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Among blue carbon ecosystems, seagrass meadows have been highlighted for their contribution to the ocean carbon cycle and climate change mitigation derived from their capacity to store large amounts of carbon over long periods of time in their sediments. Most of the available estimates of carbon stocks beneath seagrass meadows are based on the analysis of short sediment cores in very limited numbers. In this study, high-resolution seismic reflection techniques were applied to obtain an accurate estimate of the potential size of the organic deposit underlying the meadows of the Mediterranean seagrass Posidonia oceanica (known as ‘matte’). Seismic profiles were collected over 1380 km of the eastern continental shelf of Corsica (France, Mediterranean Sea) to perform a large-scale inventory of the carbon stock stored in sediments. The seismic data were ground-truthed by sampling sediment cores and using calibrated seismo-acoustic surveys. The data interpolation map highlighted a strong spatial heterogeneity of the matte thickness. The height of the matte at the site was estimated at 251.9 cm, being maximum in shallow waters (10–20 m depth), near river mouths and lagoon outlets, where the thickness reached up to 867 cm. Radiocarbon dates revealed the presence of seagrass meadows since the mid-Holocene (7000–9000 cal yr BP). Through the top meter of soil, the matte age was estimated at 1656 ± 528 cal yr BP. The accretion rate showed a high variability resulting from the interplay of multiple factors. Based on the surface area occupied by the meadows, the average matte thickness underneath them and the carbon content, the matte volume and total Corg stock were estimated at 403.5 ± 49.4 million m3 and 15.6 ± 2.2 million t Corg, respectively. These results confirm the need for the application of large-scale methods to estimate the size of the carbon sink associated with seagrass meadows worldwide., This work would not have been possible without the participation of the oceanographic research vessel L'Europe (IFREMER) and its crew (GENAVIR), provided by the Flotte Océanographique Française for the CoralCorse, PosidCorse and Carbonsink surveys. This research was financially supported by the Office Français de la Biodiversité (AAMP/15/065 and UCPP 2510-AFB/2018/274), the Collectivité de Corse (PADDUC-CHANGE program; 17-DESR-SR-87), the Office de l’Environnement de la Corse (UCPP 2019-156) and the Direction Régionale de l’Environnement, de l’Aménagement et du Logement de Corse (2015073–0001). This research was part of the Interreg Italy-France Marittimo 2014–2020 cooperation program - GIREPAM project (E76J16001050007). The authors would like to express their gratitude to PhD researcher C. Luzzu (Biosurvey Company), and researchers from the University of Palermo for their contributions to the seismic acquisition during Sismat survey as well as the University Grant program of the Information Handling Services company (IHS Inc. www.ihs.com) by providing the access to the Kingdom PAKaged Suite + software.

Diatoms are the dominant algal group that cycles dissolved silicic acid in the ocean; they also play an important role in the oceanic carbon cycle. It is therefore important to quantify the spatial distribution of silica cycling for defining global ocean biogeochemical cycles. On the research cruise CDisK-IV, water samples and sediment cores were collected at 5 stations along a North Pacific transect near 150oW from 22oN to 50oN to evaluate benthic remineralization rates of biogenic silica (bSi). Two independent methods, core incubation and diffusive transport based on porewater profiles, were utilized to estimate benthic silicic acid fluxes, and these independent methods yield fluxes that agree within uncertainties. The benthic fluxes are reported as 0.04 ± 0.01, 0.04 ± 0.01, 0.05 ± 0.01, 0.67 ± 0.14, 0.40 ± 0.08 mmol Si m^(−2) day^(−1) for Stations 1 to 5, south to north, respectively. Burial fluxes were estimated using measurements of solid phase bSi in sediments and literature values of sediment accumulation rate. Burial efficiencies of bSi at all stations were

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.

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%.

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 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.

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.

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.

