anomaly of the gravitational field to -60 mGal. In the process of endogenous evolution of the region, the relief became more complex. The primary endogenous continental slope was almost everywhere transformed by exogenous processes. Within the shelf, the filling of the basin led to the unification of all its projections into a single raised base. A significant role in the formation of the modern relief was played by large-scale repeated sea regressions, as a result of which the formation of the sedimentary cover was interrupted by periods of its erosion with the formation of planation surfaces. The Late Pleistocene peak of the regression (1618 thousand years BP), which caused the complete drainage of the shelf areas of the region, caused the erosion of the shelf and the movement of coarse clastic material through submarine canyons and valleys.

Seismic analysis of the gas hydrate system at Pointer Ridge offshore SW Taiwan

Pointer Ridge is a gas hydrate prospect on the South China Sea continental slope offshore SW Taiwan. It is characterized by densely distributed bottom simulating reflections (BSRs), active gas seepage, and potential sandy gas hydrate reservoirs. To understand how the fluids have migrated toward the seafloor, and the role of geological processes in the gas hydrate system, we have collected and analyzed high-quality 2D and 3D reflection seismic data. We first mapped the spatial distribution of the BSRs, and interpreted a major normal fault, Pointer Ridge Fault (PR Fault). The NE-SW trending fault dips to the east, and separates the erosional regime to the west from the depositional regime to the east. One active vent site was identified directly above the PR Fault, while another is located on a topographic high to the west of the fault. On the hanging block of the fault we found at least one major unconformity. The seismic data indicate refilled channels with coarser-grained sediments in the hanging wall of the normal fault. Seismic attribute analysis shows subsurface fluid conduits and potential gas hydrate reservoirs. We propose two types of gas chimneys, which are separated by the fault. Gas plumes derived from hydroacoustic data are mostly from the footwall block of the fault. We infer that fluid flow is more active in the erosional environment compared to the depositional one, and that this is the result of reduced overburden. The methane-bearing fluids migrate upward along the PR Fault and chimneys and form hydrates above the base of the gas hydrate stability zone. Based on seismic interpretation and seismic attribute analysis, we postulate that the channel infill constitutes the most promising hydrate reservoirs in this geological setting. In the surveyed area of Pointer Ridge these channels occur mainly below the gas hydrate stability zone.

Analyzing amplitudes of internal solitary waves in the northern South China Sea by use of seismic oceanography data

In the northern South China Sea, numerous multichannel seismic reflection sections are used to identify internal solitary wave (ISW) packets and extract wave amplitudes and corresponding water depths. The analyzed data show that these depression ISWs occur on the upper continental slope at water depths between 263 m and 740 m, with maximum amplitudes ranging from 35 m to 128 m. Our results, in conjunction with previous studies, suggest that the maximum amplitudes of the ISWs on the northern South China Sea continental slope are highly correlated with seafloor depths and that they have a logarithmic function relationship. The maximum amplitudes decrease with decreasing water depths. Interactions between the ISWs and the seafloor play a crucial role in decreasing ISW maximum amplitudes, especially in shallow areas. In addition, we compare the observed vertical amplitude distribution with theoretical results and find that they are concordant. The “bottom depth” (H) in the boundary conditions of eigenfunctions represents the extension depth of the ISW rather than the actual seafloor depth. Here, the ISW extension depth is where the ISW amplitude becomes zero and the seafloor depth is just under the ISW. If the ISW interacts intensely with the seafloor, its observed vertical amplitude distribution may exhibit prominent differences from the theoretical result.

