Deep South China Sea Research Articles

Seasonal variability of tides in the deep northern South China Sea

Tides are the major energy source for ocean mixing, regulating the variation of oceanic circulation and sediment transport in the deep sea. Here twenty months of high-resolution current profiles, which were observed via a mooring system at a water depth of 2100 m in the northern South China Sea (SCS), are used to investigate seasonal variability in deep-sea tides. Spectral analysis shows that tides in this region are dominated by diurnal tide, and both diurnal and semidiurnal tide are vertical mode-1 dominant. Baroclinic diurnal tidal current exhibits pronounced seasonal variability, showing its kinetic energy was the strongest in summer, and the maximum depth-averaged value was up to 86.7 cm2 s−2, which was about 1.5 times of that in winter and twice that in spring and autumn. In contrast, baroclinic semidiurnal tide displays no evident seasonal variability. Such seasonal variability in baroclinic tide was mainly modulated by the barotropic forcing from the Luzon Strait. On the other hand, two anticyclonic eddies and one cyclonic eddy, which originated off southwestern Taiwan in winter, crossed the mooring system. The cyclonic eddy had weak impact on current velocity in the deep sea, but the two deep-reaching anticyclonic eddies enhanced the current velocity through the full-water column by inducing strong subinertial flows. Consequently, the kinetic energy of tides was strengthened and the incoherent variance of baroclinic diurnal tide increased in winter, which contributed ~85% of the variability in diurnal tide. Meanwhile, the velocity of baroclinic diurnal tide was reduced in winter, which was attributed to the weakened stratification induced by the passage of anticyclonic eddies in the deep sea. The seasonal variability of tides in the deep northern SCS can provide a dynamic mechanism for interpreting sediment transport processes in the deep sea on different time scales.

Elevated carbon flux in deep waters of the South China Sea

We measured particulate organic carbon (POC) fluxes from the euphotic zone into the twilight zone and deep waters (>1000 m) that occurred between the shelf and the basin in the South China Sea (SCS) and at the SouthEast Asia Time Series Station (SEATS) using floating sediment trap arrays. Additionally, selected sinking particles were imaged by scanning electron microscope (SEM) to reveal particle morphology and composition. Results showed large variations in POC fluxes with elevated values (32–104 mg-C m−2 d−1) below the euphotic zone and a trend towards lower values in the deep SCS. Vertical POC fluxes measured in deep waters between the shelf and the SCS basin were much higher than those estimated by Martin’s attenuation equation. These elevated POC fluxes in deep waters were attributed to lateral particle transport as opposed to enhanced settling out of the euphotic zone. SEM images of sinking particles at 150 m show abundant marine biogenic detritus, while those in deep waters contained a higher proportion of lithogenic material. A great deal of the spatial variability in POC fluxes across the twilight zone and deep waters of the SCS cannot be represented by current biogeochemical models.

Tectonic and paleoceanographic conditions during the formation of ferromanganese nodules from the northern South China Sea based on the high-resolution geochemistry, mineralogy and isotopes

Deep-water ferromanganese (Fe-Mn) nodules cumulate trace elements from the underlying sediment pore-water and seawater during the concentric mineral layers' growth process over millions of years. They can record long-term tectonic movements and paleoceanographic changes. To systematically ascertain paleoceanographic evolution of the deep South China Sea (SCS) since the last 3.2 Ma, we comprehensively investigated microstructure, mineralogy, geochemical element concentrations as well as Sr, Nd and Pb isotopic compositions in the Fe-Mn nodules nearby the Dongsha Islands in the northern SCS. Results indicate that these deposits were formed in a continental marginal environment by a mixture of diagenetic and hydrogenetic processes. Overall, the studied nodules can be divided into three stages with different microstructures and chemical compositions, which indicate different formation environments. Stage 1a is the innermost and oldest layer with a porous mottled texture, and has the lowest Mn/Fe ratio, Cu, Ni and Zn, which implies comparatively high oxidization conditions. In contrast, the Mn mineralogy of Stage 2 shows conditions similar to those of buserite formed under less oxidative conditions. Between Stage 1a and 2, Stage 1b represents a mixture of Stage 1a and 2 and has inhomogeneous growth structures. Based on the Nd and Sr isotopic composition of the detrital phases and Pb isotopes, three potential terrigenous source regions including South China, Luzon and Taiwan that discharge into the northern SCS from 3.2 Ma to 1.06 Ma. After 1.06 Ma, the terrigenous fraction in the Fe-Mn deposits on the northern slope of the SCS is a two end-members mixture of fluvial input from Taiwan Islands and eolian dusts from the Chinese loess. Our study indicates that SCS bottom water variation occurred at ~3.2 Ma, 2.1 Ma, and 1.06 Ma, respectively, which could be attributed to global cooling and the influence of Luzon arc-forearc uplift.

