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International audience; Chaotic intrinsic variability (CIV) emerges spontaneously from nonlinear ocean dynamics even without any atmospheric variability. Eddy-permitting numerical simulations suggest that CIV is a significant contributor to the interannual to decadal variability of physical properties. Here we show from an ensemble of global ocean eddy-permitting simulations that large-scale interannual CIV propagates from physical properties to sea-air CO 2 fluxes in areas of high mesoscale eddy activity (e.g., Southern Ocean and western boundary currents). In these regions and at scales larger than 500 km (~5°), CIV contributes significantly to the interannual variability of sea-air CO 2 fluxes. Between 35°S and 45°S (midlatitude Southern Ocean), CIV amounts to 23.76 TgC yr −1 or one half of the atmospherically forced variability. Locally, its contribution to the total interannual variance of sea-air CO 2 fluxes exceeds 76%. Outside eddy-active regions its contribution to total interannual variability is below 16%. Plain Language Summary Sea-air CO 2 fluxes undergo substantial regional and interannual fluctuations. These fluctuations are mostly forced by changes in large-scale atmospheric patterns, but ocean internal dynamics could also contribute to them. This study quantifies these two sources of variability and their contributions to fluctuations of sea-air CO 2 fluxes over large oceanic regions. It relies on the analyses of three ocean numerical simulations driven by the same atmospheric forcing but starting from small differences in initial conditions, and including a simplified representation of marine ecosystems. Simulations are run at a horizontal resolution allowing to model part of the effect of ocean mesoscale activity on physical and chemical tracers. We demonstrate that nonlinear oceanic processes drive fluctuations of sea-air CO 2 fluxes at interannual timescales that are inherently random. The magnitude of these fluctuations is substantial over areas of high kinetic energy and locally exceeds 76% of the total interannual variance of sea-air CO 2 fluxes.

Quantifying variability in the ocean carbon sink remains problematic due to sparse observations and spatiotemporal variability in surface ocean pCO2. To address this challenge, we have updated and improved ECCO‐Darwin, a global ocean biogeochemistry model that assimilates both physical and biogeochemical observations. The model consists of an adjoint‐based ocean circulation estimate from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium and an ecosystem model developed by the Massachusetts Institute of Technology Darwin Project. In addition to the data‐constrained ECCO physics, a Green's function approach is used to optimize the biogeochemistry by adjusting initial conditions and six biogeochemical parameters. Over seasonal to multidecadal timescales (1995–2017), ECCO‐Darwin exhibits broad‐scale consistency with observed surface ocean pCO2 and air‐sea CO2 flux reconstructions in most biomes, particularly in the subtropical and equatorial regions. The largest differences between CO2 uptake occur in subpolar seasonally stratified biomes, where ECCO‐Darwin results in stronger winter uptake. Compared to the Global Carbon Project OBMs, ECCO‐Darwin has a time‐mean global ocean CO2 sink (2.47 ± 0.50 Pg C year−1) and interannual variability that are more consistent with interpolation‐based products. Compared to interpolation‐based methods, ECCO‐Darwin is less sensitive to sparse and irregularly sampled observations. Thus, ECCO‐Darwin provides a basis for identifying and predicting the consequences of natural and anthropogenic perturbations to the ocean carbon cycle, as well as the climate‐related sensitivity of marine ecosystems. Our study further highlights the importance of physically consistent, property‐conserving reconstructions, as are provided by ECCO, for ocean biogeochemistry studies.

Extensive studies in the 1980s–1990s led to the characterization of latitudinal variations in sea surface δ13C values of particulate organic carbon (δ13CPOC), and relationships were found with CO2 concentrations, temperature, growth rates, and cell geometries. Surprisingly, no large-scale efforts have been made to describe variations in δ13CPOC values over depth in the water column. Here we compile published examples demonstrating a widespread isotopic pattern in particulate organic carbon (POC) of the upper water column. In 51 vertical profiles, δ13CPOC values in the lower euphotic zone on average are 1.4‰ lower than δ13CPOC values in the upper euphotic zone of open ocean settings. In a majority of locations this vertical decrease in δ13CPOC values is >2‰ and up to 5‰, larger than the commonly recognized vertical δ13C variation in dissolved inorganic carbon over the same depths. We briefly review hypotheses and supporting evidence offered by previous studies of individual water columns: The observed patterns could result from vertical differences in photosynthetic growth rates or community composition, biochemical composition of organic matter due to degradation, isotopic disequilibrium within the dissolved inorganic carbon pool, particle dynamics, or seasonal vertical mixing. Coordinated isotopic, biological, and seawater chemistry data are sparse, and consistent drivers of this widespread isotopic pattern are currently elusive. Further work is needed to adequately characterize the environmental conditions coinciding with this pattern, to test its origins, and to determine if the magnitude of upper water column δ13CPOC variations could be a useful marker of upper ocean carbon cycle dynamics.

Despite the potential importance in the oceanic carbon cycle and benthic ecosystem, global feature of lateral supply of aged organic matter hosted on lithogenic particles derived from sediment resuspension has not been systematically examined. We compiled concentrations and fluxes of lithogenic material in the ocean in a global-scale by using literature data of sediment trap studies to understand the contribution of resuspended sediment to sinking particulate matter. We find that these contributions are significant in various oceanic settings, particularly over continental margins. Lithogenic material flux decreased with increasing distance from the margins and above the seafloor. Examination of Δ14C values of sinking POC revealed strong relationships with parameters that represent contribution of resuspended sediment. We then derive estimates for the contribution of aged POC from sediment resuspension to sinking POC based on these relationships and global lithogenic material flux data.

Plankton respiration (R) is a key factor governing the ocean carbon cycle. However, although the ocean supports respiratory activity throughout its entire volume, to our knowledge there are no studies that tackle both the spatial and temporal variability of respiration in the dark ocean and its dependence on organic carbon sources. Here, we have studied the variability of epipelagic and mesopelagic R via the enzymatic activity of the electron transport system (ETS) in microbial communities, along two zonal sections (21°N and 26°N) extending from the northwest African coastal upwelling to the open-ocean waters of the North Atlantic subtropical gyre, during the fall 2002 and the spring 2003. Overall, integrated R in epipelagic (Repi; 0–200 m) waters, was similar during the two periods, while integrated mesopelagic respiration (Rmeso; 200–1000 m) was >25% higher in the fall. The two seasons, however, exhibited contrasting zonal and meridional patterns of ETS distribution in the water column, largely influenced by upwelling effects and associated mesoscale variability. Multiple linear regression between average R and average concentrations of dissolved organic carbon (DOC) and slow-sinking (suspended) particulate organic carbon (POCsus) indicates that POCsus is the main contributor to Rmeso, supporting previous results in the same area. Rmeso exceeded satellite-derived net primary production (NPP) at all stations except at the most coastal ones, with the imbalance increasing offshore. Moreover, the export flux of sinking POC collected at 200 m with sediment traps, represented on average less than 6% of the NPP. All this indicates that Rmeso depends largely on small particles with low sinking rates, which would be laterally advected at mid water depths from the continental margin toward the open ocean, or transported by mesoscale features from the surface to the mesopelagic ocean, providing support to inferences from modeling studies in the region.