Terrigenous sediment input responding to sea level change and East Asian monsoon evolution since the last deglaciation in the southern South China Sea

Abstract Siliciclastic grain size, clay mineralogy, and major element geochemistry of Core MD05–2893 from the lower continental slope off the Sunda Shelf in the southern South China Sea were investigated to assess the response of terrigenous sediment input to sea-level and climatic changes since the last deglaciation. The chronology is based on foraminiferal AMS 14C dates and oxygen isotope records combined with carbonate stratigraphy. In this study, we reconstruct variations in terrigenous sediment input using clay/silt, TiO2/CaO, SiO2/Al2O3, Al2O3/K2O, and smectite/(illite + chlorite) proxies. Clay/silt, TiO2/CaO, and SiO2/Al2O3 ratios are clearly correlated to sea-level change, showing high values during the early phase of deglacial sea-level rise (17.2–14.5 cal ka BP, Stage I) and rapidly decreasing values during the meltwater pulse 1A and further sea-level rise (14.5–11.1 cal ka BP, Stage II). Meanwhile, variations in Al2O3/K2O and smectite/(illite + chlorite) correspond well to monsoon rainfall variability during the late phase of deglacial sea-level rise and Holocene sea-level highstand (11.1–1.5 cal ka BP, Stage III). Based on these records, we recognize three stages of evolution of terrigenous sediment input to the southern South China Sea continental slope. Dominance of terrigenous sediment input occurred during Stage I, when most part of the Sunda Shelf was still exposed and drained by several rivers. The stepwise elevated sea level by meltwater pulse 1A and subsequent rising sea-level after 14.5 cal ka BP could have led to a drop in terrigenous sediment input to this area due to regression of the paleo-Sunda river systems (Stage II). Thereafter during last phase of sea-level rise and sea-level highstand with the evolution of the modern coast, sediments have a clear fingerprint of chemical weathering induced by a strengthening of the East Asian summer monsoon (Stage III). Our findings highlight that the variability of terrigenous sediment input to the lower continental slope in the southern South China Sea was first in the late Pleistocene mainly driven by sea-level change and later in the Holocene by a strengthened East Asian summer monsoon.

Sediment dynamics driven by contour currents and mesoscale eddies along continental slope: A case study of the northern South China Sea

Based upon 3-D seismic reflection data, published drill core studies, oceanographic observations and numerical modelling, this study identifies a small scale contourite depositional system that is locally confined by canyons, slope failures, erosional terraces and erosional slopes on the Jianfeng slope, northern South China Sea (~1500 to ~2300 m water depth). Possible oceanic driving mechanisms for the spatial heterogeneity in sedimentation and erosion within the Quaternary deposits in this complex slope system are investigated. Numerical simulations show that the quasi-steady South China Sea deep water circulation, which affects the study area, is characterised by low velocities (maximum 4 cm/s), with a slight modulation by tides (±1 cm/s). This quasi-steady deep-water thermohaline circulation is able to keep sediment particles in suspension, rather than to erode seafloor sediment. The bottom hydrodynamic regime becomes energetic when mesoscale eddies (horizontal scale of 10 to 100 km) approach.Our modelling study indicates that both surface and bottom mesoscale eddies are essential to account for the spatial heterogeneity in sedimentation pattern along the northern South China Sea continental slope. According to simulation, the eddy front contains the highest flow velocity over a mesoscale eddy cycle (45 days), exceeding the threshold for resuspension of unconsolidated sediment (15 cm/s). As a consequence massive resuspension is produced at various sites, and redistributed by sub-mesoscale (horizontal scale of 1 to 10 km) circulations originated from an eddy-topography interaction. In contrast to the surrounding areas, which are subject to erosional forcing for a relatively long part (>7% of an eddy cycle), both the locally-confined drift and a further upstream large elongated-mounded drift experience little erosion (<2.5% of an eddy cycle), and serve as depositional centres for sediment from remote areas and erosion from adjacent areas. Our study demonstrates a promising new perspective for bridging the scales between short-term sediment dynamics and long-term sedimentation through a comparison of modelled scenarios between normal conditions and energetic events.

