Export Flux Of Organic Carbon Research Articles

Particulate organic carbon export fluxes across the Seychelles-Chagos thermocline ridge in the western Indian Ocean using 234Th as a tracer

We investigated the export flux of particulate organic carbon (POC) using 234Th as a tracer in the western Indian Ocean along 60°E and 67°E transects in 2017 and 2018. The Seychelles-Chagos Thermocline Ridge (SCTR), where production is relatively high due to nutrient replenishment by upwelling of subsurface water, was observed at 3°S – 12°S in 2017 and 4°S – 13°S both 60°E and 67°E in 2018. POC fluxes in 2017 showed no differences between the SCTR and non-SCTR regions. However, in 2018, the POC fluxes in the SCTR regions (8.52 ± 7.89 mmol C m–2 d–1) were one order of magnitude higher than those observed in the non-SCTR regions (0.63 ± 0.07 mmol C m–2 d–1), which appeared to be related to the strong upwelling of subsurface water. These POC fluxes were comparable to those observed under bloom conditions, and thus, are important for estimating the efficiency of carbon sequestration in the ocean.

Particulate organic carbon export fluxes estimates by 234Th[sbnd]238U disequilibrium in the oxygen minimum zone off the Peruvian coast

The Peruvian upwelling zone is one of the most productive marine ecosystems in the world with a spectacular, pronounced oxygen minimum zone (OMZ). Globally OMZs are increasing in size and intensity with far-reaching consequences for the marine biological carbon pump and carbon export; thus, these zones need to be carefully monitored to be able to understand future climate change impacts. The current study was carried out in 2013 and 2017 to quantify the vertical flux of organic matter exported out of the productive surface layer by measuring 234Th238U disequilibria in the water column. Samples were collected in January 2013 and May 2017 along an identical transect located at 12°S off the Peruvian coast near Lima, Peru. Th-234 fluxes ranged from 0 to 2088 ± 95 dpm m−2 d−1 in 2013 and 698 ± 63 to 3648 ± 113 dpm m−2 d−1 in 2017. The corresponding POC fluxes varied between 0 and 164.2 ± 7.9 mg C m−2 d−1 in 2013 and 22.7 ± 2.7 to 133.1 ± 15.2 mg C m−2 d−1 in 2017, with POC fluxes gradually decreasing with distance from the coast. Despite higher POC fluxes, the export efficiencies were found to be extremely low due to high particle remineralization rates observed within the euphotic zone.

210Po-210Pb Disequilibrium in the Western North Pacific Ocean: Particle Cycling and POC Export

Estimating the particulate organic carbon (POC) export flux from the upper ocean is fundamental for understanding the efficiency of the biological carbon pump driven by sinking particles in the oceans. The downward POC flux from the surface ocean based on 210Po-210Pb disequilibria in seawater samples from the western North Pacific Ocean (w-NPO) was measured in the early summer (May-June) of 2018. All the profiles showed a large 210Po deficiency relative to 210Pb in the euphotic zone (0–150 m), while this 210Po deficiency vanished below ∼500 m (with 210Po/210Pb ∼1 or > 1). A one-dimensional steady-state irreversible scavenging model was used to quantify the scavenging and removal fluxes of 210Po and 210Pb in the euphotic zone of the w-NPO. In the upper ocean (0–150 m), dissolved 210Po (D-Po) was scavenged into particles with a residence time of 0.6–5.5 year, and the 210Po export flux out of the euphotic zone was estimated as (0.33–3.49) × 104 dpm/m2/year, resulting in a wide range of particulate 210Po (P-Po) residence times (83–921 days). However, in the deep ocean (150–1,000 m), 210Po was transferred from the particulate phase to the dissolved phase. Using an integrated POC inventory and the P-Po residence times (Eppley model) in the w-NPO euphotic zone, the POC export fluxes (mmol C/m2/d) varied from 0.6 ± 0.2 to 8.8 ± 0.4. In comparison, applying the POC/210Po ratio of all (>0.45 μm) particles to 210Po export flux (Buesseler model), the obtained POC export fluxes (mmol C/m2/d) ranged from 0.7 ± 0.1 to 8.6 ± 0.8. Both Buesseler and Eppley methods showed enhanced POC export fluxes at stations near the continental shelf (i.e., Luzon Strait and the Oyashio-Kuroshio mixing region). The Eppley model-based 210Po-derived POC fluxes agreed well with the Buesseler model-based fluxes, indicating that both models are suitable for assessing POC fluxes in the w-NPO. The POC export efficiency was < 15%, suggesting a moderate biological carbon pump efficiency in the w-NPO. These low export efficiencies may be associated with the dominance of smaller particles and the processes of degradation and subsequent remineralization of these small particles in the euphotic zone of oligotrophic regions in the w-NPO.