Modern observations indicate that variations in marine phytoplankton stoichiometry correlate with the boundaries of major surface waters. For example, phytoplankton in the oligotrophic subtropical gyres typically have much higher C:N:P ratios (i.e., higher C:P and higher N:P ratios) than those in eutrophic upwelling regions and polar regions. Such a spatial pattern points to nutrient availability as a key environmental driver of stochiometric flexibility. Environmental dependence of phytoplankton C:N:P opens unexplored possibilities for modifying the strength of the biological pump under different climate conditions. Here we present a power law formulation of C:N:P flexibility that is driven by nutrients, temperature, and light. We embed the formulation in a global ocean carbon cycle model with multiple phytoplankton types and explore biogeochemical implications under glacial conditions. We find three key controls on export C:N:P ratio: phytoplankton physiology and community structure as well as the balance in regional production at the global level. Glacial inputs of iron and sea ice expansion are important modifiers of these three controls. We also find that global export C:N:P increases substantially under glacial conditions, and this strongly buffers global carbon export against decrease and draws down approximately 20 μatm of atmospheric CO2. These results point to the importance of including phytoplankton C:N:P flexibility in a mix of mechanisms that drive atmospheric CO2 over glacial-interglacial time scale. Finally, our simulations indicate decoupling of nutrients, which may provide a resolution to the longstanding disagreement regarding nutrient utilization in the glacial Southern Ocean derived from different nutrient proxies.

Ocean biology helps regulate global climate by fixing atmospheric CO2 and exporting it to deep waters as sinking detrital particles. New observations demonstrate that particle fragmentation is the principal factor controlling the depth to which these particles penetrate the ocean's interior, and hence how long the constituent carbon is sequestered from the atmosphere. The underlying cause is, however, poorly understood. We speculate that small, particle-associated copepods, which intercept and inadvertently break up sinking particles as they search for attached protistan prey, are the principle agents of fragmentation in the ocean. We explore this idea using a new marine ecosystem model. Results indicate that explicitly representing particle fragmentation by copepods in biogeochemical models offers a step change in our ability to understand the future evolution of biologically-mediated ocean carbon storage. Our findings highlight the need for improved understanding of the distribution, abundance, ecology and physiology of particle-associated copepods.

International audience; Planktonic archaea include predominantly Marine Group I Thaumarchaeota (MG I) and Marine Group II Euryarchaeota (MG II), which play important roles in the oceanic carbon cycle. MG I produce specific lipids called isoprenoid glycerol dibiphytanyl glycerol tetraethers (GDGTs), which are being used in the sea surface temperature proxy named TEX86. Although MG II may be the most abundant planktonic archaeal group in surface water, their lipid composition remains poorly characterized because of the lack of cultured representatives. Circumstantial evidence from previous studies of marine suspended particulate matter suggests that MG II may produce both GDGTs and archaeol-based lipids. In this study, integration of the 16S rRNA gene quantification and sequencing and lipid analysis demonstrated that MG II contributed significantly to thepool of archaeal tetraether lipids in samples collected from MG II-dominated surface waters of the Northwestern Pacific Ocean (NWPO). The archaeal lipid composition in MG II-dominated NWPO waters differed significantly from that of known MG I cultures, containing relatively more 2G-OH-, 2G- and 1G- GDGTs, especially in their acyclic form. Lipid composition in NWPO waters was also markedly different from MG I-dominated surface water samples collected in the East China Sea. GDGTs from MG II-dominated samples seemed to respond to temperature similarly to GDGTs from the MG I-dominated samples, which calls for further study using pure cultures to determine the exact impact of MG II on GDGT-based proxies.

Less than three decades ago only a small fraction of the Arctic Ocean (AO) was ice free and then only for short periods. The ice cover kept sea surface pCO2 at levels lower relative to other ocean basins that have been exposed year round to ever increasing atmospheric levels. In this study, we evaluate sea surface pCO2 measurements collected over a 6‐year period along a fixed cruise track in the Canada Basin. The measurements show that mean pCO2 levels are significantly higher during low ice years. The pCO2 increase is likely driven by ocean surface heating and uptake of atmospheric CO2 with large interannual variability in the contributions of these processes. These findings suggest that increased ice‐free periods will further increase sea surface pCO2, reducing the Canada Basin's current role as a net sink of atmospheric CO2., Key Points The Arctic Ocean is losing ice cover at a rapid rateThe loss of ice cover is leading to heating and uptake of atmospheric CO2, increasing sea surface CO2 levelsContinued increase of sea surface CO2 will likely reduce the uptake of atmospheric CO2 in the future

The role of zooplanktivorous mesopelagic fish in the ocean carbon cycle is attracting increasing attention. However, little information is available regarding the carbon budget of marine zooplankti...

The deglacial atmospheric CO2 increase has been attributed to a combination of mechanisms, many of which relate to the ocean outgassing triggered by changing marine physical and biogeochemical states. To quantify the impact of proposed processes and feedback on the deglacial CO2 rise, previous modelling studies mostly conducted time-slice sensitivity experiments. Here, we present results from a transient deglaciation simulation (24 kB.P. - 1850) using the comprehensive Max Planck Institute Earth System Model (MPI-ESM). We force the model with the deglacial atmospheric greenhouse gases (CO2, CH4, N2O) concentrations, obital parameters, ice sheet reconstruction and transient dust deposition. The ocean biogeochemical component of MPI-ESM is using the same automatical adjustment of bathymetry and land-sea mask in response to deglacial continental runoff and melt water discharge. In and around the areas of changing land-sea mask, we redistribute the marine biogeochemical tracers in accord with the simulated salinity. Terrestrial organic matter is transferred from flooded land areas to the ocean, which guarantees mass conservation with respect to carbon. We also include 13C tracers in the ocean biogeochemical component to evaluate the simulated ocean state against proxy data. The initial marine nutrients and carbon inventories are set the same as those in the present-day ocean. During the first 3 kyr, the climate and ocean state show, as expected, only modest variations. Some flooding events of coastal areas bring terrestrial organic matter to the ocean and lead locally to CO2 outgassing for several decades. Terrestrial organic matter has a higher carbon to nutrient stoichiometry as compared to marine organic matter, thus its remineralization favours CO2 outgassing. Additionally, the accumulation of terrestrial organic matter in the top layers of the marine sediment reduces the replenishment of the water-column nutrients by the re-flux of remineralization products from marine sediment. Consequently, the strength of the local biological pump decreases. Further results will be presented and discussed.

Mineral dust plays an important role in the atmospheric radiation budget as well as in the ocean carbon cycle through fertilization and by ballasting of settling organic matter. However, observational records of open-ocean dust deposition are sparse. Here, we present the spatial and temporal evolution of Saharan dust deposition over 2 years from marine sediment traps across the North Atlantic, directly below the core of the Saharan dust plume, with highest dust fluxes observed in summer. We combined the observed deposition fluxes with model simulations and satellite observations and argue that dust deposition in the Atlantic is predominantly controlled by summer rains. The dominant depositional pathway changes from wet deposition in summer to dry deposition in winter. Wet deposition has previously been suggested to increase the release of dust-derived nutrients and their bioavailability, which may be a key contributor to surface-ocean productivity in remote and oligotrophic parts of the oceans.