Seasonal and interannual variability of coccolithophore flux in the northern South China Sea

Coccolithophorids are key player in the marine biological pump and marine carbon cycle, as their production and community-structure are crucial for export and sequestration of carbon from the atmosphere to the deep sea with important implications to climatic trends. Variations in the species composition of coccolithophore communities largely reflect environmental changes and are therefore fundamental for palaeoceanographic reconstructions. The South China Sea (SCS) is an ideal area to study the response of coccolithophores to environmental change because of the remarkable seasonal and interannual variations of the monsoonal climate and the hydrography. However, to date only limited studies on the temporal changes of coccolithophores in the modern SCS have been reported. In the present work, coccolithophores in the northern SCS were investigated using time-series sediment traps during 2009–2010 and 2011–2012. Extinct coccoliths which had their last occurrence in the Miocene (e.g. Triquetrorhabdulus longus, Reticulofenestra floridanus), Pliocene (e.g. Reticulofenestra pseudoumbilica, Discoaster tamilis), and Pleistocene (e.g. Pseudoemiliania locunosa, Discoaster variabilis) were a frequent component of the coccoliths throughout all the seasons and provided, for the first time, a strong micropaleontological evidence for lateral advective transport in the deep SCS. The source of the fossil coccoliths most likely were the reworked Pleistocene sands which cover the outer shelf and upper slope to the west and south of the Dongsha Islands between 20 m and 600 m water depth. These sediments contain limestone fragments with foraminiferal assemblages of Miocene to Pliocene age, and, to the north and south of the islands, Miocene strata are exposed on the sea floor. Mesoscale eddies, both cyclonic and anticyclonic, were probably the main agent for resuspending and transporting the coccoliths as they propagated westwards along-slope from the Dongsha area to the mooring site. Extant coccolithophore were composed of 31 taxa with Florisphaera profunda, Gephyrocapsa oceanica, and Emiliania huxleyi contributing 91.4% and 83.8% of the annual coccolithophore export flux in 2009–2010 and 2011–2012, respectively; F. profunda was the predominant species. Enhanced fluxes of extant coccoliths occurred in the summer of 2009/2010, spring 2010, autumn 2011 and winter 2011/2012, but varied in phase with the extinct species. Therefore, lateral advection of extant taxa may have also taken place but the extent to which this may have masked primary signals from in-situ coccolithophore production remains open. The low fluxes of coccoliths in the winter of 2009/2010 in association with relatively reduced wind strength, higher SST and a shallower mixed layer might compared to those of 2011/2012 have been driven by the weak El Niño event, which affected the northern SCS during that season.

Polycyclic and organochlorine hydrocarbons in sediments of the northern South China Sea

We investigated the concentration distribution and composition of organic pollutants in sediments of the shelf and the deep northern South China Sea (NSCS). Concentrations of polycyclic aromatic hydrocarbons (Σ15PAH; 10.69–66.45 ng g−1), Dichlorodiphenyltrichloroethane (Σ4DDT; 0–0.82 ng g−1), and polychlorinated biphenyls (Σ24PCB; 0–0.12 ng g−1) are below established sediment quality guidelines, suggesting no environmental risk. Surprisingly, concentrations increase from the shelf to the deep NSCS, and are higher in the east of the study area. The organic pollutant composition indicates PAH mainly derived from pyrogenic sources, and mostly degraded DDT and PCB. However, in the deep NSCS, considerable contribution of petrogenic PAH, low chlorinated PCB and p,p′-DDT suggest more recent input from different sources compared to the shelf. From these results we infer that organic pollution in the NSCS does not originate from the Pearl River Estuary but from the NE SCS, SW of Taiwan.