Thaumarchaeota, the major sources of marine glycerol dibiphytanyl glycerol tetraether lipids (GDGTs), are believed to fix the majority of their carbon directly from dissolved inorganic carbon (DIC). The δ13C values of GDGTs (δ13CGDGT) may be powerful tools for reconstructing variations in the ocean carbon cycle, including paleoproductivity and water mass circulation, if they can be related to values of δ13CDIC. To date, isotope measurements primarily are made on the C40 biphytane skeletons of GDGTs, rather than on complete tetraether structures. This approach erases information revealed by the isotopic heterogeneity of GDGTs within a sample and may impart an isotopic fractionation associated with the ether cleavage. To circumvent these issues, we present δ13C values for GDGTs from twelve recent sediments representing ten continental margin locations. Samples are purified by orthogonal dimensions of HPLC, followed by measurement of δ13C values by Spooling Wire Microcombustion (SWiM)-isotope ratio mass spectrometry (IRMS) with 1σ precision and accuracy of ±0.25‰. Using this approach, we confirm that GDGTs, generally around −19‰, are isotopically “heavy” compared to other marine lipids. However, measured δ13CGDGT values are inconsistent with predicted values based on the 13C content of DIC in the overlying water column and the previously-published biosynthetic isotope fractionation for a pure culture of an autotrophic marine thaumarchaeon. In some sediments, the isotopic composition of individual GDGTs differs, indicating multiple source inputs. The data appear to confirm that crenarchaeol primarily is a biomarker for Thaumarchaeota, but its δ13C values still cannot be explained solely by autotrophic carbon fixation. Overall the complexity of the results suggests that both organic carbon assimilation (ca. 25% of total carbon) and multiple source(s) of exogenous GDGTs (contributing generally

The Ediacaran-Cambrian transition was a critical period of Earth history because of the coincidence among climate change, bioevolution, and tectonic activity. The nature of carbon and sulfur-cycling processes spanning the Ediacaran-Cambrian boundary remains debatable. This study considers high-resolution stable isotope chemostratigraphy of the Sugetbrak and Yutixi sections of the Keping area of the Tarim Basin, northwestern China. The sections display a pronounced negative δ13Ccarb shift (the Basal Cambrian Carbon Isotope Excursion) that spans the Ediacaran-Cambrian boundary. Deposits overlying the negative excursion are characterized by heavier δ13Ccarb values ranging from −7.8‰ to +4.1‰. The biostratigraphically calibrated δ13Ccarb profiles likely reflect widespread perturbation of the global oceanic carbon cycle and associated mass extinction and subsequent biological evolution. The δ34Spy profiles of the studied sections display a wide distribution of values, ranging from −16.4‰ to 23.8‰. The variable δ34Spy signatures of these sections are partially controlled by microbial sulfate reduction rates, which are sensitive to local sulfate concentration, availability of organic matter and the marine redox state. The strong coupling between enhanced organic carbon and pyrite burial, the appearance of Cambrian-type animals and a shift to a more oxidizing marine environment might suggest a temporal causality of higher oxygen levels of the atmosphere-ocean realm with bio-events during the Ediacaran–Cambrian transition. The development of ecosystems across the Ediacaran–Cambrian transition may have significantly impacted the oceanic carbon and sulfur biogeochemical processes in Earth's history and, in turn, affecting themselves.