Hydrothermal plumes as hotspots for deep-ocean heterotrophic microbial biomass production

Carbon budgets of hydrothermal plumes result from the balance between carbon sinks through plume chemoautotrophic processes and carbon release via microbial respiration. However, the lack of comprehensive analysis of the metabolic processes and biomass production rates hinders an accurate estimate of their contribution to the deep ocean carbon cycle. Here, we use a biogeochemical model to estimate the autotrophic and heterotrophic production rates of microbial communities in hydrothermal plumes and validate it with in situ data. We show how substrate limitation might prevent net chemolithoautotrophic production in hydrothermal plumes. Elevated prokaryotic heterotrophic production rates (up to 0.9 gCm−2y−1) compared to the surrounding seawater could lead to 0.05 GtCy−1 of C-biomass produced through chemoorganotrophy within hydrothermal plumes, similar to the Particulate Organic Carbon (POC) export fluxes reported in the deep ocean. We conclude that hydrothermal plumes must be accounted for as significant deep sources of POC in ocean carbon budgets.

Apparent oxygen utilization rates based on tritium-helium dating in the South China Sea: Implications for export production

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.

Oxygen Isotope Offsets in Deep-Water Benthic Foraminifera

ABSTRACT Despite the extensive use of the benthic foraminiferal oxygen isotope composition (δ18O) as a proxy for paleoclimatic reconstructions, uncertainties remain regarding the consistency of interspecies offsets and the environmental factors controlling 18O fractionation. We investigated δ18O offsets of some frequently used Uvigerina, Bulimina, and Cibicidoides species in core top samples from different hydrographic and sedimentary regimes in the South China Sea, Makassar Strait, and Timor Strait/Eastern Indian Ocean. The δ18O values of the epifaunal taxa Cibicidoides mundulus and Cibicidoides wuellerstorfi showed no significant offset in all investigated regions, whereas shallow infaunal Cibicidoides species exhibited higher variability and were less reliable. We found no offsets between species of Uvigerina and Bulimina and assume that these genera can be measured together and/or substituted. Our results show that epifaunal taxa are close to equilibrium with ambient seawater and thus provide more reliable records of past ice volume and/or bottom water temperature variations than infaunal taxa. Offsets among equilibrium calcite, epifaunal taxa, and infaunal taxa are not constant “vital effects” but are influenced by changing gradients in bottom to pore water pH and carbonate ion concentrations that depend on deep-water ventilation and export flux of particulate carbonate and organic carbon. Offsets between epifaunal and infaunal taxa varied between 0.58 and 0.73‰, depending on regional bottom and pore water conditions. Our findings highlight the importance of regional and temporal variations in organic carbon flux/degradation and dissolution of calcite that may lead to slight under- or overestimates of the amplitude of δ18O fluctuations, especially during times of rapidly changing calcite-saturation of bottom and pore water.

Southwest monsoon-driven changes in the phytoplankton community structure in the central Arabian Sea (2017–2018): After two decades of JGOFS