Contrasting Fates of Petrogenic and Biospheric Carbon in the South China Sea

AbstractA synthesis of published and newly acquired stable and radiocarbon isotope data from soil, river, and marine particulate organic carbon (OC) from the South China Sea drainage and sedimentary basin reveals that OC derived from bedrock‐erosion (petrogenic OC) and marine productivity comprises the major contributors to bulk OC in particulate matter reaching abyssal depths, while soil‐derived OC appears negligible. Aluminum‐radiocarbon relationships of sediments suggest that soil OC initially associated with detrital terrestrial minerals is lost and replaced by marine OC during transport beyond the continental shelf. We estimate that petrogenic OC sinking to a ~30,000 km2 region of the deep northeastern South China Sea accounts for 0.6% of global petrogenic OC burial. The basin‐wide OC isotope patterns coupled with sediment trap observations highlight both the spatial variabilities of OC components as they propagate from source to sedimentary sink and the significance of petrogenic OC to deep ocean sediments.

Changes in Deep Water Oxygenation of the South China Sea Since the Last Glacial Period

AbstractThe Pacific meridional overturning circulation is thought to have a significant influence on global climate. However, the extent to which intermediate and deep circulations have changed in the Pacific Ocean since the Last Glacial Maximum (LGM) is not well known. At present, the South China Sea Deep Water (SCSDW) is fed by the upper Pacific Deep Water. Here we present new benthic foraminiferal δ13C and redox‐sensitive elemental data from a sediment core retrieved from the southern deep SCS to reconstruct the oxygenation history of the SCSDW since the LGM. Oxygenation records from the deep SCS and intermediate and deep waters in the Pacific Ocean demonstrate that the SCSDW deeper than 1,600 m has been sourced by the Pacific Deep Water since the LGM. Our data suggest that the well‐ventilated North Pacific Intermediate Water would not have influenced the SCSDW during cold stadials of the last deglacial period.

Deep water characteristics and circulation in the South China Sea

This study investigates the deep circulation in the South China Sea (SCS) using oceanographic observations combined with results from a bottom layer reduced gravity model. The SCS water, 2000 m below the surface, is quite different from that in the adjacent Pacific Ocean, and it is characterized by its low dissolved oxygen (DO), high temperature and low salinity. The horizontal distribution of deep water properties indicates a basin-scale cyclonic circulation driven by the Luzon overflow. The results of the bottom layer reduced gravity model are consistent with the existence of the cyclonic circulation in the deep SCS. The circulation is stronger at the northern/western boundary. After overflowing the sill of the Luzon Strait, the deep water moves broadly southwestward, constrained by the 3500 m isobath. The broadening of the southward flow is induced by the downwelling velocity in the interior of the deep basin. The main deep circulation bifurcates into two branches after the Zhongsha Islands. The southward branch continues flowing along the 3500 m isobath, and the eastward branch forms the sub-basin scale cyclonic circulation around the seamounts in the central deep SCS. The returning flow along the east boundary is fairly weak. The numerical experiments of the bottom layer reduced gravity model reveal the important roles of topography, bottom friction, and the upwelling/downwelling pattern in controlling the spatial structure, particularly the strong, deep western boundary current.

Preface: Evolution of the deep South China Sea: Integrated IODP Expedition 349 results

The offshore Chinese basins are located in the transition zone from the East Asia to the Western Pacific plate. The Cenozoic evolution of these basins reflects the geodynamic processes in East Asia. However, the coupling of the evolution of the offshore Chinese basins to the geodynamic processes occurring in the East Asian continental margin is still poorly understood. Here, the rifting processes of the offshore Chinese basins and the relationship between the basin evolution and the geodynamic processes of the East Asia Plate were analyzed using seismic data, fault activity, syn-sedimentary fault maps, and isopach maps. In the Offshore Bohai Bay Basin, the regional extension and local shearing occurred simultaneously. Accordingly, the regional NNW-SSE extension and local NNE-strike-slip shearing resulted in the NNE-striking graben-horst structure in the Liaodong Bay area while the NW-striking extensional faults and NNE-striking transtensional faults jointly control the structure of the Bozhong area. In addition, the NNE-striking branches of the Tan-lu Fault Zone led to the pull-apart structure in the Bonan area, and the NW-striking normal faults governed the half-graben structure in the Boxi area. Two stages of eastward movement of the rifting center in the East China Sea Basin were triggered by a change in the direction of subduction of the Pacific plate at ~43 Ma, and subduction of the Philippine Sea plate at ~20 Ma, respectively. In the South China Sea (SCS), Paleocene to E. Eocene initial extension occurred in the present northern and southern marginal basins of the SCS. Subsequently, the Ailao Shan Red River shear zone (ASRRSZ) traversed the eastern margin of the Indochina block, and pull-apart rifting dominated in the basins along this fault. Combined with the southward slab pull of proto-SCS and the strike-slip movement of ASRRSZ, sea floor spreading occurred, and the southern marginal basins drifted southerly and apart from the South China from 33 to 16 Ma.

