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Salinity, as one of the essential physical properties of seawater, is a common tracer differentiating water masses in the ocean, which often require relatively high-resolution datasets. Limited by the coverage of direct observations, however, high-resolution spatial and temporal salinity data are not always available, which hinders the fine application of salinity data in discerning ocean processes and improved modeling of ocean physics and biogeochemistry. To supplement the salinity database, we reconstructed sea surface salinity (SSS) with reasonably high spatial resolution (0.05° × 0.05°) over 2003–2020 in the South China Sea (SCS) with a machine learning algorithm based on a combination of MODIS-Aqua remote sensing data and a large cruise observation-based dataset. The reconstructed SSS has a mean absolute error (MAE) of 0.2 when compared with our underway observations with a corresponding root mean square error (RMSE) of 0.3. The MAE between station-based observations and our reconstruction was 0.5, and the RMSE was 0.7. These validations strongly suggest that our reconstruction is highly adequate, representing at most a quarter of the identified discrepancies compared to the remote sensing SSS or two other prevalent model-derived datasets. Based on our reconstruction, the SSS in the SCS is relatively low in coastal waters, but high in the ocean basin, with a seasonal pattern with a minimum in the summer and a maximum in the winter. This spatio-temporal distribution is well consistent with the observations and is affected by the Pearl River plume, sea surface circulation, and precipitation. Using our reconstructed SSS, we were able to successfully characterize the spreading of the Pearl River and Mekong River plumes and the intrusion of the Kuroshio Current from the Pacific Ocean into the SCS. [ABSTRACT FROM AUTHOR]

Reconstructing past changes in the oceanic nitrate inventory with sedimentary N records in the South China Sea (SCS), which is the terminal of North Pacific Intermediate Water (NPIW), has regional and global implications. However, water‐column nitrate cycling that affects N isotope preservation remains poorly understood in the SCS. We present a new data set of nitrate isotopes (δ15NNO3 ${\delta }^{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and δ18ONO3 ${\delta }^{18}{\mathrm{O}}_{{\text{NO}}_{3}}$) to elucidate nitrate dynamics in the SCS and the adjoining western North Pacific Ocean (wNPO). Greater increases in δ18ONO3 ${\delta }^{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ than in δ15NNO3 ${\delta }^{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ are observed in the SCS euphotic zone, suggesting a combined effect of partial nitrate assimilation and nitrification. In the subsurface and thermocline waters of both regions, upward disproportional decreases in δ15NNO3 ${\delta }^{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and δ18ONO3 ${\delta }^{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ accompanied by elevated nitrate anomalies (N*) indicate an accumulation of external N. Such changes are less significant in the SCS due to higher nitrate concentrations therein, although external N influxes are comparable in both regions. High δ15NNO3 ${\delta }^{15}{\mathrm{N}}_{{\text{NO}}_{3}}$ and δ18ONO3 ${\delta }^{18}{\mathrm{O}}_{{\text{NO}}_{3}}$ values in the wNPO intermediate water result from the lateral transport of NPIW with isotopically more enriched nitrate from the remote denitrification zones followed by mixing with overlying water containing isotopically depleted nitrate. As NPIW flows into the SCS, its isotopically enriched signal is further diluted by strong vertical mixing with overlying and underlying waters in the interior. Compared to its source water from the wNPO, the SCS deep water has consistent nitrate isotopic compositions but significantly lower N*, indicating increased benthic denitrification at the wNPO margins with an estimated rate of 0.26–0.41 mmol N m−2 day−1. Plain Language Summary: The South China Sea (SCS) is an ideal region to study past changes in the oceanic N pool using sedimentary N records. These records are sensitive to water‐column nitrate cycling. This study presents a new data set of nitrate isotopes in the SCS and the neighboring western North Pacific Ocean, the aim of which is to determine major processes controlling their distribution. Partial nitrate consumption by phytoplankton and nitrate production from ammonium/nitrite oxidation by microorganisms together determine nitrate isotopes in the upper 100 m of the SCS. Atmospheric‐derived and biologically fixed N that provides lighter nitrate isotopes mainly accumulates in the subsurface water (100–400 m). In deeper waters, the distribution of nitrate isotopes in the SCS is regulated by a vertical sandwich‐like water flow structure through the Luzon Strait, that is westward inflows of the North Pacific waters in the upper (<750 m) and deep (>1,500 m) layers and an eastward outflow of SCS waters between the two layers. Strong vertical mixing in the SCS interior further smooths the original nitrate isotopes. Our findings highlight the important role of these biogeochemical and physical processes in governing water‐column nitrate isotopes and thus sedimentary N records in the SCS. Key Points: Nitrate δ15N and δ18O increase unequally in the South China Sea (SCS) surface water due to partial nitrate assimilation and nitrificationExternal N inputs lower δ15N and δ18O of subsurface nitrate to a lesser extent in the SCS than in the western North Pacific Ocean (wNPO)Lateral exchange between SCS and wNPO intermediate and deep waters along with vertical mixing modifies their nitrate isotopic composition [ABSTRACT FROM AUTHOR]

