Abstract The role of phytoplankton, particularly diazotrophs, in affecting in situ 210Po scavenging has been little investigated. Here, we first report an enhanced 210Po deficit in a cyclonic eddy core relative to its edge in the subtropical Pacific, coinciding with an elevated integrated nitrogen fixation rate (NFR). In contrast, particulate organic carbon, chlorophyll a, total particulate matter, and 210Pb show no core‐edge difference, revealing a selective fractionation between 210Po and 210Pb during scavenging. The partition coefficient of 210Po, but not 210Pb, correlates with NFR. The 210Po/210Pb fractionation factor also scales with NFR. These relationships indicate diazotrophs' uptake of 210Po and provide a field data‐based algorithm linking 210Po disequilibria to NFR in (sub)tropical oceans. Our findings suggest that 210Po offers a potential radiotracer for quantifying marine NFR, highlighting a new insight to constrain nitrogen cycling in oligotrophic oceans.

In the ocean, the depleted signal of riverine black carbon (BC) has challenged our knowledge of the BC origin, largely due to a poor understanding of the BC fate in the river-to-coast continuum. Here, the time-series observations of particulate soot BC (PBC), bulk POC, delta C-13, and Th-234 were conducted in the river-dominated Xiamen Bay to examine the export of PBC to the Taiwan Strait. delta C-13(PBC) (-20.5 parts per thousand similar to -15.0 parts per thousand) are different from those of PBC-excluded POC (-23.3 parts per thousand similar to -20.2 parts per thousand), characterizing the main C4-plant origin of PBC. PBC ranged from 0.41 to 7.10 mu mol L-1. In the surface water, PBC, together with temperature and salinity, displayed cyclic variations with the tidal cycles, ascribing to the freshwater-seawater mixing. However, the cyclic variation in PBC pointed to tide-induced resuspension in the bottom water. The higher PBC at the bay mouth than that in the open seawater supported the net export of PBC to the Taiwan Strait with the tidal fluctuation. A correlation between the exchange flux of Th-234 and the tidal variation revealed that the 'tidal pump' drove the exchange of particulate matter. Based on the non-steady state model of Th-234, the export and import fluxes of PBC were 1.15-4.32 mmol m(-2) d(-1) and < 8.24 mmol m(-2) d(-1), respectively. A preliminary estimate indicated that 15 %-33 % of riverine PBC is exported to the Taiwan Strait annually. This study discovered the tidal pump mechanism of the PBC transport and shed new light on the fate of PBC in the river-to-coast continuum.

The lateral transport of sediments from the shelf to the ocean interior is ubiquitous on continental margins, exerting great impacts on the characteristics and flux of sinking particles in the mesopelagic ocean. In this study, we determined the biogenic components and stable carbon isotopes in sinking particles collected from the slope of the northern South China Sea using a time series sediment trap. Laterally derived particulate organic carbon (POC lateral ), which originated primarily from marine sources, as indicated by the ratio of POC to particulate nitrogen ratio and POC‐δ 13 C, accounted for 31 ± 3% of the total POC flux during the one‐and‐a‐half‐year deployment. High fluxes of POC lateral occurred in April and May 2021, comprising 69 ± 12% and 53 ± 9% of the total POC flux, respectively. This was likely due to the lateral input of shelf sediments entrained by a westward anticyclonic eddy. These allochthonous particles were mainly composed of small (<53 μm) sediments and were depleted in biogenic components, such as POC and biogenic silica (bSi). In contrast, the flux of vertically derived POC (POC vertical ), that is, local biogenic POC, was strongly correlated with the surface Chl‐ a concentration ( R 2 = 0.62, p < 0.001) and bSi flux ( R 2 = 0.83, p < 0.001), indicating that the POC vertical flux in the mesopelagic zone was governed by both primary productivity and the growth of diatoms, with the latter being the main controlling factor. Our results highlight that the POC vertical flux was controlled by the seasonal variations in primary productivity and the phytoplankton community structure, whereas the POC lateral flux was closely linked to hydrodynamic processes, such as mesoscale eddies.