The Arabian Sea, a land-locked low latitude high productive oceanic province, is characterized by strong seasonality due to the reversal of monsoon winds. “Open ocean upwelling” occurs during the southwest monsoon (SWM) in the northern part of the Findlater Jet, (a low-level atmospheric jet that forms in the Arabian Sea), and consequently, the southern part experiences downwelling. Moreover, the hydrography of this region can be further modulated by coupled ocean-atmospheric events such as ENSO and IOD which eventually lead to changes in biological production. After two decades of joint global ocean flux studies (JGOFS), this is the first report on the phytoplankton community from the central Arabian Sea during the SWM. We tried to interlink the north-south gradient in hydrography as well as the physicochemical features with the phytoplankton community structure (within 60 m) along 64° E (11–21° N) during two consecutive SWM cruises (August 2017 and 2018). The variability in surface wind forcing directly influenced the nutrient stoichiometry and phytoplankton dynamics, through the changes in open ocean upwelling and mixing. Significant spatial and inter-annual variability in phytoplankton abundance, diversity, ratios of diatom: dinoflagellate and centric: pennate diatoms were noticed and were closely coupled with the nutrient availability (bottom-up control). Relatively low phytoplankton cell density in 2017 was coupled with high zooplankton biomass (top-down control). During 2018, higher dissolved silicate (DSi) supply due to stronger open-ocean upwelling along with the advection of upwelled water from the Somali coast likely promoted larger diatoms capable of escaping grazing and have led to higher export production. The large centric diatoms occupied the upwelled regions (21–20° N) (e. g. Rhizosolenia imbricata, R. hebetata and Coscinodiscus radiatus) whereas, small pennate diatoms (Pseudo-nitzschia spp., Navicula spp. Nitzschia spp.) and dinoflagellates (Gymnodinium spp.) were pronounced in the south (11–15° N). And while comparing the trends with the JGOFS reports, no considerable community shift was noticed. However, in the context of ongoing climate change, the rapid warming of the Arabian Sea may potentially influence the hydrography and biogeochemistry of this region and thus may impact phytoplankton community structure, trophic transfer, and organic carbon export flux.

Molecular Level Characterization of Diatom and Coccolithophore-Associated Biopolymers That Are Binding 210Pb and 210Po in Seawater

Through a combination of selective extractions and molecular characterization techniques including Isoelectric Focusing Chromatography and Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass spectrometry, molecular structures of diatom (Phaeodactylum tricornutum) and coccolithophore (Emiliania huxleyi)-associated biopolymers that are responsible for the distinct partitioning behavior between 210Pb and 210Po were determined. Our results show that diatom-derived biopolymers have distinctive elemental grouping distributions as compared to those excreted by the coccolithophore, with the former consisting of more heterogeneous elements (i.e., nitrogen, sulfur and phosphorus-containing organic compounds). For the coccolithophore culture, two 210Pb-enriched biopolymers (non-attached exopolymeric substances and coccosphere shell-associated biopolymers) have a higher abundance of CHO-type compounds, suggesting CHO-only-type compounds as the main binding moieties for 210Pb. In contrast, such association was not evident in the diatom culture. Different with 210Pb, 210Po enrichment in coccolithophore-derived attached exopolymeric substances and Fe-Mn-associated metabolites coincided with the higher abundance of nitrogen/sulfur-containing organic compounds in these two biopolymer fractions, suggesting the strong parallel of Po with the production of nitrogen-rich organic matter as well as sulfur-containing amino acids. These different associations between 210Pb/210Po and organic functional groups were further explored by separating 210Pb or 210Po-labeled coccolithophore-derived biopolymers via isoelectric focusing. This technique suggests that phosphate group-containing molecules but not the other molecules that contain heterogeneous elements (e.g., CHONS, CHON, and CHOS) as the strongest binding agents for 210Pb, while the more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties acted as the main 210Po-binding ligands. It is concluded that the deficiency of 210Po with respect to 210Pb can be influenced by the relative abundance of nitrogen/sulfur-enriched organic moieties to the nitrogen/sulfur-depleted organic compounds in the water column. This behavior constrains the application of 210Po-210Pb approach to quantify the particulate organic carbon (POC) export flux in the ocean. It also explains that differences in chemical binding of the 210Po as compared to those of other radionuclides (e.g., thorium-234) as the main factor. That suggests that differences in decay half-lives or physical factors are less important when these nuclides are applied to estimate the POC flux in the ocean.

Distribution, associations and role in the biological carbon pump of Pyrosoma atlanticum (Tunicata, Thaliacea) off Cabo Verde, NE Atlantic

