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Through the constant rain of sinking marine particles in the ocean, carbon (C) trapped within is exported into the water column and sequestered when reaching depths below the mesopelagic zone. Atmospheric CO2 levels are thereby strongly related to the magnitude of carbon export fluxes in the mesopelagic zone. Sinking particles represent the main source of carbon and energy for mesopelagic organisms, attenuating the C export flux along the water column. Attempts to quantify the amount of C exported versus C consumed by heterotrophic organisms have increased in recent decades. Yet, most of the conducted estimations have led to estimated C demands several times higher than the measured C export fluxes. The choice of parameters such as growth efficiencies or various conversion factors is known to greatly impact the resulting C budget. In parallel, field or experimental data are sorely lacking to obtain accurate values of these crucial overlooked parameters. In this study, we identify the most influential of these parameters and perform inversion of a mechanistic model. Further, we determine the optimal parameter values as the ones that best explain the observed prokaryotic respiration, prokaryotic production and zooplankton respiration. The consistency of the resulting C budget suggests that such budgets can be adequately balanced when using appropriate parameters.

Together with T–S properties, particle abundance in situ measurements are useful to discriminate water masses and derive circulation patterns. In the upper layers of the Ionian Sea, the fresher Atlantic Waters (AW) recently crossing the Sicily Channel meet the resident and saltier AW, which circulated cyclonically in the eastern basin and modified after evaporation and eventually cooling. In May 2017, during the PEACETIME cruise, fluorescence and particle abundance sampled at high resolution revealed unexpected heterogeneity in the central Ionian Sea. Surface salinity measurements, together with altimetry-derived and hull-mounted acoustic Doppler current profiler (ADCP) currents, describe a zonal pathway of AW entering the Ionian Sea, consistent with the so-called cyclonic mode in the North Ionian Gyre. The ION-Tr transect, located between 19–20∘ E at approximately 36∘ N, turned out to be at the crossroads of three water masses, mostly coming from the west, north and an isolated anticyclonic eddy northeast of ION-Tr. Using Lagrangian numerical simulations, we suggest that the contrast in particle loads along ION-Tr originates from particles transported from these three different water masses. Waters from the west, identified as AW carried by a strong southwestward jet, were moderate in particle load, probably originating from the Sicily Channel. The water mass from the north, carrying abundant particles, probably originated in the northern Ionian Sea, or further away from the south Adriatic Sea. Waters from the eddy, depleted in particles and chl a, may originate from south of the Peloponnese, where the Pelops eddy forms. The central Ionian Sea hence appears as a mosaic area, where waters of contrasted biological history meet. This contrast is particularly clear in spring, when blooming and non-blooming areas co-occur. Interpreting the complex dynamics of physical–biogeochemical coupling from discrete measurements made at isolated stations at sea is a challenge. The combination of multiparametric in situ measurements at high resolution with remote sensing and Lagrangian modeling appears as one adequate way to address this challenge.

We report on the sub-basin variability in particulate organic carbon (POC) remineralization in the western and central Mediterranean Sea in late spring during the PEACETIME (ProcEss studies at the Air–sEa Interface after dust deposition in the MEditerranean sea) cruise. POC remineralization rates were estimated using the excess biogenic particulate barium (Baxs) inventories in the mesopelagic layers (100–1000 m depth) and compared with prokaryotic heterotrophic production (PHP). Baxs-based mesopelagic remineralization rates (MRs) ranged from 25±2 to 306±70 mgCm-2d-1. MRs were larger in the Algero-Provençal (ALG) Basin than in the Tyrrhenian (TYR) and Ionian (ION) basins. Our Baxs inventories and integrated PHP data also indicated that significant mesopelagic remineralization occurred down to 1000 m depth in the ALG Basin in contrast to the ION and TYR basins, where remineralization was mainly located above 500 m depth. We propose that the higher and deeper MRs in the ALG Basin were sustained by an additional particle export event driven by deep convection. The TYR Basin (in contrast to the ALG and ION basins) presented the impact of a previous dust event, as reflected by our particulate Al water column concentrations. The ION and TYR basins showed small-scale heterogeneity in remineralization processes, reflected by our Baxs inventories and integrated PHP data at the Tyrr long-duration station. This heterogeneity was linked to the mosaic of blooming and non-blooming patches reported in this area during the cruise. In contrast to the western Mediterranean Sea (ALG Basin), the central Mediterranean Sea (ION and TYR basins) showed lower remineralization rates restricted to the upper mesopelagic layer during the late spring PEACETIME cruise.