The Mediterranean Sea is an oligotrophic semi-closed environment where atmospheric deposition is expected to be an important driver for biological activity. The present study uses a three-dimensional atmospheric chemistry transport model to evaluate the nitrogen (N), phosphorus (P) and iron (Fe) atmospheric deposition fluxes to the Mediterranean Sea. The model takes into account both the inorganic and organic fractions of these fluxes and compares them to other sources of these nutrients that are external to the ocean. The estimated atmospheric deposition fluxes of soluble nutrients amount to 1281 Gg-N y−1, 4.31 Gg-P y−1 and 6.32 Gg-Fe y−1 for the present day, and are within the range of the few estimates available in literature. An almost 6-fold increase in the atmospheric deposition of soluble N is also calculated to be the result of the increase in anthropogenic and biomass burning emissions since 1850, while soluble P and Fe deposition fluxes have increased by 59% and 114%, respectively. For future (2100) emissions, however, N deposition is projected to increase only slightly (4%) while soluble P and Fe fluxes will decrease by 34% and 32% compared to current estimates. The soluble organic N and P annual deposition fluxes are calculated to be 12% and 28–83% of total soluble N and P present-day annual deposition fluxes into the Mediterranean Sea, respectively. However, to reconcile with the observed fluxes in the west and the east Mediterranean, ∼14 times higher flux of soluble P, in particular organic P, and at least 2.5 times higher flux of soluble Fe need to be considered in the model. Such high fluxes can be due to higher combustion emissions of soluble Fe and P, to higher dust emissions or solubilisation of Fe and P contained in dust aerosols, and also higher organic P flux associated with bioaerosols than currently used in the global models. Overall, the calculated deposition fluxes provide an integrated spatially complete picture of the atmospheric inputs to the Mediterranean marine ecosystem, which are potentially important for net primary production and the ocean carbon cycle.

The NASA Carbon Monitoring System (CMS) Flux Project aims to attribute changes in the atmospheric accumulation of carbon dioxide to spatially resolved fluxes by utilizing the full suite of NASA data, models, and assimilation capabilities. For the oceanic part of this project, we introduce ECCO2-Darwin, a new ocean biogeochemistry general circulation model based on combining the following pre-existing components: (i) a full-depth, eddying, global-ocean configuration of the Massachusetts Institute of Technology general circulation model (MITgcm), (ii) an adjoint-method-based estimate of ocean circulation from the Estimating the Circulation and Climate of the Ocean, Phase II (ECCO2) project, (iii) the MIT ecosystem model “Darwin”, and (iv) a marine carbon chemistry model. Air–sea gas exchange coefficients and initial conditions of dissolved inorganic carbon, alkalinity, and oxygen are adjusted using a Green’s Functions approach in order to optimize modeled air–sea CO2 fluxes. Data constraints include observations of carbon dioxide partial pressure (pCO2) for 2009–2010, global air–sea CO2 flux estimates, and the seasonal cycle of the Takahashi et al. (2009) Atlas. The model sensitivity experiments (or Green’s Functions) include simulations that start from different initial conditions as well as experiments that perturb air–sea gas exchange parameters and the ratio of particulate inorganic to organic carbon. The Green’s Functions approach yields a linear combination of these sensitivity experiments that minimizes model-data differences. The resulting initial conditions and gas exchange coefficients are then used to integrate the ECCO2-Darwin model forward. Despite the small number (six) of control parameters, the adjusted simulation is significantly closer to the data constraints (37% cost function reduction, i.e., reduction in the model-data difference, relative to the baseline simulation) and to independent observations (e.g., alkalinity). The adjusted air–sea gas exchange parameter differs by only 3% from the baseline value and has little impact ( − 0.1 %) on the cost function. The particulate inorganic to organic carbon ratio was increased more than threefold and reduced the cost function by 22% relative to the baseline integration, indicating a significant influence of biology on air–sea gas exchange. The largest contribution to cost reduction (35%) comes from the adjustment of initial conditions. In addition to reducing biases relative to observations, the adjusted simulation exhibits smaller model drift than the baseline. We estimate drift by integrating the model with repeated 2009 atmospheric forcing for seven years and find a volume-weighted drift reduction of, for example, 12.5% for nitrate and 30% for oxygen in the top 300 m. Although there remain several regions with large model-data discrepancies, for example, overly strong carbon uptake in the Southern Ocean, the adjusted simulation is a first step towards a more accurate representation of the ocean carbon cycle at high spatial and temporal resolution.