Gelatinous zooplankton are increasingly acknowledged to contribute significantly to the carbon cycle worldwide, yet many taxa within this diverse group remain poorly studied. Here, we investigate the pelagic tunicate Pyrosoma atlanticum in the waters surrounding the Cabo Verde Archipelago. By using a combination of pelagic and benthic in situ observations, sampling, and molecular genetic analyses (barcoding, eDNA), we reveal that: P. atlanticum abundance is most likely driven by local island-induced productivity, that it substantially contributes to the organic carbon export flux and is part of a diverse range of biological interactions. Downward migrating pyrosomes actively transported an estimated 13% of their fecal pellets below the mixed layer, equaling a carbon flux of 1.96–64.55 mg C m−2 day−1. We show that analysis of eDNA can detect pyrosome material beyond their migration range, suggesting that pyrosomes have ecological impacts below the upper water column. Moribund P. atlanticum colonies contributed an average of 15.09 ± 17.89 (s.d.) mg C m−2 to the carbon flux reaching the island benthic slopes. Our pelagic in situ observations further show that P. atlanticum formed an abundant substrate in the water column (reaching up to 0.28 m2 substrate area per m2), with animals using pyrosomes for settlement, as a shelter and/or a food source. In total, twelve taxa from four phyla were observed to interact with pyrosomes in the midwater and on the benthos.

Impact of Atmospheric Deposition on Carbon Export to the Deep Ocean in the Subtropical Northwest Pacific

AbstractThe impact of atmospheric deposition on deep‐ocean carbon export in the subtropical Northwest Pacific remains poorly evaluated. Using sediment trap data and a newly improved biogeochemical model, we show that iron deposition in winter and spring and nitrogen deposition in summer and fall are important drivers for the seasonal variability of deep‐ocean particulate organic carbon (POC) export flux. Nitrogen deposition can stimulate pico‐plankton growth in summer and fall, which leads to increases of microzooplankton and mesozooplankton. The increase of mesozooplankton exerts higher grazing pressure on diatoms in winter and early spring, which then reduces deep‐ocean POC export due to the reduction of ballasting mineral of opal. Iron deposition only affects the region in winter and spring when nitrogen is not a limiting factor for phytoplankton growth; and it can increase deep‐ocean POC export by stimulating diatom growth and opal production.

Carbon Export in the Seasonal Sea Ice Zone North of Svalbard From Winter to Late Summer

Phytoplankton blooms in the Arctic Ocean's seasonal sea ice zone are expected to start earlier and occur further north with retreating and thinning sea ice cover. The current study is the first compilation of phytoplankton bloom development and fate in the seasonally variable sea ice zone north of Svalbard from winter to late summer, using short-term sediment trap deployments. Clear seasonal patterns were discovered, with low winter and pre-bloom phytoplankton standing stocks and export fluxes, a short and intense productive season in May and June, and low Chl a standing stocks but moderate carbon export fluxes in the autumn post-bloom conditions. We observed intense phytoplankton blooms with Chl a standing stocks of >350 mg m−2 below consolidated sea ice cover, dominated by the prymnesiophyte Phaeocystis pouchetii. The largest vertical organic carbon export fluxes to 100 m, of up to 513 mg C m−2 day−1, were recorded at stations dominated by diatoms, while those dominated by P. pouchetii recorded carbon export fluxes up to 310 mg C m−2 day−1. Fecal pellets from krill and copepods contributed a substantial fraction to carbon export in certain areas, especially where blooms of P. pouchetii dominated and Atlantic water advection was prominent. The interplay between the taxonomic composition of protist assemblages, large grazers, distance to open water, and Atlantic water advection was found to be crucial in determining the fate of the blooms and the magnitude of organic carbon exported out of the surface water column. Previously, the marginal ice zone was considered the most productive region in the area, but our study reveals intense blooms and high export events in ice-covered waters. This is the first comprehensive study on carbon export fluxes for under-ice phytoplankton blooms, a phenomenon suggested to have increased in importance under the new Arctic sea ice regime.

Southern Ocean carbon export efficiency in relation to temperature and primary productivity

Satellite remote sensing and numerical models are widely used to estimate large-scale variations in ocean carbon export, but the relationship between export efficiency (e-ratio) of sinking organic carbon out of the surface ocean and its drivers remains poorly understood, especially in the Southern Ocean. Here, we assess the effects of temperature and primary productivity on e-ratio by combining particulate organic carbon export flux from in situ measurements during 1997–2013, environmental parameters from satellite products, and outputs from ocean biogeochemical models in the Southern Ocean. Results show that “High Productivity Low E-ratio” (HPLE) is a common phenomenon in the Subantarctic Zone and the Polar Frontal Zone, but not the Antarctic Zone. The e-ratio shows little dependence on temperature below 6 °C. Our results support the hypothesis that the HPLE phenomenon is due to the large contribution of non-sinking organic carbon. Both temperature and ballast minerals play less important roles in controlling e-ratio than ecosystem structure at low temperatures. These findings suggest that non-sinking organic carbon, ecosystem structure, and region-specific parameterizations of e-ratio are key factors to quantify the carbon export in the Southern Ocean.