Given their ephemeral nature, eddies have proven difficult to study, with contrary results from field observations that typically sample at the center during a specific stage of an eddy lifespan. Using the natural occurring radionuclide 234Th, we examined particle fluxes within two cyclonic eddies (CEs) at different evolutionary stages (mature stage eddy, C2, and a decay stage eddy, C1) in the oligotrophic western South China Sea (SCS). Using a 1‐D steady state model, 234Th derived particulate organic carbon (POC) and opal fluxes at 100 m were determined. Both POC and opal fluxes were significantly higher at the eddy edges relative to the eddy cores, with integrated eddy POC and opal fluxes of 6.2 (2.1) mmol C m−2 day−1 and 1.5 (0.28) mmol Si m−2 day−1 in C2 (C1). When compared to non‐eddy regimes, both POC (by 2.6‐fold) and opal fluxes (by 7.5‐fold) in C2 were enhanced; they were reduced and decoupled (0.9‐fold and 1.4‐fold) in C1. The difference in enhancement of particle fluxes and the coupling/decoupling between POC and opal flux likely reflects changes in phytoplankton community structures resulting from eddy evolution. Scaling these results to the entire SCS basin suggests that CEs contribute <4% of the net POC flux but >15% of the opal flux. Therefore, CEs may regulate the biogeochemical cycling of silica to a much greater extent than carbon in the ocean. Key Points: Carbon and opal export are concentrated at the edges of eddies and during their "mature" stageCarbon and opal export wane and become decoupled as the eddies decayMesoscale features disproportionately impact opal export relative to carbon export [ABSTRACT FROM AUTHOR]

Abstract: Lateral input of dissolved organics may play a significant role to support productivity in oligotrophic ocean although associated biogeochemical evidences are lacking in the field. Ammonia oxidation (AO), the first step of nitrification that bridges organic remineralization and nitrate, is potentially an immediate responder. By using 15N‐NH4+, the spatial distribution of AO was investigated in the northern South China Sea, where Kuroshio Current intrudes frequently. AO ranged widely (0.001 to 134 nmol · L−1 · day−1) in space and the depth‐integrated (200 m) AO peaked where the Kuroshio influence is moderate suggesting that enhanced AO had occurred due to lateral mixing. Since oligotrophic Kuroshio is characterized by high dissolved organic nitrogen (DON), such lateral mixing not only introduces external DON into the northern South China Sea but also enhances NH4+ regeneration and subsequent oxidation to complicate the conventional new production in the boundary zone with DON gradient. [ABSTRACT FROM AUTHOR]

In order to examine the sources of plutonium (Pu) and elaborate its scavenging and accumulation processes, 240 Pu/ 239 Pu atom ratios and 239 + 240 Pu activities in the water column of the South China Sea (SCS) were determined and compared with our previously reported data for the sediments. Consistently high 240 Pu/ 239 Pu atom ratios that ranged from 0.184–0.250 (average = 0.228 ± 0.015), indicative of non-global fallout Pu sources were observed both in the surface water and at depth during 2012–2014. The spatial distribution of the 240 Pu/ 239 Pu atom ratio in the SCS showed a decreasing trend away from the Luzon Strait, which was very consistent with the introduction pathway of the Kuroshio Current. The Kuroshio had an even heavier Pu isotopic ratio ranging from 0.250–0.263 (average = 0.255 ± 0.006), traceable to the non-global fallout Pu signature from the Pacific Proving Grounds (PPG). Using a simple two end-member mixing model, we further revealed that this PPG source contributed 41 ± 17% of the Pu in the SCS water column. The 239 + 240 Pu activities in the SCS surface seawater varied from 1.59 to 2.94 mBq m − 3 , with an average of 2.34 ± 0.38 mBq m − 3 . Such an activity level was ~ 40% higher than that in the Kuroshio. The distribution of 239 + 240 Pu in the surface seawater further showed a general trend of increase from the Kuroshio to the SCS basin, suggesting significant accumulation of Pu within the SCS. The 239 + 240 Pu inventory of the water column in the SCS basin at the SEATS station with a total depth of ~ 3840 m was estimated to be ~ 29 Bq m − 2 , which was substantially higher than the sediment core estimates made for the SCS basin (3.75 Bq m − 2 ) but much lower than the sediment core estimates made for the shelf of the northern SCS (365.6 Bq m − 2 ). Such differences were determined by the lower scavenging efficiency of Pu in the SCS basin compared to the northern SCS shelf. [ABSTRACT FROM AUTHOR]

Nutrients from depth have been hypothesized as a primary source of new nutrients that sustain new productivity in oligotrophic oceans; however, the flux is challenging to quantify. Here we show for a first time in the oligotrophic South China Sea an extremely low diapycnal dissolved inorganic nitrogen (DIN) flux as 1.8 × 10−4 mmol m−2 d−1 in the nutrient-depleted layer (NDL) above the nutricline, where other nutrient supplies sustain the new production. Here higher phosphate and silicate fluxes relative to DIN than Redfield stoichiometry further indicate N-limited biological productivity and additional removal of DIN by diatoms. Below the NDL across the nutricline to the base of euphotic zone, termed as nutrient replete layer, the DIN flux is three orders of magnitude larger and sufficient in supporting the export production therein. Here higher DIC flux relative to DIN than Redfield stoichiometry further infers DIC excess in the upper ocean. [ABSTRACT FROM AUTHOR]

Nitrogen sources and dynamics, one of the key issues in marine biogeochemical cycles, remain poorly constrained in marginal seas. Here we examine the nitrogen cycle in the northern South China Sea (SCS) by combining data from previous reports with a new data set of N isotopic compositions (δ15N) of nitrate, zooplankton, and sinking particles. Average δ15N in subsurface nitrate is 4.8 ± 0.3‰, similar to that of sinking particles (δ15Nsink of 4.4‰) through the euphotic zone (EZ) collected by floating traps and to documented mean (4.6‰) for long-term mooring traps at 200 m. This along with oft-observed shallow nitracline (<100 m) suggests that subsurface nitrate is the primary source of new N to support export production. Moreover, δ15Nsink at the bottom of the EZ resembles those of suspended particles (4.2 ± 1.0‰) and zooplankton (5.4 ± 1.0‰) inside the EZ. High similarity in δ15N among various types of particles including zooplankton in different size fractions in the EZ implies rapid N turnover in the ecosystem. In deep waters at 2000-3000 m, however, additional particulate N fluxes are found due to lateral transport, which contain 15N-depleted particles, resulting in a downward decreasing trend of δ15Nsink. Incorporation of lighter N by bacteria and selective export of picoplankton are proposed as alternative mechanisms contributing to low δ15Nsink in the deep waters. The significant δ15Nsink change in the deep water column makes the SCS different from most other marginal seas; thus, caution should be made when using sedimentary δ15N to reconstruct paleonitrogen processes. [ABSTRACT FROM AUTHOR]