Sources and transformations of nitrogen in the South China Sea: insights from nitrogen isotopes

To elucidate the sources and transformations of nitrogen in the South China Sea (SCS), the nitrogen isotopic composition of nitrate ( $${\updelta }^{ 1 5} {\text{N}}_{{{\text{NO}}_{ 3} }}$$ ) was measured in seawater samples from the water column of this marginal sea and the adjacent western North Pacific Ocean (WNP). Comparison of the isotopic signatures from these two locations suggests that the main source of nitrogen into the SCS was nitrate that entered from the WNP through the Luzon Strait. Values of $${\updelta }^{ 1 5} {\text{N}}_{{{\text{NO}}_{ 3} }}$$ were generally lower in the SCS than in the WNP, and the $${\updelta }^{ 1 5} {\text{N}}_{{{\text{NO}}_{ 3} }}$$ maximum observed in the SCS intermediate water was lower than the corresponding WNP maximum. This pattern is attributed to mixing within the SCS in combination with the outflow of SCS intermediate water to the WNP. A mass balance model indicates that atmospherically derived N (a combined input of new nitrogen from marine N2 fixation and atmospheric deposition) supplied approximately 6% of the particulate nitrogen exported from the euphotic zone to the deep SCS. This supply of isotopically light nitrogen cannot, however, explain the low and downward-decreasing δ15N that has been previously observed in sinking particles of the deep SCS. We propose that an alternative explanation might be a downward-increasing ratio of isotopically light NH4 +-N to organic N due to the degradation of organic N within the sinking particles (i.e., relative enrichment of the NH4 +) and also particle incorporation of excreted ammonium from zooplankton.

Effect of potential vorticity flux on the circulation in the South China Sea

AbstractThis study analyzes temperature and salinity products from the U.S. Navy Generalized Digital Environment Model. To avoid the fictitious assumption of no‐motion reference level, a P‐vector inverse method is employed to derive geostrophic velocity. Line integral of geostrophic velocity shows evidence for the existence of a sandwiched circulation in the South China Sea (SCS), i.e., cyclonic circulation in the subsurface and deep layers and anticyclonic in the intermediate layer. To reveal the factors responsible for the sandwiched circulation, we derive the potential vorticity equation based on a four‐and‐a‐half‐layer quasi‐geostrophic model and apply theoretical potential vorticity constraint to density layers. The result shows that the sandwiched circulation is largely induced by planetary potential vorticity flux through lateral boundaries, mainly the Luzon Strait. This dynamical mechanism lies in the fact that the net potential vorticity inflow in the subsurface and deep layers leads to a positive layer‐average vorticity in the SCS basin, yielding vortex stretching and a cyclonic basin‐wide circulation. On the contrary, the net potential vorticity outflow in the intermediate layer induces a negative layer‐average vorticity, generating an anticyclonic basin‐wide circulation in the SCS. Furthermore, by illustrating different consequence from depth/density layers, we clarify that density layers are essential for applying theoretical potential vorticity constraint to the isolated deep SCS basin.

South China Sea: the gateway to China's deep-sea ambitions

Abstract For most parts of its history, China has largely turned its back on ocean exploration. Even after it started oceanographic research in the 1950s, the focus was mostly on coastal and offshore waters. But this changed a decade ago when the country began to invest heavily on deep-sea research—resulting in the launch in 2011 of its first multi-disciplinary deep-sea research programme called the South China Sea (SCS)-Deep to probe the mystery of this marginal sea. Covering an area of 3.5 million square kilometres and with a maximum depth of 5500 metres, SCS occupies a scientifically interesting position between the world's highest mountains, the Himalayas, and the deepest point on Earth, the Mariana Trench in the western Pacific Ocean. In a forum organized by National Science Review at the Annual Conference of the South China Sea-Deep Programme held in January in Shanghai, a panel of scientists explained what China's deep-sea ambitions are, why SCS is a fantastic natural laboratory, the importance of international collaboration, what China needs to do to develop cutting-edge marine technologies and how SCS could be an ideal platform for regional cooperation. Nianzhi Jiao an ecologist at Xiamen University, Xiamen, China Dongxiao Wang a physical oceanographer at South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Jian Lin a marine geophysicist at Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA, and South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China Pinxian Wang a paleoceanographer at Tongji University, Shanghai, China Jiwei Tian a physical oceanographer at Ocean University of China, Qingdao, China Aiqun Zhang chief engineer at the Institute of Deep Sea Science and Engineering, Chinese Academy of Sciences, Sanya, China