The Cenozoic Era has been characterized by large perturbations to the oceanic carbon cycle and global climatic changes, but quantifying the magnitude and cause of these shifts is still subject to considerable uncertainty. The boron/calcium (B/Ca) ratio of fossil planktic foraminifera shells is a promising tool for reconstructing surface ocean carbonate chemistry during such events. Previous studies indicate that symbiont-bearing, planktic foraminiferal B/Ca depends on the [B(OH) 4 − /DIC] ratio of seawater and potentially, when combined with foraminiferal δ 11 B proxy reconstructions of B(OH) 4 − , an opportunity to reconstruct surface ocean DIC in the geologic past. There are, however, two barriers towards interpreting records from the pre-Pleistocene era: (1) changes in seawater major ion chemistry in the past might have affected foraminiferal B/Ca; and (2) modern foraminifera species show variable B/Ca calibration sensitivities that cannot be constrained in now-extinct species. Here we address these challenges with new experiments in which we have cultured modern, symbiont-bearing foraminifera Globigerinoides ruber (pink) and Trilobatus sacculifer in seawater with simulated early Cenozoic seawater chemistry (high [Ca], low [Mg], and low [B]T). We explore mechanisms that can account for the inter-species trends that are observed in foraminiferal B/Ca, and propose a framework that can be used to apply B/Ca calibrations to now-extinct species for reconstructing climate perturbations under varying seawater chemistries.

Evaluating carbonate dissolution in deep sea sediments is of key importance in understanding the variation of the carbonate compensation depth and the ocean carbon cycle in the geological past. Since coccoliths are one of the main contributors to oceanic CaCO3, their dissolution and preservation degrees in sediments can be a useful indicator for deep sea carbonate chemistry. Varying coccolith preservation conditions have been found due to dissolution caused by organic matter degradation in the recent surface sediments of the East China Sea, which provides a good basis for the study of coccolith morphological and assemblage responses to dissolution. We measured the coccolith weight, thickness, and length of Gephyrocapsa spp. (>3 μm) using a circularly polarized light microscope. It has been found that Gephyrocapsa spp. (>3 μm) coccoliths become thinner and lighter in response to dissolution, and coccolith assemblages are also altered in poorly preserved sediments. This phenomenon was confirmed by an acidification experiment on a sediment sample, which also showed that coccoliths became thinner and lighter under increasingly acidified conditions. There is selective dissolution, i.e., Emiliania huxleyi coccoliths are most dissolution-prone, followed by Gephyrocapsa spp. ( 3 μm), and Helicosphaera spp.. Coccolith morphological parameters can be used to quantitatively evaluate coccolith preservation and dissolution in sediment samples. We suggest that using size-normalized weight, a mean coccolith weight loss of ~30–50% can be assigned to moderate-poor preservation for coccoliths, as reflected by the measured coccolith morphological changes in the surface sediments and in the acidification experiment.

Vertical distributions of dissolved organic carbon (DOC) were determined in different seasons in the southern Adriatic and Tyrrhenian Seas to study the role of stratification in DOC patterns. These two seas are located at similar latitude but differ in extents of vertical stratification. Stratification affects DOC dynamics in both basins, but with interesting differences. In the Tyrrhenian Sea, where the upper water column was stratified during all cruises, DOC showed high surface layer values without substantial seasonality. By contrast, in the southern Adriatic Sea, the seasonal cycle of stratification forced opposite trends in the 0–50 m and 50–800 m stocks, with DOC removal from the upper 50 m associated with DOC increase below 50 m. Regarding DOC export via deep water formation in the southern Adriatic Sea, we estimate that 0.19 Tg C yr−1 was exported to the 50–800 m layer by convective overturn, while 0.85–1.19 Tg C yr−1 was sequestered below 1000 m due to a continental shelf pump mechanism. We hypothesize that enhanced stratification associated with a warmer ocean could further increase DOC concentrations in the mixed layer, changing the role of DOC in the oceanic carbon cycle.