This study shows that the response of satellite-observed chlorophyll (CHL) to eddy motion varies seasonally in the South China Sea (SCS). The spatial pattern of the CHL anomaly composite for eddies is a dipole in summer and a monopole in winter, indicating that sea surface CHL is largely regulated by the horizontal rotational velocity of the eddy in summer and by eddy pumping and trapping in winter. The dipole pattern for anticyclonic eddies was confirmed by in situ observations, which also show that the dipole pattern is mainly restricted to the mixed layer. The underlying mechanism was further investigated with a coupled physical-biogeochemical model. The key driver leading to the seasonal variation of the eddy effect is found to be the seasonal variation of the mixed layer depth. In summer when the mixed layer is shallow, the monopole nutrient change induced by eddy is restricted to the subsurface. The sea surface CHL distribution is thus mostly affected by eddy advection. In winter, the deepening of the mixed layer mixes the nutrients from subsurface where eddy significantly changes the nutrient levels, allowing the monopole pattern to be observed in the sea surface CHL anomaly. [ABSTRACT FROM AUTHOR]

Abstract: It is increasingly recognized that mesoscale eddies play an important role in modulating the variability of ocean biogeochemistry. It is commonly believed that contrary to cyclonic eddies, anticyclonic eddies are characterized by downwelling at their core regime, which may suppress particle export. Here, by considering submesoscale domains we demonstrate that particle export might be alternatively enhanced in anticyclonic eddies on the basis of a study carried out in the oligotrophic northern South China Sea basin. We examined particle fluxes associated with three coherent anticyclonic eddies using the naturally occurring radionuclide 234Th. When applying a 1D steady-state model, 234Th and its derived particulate organic carbon (POC) fluxes in all three eddy cores were 1.9- and 1.6-fold higher, respectively, relative to those in the non-eddy region. However, an eddy-resolving circulation numerical model showed complex submesoscale circulations associated with the anticyclonic eddy. Notably, dynamic interactions occurred at submesoscales that might induce advection into the eddy core from the edge, where the 234Th deficit was elevated owing to higher particle production and export, probably stimulated by upwelling at the edges. We suggest therefore that enhanced particle fluxes derived from the 1D model along the vertical horizon at eddy cores only appeared to be changes, and that horizontal advection between the eddy core and edge should be taken into consideration in the flux estimation. Indeed, by integrating the 234Th deficit among multiple profiles in the entire anticyclonic eddy system, we derived an average 234Th flux of at the 100-m horizon, equivalent to a POC flux of . This export level was 1.6-fold higher than that from the reference sites. [Copyright &y& Elsevier]

Abstract: We present a time-series analysis of nutrient and pCO2 (partial pressure of CO2) levels in an oligotrophic coastal ecosystem (Gan Bay), which was likely to be influenced by upwelled subsurface water. Gan Bay is off Currimao Harbor, in the northwest Philippines and is located at the boundary of the South China Sea (SCS). This 42-h time-series observation was conducted in December 2006. In addition to continuous observations of dissolved oxygen (DO) and pCO2, discrete samples were collected at a depth of 5m every 3h for measurements of nutrients, including soluble reactive phosphorus (SRP) and inorganic nitrogen (NO3 − +NO2 −) in order to examine their dynamics and possible physical and biological controls. We observed remarkably large short-term variations in the surface water, spanning a 10-fold change for SRP (32–330nM) and from <0.3μM to 4.3μM for NO3 − +NO2 −. DO also varied substantially from a lower end of 171 to 205μM O2. Surface water pCO2 changed from an equilibrium stage with the atmosphere (∼386μatm) to a stage where it was a significant source for the atmospheric CO2 (seawater pCO2 ∼469μatm). We found that the variation of nutrients was driven neither by tidal mixing nor by biological activities, as was suggested by the variations in the total bacterial abundance and chlorophyll a. Instead, our inverse T–S relationship suggested a two end-member mixing process during the observation period. The N:P ratio throughout the observation period was ∼13.2, which is characteristic of SCS subsurface and deep waters. Moreover, pCO2 was correlated inversely with the sea surface temperature. It is likely, therefore, that an upwelled subsurface cold water with high nutrients, low-temperature and high-pCO2 existed. It should be noted that this upwelled cold water did not appear to impact the entire observation period (approximately 35h of 42h), which might suggest an extremely dynamic nature for this upwelled cold water mass. [Copyright &y& Elsevier]

Abstract: The distribution of pCO2 in the surface waters of the northern South China Sea (NSCS) was examined in the summer of 2000, the spring of 2001 and the late fall of 2002. For the offshore region >100 km away from the coastline, surface water pCO2 varied within a range of 360–450 μatm during all the surveys. Nevertheless, they were generally higher than atmospheric pCO2. Sea–air ΔpCO2 ranged in 50–100 μatm in the summer, 0–50 μatm in the spring and 0–90 μatm in the late fall. Average sea-to-air CO2 flux was 7 mmol CO2 m−2 day−1 in the summer and 1–3 mmol CO2 m−2 day−1 in the spring and fall. Nearshore pCO2 showed a very dynamic pattern likely associated with the regional hydrodynamic settings, yet again pCO2 in the surface water overall exceeded the air pCO2. Data from this study thus suggests that the NSCS served as a source of atmospheric CO2. Seasonal variations of the pCO2 in the NSCS outer shelf and slope surface waters were significantly influenced by sea surface temperature. [Copyright &y& Elsevier]