In the ocean, remineralization rate associated with sinking particles is a crucial variable. Since the 1990s, particulate biogenic barium (Baxs) has been used as an indicator of carbon remineralization by applying a transfer function relating Baxs to O2 consumption (Dehairs's transfer function, Southern Ocean-based). Here, we tested its validity in the Mediterranean Sea (ANTARES/EMSO-LO) for the first time by investigating connections between Baxs, prokaryotic heterotrophic production (PHP) and oxygen consumption (JO2-Opt; optodes measurement). We show that (1) higher Baxs (409 pM; 100–500 m) occurs in situations where integrated PHP (PHP100/500=0.90) is located deeper, (2) higher Baxs occurs with increasing JO2-Opt, and (3) there is similar magnitude between JO2-Opt (3.14 mmol m−2 d−1; 175–450 m) and JO2-Ba (4.59 mmol m−2 d−1; transfer function). Overall, Baxs, PHP and JO2 relationships follow trends observed earlier in the Southern Ocean. We conclude that such a transfer function could apply in the Mediterranean Sea.

The eastern boundary region of the southeastern Pacific Ocean hosts one of the world's most dynamic and productive upwelling systems with an associated oxygen minimum zone (OMZ). The variability in downward export fluxes in this region, with strongly varying surface productivity, upwelling intensities and water column oxygen content, is however poorly understood. Thorium-234 (234Th) is a powerful tracer to study the dynamics of export fluxes of carbon and other elements, yet intense advection and diffusion in nearshore environments impact the assessment of depth-integrated 234Th fluxes when not properly evaluated. Here we use vessel-mounted acoustic Doppler current profiler (VmADCP) current velocities, satellite wind speed and in situ microstructure measurements to determine the magnitude of advective and diffusive fluxes over the entire 234Th flux budget at 25 stations from 11 to 16∘ S in the Peruvian OMZ. Contrary to findings along the GEOTRACES P16 eastern section, our results showed that weak surface wind speed during our cruises induced low upwelling rates and minimal upwelled 234Th fluxes, whereas vertical diffusive 234Th fluxes were important only at a few shallow shelf stations. Horizontal advective and diffusive 234Th fluxes were negligible because of small alongshore 234Th gradients. Our data indicated a poor correlation between seawater 238U activity and salinity. Assuming a linear relationship between the two would lead to significant underestimations of the total 234Th flux by up to 40 % in our study. Proper evaluation of both physical transport and variability in 238U activity is thus crucial in coastal 234Th flux studies. Finally, we showed large temporal variations on 234Th residence times across the Peruvian upwelling zone and cautioned future carbon export studies to take these temporal variabilities into consideration while evaluating carbon export efficiency.

The eastern tropical South Pacific (ETSP) represents one of the most productive areas in the ocean that is characterised by a pronounced oxygen minimum zone (OMZ). Particulate organic matter (POM) that sinks out of the euphotic zone is supplied to the anoxic sediments and utilised by microbial communities, and the degradation of POM is associated with the production and reworking of dissolved organic matter (DOM). The release of DOM to the overlying waters may, therefore, represent an important organic matter escape mechanism from remineralisation within sediments but has received little attention in OMZ regions so far. Here, we combine measurements of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) with DOM optical properties in the form of chromophoric (CDOM) and fluorescent (FDOM) DOM from pore waters and near-bottom waters of the ETSP off Peru. We evaluate diffusion-driven fluxes and net in situ fluxes of DOC and DON to investigate processes affecting DOM cycling at the sediment–water interface along a transect at 12∘ S. To our knowledge, these are the first data for sediment release of DON and pore water CDOM and FDOM for the ETSP off Peru. Pore water DOC accumulated with increasing sediment depth, suggesting an imbalance between DOM production and remineralisation within sediments. High DON accumulation resulted in very low pore water DOC ∕ DON ratios (≤1) which could be caused by an “uncoupling” in DOC and DON remineralisation. Diffusion-driven fluxes of DOC and DON exhibited high spatial variability and ranged from 0.2±0.1 to 2.5±1.3 mmolm-2d-1 and from -0.04±0.02 to 3.3±1.7 mmolm-2d-1, respectively. Generally low net in situ DOC and DON fluxes, as well as a steepening of spectral inclination (S) of CDOM and an increase in humic-like DOM at the sediment–water interface over time, indicated active microbial DOM utilisation. The latter may potentially be stimulated by the presence of nitrate (NO3-) and nitrite (NO2-) in the water column. The microbial DOC utilisation rates, estimated in our study, are potentially sufficient to support denitrification rates of 0.2–1.4 mmolm-2d-1, suggesting that the sediment release of DOM may on occasion contribute to nitrogen loss processes in the ETSP off Peru.