Microbially mediated decomposition of particulate organic carbon (POC) is a central component of the oceanic carbon cycle, controlling the flux of organic carbon from the surface ocean to the deep ocean. Yet, the specific microbial taxa responsible for POC decomposition and degradation in the deep ocean are still unknown. To target the active microbial lineages involved in these processes, 13C‐labeled particulate organic matter (POM) was used as a substrate to incubate particle‐attached (PAM) and free‐living microbial (FLM) assemblages from the epi‐ and bathypelagic zones of the New Britain Trench (NBT). By combining DNA stable‐isotope probing and Illumina Miseq high‐throughput sequencing of bacterial 16S rRNA gene, we identified 14 active bacterial taxonomic groups that implicated in the decomposition of 13C‐labeled POM at low and high pressures under the temperature of 15°C. Our results show that both PAM and FLM were able to decompose POC and assimilate the released DOC. However, similar bacterial taxa in both the PAM and FLM assemblages were involved in POC decomposition and DOC degradation, suggesting the decoupling between microbial lifestyles and ecological functions. Microbial decomposition of POC and degradation of DOC were accomplished primarily by particle‐attached bacteria at atmospheric pressure and by free‐living bacteria at high pressures. Overall, the POC degradation rates were higher at atmospheric pressure (0.1 MPa) than at high pressures (20 and 40 MPa) under 15°C. Our results provide direct evidence linking the specific particle‐attached and free‐living bacterial lineages to decomposition and degradation of diatomic detritus at low and high pressures and identified the potential mediators of POC fluxes in the epi‐ and bathypelagic zones., By combining DNA stable‐isotope probing and Illumina Miseq high‐throughput sequencing, we identified 14 bacterial taxonomic groups actively involved in the decomposition of particulate organic matter (POM) at different pressures. Both particle‐attached and free‐living microorganisms were able to decompose POM and assimilate the released dissolved organic matter, suggesting the decoupling between microbial lifestyles and ecological functions. Our results provide direct evidence linking the specific microbial lineages to decomposition and degradation of POM and identified the potential mediators of POM fluxes in the ocean.

Optical particle measurements are emerging as an important technique for understanding the ocean carbon cycle, including contributions to estimates of their downward flux, which sequesters carbon dioxide (CO2) in the deep sea. Optical instruments can be used from ships or installed on autonomous platforms, delivering much greater spatial and temporal coverage of particles in the mesopelagic zone of the ocean than traditional techniques, such as sediment traps. Technologies to image particles have advanced greatly over the last two decades, but the quantitative translation of these immense datasets into biogeochemical properties remains a challenge. In particular, advances are needed to enable the optimal translation of imaged objects into carbon content and sinking velocities. In addition, different devices often measure different optical properties, leading to difficulties in comparing results. Here we provide a practical overview of the challenges and potential of using these instruments, as a step toward improvement and expansion of their applications.

Breaking up is easy to do Sinking particles transport carbon to the seafloor, where they are buried in sediments and either provide food for benthic organisms or sequester the carbon they contain. However, only ∼30% of the maximum flux reaches depths of a kilometer. This loss cannot be fully accounted for by current measurements. Briggs et al. used data collected by robotic Biogeochemical-Argo floats to quantify total mesopelagic fragmentation and found that this process accounts for roughly half of the observed flux loss (see the Perspective by Nayak and Twardowski). Fragmentation is thus perhaps the most important process controlling the remineralization of sinking organic carbon. Science , this issue p. 791 ; see also p. 738

The ocean carbon cycle is a key player in the climate system through its role in regulating the atmospheric carbon dioxide concentration and other processes that alter the Earth's radiative balance. In the second version of the Norwegian Earth System Model (NorESM2), the oceanic carbon cycle component has gone through numerous updates that include, amongst others, improved process representations, increased interactions with the atmosphere, and additional new tracers. Oceanic dimethyl sulfide (DMS) is now prognostically simulated and its fluxes are directly coupled with the atmospheric component, leading to a direct feedback to the climate. Atmospheric nitrogen deposition and additional riverine inputs of other biogeochemical tracers have recently been included in the model. The implementation of new tracers such as “preformed” and “natural” tracers enables a separation of physical from biogeochemical drivers as well as of internal from external forcings and hence a better diagnostic of the simulated biogeochemical variability. Carbon isotope tracers have been implemented and will be relevant for studying long-term past climate changes. Here, we describe these new model implementations and present an evaluation of the model's performance in simulating the observed climatological states of water-column biogeochemistry and in simulating transient evolution over the historical period. Compared to its predecessor NorESM1, the new model's performance has improved considerably in many aspects. In the interior, the observed spatial patterns of nutrients, oxygen, and carbon chemistry are better reproduced, reducing the overall model biases. A new set of ecosystem parameters and improved mixed layer dynamics improve the representation of upper-ocean processes (biological production and air–sea CO2 fluxes) at seasonal timescale. Transient warming and air–sea CO2 fluxes over the historical period are also in good agreement with observation-based estimates. NorESM2 participates in the Coupled Model Intercomparison Project phase 6 (CMIP6) through DECK (Diagnostic, Evaluation and Characterization of Klima) and several endorsed MIP simulations.

Oxygen minimum zones (OMZs) in the ocean are expanding. This expansion is attributed to global warming and may continue over the next 10 to 100 kyrs due to multiple climate CO$_\text{2}$-driven factors. The expansion of oxygen-deficient waters has the potential to enhance organic carbon burial in marine sediments, thereby providing a negative feedback on global warming. Here, we study the response of dissolved oxygen in the ocean to increased phosphorus and iron inputs due to CO$_\text{2}$-driven enhanced weathering and increased dust emissions, respectively. We use an ocean biogeochemical model coupled to a general ocean circulation model (the Hamburg Oceanic Carbon Cycle model, HAMOCC 2.0) to assess the impact of such regional deoxygenation on organic carbon burial in the modern ocean on time scales of up to 200~kyrs. We find that an increase in input of phosphorus and iron leads to an expansion of the area of the OMZ impinging on continental margin sediments and a significant decline in bottom water oxygen in the open ocean relative to pre-industrial conditions. The associated increase in organic carbon burial could contribute to the drawdown of $\sim$1600~Gt of carbon, which is equivalent to the total amount of CO$_\text{2}$ in the atmosphere predicted for the year 2100 in a business as usual scenario, on time scales of up to 50 kyrs. The corresponding areal extent of sediments overlain by bottom waters with little or no oxygen as estimated by the model is not very different from the minimum area estimated for two major oceanic anoxic events in Earth's past. Such events were associated with major perturbations of the oceanic carbon cycle, including high rates of organic carbon burial. We conclude that organic carbon burial in low oxygen areas in the ocean could contribute to removal of anthropogenic CO$_\text{2}$ from the atmosphere on long time scales.

Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open-ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in delta C-13 values of 0.8 parts per thousand-2.5 parts per thousand were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel-derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in C-13-rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna delta C-13 values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.