Recent observations of calcium carbonate (CaCO 3 ) mineral undersaturations on the Bering Sea shelf have prompted new interest in the physical and biological factors that control the inorganic carbon system in the region. Understanding of the dynamics that influence the spatio-temporal variability of total alkalinity (TA) – one major component of the seawater carbonate system – has been constrained by limited historical data collected across the shelf, and the consensus has been that TA is largely conservative. However, the recently documented undersaturated conditions have the potential to cause substantial non-conservative variability in TA in this region through the dissolution of carbonate minerals. In order to quantify the contribution of carbonate mineral precipitation and dissolution to variability in TA on the southeastern Bering Sea shelf, we examined seasonal observations of TA that were made between 2008 and 2010 as part of the BEST-BSIERP Bering Sea Project. Conservative influences accounted for most of the variability in TA concentrations, with well-constrained mixing dominating in spring and summer of 2008. Bering Shelf Water (BSW) contained a constant ratio of TA to salinity, while river discharge (RW) added TA relative to salinity at a predictable rate. Although substantial organic carbon production and denitrification can cause some non-conservative variation in TA concentrations (a maximum of ~ 15 μmol kg SW − 1 combined), carbonate mineral dissolution and precipitation were shown to be the most important processes responsible for non-conservative TA–salinity relationships. CaCO 3 uptake by the dominant pelagic phytoplankton calcifier (i.e., coccolithophores) was shown to alter TA concentrations by as much as 59 μmol kg SW − 1 . Evidence for shallow-water CaCO 3 mineral dissolution was also observed, which caused TA concentrations to increase by as much as 36 μmol kg SW − 1 . Therefore, contrary to our previous understanding, the non-conservative physico-biogeochemical factors observed in this study play an important role in controlling the ocean carbon cycle of the Bering Sea shelf.

A pronounced negative δ 13 C shift that can be potentially correlated with the Shuram excursion has been reported from middle Ediacaran strata in the Yangtze Gorges area of South China. Whether it represents a perturbation to the ocean carbon cycle or a record of post-depositional alteration is still open to debate. Resolving this controversy will help clarify if δ 13 C variations can be used for chemostratigraphic correlation of Ediacaran successions. To further understand the regional pattern of Ediacaran carbon isotopic excursions in the Yangtze platform, we carried out a detailed δ 13 C analysis of the Lianghong section in the western part of the Yangtze platform. The Ediacaran System at Lianghong is overlain by the Maidiping Formation yielding early Cambrian small shelly fossils and underlain by the Cryogenian Lieguliu Formation diamictite and tuffaceous siltstones. It comprises the Guanyinya and Hongchunping formations, which have been traditionally correlated with the Doushantuo and Dengying formations, respectively, in the Yangtze Gorges area. Two negative δ 13 C excursions occur in the Lianghong section. The lower one at the uppermost Guanyinya Formation, with a nadir at − 8.6‰, may be correlated with the pronounced negative δ 13 C shift (EN3) in the uppermost Doushantuo Formation in the Yangtze Gorges area and possibly with the well known Shuram event in Oman. The upper negative δ 13 C excursion occurs in the upper Hongchunping Formation and may be correlated with negative excursions (EN4) near the Ediacaran/Cambrian boundary. Other negative δ 13 C excursions (e.g., EN1 and EN2) are not expressed in the Lianghong section because the lower Guanyinya Formation is dominated by siliciclastic rocks. Combined with previously published Ediacaran δ 13 C profiles, our results indicate that the EN3 excursion (likely a Shuram equivalent) may occur widely in South China and can be a useful chemostratigraphic feature for regional and global stratigraphic correlation.