Species composition and biomass density of mesopelagic nekton of the South China Sea continental slope

Two surveys using a combination of midwater trawl and acoustics were undertaken in October 2014 and June 2015 to investigate species composition and biomass density of mesopelagic nekton over the continental slope of the South China Sea (CSSCS). In two surveys, diversity and dominant species of nektons (Chondrichthyes, Osteichthyes and Cephalopoda) showed obviously diel and depth–dependent difference. In 2014, in general species richness and diversity of nektons in the 0–100 m at night were greater than that in the 350–650 m during the day and 350–650 m at night. In Jun. 2015, species richness and diversity of nektons in the 350–650 m during the day were much greater than that in the 350–650 m at night and 0–100 m at night.Myctophidae and Diaphus are the most common fishes in the CSSCS mesopelagic zone. Jellyfishes could cause the nautical area scattering coefficient (NASC) to increase, but the effect of jellyfishes on NASC could vary over the light–night cycle and trawl depth. Catch rates of different nektonic categories varied by time of day and depth. Night–time catch rate at trawl depths < 100 m were greatest, with Myctophidae the most abundant category. In 2014, from 350 to 650 m, day and night mean catch rates of mytophids were similar, however, in 2015, mean day catch rates were 64.1% night values. Cephalopoda are an important component of mesopelagic nekton. At escape rates of 0.95 and 0.99, daytime trawl–based biomass densities in the 350–650 m depth range in 2015 were 0.339 g m−2 and 1.693 g m−2, respectively. Mean daytime NASC from 200 to 1000 was 3163 ± 1223 m2 nmi−2 (2014) and 2503 ± 698 m2 nmi−2 (2015); corresponding acoustic and trawling data produced mesopelagic nekton biomass density estimates of 9.80 g m−2 (2014) and 28.45 g m−2 (2015). More fishes with swimbladders (mainly small sized lanternfishes of light weight) dominated catch in 2014 than in 2015.

Geothermal analysis of boreholes in the Shenhu gas hydrate drilling area, northern South China Sea: Influence of mud diapirs on hydrate occurrence

Mud diapirs developed on the northern South China Sea continental slope are important indicators for gas hydrate exploration. The Guangzhou Marine Geological Survey (GMGS) of the Chinese Ministry of Land and Resources targeted mud diapirs and deployed gas hydrate drilling in the Shenhu area. Although an obvious bottom-simulating reflection (BSR) was detected below borehole number SH5, no gas hydrates were found at this borehole. Scientists encountered several contradictions when they analyzed the formation mechanism of hydrates related to mud diapirs. It was important to resolve this problem by sophisticated detection and thermodynamic analysis. In this study, we analyzed the thermodynamic mechanism of mud diapirs and their relationship to the occurrence of gas hydrates based on seismic profiles, mineral identification, sediment thermal conductivity measurements, and other available datasets. Our study concludes that gas hydrates were not detected at SH5 because a mud diapir pierced into the gas hydrate stability zone, leading to the decomposition of hydrates. The evolution of a mud diapir could be divided into three stages with a continuous geological process that controlled the accumulation of gas hydrates. The seismic profile and lithologic analysis showed that the mud diapir below SH5 was in the late stage of mud diapir evolution. The gas leakage system of the mud diapir was evident in the seismic profile, and the typical V-AMP (velocity-amplitude anomaly) structure displayed free gas accumulation under the BSR. The anaerobic oxidation of leaking methane promoted the formation of authigenic carbonate minerals. Thus, the deep layer of SH5 contained a relatively high percentage of carbonates and had a low thermal conductivity. In this stage, higher-temperature liquid migrated into the gas hydrate stability zone. The in situ temperature measured in this borehole was higher than the critical temperature of hydrate formation and decomposition. The liquid changed the temperature field of the region and altered the geothermal environment of hydrate accumulation, which led to the dissociation of hydrates.