The Indo-Pacific Intersection (IPI), centered in the Indonesian Seas (IS) and South China Sea (SCS), is a convergence zone of both hydrological and atmospheric circulations and plays an important role in the water cycle and global climatic system. This study presents a unique dataset of both 239+240Pu activities and 240Pu/239Pu atom ratios to trace Plutonium (Pu) source terms and their transport in the IPI, to examine constituent water masses, notably freshwater fluxes. The 240Pu/239Pu atom ratios in surface IPI seawater ranged from 0.194 to 0.258 with an average of 0.223 ± 0.021, values that were consistently higher than global fallout (~0.18). 239+240Pu activities ranged widely from 0.55 to 2.8 mBqm−3 with a generally accumulating trend from the western Pacific Ocean (WP) to the IPI as previously observed. We further reveal, using a two end-member mixing model between global fallout and Pacific Proving Grounds (PPG), that the PPG source contributes 34 ± 8% of the Pu to the IS. In the IS, both 239+240Pu activities and 240Pu/239Pu atom ratios further show values between those in the WP and SCS, indicating a two-endmember mixing scheme between the SCS (characteristically lower-salinity but higher PPG Pu-fraction) and the WP (higher-salinity and higher PPG Pu-fraction). We estimate the fractional contribution of the SCS water mass as 35 ± 16% in the surface mixed layer and 49 ± 14% in the IS 50–235 m depth interval. This water mass transport is equivalent to a freshwater influx of 80 ± 47 mSv from the SCS to the IS (known as the South China Sea Throughflow (SCSTF), a branch of the Indonesian Throughflow (ITF)). It accounts for over half (53 ± 31%) of the total freshwater flux transported by the ITF into the Indian Ocean and supplies ~24% the freshwater net evaporation in the Indian Ocean over the 8°N – 32°S latitudinal range, and it is thus crucial in maintaining the fresh water balance therein. This freshwater influx to the IS from the SCS also leads to a ~ 0.2 kg m−3 decrease in the mean density of the upper 235 m water layer, which serves to inhibit the transport of upper layer warm water from the western tropical Pacific warm pool into the Indian Ocean. Therefore, the SCS, together with the SCSTF, play a critical role in regulating freshwater and heat balance in the IPI and Indian Ocean. These findings have strong implications for the integrative role of the SCS and SCSTF on the Indo-Pacific climate and water cycle. • The isotopic ratio of 240Pu/239Pu in the Indo-Pacific Intersection (IPI) ranges 0.194–0.258. • Pu isotopes in the IPI suggest mixed sources from both global and Pacific Proving Grounds close-in fallouts. • The freshwater flux from the SCS to the IS amounts to 80 ± 47 mSv, contributing over half of the total ITF freshwater flux. [ABSTRACT FROM AUTHOR]

• We show spatial and seasonal patterns of the carbonate system in the northern basin of the South China Sea (SCS) based, on a thus far, the largest dataset in a major world marginal sea. • Exchange fluxes of the carbonate variables between the northern SCS and West Philippine Sea (WPS) via the Luzon Strait are constrained in the upper layer by Kuroshio intrusion and over the full depth range. • We reveal significant spatial and seasonal changes in the carbonate chemistry of the deep SCS below 2000 db, attributable likely to spatial and seasonal variations in organic matter decomposition. Using a most comprehensive, high quality, high-resolution dataset for any marginal sea up to depths >2000 db, we examined the seasonality of the carbonate system in the northern South China Sea (nSCS) and exchange with the West Philippine Sea (WPS) during 2009–2011. The carbonate system dynamics demonstrated evident spatial and seasonal variations. Winter exhibited the highest average surface dissolved inorganic carbon (DIC) concentrations (1936 ± 15 μmol kg−1), and summer had the lowest (1882 ± 12 μmol kg−1), primarily associated with more abundant freshwater inputs in summer. At 100 db depth, decreased DIC and total alkalinity (TA) values were observed within the Luzon Strait vicinity due to the influence of WPS waters. Higher DIC and TA concentrations were found within the central nSCS basin. The average Kuroshio contribution to the DIC inventory in the upper 150 db was seasonally significant, ranging between 11 and 32%, with the highest contributions during spring and winter. Below 2000 db, nSCS basin-averaged DIC was significantly higher than WPS-averaged DIC (∼23 µmol kg−1 difference) due to more organic matter decomposition in the nSCS basin. Within the basin, average deep water DIC values were highest in autumn, and averaged concentrations at >18.5°N were lower than at ≤18.5°N. Our datasets and analysis imply that (i) the significant seasonal and spatial patterns of carbonate chemistry in the nSCS are controlled by a combination of large-scale and smaller mesoscale physical processes; (ii) extrinsically from Asian monsoons via seasonal freshwater discharge and dynamic exchanges with open ocean waters; and (iii) intrinsically, through seasonal vertical mixing as well as mesoscale processes and their subsequent new productions. The seasonal and spatial variability in carbonate parameters established here serves as an essential baseline to monitor future changes to the nSCS and to compare with other marginal sea systems. [ABSTRACT FROM AUTHOR]