The geochemical behavior of black carbon (BC) in the estuary system controls the magnitude of riverine BC into coastal seas and thus the budget of BC in the ocean. However, the poor understanding of BC behavior in estuaries prevents us from well quantifying the riverine BC export. Here, eight cruises were conducted in the Jiulong River estuary to delineate the behavior of dissolved black carbon (DBC) determined using the benzenepolycarboxylic acid (BPCA) method, as well as its influence on riverine DBC estimation. DBC showed a range of 1.16-7.88 mu mol L-1. On an estuarine scale, DBC correlated negatively with salinity in all cruises (p < 0.05), revealing a dominant dilution effect of seawater during the transport of riverine DBC to the Taiwan Strait. Notably, the continuous addition behavior of DBC was identified in the low salinity zones based on high-solution samplings, attributing to the desorption of soluble BC from particulate matter. Simulation analysis indicated that the desorption followed a pseudo-first-order kinetic law. The kinetic constants varied from 0.053 h(-1) to 0.84 h(-1) with the maximum in summer. Incorporating desorbed DBC, the annual export rate of riverine DBC was 1.1 Gg yr(-1), about 26 % higher than 0.9 Gg yr(-1) estimated based on the traditional algorithm (i.e., without addition). The first examination of DBC desorption kinetics provided insights into the behavior of BC in the estuary and the riverine DBC magnitude into the ocean.

Dissolved black carbon (DBC) plays a key role in global carbon cycle and pollutant transport. However, the time‐consuming and labor‐intensive chemical analysis limits its spatiotemporal resolution. Here, we developed models to predict DBC from chromophoric dissolved organic matter (CDOM) measurements across the land‐to‐ocean continuum. We found that the mean ratio of DBC to light absorbance at 254 nm ( a 254 ) changed <20% among different environments. However, a single‐wavelength model is inadequate for precise prediction due to microbial production of CDOM. Incorporating longer wavelengths using multiple linear regression improves model performance. Random Forest Regression using the full spectral range performed even better at all environments, including the open ocean, achieving a root mean square logarithmic error of <0.15, median symmetric accuracy of <10%, and R 2 of >0.85. This study demonstrates the feasibility of using CDOM to predict DBC concentrations and highlights the potential for in situ monitoring and remote sensing applications.

Estuarine processes regulate the transport of dissolved black carbon (DBC) and associated contaminants to the ocean. However, there is limited understanding of the geochemical behavior of DBC in estuaries. In this study, DBC in the Pearl River Estuary (PRE) and the northern shelf of the South China Sea were examined using the benzene polycarboxylic acid (BPCA) method. DBC, bulk dissolved organic carbon (DOC), and chromophoric dissolved organic matter (CDOM) exhibited distinct behaviors during their transport from the PRE to the sea. DOC and CDOM decreased during the initial mixing of river water and seawater but increased at the lower PRE. In contrast, high aromaticity DBC inputs were observed throughout the PRE, likely originating from local terrestrial sources, such as runoff from nearby islands, as indicated by the high R H/L values (i.e., the ratio of BPCA containing 5 and 6 carboxyl groups to that containing 3 and 4 carboxyl groups; 2.03–2.30). In the Pearl River-plume zone (salinity < 33.0), DOC, CDOM, and DBC showed quasi-conservative behaviors against salinity, indicating that their geochemical behaviors were primarily governed by physical mixing between plume water and seawater. Using a flux model, it was estimated that the discharge of riverine DBC from the Pearl River Delta ranged from 11.2 to 16.3 Gg year −1 , representing an important source of bio-resistant DOC to the northern South China Sea.

Atmospheric deposition of 210Pb and 210Po from Nov. 2010 to Jan. 2012 were analyzed to reveal their temporal variations, as well as applications in constraining the residence times of aerosol. The monthly depositional fluxes varied from 2.42 to 29.31 Bq/m2/mon and from 0.160 to 3.388 Bq/m2/mon for 210Pb and 210Po, respectively. High fluxes of 210Pb and 210Po were observed in the southwest monsoon prevailing months, while low fluxes corresponded to the northeast monsoon seasons, revealing the monsoon control over 210Pb and 210Po deposition on seasonal timescales. There were significant positive linear correlations between the daily depositional fluxes and precipitation, supporting the predominant removal passage of 210Pb and 210Po through rainfall. The inverse relations between specific activities and precipitation indicated that the removal efficiencies were much higher at the beginning of rainfall. In contrast, the dry deposition only accounted for 22