Authigenic carbonates from an active cold seep of the northern South China Sea: New insights into fluid sources and past seepage activity

Site F (also named Formosa ridge) represents the most vigorous cold seep on the northern South China Sea continental slope. In order to constrain the fluid sources and intensities of seepage, we investigated the petrography, mineralogy, stable carbon and oxygen isotopic compositions, element geochemistry and radiocarbon dating of authigenic carbonate rocks retrieved from the seafloor. Carbonate rocks mainly occurred as crusts, nodules, and nodular masses incorporated in carbonate breccias. The carbonates were comprised mainly of high-Mg calcite and aragonite. The δ13C of authigenic carbonate varied from −55.3‰ to −34.3‰ (mean: −48.5‰; n=47) vs. V-PDB, suggesting biogenic methane is the dominant carbon source fuelling the system. The δ18OCarbonate values were from +3.6‰ to +4.8‰ (mean: +3.9‰; n=47). The observed 18O-enrichement in relation to calculated equilibrium values in the carbonates probably reflects dissolution of gas hydrates. Combination of seafloor observations and the obtained AMS 14C ages suggest that (1) initiation of methane seepage from at least 10.6ka ago; (2) environmental conditions may have been favorable for enhanced fluid seepage around 6ka BP and (3) relatively low intensity of seepage from 2ka BP till today.

Molecular and stable carbon isotopic compositions of hopanoids in seep carbonates from the South China Sea continental slope

The lipid biomarkers of hopanoids in cold seep carbonates from the South China Sea continental slope were investigated by gas chromatography–mass spectrometer (GC–MS) and gas chromatography-isotope ratio-mass spectrometer (GC-ir-MS). The distribution of hopanes/hopenes shows a preference for the ‘biological’ 17β(H), 21β(H)-over the ‘geological’ 17α(H), 21β(H)-configuration. This interpretation is in agreement with the strong odd–even preference of long-chain n-alkanes in those samples, suggesting that the ββ hopanes may be the early diagenetic products of biohopanoids and the αβ, βα configurations of hopanes were mainly derived from allochthonous sources contributing to the organic matter of the carbonates. In terms of hopanoid acids, the C30 to C33 17β(H), 21β(H)-hopanoid acids were detected with C32 17β(H), 21β(H)-hopanoid acid being the most abundant. However, there is a significant difference in stable carbon isotopic compostions of the C32 17β(H), 21β(H)-hopanoic acid among samples (−30.7‰ to −69.8‰). The δ13C values match well with the carbon isotopic compositions of SRB-derived iso-/anteiso-C15:0 fatty acids in the samples, which strongly depend on the carbon utilization types by microbe. The most abundant compound of hopanols detected in the samples, C30-17β(H), 21β(H)-hopanol, may be a good indicator of diagenetic product of type I methanotrophs. The molecular and carbon isotopic compositions of hopanoids demonstrate clearly that there is a combination contribution of both SRB and type I or type X methanotrophs to the source organism in the seep carbonates from the South China Sea continental slope.

Factors controlling the types of microbial consortia in cold-seep environments: A molecular and isotopic investigation of authigenic carbonates from the South China Sea

Authigenic carbonates were recovered from several locations of the South China Sea continental slope at a 470m to 770m water depth. The sources of seeping fluids, the time involved in carbonate formation and the potential driving forces are increasingly understood, but little is known about the biogeochemical processes that occur during this type of carbonate precipitation. Here, a molecular and isotopic investigation of authigenic carbonates was conducted to determine the factors controlling the types of microbial consortia in cold-seep environments. The studied carbonates were one aragonitic crust and three high-Mg calcitic tubular concretions from specific sites. A significant input of biogenic methane to the carbon pool during the precipitation of the carbonates was suggested by the presence of 13C-depleted molecular fossils of methanotrophic archaea (with δ13C values as negative at −141.5‰) and sulfate-reducing bacteria (with δ13C values as negative at −128.5‰). The methane was primarily oxidized in an anaerobic process. The predominant consortia were ANME-2 assemblages in the aragonitic carbonate (sample TVG3-C2), whereas high-Mg calcitic carbonates (samples TVG14-C2 and TVG8-C5) were dominated by ANME-1 assemblages according to the specific biomarkers observed. Taking the favored environments for specific AOM consortia and crystallization conditions for aragonite and calcite together, the factors selecting for specific AOM communities appear to be constrained by their biogeochemical settings. Accordingly, higher alkalinity, abundant sulfate concentration and being closer to the water–sediment interface conditions favor the precipitation of aragonite over high-Mg calcite.