Transparent exopolymer particles (TEPs) have drawn extensive attention in recent decades due to their crucial role in the biogeochemical and ecological processes of the ocean. However, TEP distribution and fluxes are relatively less addressed in the shelf-seas, where its variability can be affected by not only biology but also complex physical dynamics. Here, we present a comprehensive study of TEP from the coast to the basin (12 sampling sites) of the northern South China Sea (NSCS). We found a large TEP variability from 0.6 to 78.6 μg Xeq. L−1 with higher levels in the coastal waters than the offshore epipelagic waters and the deep waters. In addition, the spatial distribution of TEP was significantly correlated to the cross-shelf change of temperature, salinity, and chlorophyll-a, revealing the complex physical-biogeochemical controls on TEP variability. We found the TEP dynamics nearshore largely influenced by the sedimentation and transportation of TEP-rich aggregates from the river plume. The contribution of TEP to particulate organic carbon (POC) increased gradually when approaching the shore from the sea, suggesting an elevated role of TEP in the coastal carbon cycle. Finally, a good correlation of particle-attached bacteria (PAB) with TEP but not POC revealed a preferential utilization of TEP by PAB. Thus, TEP may play an essential role in the recycling of carbon and nitrogen in the shelf-sea. These findings are crucial for understanding of the TEP dynamics under a changing environment and the associated impacts on the oceanic carbon cycle.

The whole story An accurate, precise record of the carbon-14 ( 14 C) content of the atmosphere is important for developing chronologies in climate change, archaeology, and many other disciplines. Cheng et al. provide a record that covers the full range of the 14 C dating method (∼54,000 years), using paired measurements of 14 C/ 12 C and thorium-230 ( 230 Th) ages from two stalagmites from Hulu Cave, China. The advantage of matching absolute 230 Th ages and 14 C/ 12 C allowed the authors to fashion a seamless record from a single source with low uncertainties, particularly in the older sections. Science , this issue p. 1293

The biological carbon pump exports carbon fixed by photosynthesis out of the surface ocean and transfers it to the deep, mostly in the form of sinking particles. Despite the importance of the pump in regulating the air‐sea CO2 balance, the magnitude of global carbon export remains unclear, as do its controlling mechanisms. A possible sinking flux of carbon to the mesopelagic zone may be via the mixed‐layer pump: a seasonal net detrainment of particulate organic carbon (POC)‐rich surface waters, caused by sequential deepening and shoaling of the mixed layer. In this study, we present a full year of daily small‐particle POC concentrations derived from glider optical backscatter data, to study export variability at the Porcupine Abyssal Plain (PAP) sustained observatory in the Northeast Atlantic. We observe a strong seasonality in small‐particle transfer efficiency, with a maximum in winter and early spring. By calculating daily POC export driven by mixed‐layer variations, we find that the mixed‐layer pump supplies an annual flux of at least 3.0 ± 0.9 g POC·m−2·year−1 to the mesopelagic zone, contributing between 5% and 25% of the total annual export flux and likely contributing to closing a gap in the mesopelagic carbon budget found by other studies. These are, to our best knowledge, the first high‐frequency observations of export variability over the course of a full year. Our results support the deployment of bio‐optical sensors on gliders to improve our understanding of the ocean carbon cycle on temporal scales from daily to annual., Key Points Gliders captured a full annual cycle of high‐frequency POC variability over the upper mesopelagic zoneSmall‐particle transfer efficiency displayed a strong seasonality, peaking in winter and early springThe data indicate a significant annual carbon supply to the mesopelagic zone by small particles through the mixed‐layer pump

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.

U.S. National Science Foundation's Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project under the NSF [PLR-1425989]; NASA [NNX14AP49G]; U.S. Argo through NOAA/JISAO [NA17RJ1232]; Pacific Marine Environmental Laboratory of NOAA; Ocean Observations and Monitoring Division, Climate Program Office, National Oceanic and Atmospheric Administration, United States; ARCS Foundation Oregon Chapter

Dissolved organic carbon (DOC) plays an important role in the oceanic carbon cycle , with its distribution and variation primarily controlled by in situ productivity. The vertical and spatial distribution of DOC measured during two cruises has been investigated to decipher the processes controlling DOC in the Bay of Bengal (BoB). The terrestrial DOC flux to the BoB constitutes a significant fraction of the total DOC and has implications for the carbon budget of the northern Indian Ocean. High concentrations of DOC were observed in the northern BoB, which is primarily due to high flux of terrestrially derived fresh water from various rivers draining to the BoB. Compared to surface DOC concentration of the world oceans, BoB surface water generally has a higher DOC concentration (75–100 μM), which can be attributed to the high DOC content of the riverine flux. Stratification of surface water helps to maintain DOC content due to inhibited vertical mixing in the BoB as a result of the fresh water lens. High subsurface DOC concentration is due to the remineralisation of biogenic sinking particles brought along with the sediments. Subsurface DOC concentrations in the BoB exhibit a conspicuous decreasing trend southwards due to reduced influence of riverine biogenic flux. This study demonstrates that the average contribution of apparent oxygen utilization (AOU) to the DOC remineralisation in the BoB was ~18%.

Human-induced changes in carbon fluxes across the land-ocean interface can influence the global carbon cycle, yet the impacts of rapid urbanization and establishment of wastewater treatment plants (WWTPs) on coastal ocean carbon cycles are poorly known. This is unacceptable as at present ∼64% of global municipal wastewater is treated before discharge. Here, we report surface water dissolved inorganic carbon (DIC) and sedimentary organic carbon concentrations and their isotopic compositions in the rapidly urbanized Jiaozhou Bay in northeast China as well as carbonate parameters in effluents of three large WWTPs around the bay. Using DIC, δ13CDIC and total alkalinity (TA) data and a tracer model, we determine the contributions to DIC from wastewater DIC input, net ecosystem production, calcium carbonate precipitation, and CO2 outgassing. Our study shows that high-DIC and low-pH wastewater effluent represents an important source of DIC and acidification in coastal waters. In contrast to the traditional view of anthropogenic organic carbon export and degradation, we suggest that with the increase of wastewater discharge and treatment rates, wastewater DIC input may play an increasingly more important role in the coastal ocean carbon cycle.

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.

Due to the combined effects of natural processes and human activities, carbon source/sink processes and mechanisms in the coastal ocean are becoming more and more important in current ocean carbon cycle research. Based on differences in the ratio of total alkalinity (TA) to dissolved inorganic carbon (DIC) associated with terrestrial input, biological process (production and respiration), calcium carbonate (CaCO 3 ) process (precipitation and dissolution) and CO 2 evasion/invasion, we discuss the mechanisms controlling the surface partial pressure of CO 2 ( p CO 2 ) in Jiaozhou Bay (JZB) during summer and the influence of heavy rain, via three cruises performed in mid-June, early July and late July of 2014. In mid-June and in early July, without heavy rain or obvious river input, sea surface p CO 2 ranged from 521 to 1080 μatm and from 547 to 998 μatm, respectively. The direct input of DIC from sewage and the intense respiration produced large DIC additions and the highest p CO 2 values in the northeast of the bay near the downtown of Qingdao. However, in the west of the bay, significant CaCO 3 precipitation led to DIC removal but no obvious increase in p CO 2 , which was just close to that in the central area. Due to the shallow depth and longer water residence time in this region, this pattern may be related to the sustained release of CO 2 into the atmosphere. In late July, heavy rain promoted river input in the western and eastern portions of JZB. Strong primary production led to a significant decrease in p CO 2 in the western area, with the lowest p CO 2 value of 252 μatm. However, in the northeastern area, the intense respiration remained, and the highest p CO 2 value was 1149 μatm. The average air–sea CO 2 flux in mid-June and early July was 20.23 mmol m − 2 d − 1 and 23.56 mmol m − 2 d − 1 , respectively. In contrast, in late July, sources became sinks for atmospheric CO 2 in the western and central areas of the bay, halving the average air–sea CO 2 flux to a value of 10.58 mmol m − 2 d − 1 . Therefore, without considering the impact of heavy rains, the estimated air–sea CO 2 flux is likely inaccurate in coastal waters. Our study implies that more studies in the coastal ocean are needed to determine the duration and intensity of the CO 2 sink after the occurrence of heavy rain as well as the magnitudes of the CO 2 sink associated with varying rainfall intensities.