Global climate and environmental change affect the biogeochemistry and ecology of aquatic systems mostly due to a combination of natural and anthropogenic factors. The latter became more and more important during the past few thousand years and particularly during the ‘Anthropocene’. However, although they are considered important in this respect as yet much less is known from tropical than from high latitude coasts. Tropical coasts receive the majority of river inputs into the ocean, they harbor a variety of diverse ecosystems and a majority of the population lives there and economically depends on their natural resources. This review delineates the biogeochemical response of coastal systems to environmental change and the interplay of natural and anthropogenic control factors nowadays and in the recent geological past with an emphasis on tropical regions. Weathering rates are higher in low than in high latitude regions with a maximum in the SE Asia/Western Pacific region. On a global scale the net effect of increasing erosion due to deforestation and sediment retention behind dams is a reduced sediment input into the oceans during the Anthropocene. However, an increase was observed in the SE Asia/Western Pacific region. Nitrogen and phosphorus inputs into the ocean have trebled between the 1970s and 1990s due to human activities. As a consequence of increased nutrient inputs and a change in the nutrient mix excessive algal blooms and changes in the phytoplankton community composition towards non-biomineralizing species have been observed in many regions. This has implications for foodwebs and biogeochemical cycles of coastal seas including the release of greenhouse gases. Examples from tropical coasts with high population density and extensive agriculture, however, display deviations from temperate and subtropical regions in this respect. According to instrumental records and observations the present-day biogeochemical and ecological response to environmental change appears to be on the order of decades. A sediment record from the Brazilian continental margin spanning the past 85,000 years, however, depicts that the ecosystem response to changes in climate and hydrology can be on the order of 1000–2000 years. The coastal ocean carbon cycle is very sensitive to Anthropocene changes in land-derived carbon and nutrient fluxes and increasing atmospheric carbon dioxide. As opposing trends in high latitude regions tropical coastal seas display increasing organic matter inputs and reduced calcification rates which have important implications for calcifying organisms and the carbon source or sink function of the coastal ocean. Particularly coral reefs which are thriving in warm tropical waters are suffering from ocean acidification. Nevertheless, they are not affected uniformly and the sensitivity to ocean acidification may vary largely among coral reefs. Therefore, the prediction of future scenarios requires an improved understanding of present and past responses to environmental change with particular emphasis put on tropical regions.

Moored sediment traps were deployed from January 2004 through December 2007 at depths of 550 and 800 m in San Pedro Basin (SPB), CA (33133.0 0N, 118126.50W). Additionally, floating sediment traps were deployed at 100 and 200 m for periods of 12–24 h during spring 2005, fall 2007, and spring 2008. Average annual fluxes of mass, particulate organic carbon (POC), d 13 Corg, particulate organic nitrogen (PON), d 15 N-PON, biogenic silica (bSiO2), calcium carbonate (CaCO3), and detrital material (nonbiogenic) were coupled with climate records and used to examine sedimentation patterns, vertical flux variability, and organic matter sources to this coastal region. Annual average flux values were determined by binning data by month and averaging the monthly averages. The average annual fluxes to 550 m were 516742 mg/m 2 d for mass (sdom of the monthly averages, n¼117), 3.1870.26 mmol C/m 2 d for POC (n¼ 111), 0.7070.05 mmol/m 2 d for CaCO3 (n¼ 110), 1.3170.21 mmol/m 2 df or bSiO 2 (n¼115), and 0.3570.03 mmol/m 2 df or PON (n¼ 111). Fluxes to 800 and to 550 m were similar, within 10%. Annual average values of d 13 Corg at 550 m were � 21.870.2% (n¼ 108), and d 15 N averages were 8.970.2% (n¼ 95). The timing of both high and low flux particle collection was synchronous between the two traps. Given the frequency of trap cup rotation (4–11 days), this argues for particle settling rates Z83 m/d for both high and low flux periods. The moored traps were deployed over one of the wettest (2004–2005