Impact of Tidal Mixing on Water Mass Transformation and Circulation in the South China Sea

AbstractUsing a high-resolution regional ocean model, the impact of tidal mixing on water mass transformation and circulation in the South China Sea (SCS) is investigated through a set of numerical experiments with different configurations of tide-induced diapycnal diffusivity. The results show that including tidal mixing in both the Luzon Strait (LS) and SCS has significant impact on the LS transport and the intermediate–deep layer circulation in the SCS Basin. Analysis of the density field indicates that tidal mixing in both the LS and SCS are essential for sustaining a consistent density gradient and thus a persistent outward-directed baroclinic pressure gradient both between the western Pacific and LS and between the LS and SCS Basin, so as to maintain the strong deep-water transport through the LS. Further analysis of water mass properties suggests that tidal mixing in the deep SCS would strengthen the horizontal density gradient, intensify the basin-scale cyclonic circulation, induce more vigorous overturning, as well as generate the subbasin-scale eddies in the abyssal SCS. The results imply that tidal mixing in both the LS and SCS plays a key dynamic role in controlling water mass properties and deep circulation features in the SCS and thus need to be deliberately parameterized in ocean circulation models for this region.

Riverine influence on ocean color in the equatorial South China Sea

Analysis of SeaWiFS data off Northwest Borneo reveals coastal chlorophyll bloom extending more than 200km to deep South China Sea during winter. A combination of remote sensing products is used to untangle its statistical relation with various forcing factors. River discharge rate is estimated from satellite measurements of land rainfall, and surface chlorophyll over the middle shelf is shown to vary with river discharge and lag by one month. Strong (weak) chlorophyll blooms tend to occur in La Niña (El Niño) years. The study provides evidence that river runoff from maritime continent has major influence on equatorial ocean color.

A foraminiferal study on Middle Eocene-Oligocene break-up unconformity in northern Taiwan and its correlation with IODP Site U1435 to constrain the onset event of South China Sea opening

Taiwan is situated in the SE Asian continental margin where lower Cenozoic strata are well exposed. Its geological and tectonic setting provides a unique opportunity to study the time constraint of the break-up unconformity relevant to the opening of the South China Sea. A detailed foraminiferal study on four land sections and a deep core across the boundary between the Upper and Lower Wulai Groups in the Hsüehshan Range, northern Taiwan, was conducted. Results show remarkable changes of depositional environment and lithology from the amalgamated fluvial dominant sandstones (>2000m) of Middle Eocene in the Lower Wulai Group to the overlying inner neritic shales and siltstones (>2000m) of Oligocene in the Upper Wulai Group. Integrated sedimentological and foraminiferal data further suggest an occurrence of a depositional hiatus between 32 ∼33 and 40Ma across the boundary between the Kankou Formation of the Upper Wulai Group and the Szeleng Sandstone of the Lower Wulai Group in the northern Hsüehshan Range, as same as the scenario seen in the Western Foothills (25–39Ma) of central Taiwan and the Penghu Basin (25–40Ma) in the Taiwan Strait.This stratigraphic hiatus in the Taiwan mountain belt marks a regional break-up unconformity between the Middle Eocene syn-rift strata and the overlying Oligocene post-rift sequences in the NE South China Sea. The break-up unconformity can be correlated with those commonly recognized in the seismic profiles of the Pearl River Mouth Basin and drilling of the IODP site U1435 (Expedition 349) in the northern shelf-slope of the South China Sea. The upper time limit of the breakup unconformity 32Ma could mark the tectonic change from rifting to depression in the Paleogene Asian continental margin which eventually led to opening of the deep South China Sea in the Early Oligocene (∼33Ma).

Leeuwenhoekiella nanhaiensis sp. nov., isolated from deep-sea water.