Abstract: This paper describes the simulated temporal variation of surface seawater CO2 partial pressure (pCO2) in the northern South China Sea. We produced the simulations with a one-dimensional (1-D) coupled physical–biogeochemical model that had high-frequency, time-dependent atmospheric forcing and that were validated with field observations. We also examined the associated processes that modulate seawater pCO2 at different time scales, from diurnal to seasonal, using a series of process-oriented experiments. At seasonal time scales, we revealed that the sea–air CO2 exchange was a primary process that modulated surface pCO2 and exceeded the role of sea surface temperature (SST) even though the phase of the pCO2 variation generally followed the strong seasonal cycle of SST. This was because sea–air CO2 exchange is a slow process and has an accumulative effect on surface water pCO2 due to the buffering effect of the carbonate system once CO2 has dissolved in the seawater, which leads to a long equilibration time of CO2 between the atmosphere and seawater. The mixing effect on pCO2 induced by total alkalinity and dissolved inorganic carbon variations was, generally, positively correlated with the seasonal evolution of wind speed. Biological processes were the smallest contributors to pCO2 variations at the seasonal scale because of the oligotrophic characteristic of the region. At diurnal time scales, the dominant pCO2 controlling factor was mainly associated with the local physical and biological conditions. Temperature and wind-induced vertical mixing played major roles in pCO2 when the winter heat flux and upward transport of low temperature and high pCO2 in deep water were intensified. Phytoplankton blooms generally occur after a period of strong wind, as a result, biological metabolism becomes the most important pCO2 regulator when the surface chlorophyll-a reached its highest level. Unlike that in the seasonal scale, the effect of sea–air CO2 exchange was minor at diurnal time scales due to the long equilibration time of CO2 between the atmosphere and seawater. We also found that the frequency of the model driving force was important in reproducing the sea surface pCO2. The high frequency forcing was important in controlling the pCO2 variation through the feedback effect to the corresponding physical and biogeochemical responses. [Copyright &y& Elsevier]

Abstract: Frequently observed subsurface chlorophyll maximum (SCM) contributes substantial biomass to the waters over the continental shelf of the northeastern South China Sea (NSCS) but it has not been sufficiently investigated. In this study, observations and a three-dimensional coupled physical–biological numerical model were utilized to investigate the characteristics of the SCM under strong controls of coastal upwelling circulation and the Pearl River plume in the NSCS. The model captures the observed characteristics of the SCM in the NSCS reasonably well. Both the depth and intensity of the SCM are spatially variable and regulated by the variable upwelling circulation and associated plume distribution over the complex shelf topography. In nearshore waters, the SCM shoals and weakens towards the coast as a result of the upwelling of high nutrient low chlorophyll deep water while surface productivity is enhanced. The intensity of the SCM weakens when the surface layer is covered by the river plume because of the substantial reduction of photosynthetic active radiation (PAR). Strengthening upwelling-favorable wind weakens the intensity of the SCM due to dilution by the enhanced mixing, but the SCM depth remains relatively stable in the offshore water as a result of no apparent shift of the nutricline. Changing incident PAR leads to an interactive response in chlorophyll concentration, nutricline, and the depth of the SCM. [Copyright &y& Elsevier]

Enabled by the success in the determination of stable barium (Ba) isotopic compositions in seawater, Ba isotopes have been suggested as a novel tool to study physical and biogeochemical processes in the present and past ocean. However, a better understanding of the fractionation of Ba isotopes during particle-seawater interactions is a prerequisite for such applications. In this study, we use an extensive data set of concentrations and isotopic compositions of dissolved Ba (DBa and δ138Ba DBa) and bulk particulate Ba (pBa bulk and δ138Ba pBabulk) collected in the northern South China Sea (NSCS) to constrain Ba isotope fractionation in the upper ocean. Seawater and suspended particle samples for Ba isotope measurements were collected in January 2010 along a transect from the outer shelf to the lower slope. The water column profiles reaching depths of 1000 m are characterized by a general decrease of δ138Ba DBa and an increase of DBa with depth. However, δ138Ba DBa signatures are essentially constant at +0.6‰ in the upper 150 m of the entire study area. The corresponding δ138Ba pBabulk , which primarily represents the isotopic compositions of oceanic or excess particulate Ba (pBa xs), is consistently lower than δ138Ba DBa but also constant at values of +0.1 to +0.2‰. This suggests that the same Ba isotope fractionation process prevails above 150 m on the NSCS outer shelf and slope resulting in a constant in situ fractionation factor of −0.5‰. This value is consistent with previously reported values of −0.4 to −0.5‰ in the upper 200 m of the open ocean and a lake. Moreover, we observe significant differences of pBa xs distributions from those of particulate calcium, particulate organic carbon and nitrogen, and biogenic silica indicating that passive adsorption onto particles rather than active biological utilization is most likely the primary process inducing Ba isotope fractionation in the upper NSCS. The constant δ138Ba pBabulk signatures suggest that particulate Ba isotopes integrate reliable information during transformation of DBa to pBa xs and are thus a more robust proxy for total particle fluxes than pBa xs concentrations, which show variable values potentially affected by other processes (e.g., particle sinking and/or zooplankton grazing) and thus reflects "snapshot" processes in the water column. We contend that biological productivity plays only a subordinate role in regulating the surface Ba isotopic composition of bulk suspended particles. The extent to which Ba isotopes may nevertheless be a reliable proxy for present and past export productivity requires further analyses of the δ138Ba signature of specific Ba carriers such as barite throughout the water column and in the sediments. [ABSTRACT FROM AUTHOR]