Mesoscale processes play fundamental roles in the dynamics and biogeochemistry of the Southern Ocean. Previous modeling studies with coarse resolution models show varying estimates of carbon uptake there due to the differences in the representation of physical circulation and mesoscale processes. We develop a new eddy-permitting carbon cycle model of the Southern Ocean with improved representation of eddies, jets, and frontal structures. The model employs physical circulation fields from a Southern Ocean State Estimate: a model solution that is constrained by a suite of remotely sensed and in situ measurements to ensure realistic hydrographic structures and transport fields. A simple biogeochemistry model is coupled to the physical model to simulate the cycling of alkalinity, carbon, and nutrients. In this paper we compare simulated biogeochemical tracers with observations, and diagnose mechanisms controlling the regional carbon cycle. Model-data comparisons in Drake Passage and CLIVAR repeat sections provide tests for the model performance in capturing observed biogeochemical properties. Temporal variability of air-sea CO2 flux is primarily controlled by surface pCO2 on seasonal timescale, modulated by rapid fluctuations in surface wind speed on a shorter timescale. Analysis of simulated carbon transport indicates that the zonal mean circulation transports carbon southward, partly compensated by the northward stationary and transient eddy fluxes. Variability of zonal mean circulation, which is strongly correlated with Ekman transport, dominates the temporal variability of meridional carbon transport across the Polar Front.

A growing number of studies investigated the feedback between the carbon cycle and the climate system. Modeling studies evolved from analysis based on simple land or ocean carbon cycle models to comprehensive Earth System Models accounting for state-of-the-art climate models coupled to land and ocean biogeochemical models. So far, there is a general agreement that climate change negatively affects the oceanic uptake of carbon. On land there was a similar agreement until recently where new studies showed that warming could reduce nitrogen limitation to growth, reducing the amplitude, or even changing the sign of, the land feedback. In parallel, alternative approaches used the observational record of atmospheric CO2 and temperature, on time scales ranging from interannual to millennial, to estimate the climate–carbon cycle feedback. These studies confirmed that at the global scale, warming leads to a release of CO2 from the land/ocean system to the atmosphere. Whether these observations can strongly constrain the magnitude of the feedback under future climate change is still under investigation.

The ongoing regression of sea ice cover is expected to significantly affect the fate of organic carbon over the Arctic continental shelves. Long-term moored sediment traps were deployed in 2005–2006 in the Beaufort Sea, Northern Baffin Bay and the Laptev Sea to compare the annual variability of POC fluxes and to evaluate the factors regulating the annual cycle of carbon export over these continental shelves. Annual POC fluxes at 200 m ranged from 1.6 to 5.9 g C m −2 yr −1 with the highest export in Northern Baffin Bay and the lowest export over the Mackenzie Shelf in the Beaufort Sea. Each annual cycle exhibited an increase in POC export a few weeks before, during, or immediately following sea ice melt, but showed different patterns over the remainder of the cycle. Enhanced primary production, discharge of the Lena River, and resuspension events contributed to periods of elevated POC export over the Laptev Sea slope. High POC fluxes in Northern Baffin Bay reflected periods of elevated primary production in the North Water polynya. In the Beaufort Sea sediment resuspension contributed to most of the large export events. Our results suggest that the outer shelf of the Laptev Sea will likely sustain the largest increase in POC export in the next few years due to the large reduction in ice cover and the possible increase in the Lena River discharge. The large differences in forcing among the regions investigated reinforce the importance of monitoring POC fluxes in the different oceanographic regimes that characterize the Arctic shelves to assess the response of the Arctic Ocean carbon cycle to interannual variability and climate change.