Atmospheric deposition of 7Be was measured at a time-series station in the southeast of China (Xiamen) from 2011 to 2013. The deposition fluxes of 7Be ranged from 0.05 Bq m−2 d−1 to 7.42 Bq m−2 d−1, averaging 1.87 ± 0.10 Bq m−2 d−1. High fluxes occurred in months with northeast monsoon, and low values were observed in the southwest monsoon prevailing months. The significant correlations between 7Be deposition and precipitation, existing in both northeast and southwest monsoon seasons, suggested the dominant removal of atmospheric 7Be via precipitation. However, the correlations showed a large slope for the northeast monsoon season, indicating higher 7Be contents in the atmosphere during the northeast monsoon prevailing months, supported by the precipitation-normalized 7Be and the temporal variability of 7Be/210Pb ratios. Such a scenario revealed more intensive exchange of air mass between the stratosphere and troposphere during the northeast monsoon prevailing months. Together with the high pollutant concentrations in ambient air observed in these seasons, the results indicated that the pollutants in Xiamen might enter into the upper troposphere via vertical air mass exchange.

The dynamics of particulate black carbon (PBC) in marine environments are poorly understood. Here, radioactive 234Th was used to constrain the resident timescale, settling speed, and sinking flux of PBC (soot) in the coastal North-eastern South China Sea (NSCS). The PBC concentration varied from 0.013 & mu;g-C L-1 to 4.340 & mu;g-C L-1. Spatially, PBC showed an exponential decrease offshore, with a coefficient of 0.030 & PLUSMN; 0.004. Compiling available data, an empirical formula of PBC = a e-0.032x (xis the distance offshore) was proposed for predicting the descent of PBC offshore in coastal seas. Residence times of 0.8-13 d indicate that PBC is retained for days, implying its limited dispersal to the open sea. For the first time, the settling speed of PBC was evaluated in seawater, which averaged 8.8 & PLUSMN; 7.1 m d-1. These results highlight that bottle-sampled PBC falls mainly into the slow-sinking particle continuum in marine envi-ronments, due to its fine size. The sinking flux of PBC averaged 4.57 mg-C m-2 d-1 in the coastal NSCS. Using the sink-ing speed, the preliminarily estimated sinking rate of PBC was 23.8-1800 Tg-C yr-1 on global shelves. The crucial dynamic parameters of PBC provide insights into its internal cycling in coastal seas and can be used as model param-eters for assessing global PBC.

Macroalgae culture–induced carbon sink in sediments has been little investigated. Here, total organic carbon (TOC), total nitrogen (TN), and δ13C were examined in sediments in a cultivation field of macroalgae (kelp and Gracilariopsis lemaneiformis) in Sansha Bay, Southeast China. Both proxies of C/N (TOC to TN ratio) and δ13C indicated a multisource of TOC. Based on a three-endmember model, macroalgae-derived TOC (TOCma) accounted for < 35

Vertical distributions of dissolved and particulate 210Po and 210Pb in the water column were examined along three transects occupied on July 2016 in the northern South China Sea (SCS) to investigate particle dynamics and export. In the mixed layer, 210Po was deficient with respect to 210Pb with a 210Po/210Pb activity ratio of 0.08-0.57 and an average of 0.31 & PLUSMN; 0.12. The 210Po/210Pb ratio increased to 0.39-0.96 with an average of 0.71 & PLUSMN; 0.16 at the base of the euphotic layer (100 m). 210Po was largely depleted relative to 210Pb again in the mesopelagic layer (100-1000 m). The activity ratio of 210Po/210Pb ranged from 0.12 to 1.04 with an average of 0.42 & PLUSMN; 0.20, suggesting an intensified particle scavenging. Meanwhile, the residence time of particulate 210Po (PPo) in the mesopelagic layer varied from 0.01 to 0.47 yr with an average of 0.11 & PLUSMN; 0.02 yr, also indicating the fast removal of particles. Both the disequilibrium between 210Po and 210Pb and the short residence time of PPo in the mesopelagic water indicated a stronger scavenging and subsequent removal of 210Po relative to 210Pb throughout the water column. Based on the observed 210Po deficit, the export flux of 210Po in the mesopelagic layer was estimated in the range of 96-625 dpm m- 2 d-1 with an average of 382 & PLUSMN; 16 dpm m- 2 d-1, which was an order of magnitude higher than that in the euphotic layer. Furthermore, the export flux of total particulate matter (TPM) in mesopelagic layer was estimated between 2.15 and 30.18 g m- 2 d-1. Comparing to the TPM export flux in the euphotic layer (0.44-2.67 g m- 2 d-1), the significantly elevated TPM export flux in the mesopelagic waters may require extra supply of particles in addition to sinking particles from the upper water column, which is likely sourced from the surrounding shelves in the northern SCS.