• Layered circulation in the SCS is key in regulating the variation of nutrients. • Upward advection of deep water compensates for the nutrient deficiency in upper layer. • Remineralization contributes most of the PO 4 in the euphotic layer. The South China Sea (SCS) is one of the largest marginal seas in the world, yet the processes that drive its nutrient distribution are not well understood. Using the three-dimensional China Sea Multiscale Ocean Modeling System (CMOMS) embedded with a nitrogen-phosphorus-phytoplankton-zooplankton-detritus ecosystem model, we investigated nutrient transport and dynamics in the SCS. The model was well validated with field and remotely sensed observations. We found that the unique layered distribution of nutrients that increases from north to south in the upper layer and increases in the opposite direction in the middle and deep layers. We found that the lateral and vertical fluxes associated with three-dimensional ocean circulation and biogeochemical processes shape the distribution. The lateral fluxes are provided extrinsically by the sandwich-like transport through Luzon Strait (LS) and the exchanges through other straits, and intrinsically by the interior vertical motion due to flow-slope topography interaction. The nutrient deficiency in the upper layer due to dilution by inflow from the Western Pacific Ocean is mainly compensated by the upward nutrient-rich deep water. The nutrient transport is jointly regulated by volume transport and the uneven spatiotemporal distribution of nutrients. We demonstrated that labile organic matter plays an important role in total nutrient budgets. In euphotic layer, biological production is the major sink, but remineralization contributions to PO 4 are more important than to NO 3 due to the higher recycling rate of PO 4. In the shelf water, coastal upwelling serves as major source for both NO 3 and PO 4 , vertical diffusion is another important source of PO 4 due to the strong downward increasing gradient. [ABSTRACT FROM AUTHOR]

Degree of oxygenation in intermediate water modulates the downward transferring efficiency of primary productivity (PP) from surface water to deep water for carbon sequestration, consequently, the storage of nutrients versus the delivery and sedimentary burial fluxes of organic matter and associated biomarkers. To better decipher the PP history of the South China Sea (SCS), appreciation about the glacial-interglacial variation of the Luzon Strait (LS) throughflow, which determines the mean residence time and oxygenation of water mass in the SCS interior, is required. Based on a well-established physical model, we conducted a 3-D modeling exercise to quantify the effects of sea level drop and monsoon wind intensity on glacial circulation pattern, thus, to evaluate effects of productivity and circulation-induced oxygenation on the burial of organic matter. Under modern climatology wind conditions, a 135 m sea-level drop results in a greater basin closeness and a ∼24% of reduction in the LS intermediate westward throughflow, consequently, an increase in the mean water residence time (from 19.0 to 23.0 years). However, when the wind intensity was doubled during glacial low sea-level conditon, the throughflow restored largely to reach a similar residence time (18.4 years) as today regardless its closeness. Comparing with present day SCS, surface circulation pattern in glacial model exhibits (1) stronger upwelling at the west off Luzon Island, and (2) an intensified southwestward jet current along the western boundary of the SCS basin. Superimposed hypothetically by stronger monsoon wind, the glacial SCS conditions facilitate greater primary productivity in the northern part. Manganese, a redox sensitive indicator, in IMAGES core MD972142 at southeastern SCS revealed a relatively reducing environment in glacial periods. Considering the similarity in the mean water residence time between modern and glacial cases, the reducing environment of the glacial southeastern SCS was thus ascribed to a productivity-induced rather than ventilation-induced consequence. [ABSTRACT FROM AUTHOR]

The distribution of hydrogen peroxide was determined at 28 stations in four transects across the No rthern So uth C hina Sea S helf-sea (NoSoCS) in June 2010. The average concentration in the top 10 m ranged between 0.063 and 0.231 µM. Among the hydrographic regimes: the inner shelf outside the influence of upwelling, the inner shelf under the influence of upwelling, the middle shelf, the outer shelf and the open South China Sea, the highest concentration was found in the day-time in the inner shelf outside of the influence of upwelling and the lowest concentration was found in the night-time in the inner shelf under the influence of upwelling. The average concentration at the day-time stations in each hydrographic regime was invariably higher than the corresponding concentration at the night-time stations. Across the shelf, the concentration was the highest in the inner shelf. The variations in the concentrations of hydrogen peroxide and total organic carbon (TOC) in the different hydrographic regimes followed a similar pattern. All these behaviors are consistent with the photochemical production of hydrogen peroxide, using dissolved organic matter as the chromophore, as a primary control on its occurrence in the surface waters. Moreover, at day-time stations at different locations in the NoSoCS and at a single location sampled through the day, the relationship between the concentration of hydrogen peroxide and the product of the concentration of TOC and the time-integrated irradiance at the time of sample collection fell on two distinct linear lines with different slopes and intercepts: one for mostly the inner and middle shelf waters and the other for mostly the water in the open northern SCS. These relationships suggest that the light history, the pre-existing concentration of hydrogen peroxide, and, the concentration and the photochemical efficiency of the dissolved organic matter in the production of hydrogen peroxide play major roles in determining the temporal and spatial variations in the concentration of hydrogen peroxide in the surface waters in the day-time. In the sub-surface, a notably large fraction, about one-third, of the profiles of hydrogen peroxide did not follow the typical distribution of a quasi-exponential decrease in concentration with depth. Almost all the profiles at the stations in the slope where the internal waves were most active were atypical. At these stations, the concentration of hydrogen peroxide was generally elevated down to about 100 m, well beyond the depths where solar irradiance was plentiful. Superimposed on these elevated concentrations was a persistent sub-surface maximum, with concentrations reaching about 0.5 µM, at 80 m. While the origin of these atypical distributions is not yet clear, dark biological production and the action of internal waves conceivably could have contributed to their occurrence. [ABSTRACT FROM AUTHOR]