The ocean's biological carbon pump (BCP) plays a major role in the global carbon cycle. A fraction of the photosynthetically fixed organic carbon produced in surface waters is exported below the sunlit layer as settling particles (e.g., marine snow). Since the seminal works on the BCP, global estimates of the global strength of the BCP have improved but large uncertainties remain (from 5 to 20 Gt C yr−1 exported below the euphotic zone or mixed-layer depth). The 234Th technique is widely used to measure the downward export of particulate organic carbon (POC). This technique has the advantage of allowing a downward flux to be determined by integrating the deficit of 234Th in the upper water column and coupling it to the POC∕234Th ratio in sinking particles. However, the factors controlling the regional, temporal, and depth variations of POC∕234Th ratios are poorly understood. We present a database of 9318 measurements of the POC∕234Th ratio in the ocean, from the surface down to >5500 m, sampled on three size fractions (∼>0.7 µm, ∼1–50 µm, ∼>50 µm), collected with in situ pumps and bottles, and also from bulk particles collected with sediment traps. The dataset is archived in the data repository PANGAEA® under https://doi.org/10.1594/PANGAEA.911424 (Puigcorbé, 2019). The samples presented in this dataset were collected between 1989 and 2018, and the data have been obtained from published papers and open datasets available online. Unpublished data have also been included. Multiple measurements can be found in most of the open ocean provinces. However, there is an uneven distribution of the data, with some areas highly sampled (e.g., China Sea, Bermuda Atlantic Time Series station) compared to some others that are not well represented, such as the southeastern Atlantic, the south Pacific, and the south Indian oceans. Some coastal areas, although in a much smaller number, are also included in this global compilation. Globally, based on different depth horizons and climate zones, the median POC∕234Th ratios have a wide range, from 0.6 to 18 µmol dpm−1.

Particle sinking is a major form of transport for photosynthetically fixed carbon to below the euphotic zone via the biological carbon pump (BCP). Oxygen (O2) depletion may improve the efficiency of the BCP. However, the mechanisms by which O2 deficiency can enhance particulate organic matter (POM) vertical fluxes are not well understood. Here, we investigate the composition and vertical fluxes of POM in two deep basins of the Baltic Sea (GB: Gotland Basin and LD: Landsort Deep). The two basins showed different O2 regimes resulting from the intrusion of oxygen-rich water from the North Sea that ventilated the water column below 140 m in GB, but not in LD, during the time of sampling. In June 2015, we deployed surface-tethered drifting sediment traps in oxic surface waters (GB: 40 and 60 m; LD: 40 and 55 m), within the oxygen minimum zone (OMZ; GB: 110 m and LD: 110 and 180 m) and at recently oxygenated waters by the North Sea inflow in GB (180 m). The primary objective of this study was to test the hypothesis that the different O2 conditions in the water column of GB and LD affected the composition and vertical flux of sinking particles and caused differences in export efficiency between those two basins. The composition and vertical flux of sinking particles were different in GB and LD. In GB, particulate organic carbon (POC) flux was 18 % lower in the shallowest trap (40 m) than in the deepest sediment trap (at 180 m). Particulate nitrogen (PN) and Coomassie stainable particle (CSP) fluxes decreased with depth, while particulate organic phosphorus (POP), biogenic silicate (BSi), chlorophyll a (Chl a) and transparent exopolymeric particle (TEP) fluxes peaked within the core of the OMZ (110 m); this coincided with the presence of manganese oxide-like (MnOx-like) particles aggregated with organic matter. In LD, vertical fluxes of POC, PN and CSPs decreased by 28 %, 42 % and 56 %, respectively, from the surface to deep waters. POP, BSi and TEP fluxes did not decrease continuously with depth, but they were higher at 110 m. Although we observe a higher vertical flux of POP, BSi and TEPs coinciding with abundant MnOx-like particles at 110 m in both basins, the peak in the vertical flux of POM and MnOx-like particles was much higher in GB than in LD. Sinking particles were remarkably enriched in BSi, indicating that diatoms were preferentially included in sinking aggregates and/or there was an inclusion of lithogenic Si (scavenged into sinking particles) in our analysis. During this study, the POC transfer efficiency (POC flux at 180 m over 40 m) was higher in GB (115 %) than in LD (69 %), suggesting that under anoxic conditions a smaller portion of the POC exported below the euphotic zone was transferred to 180 m than under reoxygenated conditions present in GB. In addition, the vertical fluxes of MnOx-like particles were 2 orders of magnitude higher in GB than LD. Our results suggest that POM aggregates with MnOx-like particles formed after the inflow of oxygen-rich water into GB, and the formation of those MnOx–OM-rich particles may alter the composition and vertical flux of POM, potentially contributing to a higher transfer efficiency of POC in GB. This idea is consistent with observations of fresher and less degraded organic matter in deep waters of GB than LD.