A novel heterotrophic, aerobic, Gram-stain-negative, rod-shaped and yellow bacterium, designated strain G18T, was isolated from a water sample collected from the deep South China Sea. Strain G18T grew at 4-40 °C (optimum 28-32 °C), at pH 6.0-8.0 (optimum pH 6.5-7.5) and with 0-12 % (w/v) NaCl (optimum 3-4 %). The organism was mesophilic and piezotolerant, its optimal growth pressure was 0.1 MPa, which was lower than that at the depth from which it was isolated. Its optimal growth temperature was higher than that at the depth of its isolation. The predominant cellular fatty acids were C15 : 0iso, C17 : 0iso 3-OH and C15 : 1iso. The major polar lipids were composed of phosphatidylethanolamine, one unknown aminolipid and one unknown polar lipid. The major respiratory quinone was menaquinone 6. The G+C content of the genomic DNA was 35 mol%. Phylogenetic analysis, based on 16S rRNA gene sequences, revealed that strain G18T clustered with species of the genus Leeuwenhoekiella with validly published names within the family Flavobacteriaceae with 95.9-98.2 % sequence similarity. DNA-DNA reassociation values ranged from 9 to 42 %. Differential phenotypic properties, together with the phylogenetic distinctiveness, suggest that strain G18T differs from species of the genus Leeuwenhoekiella with validly published names. On the basis of the polyphasic evidence, strain G18T represents a novel species, isolated from deep-sea, of the genus Leeuwenhoekiella for which the name Leeuwenhoekiella nanhaiensis sp. nov. is proposed. The type strain is G18T ( = CCTCC AB 2015204T = KCTC 42729T).

Deep-water sedimentary systems and their relationship with bottom currents at the intersection of Xisha Trough and Northwest Sub-Basin, South China Sea

Based upon 2D reflection seismic data and numerical modelling, this study confirms the presence of a complex deep-water sedimentary system on the present-day seafloor at the intersection of the Xisha Trough and the Northwest Sub-Basin (South China Sea) and investigates their relationship with bottom currents. The deep-water sedimentary system consists of submarine canyons, slides and slumps, wave-like successions, mounded drifts and two groups of marginal depressions (those with erosional features and those appearing as morphological sediment sinks). Three-dimensional process-based modelling is applied to investigate sediment dynamics induced by a combined effect of tidal currents and a quasi-steady geostrophic current (South China Sea Deep Water Circulation). Simulation results show that the South China Sea Deep Water Circulation at the southeastern flank of the seamount plateau could reach velocities of 15cm/s during flood tides, enabling erosion and transport processes. In contrast, the rest of the plateau area is favoured for deposition, since current velocities in this region are persistently lower than 10cm/s. The current velocities at the feet of the obstacles (where the morphological depressions are located) are strengthened and are several cm/s higher than that in adjacent flat areas (e.g. where the mounded drifts are located). The flow is constricted and accelerated after being deflected by the obstacles, resulting in contrasting higher sedimentation rates within the mounded sediments and lower rates at the morphological depressions. A comparison between the seismic stratigraphy and the simulated fluid dynamics enables a decoding of the pathway, identifying the current regime as well as unravelling the relationship between depositional processes and the deep-sea water circulation. This study provides new insights and exposes new challenges in understanding the dynamics of deep-sea sedimentation processes in South China Sea.

Effect of surface mesoscale eddies on deep-sea currents and mixing in the northeastern South China Sea

Recent studies suggest that deep-reaching surface-generated eddies result in anomalous current velocities in the deep sea, and ultimately lead to energy transfer from mesoscale to small-scale motions. Here we examine the influence of mesoscale eddies on deep-sea subinertial and near-inertial currents, and on possible enhanced oceanic mixing in the deep South China Sea (SCS). We analyzed current velocity data for nearly a full water column. Data were obtained using acoustic Doppler current profilers and recording current meters on a deep-sea mooring system at a depth of 2100m in the northeastern SCS from October 2012 to May 2013. A highly nonlinear southwestward-propagating anticyclonic eddy was detected via a resolved sea-surface-level anomaly. This eddy induced pronounced subinertial currents with a characteristic time scale of 1–2 months and a maximum velocity of up to 0.2ms–1 at the subsurface and 0.1ms–1 at great depth. Near-inertial energy co-occurring with subinertial flows showed a distinctive vertical propagation trend during strong subinertial oscillations in the deep sea. During periods of strong subinertial and near-inertial kinetic energy, estimates of diapycnal diffusivity in the deep ocean showed approximately 10-fold enhancement, with a mean value of 1.2×10–3m2s–1 compared to the background value of 1.4×10–4m2s–1. The results provide observational evidence of the effect of surface-observed mesoscale motions on benthic currents and ocean mixing in the deep SCS.