The implications of a mesoscale eddy for relevant properties of the Southern Ocean carbon cycle are examined with in situ observations. We explored carbon properties inside a large (~190 km diameter) cyclonic eddy that detached from the Subantarctic Front (SAF) south of Tasmania in March 2016. Based on remote sensing, the eddy was present for ∼2 months in the Subantarctic Zone (SAZ), an important region of oceanic carbon dioxide (CO2) uptake throughout the annual cycle and carbon subduction (i.e., where mode and intermediate waters form), before it was reabsorbed into the SAF. The eddy was sampled during the middle of its life, 1 month after it spawned. Comparatively, the eddy was ∼3°C colder, 0.5 practical salinity unit fresher, and less biologically productive than surrounding SAZ waters. The eddy was also richer in dissolved inorganic carbon (DIC) and had lower saturation states of aragonite and calcite than the surrounding SAZ waters. As a consequence, it was a strong source of CO2 to the atmosphere (with fluxes up to +25 mmol C m−2 d−1). Compared to the SAF waters, from which it originated, DIC concentration in the eddy was ∼20 μmol kg−1 lower, indicating lateral mixing, small-scale recirculation, or eddy stirring with lower-DIC SAZ waters by the time the eddy was observed. As they are commonly spawned from the Antarctic Circumpolar Current, and as 50% of them decay in the SAZ (the rest being reabsorbed by the SAF-N), these types of eddies may represent a significant south-north transport pathway for carbon across the ACC and may alter the carbon properties of SAZ waters.

Tintinnid ciliates are an important link in marine food webs as they feed on phytoplankton and bacteria while providing nutrients to higher trophic levels. Tintinnids are known to agglutinate mineral particles or dead biogenic material such as diatom frustules to their shell-like housing (lorica), however, reasons for this agglutination remain questioned. We report on our observation of agglomeration of the living diatoms Fragilariopsis curta, F. cylindrus, F. pseudonana and F. rhombica to loricae of the Antarctic tintinnid ciliates Laackmanniella naviculaefera and Codonellopsis gaussi. These unusual associations between living diatoms and tintinnids were exclusively observed south of 63.59°S. We discuss the significance of our new finding and generate hypotheses to be tested by future research. It remains unclear where these living diatom–tintinnid associations are initially formed (in or near sea ice or also further north when abundances of L. naviculaefera, C. gaussi, F. curta, F. cylindrus, F. pseudonana and F. rhombica happen to be relatively high); who the beneficiary is in this association; what the exact benefits are; and how they might influence the Southern Ocean carbon cycle. Nevertheless, our observation provides a key step forward towards illuminating the largely unknown ecology of two Southern Ocean-endemic tintinnid species.

Over the last decade many climate models have evolved into Earth system models (ESMs), which are able to simulate both physical and biogeochemical processes through the inclusion of additional components such as the carbon cycle. The Australian Community Climate and Earth System Simulator (ACCESS) has been recently extended to include land and ocean carbon cycle components in its ACCESS-ESM1 version. A detailed description of ACCESS-ESM1 components including results from pre-industrial simulations is provided in Part 1. Here, we focus on the evaluation of ACCESS-ESM1 over the historical period (1850–2005) in terms of its capability to reproduce climate and carbon-related variables. Comparisons are performed with observations, if available, but also with other ESMs to highlight common weaknesses. We find that climate variables controlling the exchange of carbon are well reproduced. However, the aerosol forcing in ACCESS-ESM1 is somewhat larger than in other models, which leads to an overly strong cooling response in the land from about 1960 onwards. The land carbon cycle is evaluated for two scenarios: running with a prescribed leaf area index (LAI) and running with a prognostic LAI. We overestimate the seasonal mean (1.7 vs. 1.4) and peak amplitude (2.0 vs. 1.8) of the prognostic LAI at the global scale, which is common amongst CMIP5 ESMs. However, the prognostic LAI is our preferred choice, because it allows for the vegetation feedback through the coupling between LAI and the leaf carbon pool. Our globally integrated land–atmosphere flux over the historical period is 98 PgC for prescribed LAI and 137 PgC for prognostic LAI, which is in line with estimates of land use emissions (ACCESS-ESM1 does not include land use change). The integrated ocean–atmosphere flux is 83 PgC, which is in agreement with a recent estimate of 82 PgC from the Global Carbon Project for the period 1959–2005. The seasonal cycle of simulated atmospheric CO2 is close to the observed seasonal cycle (up to 1 ppm difference for the station at Mace Head and up to 2 ppm for the station at Mauna Loa), but shows a larger amplitude (up to 6 ppm) in the high northern latitudes. Overall, ACCESS-ESM1 performs well over the historical period, making it a useful tool to explore the change in land and oceanic carbon uptake in the future.

Earth system models (ESMs) that incorporate carbon–climate feedbacks represent the present state of the art in climate modelling. Here, we describe the Australian Community Climate and Earth System Simulator (ACCESS)-ESM1, which comprises atmosphere (UM7.3), land (CABLE), ocean (MOM4p1), and sea-ice (CICE4.1) components with OASIS-MCT coupling, to which ocean and land carbon modules have been added. The land carbon model (as part of CABLE) can optionally include both nitrogen and phosphorous limitation on the land carbon uptake. The ocean carbon model (WOMBAT, added to MOM) simulates the evolution of phosphate, oxygen, dissolved inorganic carbon, alkalinity and iron with one class of phytoplankton and zooplankton. We perform multi-centennial pre-industrial simulations with a fixed atmospheric CO2 concentration and different land carbon model configurations (prescribed or prognostic leaf area index). We evaluate the equilibration of the carbon cycle and present the spatial and temporal variability in key carbon exchanges. Simulating leaf area index results in a slight warming of the atmosphere relative to the prescribed leaf area index case. Seasonal and interannual variations in land carbon exchange are sensitive to whether leaf area index is simulated, with interannual variations driven by variability in precipitation and temperature. We find that the response of the ocean carbon cycle shows reasonable agreement with observations. While our model overestimates surface phosphate values, the global primary productivity agrees well with observations. Our analysis highlights some deficiencies inherent in the carbon models and where the carbon simulation is negatively impacted by known biases in the underlying physical model and consequent limits on the applicability of this model version. We conclude the study with a brief discussion of key developments required to further improve the realism of our model simulation.