In the ocean, sinking of particulate organic matter (POM) drives carbon export from the euphotic zone and supplies nutrition to mesopelagic communities, the feeding and degradation activities of which in turn lead to export flux attenuation. Oxygen (O2) minimum zones (OMZs) with suboxic water layers (2 kg−1) show a lower carbon flux attenuation compared to well-oxygenated waters (> 100 µmol O2 kg−1), supposedly due to reduced heterotrophic activity. This study focuses on sinking particle fluxes through hypoxic mesopelagic waters (2 kg−1); these represent ∼ 100 times more ocean volume globally compared to suboxic waters, but they have less been studied. Particle export fluxes and attenuation coefficients were determined in the eastern tropical North Atlantic (ETNA) using two surface-tethered drifting sediment trap arrays with seven trapping depths located between 100 and 600 m. Data on particulate matter fluxes were fitted to the normalized power function Fz = F100 (z∕100)−b, with F100 being the flux at a depth (z) of 100 m and b being the attenuation coefficient. Higher b values suggest stronger flux attenuation and are influenced by factors such as faster degradation at higher temperatures. In this study, b values of organic carbon fluxes varied between 0.74 and 0.80 and were in the intermediate range of previous reports, but lower than expected from seawater temperatures within the upper 500 m. During this study, highest b values were determined for fluxes of particulate hydrolyzable amino acids (PHAA), followed by particulate organic phosphorus (POP), nitrogen (PN), carbon (POC), chlorophyll a (Chl a) and transparent exopolymer particles (TEP), pointing to a sequential degradation of organic matter components during sinking. Our study suggests that in addition to O2 concentration, organic matter composition co-determines transfer efficiency through the mesopelagic. The magnitude of future carbon export fluxes may therefore also depend on how organic matter quality in the surface ocean changes under influence of warming, acidification and enhanced stratification.

To examine the potentially competing influences of microzooplankton and calcite mineral ballast on organic matter remineralization, we incubated diatoms in darkness in rolling tanks with and without added calcite minerals (coccoliths) and microzooplankton (rotifers). Concentrations of particulate organic matter (POM in suspension or in aggregates), of dissolved organic matter (DOM), and of dissolved inorganic nutrients were monitored over 8 days. The presence of rotifers enhanced the remineralization of ammonium and phosphate, but not dissolved silicon, from the biogenic particulate matter, up to 40% of which became incorporated into aggregates early in the experiment. Added calcite resulted in rates of excretion of ammonium and phosphate by rotifers that were depressed by 67% and 36%, respectively, demonstrating the potential for minerals to inhibit the destruction of POM by zooplankton in the water column. Lastly, the presence of the rotifers and added calcite minerals resulted in a more rapid initial rate of aggregation, although not a greater overall amount of aggregation during the experiment.