Analysis of 210Po, 210Bi, and 210Pb in atmospheric and oceanic samples by simultaneously auto-plating 210Po and 210Bi onto a nickel disc

210Po and 210Pb are commonly measured to study particle cycling and particulate organic carbon export (POC) flux from the upper ocean. 210Bi is a potential oceanographic tracer. However, no convenient and rapid analytical method for 210Bi has been developed in the marine environment due to its short half-life (5 days). The aims are to study factors influencing the simultaneous auto-plating of 210Po and 210Bi onto nickel disc and to develop an effective ship-board procedure for the rapid measurement of 210Po, 210Bi, and 210 Pb at sea. The results suggest that the optimal conditions for auto-deposition of 210Po and 210Bi were achieved by using a 25 mm diameter nickel disc in 60 mL acidic solution (pH = 0.5) for 16 h at 25 ± 1 °C. By adding 209Po and 207Bi as yield tracers to the sediments, recoveries of Po and Bi were >90%. If 210Po or 210Bi were in equilibrium with 210Pb, 210Po and 210Bi could also be potential proxies for 210Pb in sedimentary chronology. The overall recoveries of 209Po and 207Bi were >70% for rainwaters and >60% for seawater samples, respectively, which indicates this analytical procedure is also applicable to marine and atmospheric environments. This method allows the rapid measurement of 210Bi on a ship and help to obtain its vertical profile or spatial distribution in the marine environment within 1–2 days.

Understanding the remote influences of ocean weather on the episodic pulses of particulate organic carbon flux

The biological carbon pump has been estimated to export ~5–15 Gt C yr−1 into the deep ocean, and forms the principal deep-sea food resource. Irregular, intense pulses of particulate organic carbon (POC) have been found to make up about one-third of the overall POC fluxes at a long-term deep-sea research station influenced by coastal upwelling of the California Current, Station M (34°50′N, 123° W, 4000 m depth). However, the drivers of these pulses have been challenging to quantify. It has long been recognized that ocean currents can result in particles being advected while sinking to the point of collection by a sediment trap. Thus, a sediment trap time series can incorporate material from a dynamic catchment area, a concept sometimes referred to as a statistical funnel. This concept raises many questions including: what are the day-to-day conditions at the source locations of the sinking POC? And, how might such ‘ocean weather’ and related ecosystem factors influence the intense variation seen at the seafloor? Here we analyzed three-dimensional ocean currents from a Regional Ocean Modeling System (ROMS) model from 2011 to 2017 to trace the potential source locations of particles sinking at 1000, 100, and 50 m d−1 from an export depth of 100 m. We then used regionally tailored satellite data products to estimate export flux of POC from these locations. For the 100 m d−1 speed, the particles had origins of up to ~300 km horizontal distance from the sediment trap location, moored at Station M at 3400 m depth., and nearly 1000 km for the 50 m d−1 speed. Particle tracking indicated that, there was considerable inter-annual variation in source locations. Particle source locations tended to originate from the east in the summer months, with higher export and POC fluxes. Occasionally these locations were in the vicinity of highly productive ocean features nearer to the coast. We found significant correlations between export flux of organic carbon from the estimated source locations at 100 m depth to trap-estimated POC fluxes at 3400 m depth. These results set the stage for further investigation into sinking speed distributions, conditions at the source locations, and comparisons with mechanistic biogeochemical models and between particle tracking model frameworks.