An increase in global wind speeds over time is affecting the global uptake of CO2 by the ocean. We determine the impact of changing winds on gas transfer and CO2 uptake by using the recently updated, global high-resolution, cross-calibrated multiplatform wind product (CCMP-V2) and a fixed monthly pCO(2) climatology. In particular, we assess global changes in the context of regional wind speed changes that are attributed to large-scale climate reorganizations. The impact of wind on global CO2 gas fluxes as determined by the bulk formula is dependent on several factors, including the functionality of the gas exchange-wind speed relationship and the regional and seasonal differences in the air-water partial pressure of CO2 gradient (pCO(2)). The latter also controls the direction of the flux. Fluxes out of the ocean are influenced more by changes in the low-to-intermediate wind speed range, while ingassing is impacted more by changes in higher winds because of the regional correlations between wind and pCO(2). Gas exchange-wind speed parameterizations with a quadratic and third-order polynomial dependency on wind, each of which meets global constraints, are compared. The changes in air-sea CO2 fluxes resulting from wind speed trends are greatest in the equatorial Pacific and cause a 0.03-0.04PgCdecade(-1) increase in outgassing over the 27year time span. This leads to a small overall decrease of 0.00 to 0.02PgCdecade(-1) in global net CO2 uptake, contrary to expectations that increasing winds increase net CO2 uptake. Plain Language Summary The effects of changing winds are isolated from the total change in trends in global air-sea CO2 fluxes over the last 27years. The overall effect of increasing winds over time has a smaller impact than expected as the impact in regions of outgassing is greater than for the regions acting as a CO2 sink.

The carbon isotope (δ 13 C) record from the Plio-Pleistocene shows prominent 400-kyr cycles with maximum values at eccentricity minima during the Pliocene. The period extends to 500 kyr in the Pleistocene after ~ 1.6 Ma. Five δ 13 C maxima occurred at ~ 0.2, 0.5, 1.0, 1.5 and 1.9 Ma over the last 2 Ma. Although several hypotheses have been suggested to explain why the 400–500-kyr cycles are so strong in δ 13 C records and how they may have originated, the mechanism is still not clear. The aim of this study was to test the dissolved organic carbon (DOC) hypothesis, which was proposed recently to explain this 400–500-kyr cycle in deeper time. We used an intermediate complexity box model that is computationally efficient for studies involving longer timescales. The model incorporates sophisticated microbial processes, dividing the oceanic carbon cycle into a rapid and a slow cycle. The model result suggests that when more nutrients enter the surface ocean, the rapid carbon cycle is more active, and less refractory DOC (RDOC) is produced. The opposite sequence occurs when fewer nutrients enter the ocean. The modeled RDOC concentration and the δ 13 C of dissolved inorganic carbon (DIC) are anti-correlated with riverine nutrient input. According to mass conservation, the release of isotopically lighter carbon from the RDOC pool leads to lighter DIC δ 13 C while an increase in the RDOC pool enriches it. The transient simulations produced a one-to-one correspondence between modeled and measured δ 13 C. This study supports the hypothesis that chemical weathering-induced variations in the DOC pool act as a pacemaker for δ 13 C changes over 400–500-kyr cycles.

Marine biology plays an important role in the ocean carbon cycle. However, the effect of warming-induced changes in biological rates on oceanic CO2 uptake has been largely overlooked. We use an Earth system model of intermediate complexity to investigate the effect of temperature-induced changes in biological rates on oceanic uptake of atmospheric CO2 and compare it with the effects from warming-induced changes in CO2 solubility and ocean mixing and circulation. Under the representative CO2 concentration pathway RCP 8.5 and its extension, by year 2500, relative to the simulation without warming effect on the ocean carbon cycle, CO2-induced warming reduces cumulative oceanic CO2 uptake by 469 Pg C, of which about 20% is associated with the warming-induced change in marine biological rates. In our simulations, the bulk effect of biological-mediated changes on CO2 uptake is smaller than that mediated by changes in CO2 solubility and ocean mixing and circulation. However, warming-induced changes in individual biological rates, including phytoplankton growth, phytoplankton mortality, and detritus remineralization, are found to affect oceanic CO2 uptake by an amount greater than or comparable to that caused by changes in CO2 solubility and ocean physics. Our simulations, which include only a few temperature-dependent biological processes, demonstrate the important role of biological rates in the oceanic CO2 uptake. In reality, many more complicated biological processes are sensitive to temperature change, and their responses to warming could substantially affect oceanic uptake of atmospheric CO2.

The North Pacific Subtropical Gyre (NPSG) is one of the largest biomes on Earth. It has a semi-enclosed surface area of about 2 × 107 km2 and mean depth of nearly 5 km and includes a broad range of habitats from warm, light-saturated, nutrient-starved surface waters to the cold, nutrient-rich abyss. Microorganisms are found throughout the water column and are vertically stratified by their genetically determined metabolic capabilities that establish physiological tolerances to temperature, light, pressure, as well as organic and inorganic growth substrates. Despite the global significance of the NPSG for energy and matter transformations and its role in the oceanic carbon cycle, it is grossly undersampled and not well characterized with respect to ecosystem structure and dynamics. Since October 1988, interdisciplinary teams of scientists from the University of Hawaii and around the world have been investigating the NPSG ecosystem at Station ALOHA (A Long-term Oligotrophic Habitat Assessment), a site chosen to be representative of this expansive oligotrophic habitat, with a focus on microbial processes and biogeochemistry. At the start of this comprehensive field study, the NPSG was thought to be a “Climax” community with a relatively stable plankton community structure and relatively low variability in key microbiological rates and processes. Now, after nearly three decades of observations and experimentation we present a new view of this old ocean, one that highlights temporal variability in ecosystem processes across a broad range of scales from diel to decadal and beyond. Our revised paradigm is built on the strength of high-quality time-series observations, on insights from the application of state-of-the-art –omics techniques (genomics, transcriptomics, proteomics and metabolomics) and, more recently, the discoveries of novel microorganisms and metabolic processes. Collectively, these efforts have led to a new understanding of trophic dynamics and population interactions in the NPSG. A comprehensive understanding of the environmental controls on microbial rates and processes, from genomes to biomes, will be required to inform the scientific community and the public at large about the potential impacts of human-induced climate change. The pace of new discovery, and the importance of integrating this new knowledge into conceptual paradigms and predictive models, is an enormous contemporary challenge with great scientific and societal relevance.

CFC-11 is an important tool used to assess oceanic cycle models. The CFC-11 which is dissolved in seawater can be used to analyze the ventilation of the oceans. In this study, a tracer CFC-11 module was developed based on the global oceanic carbon cycle circulation model MOM4 L40, which was developed by the National Climate Center of China Meteorological Administration. Then, the model was employed to study the distribution of CFC-11 in the global oceans, and also to assess the model's ventilation capacity. The simulated parameters, such as the global sea surfaces' CFC-11 concentrations, inventory, vertical penetration depths, and concentration distributions, were verified against the actual observations. The results showed that the model reasonably simulated the surface and vertical distribution of the CFC-11. The main storage areas of the CFC-11 were determined to be located in the Northwest Atlantic Ocean, subtropical North Pacific Ocean, and the Southern Ocean. The distributions of the CFC-11 concentrations on the oceanic surfaces were found to be remarkably affected by the sea surfaces' temperatures. The distribution of the simulated CFC-11 was found to have a high agreement with that of the actual observations, and showed an opposite gradient to the sea surfaces' temperatures. When compared with the observations of five sections located in three oceans, the simulated results of the CFC-11 were in general agreement with the observations in the majority of the areas. In addition, the distribution of the simulated CFC-11 was found to be in agreement with meridional overturning circulation in the global oceans, and an improved simulation of the Southern Ocean and deep oceans, as well as the penetration depths, were achieved. However, there were some deviations observed between the observations and simulations. For example, in the North Atlantic Ocean, where a main storage of the CFC-11 was located, the model underestimated the absorption of the CFC-11, which was found to be related to the over-transport simulations from the high latitudes to the low latitudes. These results may have been influenced by the thermohaline circulation and forced data. Overall, it was determined that the MOM L40 reasonably simulated the absorption of the total CFC-11 in the oceans, and effectively reproduced the oceans' ventilation capabilities by simulating the passive tracer CFC-11.