Dense water out of the Antarctic shelves is expected to drive the transport of carbon into the deep Southern Ocean via the formation of Antarctic Bottom Water. However, bottom water formation's capacity to sequester carbon into the deep ocean is poorly constrained. Here, dissolved organic carbon (DOC), dissolved black carbon (DBC), and particulate organic carbon (POC) were examined to reveal the influence of the Weddell Sea Deep Water (WSDW) on DOC transport during its flowing out of the Weddell Sea. High DOC concentrations (>60 mu M-C) and low DBC/DOC ratios (<1.5%) were observed in surface water near the South Orkney Islands, ascribing to sea ice melt-induced phytoplankton blooms. Seawater at the mid-deep depths exhibited a higher DOC concentration (averaging 48.1 +/- 3.7 mu M-C) than the incoming water source, resulting from the release of DOC from sinking particles. Bottom water had higher DOC concentration compared to the mid-deep layer water (t-test, p < 0.005), while the DBC concentrations were comparable. In addition, the excess DOC (relative to WSDW) in bottom water showed a close relation with POC in surface water. These results reveal a top-down control over the DOC concentration in bottom water through a quick sinking of diatom detritus and subsequently solubilization in bottom water and/or sediment. With an estimate, the WSDW carries 5.1 +/- 2.8 Tg-C/yr of excess DOC out of the Powell Basin, representing an important carbon source to the deep Southern Ocean. This study highlights the key role of the Antarctic continental shelf pump in carbon sequestration.

Macro-algae culture has recently attracted attention in China because of its capability to sequester carbon. Here, radionuclides, total organic carbon (TOC), and nitrogen (TN) were examined in a cultivation area of macro-algae in Southeast China. At the reference site, the ratio of TOC to TN (C/N, 8.1 +/- 0.2, mean +/- SD) did not exhibit discernible variation over the past 70 years. In contrast, in the cultivation area, C/N descended from 9.0 +/- 0.2 around 1960 to 8.3 +/- 0.2 between 1960 and 1990 and 7.6 +/- 0.2 after 1990, coincident with the recorded kelp production in this area, indicating an influence of macro-algae culture-associated activities on carbon origin. Using a model, algal culture-associated activities contributed 23 +/- 7 % between 1963 and 1990 and 53 +/- 8 % between 1990 and 2022 to TOC. The burial of culture-associated TOC varied from 0.15 to 1.23 mg-C cm(-2) yr(-1), implying the unneglectable influence on carbon storage.

Coastal acidification has been widely investigated in terms of its rationale and ecological effects in the last decade. However, the driving mechanism for acidification in open seawater, especially in mesopelagic water, is still poorly understood. Here, the sinking velocity and flux attenuation of particulate organic carbon (POC) were examined based upon the radioactive 210 Po- 210 Pb tracer to reveal the remineralization of POC in the mesopelagic zone in the northeastern South China Sea (SCS). Overall, the profiles of 210 Po followed those of 210 Pb, lending support to the particle sinking controlled top-down deficits of 210 Po. Using an inverse model, the sinking velocity of particles, for the first time in the SCS, was estimated to vary from 3 to 34 m d -1 with the mean value of 15 ± 9 m d -1 , indicating that the slow sinking particles largely contribute to the POC flux in the SCS. Beneath the euphotic zone, a consistent descending of the sinking speed implied continuous remineralization of sinking POC in the twilight zone. A preliminary estimate revealed that 1.9-5.4 mmol-C m -2 d -1 remineralized back to carbon dioxide within 100-500 m, representing about 70% of the exported autochthonous POC from the euphotic zone. In 100-1000 m, 2.4-6.6 mmol-C m -2 d -1 (i.e., 84%) remineralized. Thus, the upper twilight zone (i.e., 100-500 m) is the dominant layer of POC remineralization, and POC-induced acidification could be unneglectable there. These results provided insights into the POC-induced acidification mechanism in the mesopelagic water, especially in the upper mesopelagic layer.