Variability in the abundance and species composition of diatoms in sinking particles in the northern South China Sea: Results from time-series moored sediment traps

Seasonal variability of diatom abundance and species composition in sinking particles in the northern South China Sea was studied using continuous time-series sediment trap moorings during a one-year period, 2009–2010. A clear seasonal pattern of diatom abundance was observed, with higher fluxes in winter than in summer. This could be related to positive response of diatom growth to increased nutrient supply from the subsurface waters as a result of enhanced vertical mixing in winter. Mesoscale eddy activities and strong Asian dust input could also stimulate the diatom growth and increase the diatom export flux to the deep South China Sea. The strong Asian dust input during spring 2010 probably accelerated the aggregation and downward flux of diatoms in the northern South China Sea. No distinctive seasonal pattern in diatom species composition was recorded, but there was a close correlation of diatom abundances with the cell size and the degree of silification. The sinking velocity of the diatom related particles from the sea surface to the deep South China Sea has been estimated to be ~30–50md−1 by comparing the surface chlorophyll a with the diatom abundance and species in sediment traps at 1003m and 3226m depths. Strong dissolution of small-sized and low-silicified diatom species and lateral advection of particles were also indicated for the deep northern South China Sea. The present results are crucial as a basis for future understanding of fossil assemblage changes in connection with palaeoclimatic and palaeooceanographic research.

Dissolved organic carbon in the South China Sea and its exchange with the Western Pacific Ocean

Based on a large and high quality dataset of total organic carbon (TOC, an approximation of dissolved organic carbon) collected from three cruises in spring, fall and winter in 2009–2011, we examined the distribution of TOC and its seasonality in the oligotrophic regime of the Northern South China Sea (NSCS) as well as its exchanges with the West Philippine Sea (WPS) in the Northwest Pacific Ocean through the Luzon Strait, the only deep channel linking the South China Sea (SCS) and the Pacific Ocean. Surface TOC concentration in the slope and basin areas of the NSCS varied from 65 to 75μmolL−1 with relatively high values in the northeast part (southwest of Taiwan Island) in spring, and in the eastern parts of the NSCS during fall and winter. The TOC inventory in the upper 100m of the water column ranged from 6.0–7.5molm−2 with a similar distribution pattern as the surface TOC concentration. There were two most significant differences in the TOC profiles between the SCS and the WPS. One was in the upper 200m, where more TOC was accumulated in the WPS; the other was in the intermediate layer at ~1000–1500m, where the gradient of TOC concentration was still persistent below 1000m in the SCS, a feature which did not exist in the WPS. At this intermediate layer, there also appeared an excess of TOC in the SCS as compared with that in the WPS. The TOC concentration below 2000m in the SCS was identical to that in the Northwestern Pacific, both of which were ~40μmolL−1 without significant difference among stations and seasons, suggesting that this deep water TOC was homogeneously distributed in the deep SCS basin owing to the fast replenishment of the deep water from the WPS. We adopted an isopycnal mixing model to derive the water proportion contributed respectively from the SCS and Kuroshio along individual isopycnal plane and examined the impact of the Kuroshio intrusion on the TOC in the NSCS. The upper 100m TOC inventory in the NSCS was overall positively correlated with the Kuroshio water fraction, suggesting that the Kuroshio intrusion enhanced the TOC inventory thereby significantly influencing TOC distribution in the NSCS. Following the sandwich structure of water exchange through the Luzon Strait, with an inflow in the surface and deep layer but an outflow from the SCS in the intermediate layer, we conducted a first order estimation of the TOC transport fluxes based on the reported cross strait volume transport. The TOC transport flux was −107.1±54.6, 54.7±15.0 and −16.4±13.1TgCyr−1 at the upper, intermediate and deep layer, respectively. Note that the positive sign means that the flux was from the SCS to the WPS. By integrating the three-layers, the total net transport flux of TOC through the Luzon Strait would be −68.8±58.0TgCyr−1. Because of the great spatial–temporal variability of the water flow across the Luzon Strait, these first order TOC flux estimates were subject to large uncertainty. Nevertheless, because the SCS is featured by higher DOC production, the exchange of these fluxes with the open ocean interior where DOC would have experienced more degradation would have important implications for both the microbial community in the ocean interior and overall carbon cycle in the SCS.