The ocean's biological pump transfers carbon from the surface euphotic zone into the deep ocean, reducing the atmospheric CO2 concentration. Despite its climatic importance, there are large uncertainties in basic metrics of the biological pump. Previous estimates of the strength of the biological pump, as measured by the amount of organic carbon exported from the euphotic zone, range from about 4 to 12 Pg C yr−1. The fate of exported carbon, in terms of how efficiently it is transferred into the deep ocean, is even more uncertain. Here we present a new model of the biological pump that assimilates satellite and oceanographic tracer observations to constrain rates and patterns of organic matter production, export, and remineralization in the ocean. The data-assimilated model predicts a global particulate organic carbon (POC) flux out of the euphotic zone of ∼9 Pg C yr−1. The particle export ratio (the ratio of POC export to net primary production) is highest at high latitudes and lowest at low latitudes, but low-latitude export is greater than predicted by previous models, in better agreement with observed patterns of long-term carbon export. Particle transfer efficiency (Teff) through the mesopelagic zone is controlled by temperature and oxygen, with highest Teff for high-latitude regions and oxygen minimum zones. In contrast, Teff in the deep ocean (below 1000 m) is controlled by particle sinking speed, with highest deep ocean Teff below the subtropical gyres. These results emphasize the utility of both remote sensing and oceanographic tracer observations for constraining the operation of the biological pump.

Dissolved oxygen (DO) concentration in the ocean is an important component of marine biogeochemical cycles and will be greatly altered as climate change persists. In this study a global oceanic carbon cycle model (HAMOCC 2.0) is used to address how mechanisms of oxygen minimum zone (OMZ) expansion respond to changes in CO2 radiative forcing. Atmospheric pCO2 is increased at a rate of 1 % annually and the model is stabilized at 2 ×, 4 ×, 6 ×, and 8 × preindustrial pCO2 levels. With an increase in CO2 radiative forcing, the OMZ in the Pacific Ocean is controlled largely by changes in particulate organic carbon (POC) export, resulting in increased remineralization and thus expanding the OMZs within the tropical Pacific Ocean. A potential decline in primary producers in the future as a result of environmental stress due to ocean warming and acidification could lead to a substantial reduction in POC export production, vertical POC flux, and thus increased DO concentration particularly in the Pacific Ocean at a depth of 600–800 m. In contrast, the vertical expansion of the OMZs within the Atlantic is linked to increases POC flux as well as changes in oxygen solubility with increasing seawater temperature. Changes in total organic carbon and increase sea surface temperature (SST) also lead to the formation of a new OMZ in the western subtropical Pacific Ocean. The development of the new OMZ results in dissolved oxygen concentration of ≤ 50 µmol kg−1 throughout the equatorial Pacific Ocean at 4 times preindustrial pCO2. Total ocean volume with dissolved oxygen concentrations of ≤ 50 µmol kg−1 increases by 2.4, 5.0, and 10.5 % for the 2 ×, 4 ×, and 8 × CO2 simulations, respectively.

In contrast to the open ocean, the sources and sinks for atmospheric carbon dioxide (CO2) in the coastal seas are poorly constrained and understood. Here we address this knowledge gap by analyzing the spatial and temporal variability of the coastal air-sea flux of CO2 (FCO2) using a recent high-resolution (0.25°) monthly climatology for coastal sea surface partial pressure in CO2 (pCO2). Coastal regions are characterized by CO2 sinks at temperate and high latitudes and by CO2 sources at low latitude and in the tropics, with annual mean CO2 flux densities comparable in magnitude and pattern to those of the adjacent open ocean with the exception of river-dominated systems. The seasonal variations in FCO2 are large, often exceeding 2 mol C m−2 year−1, a magnitude similar to the variations exhibited across latitudes. The majority of these seasonal variations stems from the air-sea pCO2 difference, although changes in wind speed and sea ice cover can also be significant regionally. Globally integrated, the coastal seas act currently as a CO2 sink of −0.20 ± 0.02 Pg C year−1, with a more intense uptake occurring in summer because of the disproportionate influence of high-latitude shelves in the Northern Hemisphere. Combined with estimates of the carbon sinks in the open ocean and the Arctic, this gives for the global ocean, averaged over the 1998 to 2015 period an annual net CO2 uptake of −1.7 ± 0.3 Pg C year−1., SCOPUS: ar.j, info:eu-repo/semantics/published

Open-ocean biology and the horizontal circulation set the rate of carbon uptake in the subpolar Southern Ocean., Global climate is critically sensitive to physical and biogeochemical dynamics in the subpolar Southern Ocean, since it is here that deep, carbon-rich layers of the world ocean outcrop and exchange carbon with the atmosphere. Here, we present evidence that the conventional framework for the subpolar Southern Ocean carbon cycle, which attributes a dominant role to the vertical overturning circulation and shelf-sea processes, fundamentally misrepresents the drivers of regional carbon uptake. Observations in the Weddell Gyre—a key representative region of the subpolar Southern Ocean—show that the rate of carbon uptake is set by an interplay between the Gyre’s horizontal circulation and the remineralization at mid-depths of organic carbon sourced from biological production in the central gyre. These results demonstrate that reframing the carbon cycle of the subpolar Southern Ocean is an essential step to better define its role in past and future climate change.

Atmospheric deposition of aerosols to the ocean provides an important pathway for the supply of vital micronutrients, including trace metals. These trace metals are essential for phytoplankton growth, and therefore their delivery to marine ecosystems can strongly influence the ocean carbon cycle. The solubility of trace metals in aerosols is a key parameter to better constrain their potential impact on phytoplankton growth. To date, a wide range of experimental approaches and nomenclature have been used to define aerosol trace metal solubility, making data comparison between studies difficult. Here we investigate and discuss several laboratory leaching protocols to determine the solubility of key trace metals in aerosol samples, namely iron, cobalt, manganese, copper, lead, vanadium, titanium and aluminium. Commonly used techniques and tools are also considered such as enrichment factor calculations and air mass back-trajectory projections and recommendations are given for aerosol field sampling, laboratory processing (including leaching and digestion) and analytical measurements. Finally, a simple 3-step leaching protocol combining commonly used protocols is proposed to operationally define trace metal solubility in aerosols. The need for standard guidelines and protocols to study the biogeochemical impact of atmospheric trace metal deposition to the ocean has been increasingly emphasised by both the atmospheric and oceanographic communities. This lack of standardisation currently limits our understanding and ability to predict ocean and climate interactions under changing environmental conditions.