The global marine biogeochemical cycle of aluminium (Al) is believed to be driven by marine diatoms, due to the uptake of dissolved Al (DAl) by living diatoms from surface seawater. The occurrence of Al in diatom biogenic silica (BSi) can inhibit the dissolution of BSi, thus benefiting the effects of the ballast role of diatoms in the biological pump and forming a coupled Si–Al biogeochemical cycle. However, the occurrence characteristic of Al in marine diatoms is still unclear. In particular, whether or not Al is incorporated into the structure of BSi of living diatoms is unrevealed, resulting in difficulties in understanding the biogeochemical behaviours of Al. In this study, Thalassiosira weissflogii, a widely distributed marine diatom in marginal seas, was selected as the model to evaluate the occurrence of structural Al in BSi based on culturing experiments with the addition of DAl. The structural Al in BSi was detected by combining focused ion beam (FIB) scanning electron microscopy and energy-dispersive X-ray spectroscopy (EDS) mapping analysis. Visible, direct evidence of structural Al in living BSi was obtained, and the distribution and content of this Al were revealed by the EDS-mapping analysis. The effects of structural Al on BSi dissolution–inhibition are discussed based on the content of this Al. The fundamental results indicate the significant contribution of marine diatoms to the biogeochemical migration of marine Al.

Abstract 210Bi (t1/2 = 5.01 d) is theoretically a radionuclide for tracing the particle cycle over a timescale of hours to days. However, it has been rarely investigated in marine environments due to its very short half‐life and low activity. Here, 210Bi and 210Pb were examined in the water column on the shelf/slope of the northern South China Sea (SCS), as well as their atmospheric deposition. In rainwater, the 210Bi/210Pb ratio averaged 0.54 ± 0.28, indicating the influence of atmospheric deposition on the disequilibrium between 210Bi and 210Pb in surface seawater. On the shelf, 210Bi/210Pb averaged 0.73 ± 0.10 in the euphotic zone and 1.25 ± 0.10 below, supporting a quick removal of 210Bi from the euphotic zone and regeneration in the twilight zone. On the slope, deficits in 210Bi (210Bi/210Pb of 0.81 ± 0.07) were also observed in the productive low euphotic zone. The concurrence of 210Bi deficits and higher particulate organic carbon (POC) concentrations implied that POC largely dominates the deficit and excess of 210Bi. Based on a simple model, the removal fluxes of 210Bi at the euphotic base were 728 ± 73 dpm m−2 d−1 and 216 ± 89 dpm m−2 d−1 on the shelf and slope. The residence time of particulate 210Bi was 14 ± 2 d. The 210Bi‐derived export flux of POC was 1.7 ± 0.7 mmol‐C m−2 d−1 out of the euphotic zone over the slope. These results lay the foundation for 210Bi/210Pb to quantify the sinking and remineralization of particulate organic matter in coastal seas.

The 234Th–238U disequilibrium has been widely used to quantify the sinking flux of particulate organic carbon (POC) out of the upper ocean. However, the influence of the advection on the quantification is poorly understood due to the lack of in situ measured physical parameters. Here, a Lagrangian observation was deployed for 39 h to track the variability of 234Th along with the current on the slope of the northeastern South China Sea (SCS). Contrasting to the general ocean interior, 234Th showed deficits relative to 238U in the mesopelagic waters, indicating an enhanced removal of 234Th. Concurrently, elevated total particulate matter (TPM) and POC contents were observed in the mesopelagic waters, supporting the driving force of the cross-shelf dispersion of re-suspended shelf/slope sediments for the 234Th removal. The widely used 234Th-model (ignoring physical processes) produced a much lower sinking flux of POC than the sediment trap-derived POC flux during the same observation, indicating an unneglectable influence of advection and diffusion. By considering the main horizontal advection and vertical diffusion, the 234Th–238U method gave rise to comparable results to sediment trap. 234Th-derived POC fluxes showed an increased pattern from 300 to 1,000 m, consistent with the more abundant POC where. These results indicated that advection represents an unneglectable process during the quantification of the sinking flux of 234Th over the slope of the SCS.