Ammonium (NH 4 +) dynamics in the oligotrophic South China Sea (SCS) were examined using data collected during summer 2014 and spring 2016. A sensitive fluorometric technique was used to measure NH 4 + concentrations (detection limit of 0.7 nmol L−1). The NH 4 + inventory of the upper 100 m showed considerable variation between two samplings, averaging 3.46 mmol m−2 in 2014 and almost doubling in 2016. This could be attributed to seasonality or more likely to influence of Kuroshio Current intrusion, prominent within El Niño years such as 2016. Kuroshio-influenced stations exhibited elevated NH 4 + and decreased NO 3 − inventories. Higher NH 4 + concentrations at the Kuroshio-SCS frontal zone were consistent with prior observations of enhanced microbial consumption of Kuroshio-derived dissolved organic matter. Two patterns in vertical NH 4 + distributions were observed: profiles with subsurface NH 4 + maximum (30.1–241 nmol L−1), usually occurring at depths of 50–100 m that were closely coupled with the deep chlorophyll maximum and nitrite maximum, and profiles with low and generally uniform NH 4 + concentrations with depth. The depth of the NH 4 + maximum appears to be controlled by the euphotic depth, position of the nitracline, as well as the difference in the affinity to NH 4 + between phytoplankton and nitrifiers. We demonstrated that precise measurements of NH 4 + concentrations at nanomolar levels facilitate improved understanding of oceanic nitrogen biogeochemistry. • High- precision data sets for nanomolar level ammonium and nitrite in the northern South China Sea were reported. • Ammonium maximum was observed at some stations but not others while nitrite displayed subsurface maxima at all stations. • Kuroshio Current intrusion elevated ammonium inventory in the northern South China Sea. [ABSTRACT FROM AUTHOR]

We present tritium (3H) and helium isotope (δ3He) data from the South China Sea (SCS), to estimate apparent oxygen utilization rates (AOURs). The observed δ3He values are close to the theoretical solubility equilibrium value of -1.7% in the upper mixed layer, followed by an increase with depth down to ~1500 m. Below 1500 m depth, δ3He is homogenously distributed with a value of 20.9% ± 1.0%. The distribution of δ3He reveals that a significant fraction of 3He throughout the water column over the entire SCS basin is allochthonous, derived from mantle sources originated from the Pacific Ocean. By using the salinity-normalized "potential" alkalinity (NPA) as a conservative mixing tracer, 3He produced by tritium decay (tritiogenic) was separated from mantle 3He, and the apparent 3H–3He ages of the SCS water masses were subsequently calculated to be 11 ± 5 years at 100 m depth and 50 ± 4 years at 1000 m. Together with the observed dissolved oxygen concentrations, we estimated the mean AOURs and obtained values of 4.15 ± 0.27 μmol kg−1·yr−1 for the depth range of 100–500 m, and 0.81 ± 0.23 μmol kg−1·yr−1 for that between 800 and 1000 m depth. The depth-integrated AOURs between 100 and 1000 m depth, yield a spatially and temporally averaged export flux of organic carbon of 1.96 ± 0.16 mol-C·m−2·yr−1, comparable with previous estimates based on either 238U/234Th disequilibrium or nutrient and oxygen mass balance calculations in the SCS. • For the first time in a marginal sea of the Pacific Ocean, "tritiogenic" 3He was separated from mantle 3He. • Apparent 3H–3He ages of the SCS water masses range from 11 ± 5 years at 100 m to 50 ± 4 years at 1000 m. • The spatially and temporally averaged export flux of organic carbon is 1.96 ± 0.16 mol-C·m−2·year−1 in the SCS. [ABSTRACT FROM AUTHOR]

Abstract: Chlorophyll-a (Chl) concentration in the Taiwan Strait (TWS) and in the South China Sea (SCS) was estimated using time series of satellite data collected with the MODIS/Aqua and SeaWiFS instruments, and validated with in situ measurements from three cruises conducted in winter and summer 2004. For Chl between 0.1 and 10mg m−3, both SeaWiFS and MODIS agreed well with in situ data. Errors for turbid coastal waters were larger than for offshore waters but the overall RMS (root mean square) error in log scale was within 0.35. The percentage RMS error was much larger, varying between 60% and 170% for open ocean and most of the shallow (<30 m), coastal regions. However, there was no large systematic error or significant bias in either satellite data set, and these numbers were comparable to those for other global oceans and not significantly larger than the algorithm “noise” (0.22 in RMS error in log scale). Further, SeaWiFS and MODIS showed similar spatial and temporal patterns between July 2002 and October 2004, as well as nearly identical concentrations for Chl between 0.1 and 4mg m−3. RMS difference between the two data sets of monthly mean Chl for several sub-regions was generally <11% and <0.05 (after logarithmic transformation). For each individual month, the statistics (mean, mode, median) of the two data sets for the entire study region (6–9×105 satellite pixels at ∼1km resolution) were very similar, with RMS differences typically between 30% and 40% and between 0.10 and 0.15 (after logarithmic transformation), where no significant bias was found. Therefore, it would be possible to continue the time series using only one sensor such as MODIS, in the eventual absence of SeaWiFS. Further research is needed to improve the remote sensing algorithms for application in turbid coastal waters. [Copyright &y& Elsevier]

Abstract: Three cruises were carried out in the Pearl River Estuary (PRE) and adjacent northern South China Sea (SCS) in July 2000, May 2001 and November 2002 to collect seawater samples. Concentrations of dimethylsulfide (DMS), chlorophyll a (chl a), nutrients (N, P, Si), salinity and temperature in seawater were measured. The spatial and temporal distribution of DMS concentrations showed larger fluctuations compared with other estuaries reported in the literature. The mean DMS concentrations in three cruises ranged from 0.05nM (nM=10−9 moll−1) to 52.7nM (). The higher concentrations of DMS were observed at the mouth of the estuary. In wet season, high variations of environmental salinity might stimulate algae to release more DMSP to adjust osmotic pressure. Most of these DMS ‘hotspots’ were coincident with the area of high chl a concentrations, although no significant correlation between DMS and chl a was found. The values of DMS/chl a showed a clear trend along the north to the south transect, increasing sharply from estuary to shelf and open sea. There was no significant correlation between DMS and salinity in the wet season (July and May), but a significant positive correlation in the dry season. High primary production and more iron deposition implied that the NE monsoon might influence DMS production in the dry season. [Copyright &y& Elsevier]