In this paper, we present a database of the basic regimes of the carbon cycle in the ocean, the ocean carbon states, as obtained using a data mining/pattern recognition technique in observation-based as well as model data. The goal of this study is to establish a new data analysis methodology, test it and assess its utility in providing more insights into the regional and temporal variability of the marine carbon cycle. This is important as advanced data mining techniques are becoming widely used in climate and Earth sciences and in particular in studies of the global carbon cycle, where the interaction of physical and biogeochemical drivers confounds our ability to accurately describe, understand, and predict CO2 concentrations and their changes in the major planetary carbon reservoirs. In this proof-of-concept study, we focus on using well-understood data that are based on observations, as well as model results from the NASA Goddard Institute for Space Studies (GISS) climate model. Our analysis shows that ocean carbon states are associated with the subtropical–subpolar gyre during the colder months of the year and the tropics during the warmer season in the North Atlantic basin. Conversely, in the Southern Ocean, the ocean carbon states can be associated with the subtropical and Antarctic convergence zones in the warmer season and the coastal Antarctic divergence zone in the colder season. With respect to model evaluation, we find that the GISS model reproduces the cold and warm season regimes more skillfully in the North Atlantic than in the Southern Ocean and matches the observed seasonality better than the spatial distribution of the regimes. Finally, the ocean carbon states provide useful information in the model error attribution. Model air–sea CO2 flux biases in the North Atlantic stem from wind speed and salinity biases in the subpolar region and nutrient and wind speed biases in the subtropics and tropics. Nutrient biases are shown to be most important in the Southern Ocean flux bias. All data and analysis scripts are available at https://data.giss.nasa.gov/oceans/carbonstates/ (DOI: https://doi.org/10.5281/zenodo.996891).

The North Atlantic subtropical gyre (NASTG) is a model of the future ocean under climate change. Ocean warming signals are hidden within the blue color of these clear waters and can be tracked by understanding the dynamics among phytoplankton chlorophyll ([Chl]) and colored dissolved organic matter (CDOM). In NASTG, [Chl] and CDOM are strongly correlated. Yet, this unusual correlation for open oceans remains unexplained. Here, we test main hypotheses by analyzing high spatiotemporal resolution data collected by Biogeochemical‐Argo floats between 2012 and 2018. The direct production of CDOM via phytoplankton metabolism is the main occurring mechanism. More importantly, CDOM dynamics strongly depend on the abundance of picophytoplankton. Our findings thus highlight the critical role of these small organisms under the ocean warming scenario. Picophytoplankton will enhance the production of colored dissolved compounds and, ultimately, impact on the ocean carbon cycle., Key Points Colored dissolved organic matter is produced via phytoplankton metabolism in the North Atlantic subtropical gyrePicophytoplankton shapes the dynamics of colored dissolved organic matter within the mixed layer of the North Atlantic subtropical gyrePicophytoplankton may become a critical source of colored dissolved organic matter under ocean warming

Seawater total alkalinity (TA) is one important determinant used to monitor the ocean carbon cycle , whose spatial distributions have previously been characterized along the United States East Coast via discrete bottle samples. Using these data, several regional models for TA retrievals based on practical salinity (S) have been developed. Broad-scale seasonal or interannual variations, however, are not well resolved in these models and existing data are highly seasonally biased. This study reports findings from the first long duration deployment of a new, commercially available TA titrator aboard a research vessel and the continuous underway surface TA measurements produced. The instrument, operated on seven East Coast USA cruises during six months in 2017 and for two months in 2018 on the summertime East Coast Ocean Acidification survey (ECOA-2), collected a total of nearly 11,000 surface TA measurements. Data from these efforts, along with a newly synthesized set of more than 11,000 regional surface TA observations, are analyzed to re-examine distributions of TA and S along the United States East Coast. Overall, regional distributions of S and TA generally agreed with prior findings, but linear TA:S regressions varied markedly over time and deviated from previously developed models. This variability is likely due to a combination of biological, seasonal, and episodic influences and indicates that substantial errors of ±10–20 μmol kg −1 in TA estimation from S can be expected due to these factors. This finding has likely implications for numerical ecosystem modeling and inorganic carbon system calculations. New results presented in this paper provide refined surface TA:S relationships, present more data in space and time, and improve TA modeling uncertainty.

Future climate change is usually projected by coupled earth system models under specific emission scenarios designed by integrated assessment models (IAMs), and this offline approach means there is no interaction between the coupled earth system models and the IAMs. This paper introduces a new method to design possible future emission scenarios and corresponding climate change, in which a simple economic and climate damage component is added to the coupled earth system model of Beijing Normal University (BNU-ESM). With the growth of population and technological expertise and the declining emission-to-output ratio described in the Dynamic Integrated Climate-Economy model, the projected carbon emission is 13.7 Gt C, resulting in a 2.4 °C warming by the end of the twenty-first century (2080–2099) compared with 1980–1999. This paper also suggests the importance of the land and ocean carbon cycle in determining the CO2 concentration in the atmosphere. It is hoped that in the near future the next generation of coupled earth system models that include both the natural system and the social dimension will be developed.

Gephyrocapsa oceanica is a widespread species of coccolithophore that has a significant impact on the global carbon cycle through photosynthesis and calcium carbonate precipitation. We investigated combined effects of light (50 μmol m−2 s−1, 190 μmol m−2 s−1, and 400 μmol m−2 s−1) and the nitrogen sources NO3− and NH4+ on its physiological performance under nitrogen-limited conditions. The specific growth rate was highest at the mid-range light level of 190 μmol m−2 s−1, where it was further accelerated by NH4+ relative to NO3−. There were no significant growth rate differences between NO3−- and NH4+-grown cells at the two light levels either above or below this optimum irradiance. Cellular particulate organic carbon (POC) and nitrogen (PON) content were not significantly affected by different light intensities and nitrogen sources. However, both the cellular particulate inorganic carbon (PIC) content and the PIC to POC ratio were greatly decreased by increased light levels, and were further decreased by NH4+ only at the highest light level. Non-photochemical quenching (NPQ) increased with increasing light intensity, and was higher in NO3− rather than in NH4+-grown cells at medium and high light intensities. Our results demonstrate that under low, relatively realistic oceanic nitrogen concentrations, increasing light intensity and the replacement of NO3− by NH4+ would have a significant negative effect on the calcification of the coccolithophore G. oceanica. If these findings are also applicable to other coccolithophore species, the future ocean carbon cycle may be greatly affected.

In an ocean reanalysis, historical observations are combined with ocean and biogeochemical general circulation models to produce a reconstruction of the oceanic properties in past decades. This is one possible method to better constrain the role of the ocean carbon cycle in the determination of the air-sea CO2 flux. In this work, we investigate how the assimilation of physical variables and subsequently the combined assimilation of physical data and inorganic carbon variables - namely dissolved inorganic carbon (DIC) and alkalinity - affect the modelling of the marine carbonate system and the related air-sea CO2 fluxes. The performance of the two assimilation exercises are quantitatively assessed against the assimilated DIC and alkalinity data and the independent ocean surface pCO(2) observations from global datasets. We obtain that the assimilation of physical observations has contrasting effects in different ocean basins when compared with the DIC and alkalinity data: it reduces the root-mean square error against the observed pCO(2) in the Atlantic and Southern oceans, while increases the model error in the North Pacific and Indian Oceans. In both cases the corrected evaporation rates are the major factor determining the changes in concentrations. The assimilation of inorganic carbon variables on top of the physical data gives a generalized improvement in the model error of inorganic carbon variables, also improving the annual mean and spatial distribution of air-sea fluxes in agreement with other published estimates. These results indicate that data assimilation of physical and inorganic carbon data does not guarantee the improvement of the simulated pCO(2) in all the oceanic regions; nevertheless, errors in pCO(2) are reduced by a factor corresponding to those associated with the air-sea flux formulations. (C) 2016 Elsevier B.V. All rights reserved.