The provenance of black carbon (BC) and its role in affecting contaminant cycling in both the atmosphere and aquatic environments have been extensively studied. However, the fate and cycling dynamics of BC, particularly in marine environments, are poorly understood. Herein, soot BC was determined in the semi-enclosed Jiaozhou Bay to examine the seasonal variability, residence timescale in seawater, and settling flux to sediments, together with particle-reactive 234Th. Soot BC ranged from 0.39 to 4.26 μmol-C L−1. On average, spring produced the highest value of 1.88 ± 0.31 μmol-C L−1, followed by winter (1.59 ± 0.18 μmol-C L−1), summer (0.94 ± 0.09 μmol-C L−1), and autumn (0.90 ± 0.09 μmol-C L−1). The seasonality of soot BC was similar to the activity concentration of particulate 234Th (i.e., 234ThP). The close relationships between soot BC and 234ThP (p < 0.01) provide the basis for the application of 234Th to trace the fate of soot BC. Based on the 234Th deficit with respect to 238U, the residence times of soot BC were estimated to be 41 ± 6 d and 36 ± 5 d for May–August and August–November, respectively. The shorter residence times of soot BC than that of seawater indicated that soot BC delivered to Jiaozhou Bay settled in the local sediments. Furthermore, soot BC concentrations were higher in the inflow seawater from the Yellow Sea than the outflow water from Jiaozhou Bay, implying a net input of soot BC from the Yellow Sea to Jiaozhou Bay. The soot BC fluxes were 0.266 ± 0.035 mmol-C m−2 d−1 and 0.0472 ± 0.0065 mmol-C m−2 d−1 for May–August and August–November, respectively. From the bay-scale perspective, Jiaozhou Bay had buried 0.101 ± 0.010 Gg of soot BC each year. These results indicate that the semi-enclosed Jiaozhou Bay acts as an effective trap for soot BC and particle-reactive contaminants.

Nitrate concentrations and isotopic compositions (δ 15 N‐NO 3 − and δ 18 O‐NO 3 − ) in the Jiulong River Estuary (JRE) were determined in 2014 to better understand the transformation and source contributions of nitrate. In contrast to nitrate concentrations, which generally followed a conservative binary mixing pattern, both δ 15 N‐NO 3 − and δ 18 O‐NO 3 − deviated significantly from the binary isotopic mixing lines during all sampling cruises, indicating a dynamic transformation of nitrate in the JRE. The deviation of nitrate isotopic signatures from conservative mixing lines as well as the slope of the regression between N and O isotopes (δ 15 N‐NO 3 − vs. δ 18 O‐NO 3 − ) had significant seasonal variability, showing the seasonal shifting of the dominant nitrification and sedimentary denitrification in the JRE. Furthermore, the elevated nitrate concentrations and isotopic compositions in the lower JRE during the dry season, along with an isotope regression slope of 1.16, indicated a significant local input of nitrate. Nitrate source apportionment is carried out through the Stable Isotope Analyses in R model with the incorporation of nitrate isotopic fractionation factors for nitrification and denitrification in the JRE. The model results revealed that fertilizer is the largest nitrate source to the JRE, contributing 33%–55% of the nitrate throughout the sampling period. Manure is another major source and contributed 32%–39% of the nitrate to the JRE. Sewage nitrate contributions to the JRE were 7% during April and June but dramatically increased to 18% during the wet season, probably as a result of the rapid flushing of suburban and rural untreated domestic sewage into the river.

The continental shelf is crucial for the global carbon burial. However, quantifying its POC export is challenging due to the complicated hydrologic conditions and sediment resuspension. Here, seasonal variation of Th-234 and its constraint on POC export were examined in Daya Bay, the northern South China Sea (SCS) Shelf. The total Th-234 showed higher activity concentrations in summer (1.44 +/- 0.28 dpm L-1, mean +/- sd) and autumn (1.63 +/- 0.33 dpm L-1) than those in spring (1.19 +/- 0.50 dpm(-1)) and winter (1.10 +/- 0.40 dpm L-1) (t-test, p < 0.05). On average, particulate Th-234 (Th-234(P)) accounted for 48% of Th-234(T) . The significant relationships between the total suspended matter (TSM) and Th-234(P) indicated that TSM dominates the scavenging and sinking of Th-234. The model-results revealed biogenic POC and silica have stronger affinity to Th-234 than lithogenic component, and also represent the main carrier of Th-234 in most cases. Based on the Th-234 deficit, the export fluxes of POC were 11.9 +/- 1.7 mmol-C m(-2) d(-1), 11.4 +/- 1.9 mmol-C m(-2) d(-1), 7.0 +/- 1.2 mmol-C m(-2) d(-1) and 9.9 +/- 1.2 mmol-C m(-2) d(-1) for Jan.-Apr., Apr.-Aug., Aug.-Oct., and Oct.-Jan. respectively. On the annual timescale, the POC export flux accounted for 15 +/- 1% of the primary productivity, higher than the adjacent SCS, highlighting the efficient carbon sequestration in the subtropical Chinese coastal seas.