Abstract: The spatial and temporal distribution of dimethylsulfide (DMS) was investigated in surface seawater and in the marine atmosphere in the Pearl River Estuary and northern South China Sea during three cruises in July 2000, May 2001 and November 2002. Sea-to-air fluxes of DMS were subsequently estimated based upon seawater DMS concentration, temperature of surface seawater and wind speed over sea. The seawater DMS concentration of the three cruises ranged from 0.1 to 52.7 nmol l−1 (n=76). DMS concentrations showed remarkable spatial and temporal distributions and highest values were observed at the mouth of the Pearl River Estuary. Throughout the study area we observed high levels of DMS in the water with great sea-to-air flux and relatively low levels of atmospheric DMS (1.70±1.16 nmol m−3 in May 2001 and 2.25±0.38 nmol m−3 in November 2002). Aerosol components, potentially linked with DMS oxidation, were also measured. The atmospheric concentrations of nss-sulfate and nitrate were much higher in the Pearl River Estuary than in the offshore area, with mean values of 12.11 and 4.45 μg m−3 for nss-sulfate, 4.88 and 2.21 μg m−3 for nitrate. Aerosol mass and components'' concentrations decreased from the inner estuary to outer waters. High concentrations of nss-sulfate and nitrate in sea salt particles imply that oxidation of atmospheric DMS is related with anthropogenic sources and heavy ozone, NOx and SO2 pollution in the study area. [Copyright &y& Elsevier]

• Analgorithm is developed to predict nutrient concentrations in the South China Sea. • Nutrient climatology is reconstructed based on ~ 5 million data points. • Upwelling/downwelling and the Kuroshio intrusion control the nutrient distributions. Nutrients are typically the most important determinant of the productivity of marine ecosystems. Hence, nutrients have been an essential variable of ocean observations in the modern oceanography era. Understanding of marine ecosystems and biogeochemistry, however, is largely limited by the spatiotemporal coverage of nutrient data. Herein, we developed a novel algorithm based on a large observational dataset of nutrients and their relationship with water masses indexed by temperature and salinity in the South China Sea (SCS). The algorithm yielded errors of ≤ 1.3, ≤ 0.10 and ≤ 3.5 μmol L−1 for NO 3 – + NO 2 – (N + N), phosphate and silicate, respectively. It is then applied to reconstruct nutrient concentrations primarily using temperature and salinity data archived in the World Ocean Database and Argo database during 1940–2018. It increases the nutrient data to ~ 5 million, by ca. three orders of magnitude compared to direct measurements in the SCS. This allows for a full examination of the seasonal climatology of nutrients in the SCS. In summer, in the upper 200 m, nutrient concentrations in the northwest and southernmost SCS are higher than in the rest of the SCS. An overall reversed pattern is revealed in winter, when higher nutrient concentrations are found in the central basin rather than basin margins. The Kuroshio intrusion and the vertical displacement of the nutricline, driven by upwelling/downwelling induced by horizontal convergences/divergences at both meso- and basin scales, determine the spatial and seasonal variation of nutrients. Seasonally, the Kuroshio intrusion and vertical displacements of the nutricline tend to offset the spatial variation of nutrient concentrations in spring, while they show an additive effect in the fall. [ABSTRACT FROM AUTHOR]

• DIC in the SCS is modulated by intrinsic dynamics and extrinsic exchanges via straits. • 3D-DIC transports across straits and within the SCS are examined. • DIC inventory in euphotic zone decreased in spring/summer but increased in fall/winter. Using a validated, three-dimensional, coupled physical-biogeochemical model, this study examines the dynamics of dissolved inorganic carbon (DIC) in the South China Sea (SCS). The model identifies characteristic differences in DIC concentrations between the SCS and adjacent West Philippine Sea (WPS), showing an increase in DIC of up to 140 μmol kg−1 in the upper 2000 m layer from the WPS to the vicinity of the northern SCS. This increasing pattern continues from the northern to the southern SCS, but to a much lower degree (<20 μmol kg−1 increase). Overall, spatial and seasonal variation of DIC in the SCS are evidently modulated by both intrinsic dynamics (e.g., river plumes, vertical mixing, basin-scale upwelling and eddies), and extrinsic exchanges via various straits. The inputs of DIC to the SCS are mainly through the Luzon Strait in the upper layer (depth, h < 600 m) and from the deep WPS at > 1600 m. Exports of DIC occur through other straits and the SCS intermediate water outflow (600 m to 1600 m) via the Luzon Strait. The model shows upward transport of DIC in the upper 400 m and below 1100 m, in contrast to downward DIC transport between 400 m and 1100 m. Seasonally, both DIC concentrations and fluxes in the upper layer have larger dynamic ranges in winter than in summer, attributable to the combined effects of enhanced Kuroshio intrusion, stronger vertical upwelling and greater biogeochemical alterations. A balance of various constitutive DIC terms revealed that horizontal and vertical advection is an order of magnitude greater than the other terms of the DIC budget, which has an opposite sign and thus tends to neutralize each other. The model indicated a decrease in the DIC inventory in the euphotic zone in spring and summer caused by net DIC consumption, and a contrasting increase in fall and winter primarily influenced by DIC vertical transport. In intermediate and deep layers, physical transport processes play a dominant role in the seasonal variation of DIC inventories. In the upper 150 m, DIC fluxes and their stoichiometry with nutrient fluxes, which potentially impact the air-sea CO 2 fluxes, are largely driven by excess vertical DIC fluxes compared to horizontal fluxes via the Luzon Strait. [ABSTRACT FROM AUTHOR]