The oceanic biological carbon pump is an important factor in the global carbon cycle. Organic carbon is exported from the surface ocean mainly in the form of settling particles derived from plankton production in the upper layers of the ocean. The large variability in current estimates of the global strength of the biological carbon pump emphasises that our knowledge of a major planetary carbon flux remains poorly constrained. We present a database of 723 estimates of organic carbon export from the surface ocean derived from the 234Th technique. The dataset is archived on the data repository PANGEA® (www.pangea.de) under doi:10.1594/PANGAEA.809717. Data were collected from tables in papers published between 1985 and early 2013. We also present sampling dates, publication dates and sampling areas. Most of the open ocean provinces are represented by multiple measurements. However, the western Pacific, the Atlantic Arctic, South Pacific and the southern Indian Ocean are not well represented. There is a variety of integration depths ranging from surface to 300 m. Globally the fluxes ranged from 0 to 1500 mg C m−2 d−1.

Meridional and vertical distributions of several biogeochemical parameters were studied along a section in the southeastern Atlantic and the Southern Ocean south of South Africa during the austral summer 2008 of the International Polar Year to characterize the biogeochemical provinces and to assess the seasonal net diatom production. Based on analyses of macro-nutrients, ammonium (NH4), chlorophyll a, (Chl a), phaeopigments, biogenic silica (BSi), particulate inorganic carbon (PIC), and particulate organic carbon and nitrogen (POC and PON, respectively), four biogeochemical domains were distinguished along the section: the subtropical Atlantic, the confluence zone of the subtropical and subantarctic domains, the Polar Frontal Zone (PFZ) in the Antarctic Circumpolar Current (ACC), and the north-eastern branch of the Weddell Gyre. The subtropical region displayed extremely low nutrient concentrations featuring oligotrophic conditions, and sub-surface maxima of Chl a and phaeopigments never exceeded 0.5 µg L−1 and 0.25 µg L−1, respectively. The anticyclonic and cyclonic eddies crossed in the Cape Basin were characterized by a deepening and a rise, respectively, of the nutrients isoclines. The confluence zone of the subtropical domain and the northern side of the ACC within the subantarctic domain displayed remnant nitrate and phosphate levels, whereas silicate concentrations kept to extremely low levels. In this area, Chl a level of 0.4–0.5 µg L−1 distributed homogenously within the mixed layer, and POC and PON accumulated to values up to 10 µM and 1.5 µM, respectively, indicative of biomass accumulation along the confluence zone during the late productive period. In the ACC domain, the Polar Frontal Zone was marked by a post-bloom of diatoms that extended beyond the Polar Front (PF) during this late summer condition, as primarily evidenced by the massive depletion of silicic acid in the surface waters. The accumulation of NH4 to values up to 1.25 µM at 100 m depth centred on the PF and the accumulation of BSi up to 0.5 µM in the surface waters of the central part of the PFZ also featured a late stage of the seasonal diatom bloom. The silica daily net production rate based on the seasonal depletion of silicic acid was estimated to be 11.9 ± 6.5 mmol m−2 d−1 in the domain of the vast diatom post-bloom, agreeing well with the previously recorded values in this province. The Weddell Gyre occasionally displayed relative surface depletion of silicic acid, suggesting a late stage of a relatively minor diatom bloom possibly driven by iceberg drifting releases of iron. In this domain the estimated range of silica daily net production rate (e.g. 21.1 ± 8.8 mmol m−2 d−1) is consistent with previous studies, but was not significantly higher than that in the Polar Front region.

As part of the Bonus-GoodHope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. High regenerated nitrate uptake rate in the oligotrophic Subtropical Zone (STZ) resulted in low f-ratios (f = 0.2) with nitrogen uptake being dominated by ρurea, which contributed up to 70 % of total nitrogen uptake. Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (−2 d−1), mainly due to enhanced nutrient supply within an anticyclonic eddy observed in this region. A decrease in the contribution of smaller size classes to the phytoplankton community was observed with increasing latitude, concurrent with a decrease in the contribution of regenerated production. Higher f-ratios observed in the SAZ (f = 0.49), Polar Frontal Zone (f= 0.41) and Antarctic Zone (f = 0.45) relative to the STZ (f = 0.24), indicate a higher contribution of NO3−-uptake relative to total nitrogen and potentially higher export production. High ambient regenerated nutrient concentrations are indicative of active regeneration processes throughout the transect and ascribed to late summer season sampling. Higher depth integrated uptake rates also correspond with higher surface iron concentrations. No clear correlation was observed between carbon export estimates derived from new production and 234Th flux. In addition, export derived from 15N estimates were 2–20 times greater than those based on 234Th flux. Variability in the magnitude of export is likely due to intrinsically different methods, compounded by differences in integration time scales for the two proxies of carbon export.