Geochemical changes of the terrigenous sediments in the southern South China Sea and their paleoenvironmental implications during the last 31 ky

Fifty-six subsamples of the 86GC core were collected from the southern South China Sea continental slope. Major and trace elements, including rare earth ele- ments (REEs), of detrital matters in these sediments were measured. The patterns of chondrite normalized REEs and the discrimination plots based on major and trace elements, respectively, indicate that the terrigenous sediments around this area had been supplied with one dominating prove- nience during the last 31 ky. The co-operation of sea level changes with the East Asian winter monsoon-related rain- fall and sea water circulations should be the main mecha- nism causing the geochemical characteristics variations. Higher ratios of Al/Ti and some other elements to Ti, except P/Ti and Mn/Ti, are present during the last glacial sea-level low stand and the winter monsoon periods owing to increased fluvial input of material with low Ti content from the Mekong River, followed by strengthened chemi- cal weathering intensity, causing higher Al2O3 and most other elements contents. The elemental patterns imply general warm and very humid climate in the tropical areas during the last glacial times; however, the climate since the last deglaciation was very changeable, especially during the Holocene. Even so, it can still be certain that the cli- mate during the early part (29-24 ky B.P.) of the last glacial maximum was much worse than during other stages. Additionally, it is very likely that the 8 k cold event and the Younger Dryas event have been impressed in the geochemical record of this core.

Stratigraphic architecture and evolution of the continental slope system in offshore Hainan, northern South China Sea

We investigate the evolution of the stratigraphic architecture of two siliciclastic northern South China Sea continental slope systems with distinct structural styles, including rift-transform and rift segments. Our results reveal that the stratigraphic patterns of the slope system in the Yinggehai basin are mainly characterized by progradational slope clinoforms, which result in a rapid shift of shelf edges seaward and southeastward due to a high rate of sediment supply. In contrast, in the eastern Qiongdongnan area, the slope margins show a vertical stacking pattern or a slight shift seaward, characterized by dense gravitational faults, rollover structures, and slump deposits. The associated stratigraphic architecture in slope clinoforms is chaotic. Reactivation of pre-existing strike-slip faults in the Yinggehai area and extensional faults in the Qiongdongnan area resulted in the formation of initial slope clinoforms during the Miocene post-rifting stage. Dextral movement of the northeast marginal fault (No. 1 fault) since the Early Pliocene resulted in the formation of a deep embayment and early slope clinoforms along the No. 1 fault in the Yinggehai basin. The formation of initial slope clinoforms is closely related to movement along the underlying basement fault in the eastern Qiongdongnan area. Although the development of the slope architecture is influenced by tectonic movement and eustasy, the primary control of the geometry of slope clinoforms is through the effects of sediment supply. Plentiful sediment supply leads to rapid progradational slope clinoforms at a large distance of progradation in the Yinggehai and adjacent areas between the two basins. Sediment-starved margins occur in the eastern Qiongdongnan area because of a very wide shelf and insufficient sediment supply.

Internal tide characteristics over northern South China Sea continental slope

21-day in-situ ADCP current and CTD data from an anchored observational station by RN Shiyan 3 are used to investigate the characteristics of internal tide in the northern South China Sea continental slope. The results show that tide and internal tide are both mixed type in the observed region and dominated by diurnals; the maximum internal tidal major axes of K-1, O-1 were largest and reached 11 cm/s, 10 cm/s, respectively, and their current vectors rotate anticlockwise. The 300 m-deep internal tide presents obvious quasi-diurnal oscillation and its average amplitude reaches 50 m. Furthermore, internal tide carried high energy; in the upper layers above 140 m, the kinetic energy of internal tides accounts for 15%-37% of overall kinetic energy, most of internal tide energy concentrates around the thermocline.