Seasonal variability of the carbon export in the central South China Sea

The South China Sea (SCS) is strongly influenced by the East Asian monsoon system with seasonal reversal. Measurements from a 7-year continuous sediment trap located in the central SCS showed a clear seasonal pattern. The particulate organic carbon (POC) export flux at the depth of 1200 m was considerably higher in monsoon seasons. The driving dynamics leading to this seasonal variability of POC export, however, remains inadequately understood. Here, a one-dimensional physical-biogeochemical coupled model was developed to simulate the temporal variability of a lower-trophic planktonic ecosystem. The modeled POC export flux compared reasonably well with the 7-year time series from moored sediment trap. Model results showed that the POC export flux at 1200 m is highly correlated with the 0–100 m integrated primary productivity and with the export flux at 100 m, implying that the seasonal variability of sediment trap data could be induced by changes in phytoplankton production and its vertical export. Further model analysis suggested that the annual mean export ratio (e-ratio) at 100 m and transfer efficiency at 1200 m in the central SCS were 0.19 and 0.07, respectively, which are lower than those in high latitudes. The winter monsoon favors not only surface carbon fixation but also export to the deep ocean. The heat flux is the dominant factor regulating the seasonal cycle of mixed layer depth, nutrient supply, and the growth of phytoplankton in this region. The wind-driven mixing can further facilitate upward nutrient transport to the surface and amplify the seasonal amplitude of the POC export flux.

High variability of particulate organic carbon export along the North Atlantic GEOTRACES section GA01 as deduced from <sup>234</sup>Th fluxes

Abstract. In this study we report particulate organic carbon (POC) export fluxes for different biogeochemical basins in the North Atlantic as part of the GEOTRACES GA01 expedition (GEOVIDE, May–June 2014). Surface POC export fluxes were deduced by combining export fluxes of total Thorium-234 (234Th) with the ratio of POC to 234Th of sinking particles at the depth of export. Particles were collected in two size classes (>53 and 1–53 µm) using in situ pumps and the large size fraction was considered representative of sinking material. Surface POC export fluxes revealed latitudinal variations between provinces, ranging from 1.4 mmol m−2 d−1 in the Irminger basin, where the bloom was close to its maximum, to 12 mmol m−2 d−1 near the Iberian Margin, where the bloom had already declined. In addition to the state of progress of the bloom, variations of the POC export fluxes were also related to the phytoplankton size and community structure. In line with previous studies, the presence of coccolithophorids and diatoms appeared to enhance the POC export flux, while the dominance of picophytoplankton cells, such as cyanobacteria, resulted in lower fluxes. The ratio of POC export to primary production (PP) strongly varied regionally and was generally low (≤14 %), except at two stations located near the Iberian Margin (35 %) and within the Labrador basin (38 %), which were characterized by unusual low in situ PP. We thus conclude that during the GEOVIDE cruise, the North Atlantic was not as efficient in exporting carbon from the surface, as reported earlier by others. Finally, we also estimated the POC export at 100 m below the surface export depth to investigate the POC transfer efficiencies. This parameter was also highly variable amongst regions, with the highest transfer efficiency at sites where coccolithophorids dominated.

Satellite‐Based Estimation of Particulate Organic Carbon Export in the Northern South China Sea

AbstractKnowledge of particulate organic carbon export flux (EP) is the key to understanding the marine biological carbon pump. In this study, a satellite remote sensing algorithm based on the food‐web model established by Siegel et al. (2014; https://doi.org/10.1002/2013GB004743) was used to estimate EP in the northern South China Sea (NSCS), which consists of direct sinking flux of large phytoplankton and associated aggregates by gravity and flux of zooplankton feces by grazing. This is the first time that fine spatiotemporal variation of satellite‐derived EP in the NSCS is unveiled. Compared with the results on 1° spatial scale products, the model results based on the satellite products with 1/3° and 1/12° resolutions showed better consistency with the observations. Validation with in situ EP showed that the model exhibited a good performance in the NSCS basin, but the predicted EP in the shelf regions were smaller compared with measurements. The satellite‐derived annual‐mean EP in the shelf areas was 8.47 and 5.56 mmol C·m−2·d−1 in the NSCS basin, on the 1/3° spatial scale, with relative differences from observations were about −50% and −15% for the shelf and basin, respectively. The flux of feces from zooplankton grazing, especially that from microzooplankton grazing, might account for the major fraction of the EP in the NSCS. Accurate products of ocean net primary production and empirical coefficients in the food web (e.g., export efficiency of zooplankton grazing and specific mortality rate of nongrazing phytoplankton) suitable for the NSCS, are essential for performance improvement of the satellite‐based EP model.

Carbon fluxes in the China Seas: An overview and perspective

This paper aims to provide an overview of regional carbon fluxes and budgets in the marginal seas adjacent to China. The “China Seas” includes primarily the South China Sea, East China Sea, Yellow Sea, and the Bohai Sea. Emphasis is given to CO2 fluxes across the air-sea interface and their controls. The net flux of CO2 degassing from the China Seas is estimated to be 9.5±53 Tg C yr−1 . The total riverine carbon flux through estuaries to the China Seas is estimated as 59.6±6.4 Tg C yr−1 . Chinese estuaries annually emit 0.74±0.02 Tg C as CO2 to the atmosphere. Additionally, there is a very large net carbon influx from the Western Pacific to the China Seas, amounting to ~2.5 Pg C yr−1 . As a first-order estimate, the total export flux of particulate organic carbon from the upper ocean of the China Seas is 240±80 Tg C yr−1 . This review also attempts to examine current knowledge gaps to promote a better understanding of the carbon cycle in this important region.

Simulated effects of interactions between ocean acidification, marine organism calcification, and organic carbon export on ocean carbon and oxygen cycles

Ocean acidification caused by oceanic uptake of anthropogenic carbon dioxide (CO2) tends to suppress the calcification of some marine organisms. This reduced calcification then enhances surface ocean alkalinity and increases oceanic CO2 uptake, a process that is termed calcification feedback. On the other hand, decreased calcification also reduces the export flux of calcium carbonate (CaCO3), potentially reducing CaCO3-bound organic carbon export flux and CO2 uptake, a process that is termed ballast feedback. In this study, we incorporate a range of different parameterizations of the links between organic carbon export, calcification, and ocean acidification into an Earth system model, in order to quantify the long-term effects on oceanic CO2 uptake that result from calcification and ballast feedbacks. We utilize an intensive CO2 emission scenario to drive the model in which an estimated fossil fuel resource of 5000 Pg C is burnt out over the course of just a few centuries. Simulated results show that, in the absence of both calcification and ballast feedbacks, by year 3500, accumulated oceanic CO2 uptake is 2041 Pg C . Inclusion of calcification feedback alone increases the simulated uptake by 629 Pg C (31%), while the inclusion of both calcification and ballast feedbacks increase simulated uptake by 449–498 Pg C (22–24%), depending on the parameter values used in the ballast feedback scheme. These results indicate that ballast effect counteracts calcification effect in oceanic CO2 uptake. Ballast effect causes more organic carbon to accumulate and decompose in the upper ocean, which in turn leads to decreased oxygen concentration in the upper ocean and increased oxygen at depths. By year 2600, the inclusion of ballast effect would decrease oxygen concentration by 11% at depth of ca. 200 m in tropics. Our study highlights the potentially critical effects of interactions between ocean acidification, marine organism calcification, and CaCO3-bound organic carbon export on the ocean carbon and oxygen cycles.

234Th‐Based Carbon Export Flux Along the Indian GEOTRACES GI02 Section in the Arabian Sea and the Indian Ocean

Abstract234Th (t1/2 = 24.1 d), present in seawater, is a naturally occurring particle‐reactive radionuclide formed through the radioactive decay of its parent, 238U (t1/2 = 4.47 × 109 years). The 234Th:238U disequilibrium is exploited to quantify fluxes of elements moving out of the euphotic zone by attaching on to sinking particles. Under the Indian GEOTRACES programme, high‐resolution sampling in the upper 300 m depth was carried out at 11 stations in the Arabian Sea and the Indian Ocean during April–May 2014 from 17°N to 16°S along 65°E transect to estimate the 234Th‐based particulate organic carbon (POC) export flux from the upper ocean. Average 234Th fluxes integrated to 100 m depth were 2,612 and 1,968 dpm m−2 d−1 for the Arabian Sea and the Indian Ocean, respectively. The estimated POC export fluxes ranged from negligible to 9.0 mmol m−2 d−1, and the 234Th‐based POC export efficiencies were <2 to 5%. For the same season, the Arabian Sea and the Bay of Bengal showed highly contrasting carbon export trends (mean: 4.0 and 0.8 mmol C m−2 d−1, respectively). The modeled POC export fluxes from in situ and satellite‐derived primary production are higher than the 234Th‐based values for the Laws and Dunne models and are comparable for the Henson model. The modeled POC fluxes that depend on surface temperature and primary production could be further refined for the seasonal cycle in biological productivity and associated differences in trophic structure, grazing intensity, recycling efficiency, high bacterial activity, and associated dissolved organic carbon (DOC) export.