Fixed nitrogen is a key nutrient involved in regulating global marine productivity and hence the global oceanic carbon cycle. Oceanic nitrogen (N2) fixation is estimated to supply 8×1012 moles N y−1 to the ocean, approximately equal to current riverine and the atmospheric inputs of fixed N, and between 50 and 100% of current estimates of oceanic denitrification. However, the spatial and temporal variability of N2 fixation remains uncertain, mostly because of the normal low resolution sampling for diazotroph distribution and fixation rates. It is well established that N2 fixation, mediated by the enzyme nitrogenase, is a source of hydrogen (H2), but the extent to which it leads to supersaturation of H2 in oceanic waters is unresolved. Here, we present simultaneous measurements of upper ocean dissolved H2 concentration (nmol L−1), and rates of N2 fixation (μmol N m−3 d−1), determined using 15N2 tracer techniques (at 7 or 15 m), on a transect from Fiji to Hawaii. We find a significant correlation (r=0.98) between dissolved H2 and rates of N2 fixation, with the greatest supersaturation of H2 and highest rates of N2 fixation being observed in the subtropical gyres at the southern (∼18°S) and northern (18°N) reaches of the transect. The lowest H2 saturation and N2 fixation were observed in the equatorial region between 8°S and 14°N. We propose that an empirical relationship between H2 supersaturations and N2 fixation measurements could be used to guide sampling for 15N fixation measurements or to aid the spatial interpolation of such measurements.

A climatological mean distribution for the surface water pCO2 over the global oceans in non-El Nino conditions has been constructed with spatial resolution of 4°(latitude) x 5°(longitude) for a reference year 2000 based upon about 3 million measurements of surface water pCO2 obtained from 1970 to 2007. The database used for this study is about 3 times larger than the 0.94 million used for our earlier paper [Takahashi et al., 2002. Global sea-air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects. Deep-Sea Res. 11, 49, 1601-1622]. A time-trend analysis using deseasonalized surface water pCO2 data in portions of the North Atlantic, North and South Pacific and Southern Oceans (which cover about 27% of the global ocean areas) indicates that the surface water pCO2 over these oceanic areas has increased on average at a mean rate of 1.5 µatm y-1 with basin-specific rates varying between 1.2 +/- 0.5 and 2.1 +/- 0.4 µatm y-1. A global ocean database for a single reference year 2000 is assembled using this mean rate for correcting observations made in different years to the reference year. The observations made during El Nino periods in the equatorial Pacific and those made in coastal zones are excluded from the database. Seasonal changes in the surface water pCO2 and the sea-air pCO2 difference over four climatic zones in the Atlantic, Pacific, Indian and Southern Oceans are presented. Over the Southern Ocean seasonal ice zone, the seasonality is complex. Although it cannot be thoroughly documented due to the limited extent of observations, seasonal changes in pCO2 are approximated by using the data for under-ice waters during austral winter and those for the marginal ice and ice-free zones. The net air-sea CO2 flux is estimated using the sea-air pCO2 difference and the air-sea gas transfer rate that is parameterized as a function of (wind speed)2 with a scaling factor of 0.26. This is estimated by inverting the bomb 14C data using Ocean General Circulation models and the 1979-2005 NCEP-DOE AMIP-II Reanalysis (R-2) wind speed data. The equatorial Pacific (14°N-14°S) is the major source for atmospheric CO2, emitting about +0.48 Pg-C y-1, and the temperate oceans between 14° and 50° in the both hemispheres are the major sink zones with an uptake flux of -0.70 Pg-C y-1 for the northern and -1.05 Pg-C y-1 for the southern zone. The high-latitude North Atlantic, including the Nordic Seas and portion of the Arctic Sea, is the most intense CO2 sink area on the basis of per unit area, with a mean of -2.5 tons-C month-1 km-2. This is due to the combination of the low pCO2 in seawater and high gas exchange rates. In the ice-free zone of the Southern Ocean (50°-62°S), the mean annual flux is small (-0.06 Pg-C y-1) because of a cancellation of the summer uptake v flux with the winter release Of CO2 caused by deepwater upwelling. The annual mean for the contemporary net v uptake flux over the global oceans is estimated to be -1.6+/-0.9 Pg-C y-1, which includes an undersampling correction to the direct estimate of -1.4+/-0.7 Pg-C y-1. Taking the pre-industrial steady-state ocean source of 0.4+/-0.2 Pg-C y-1 into account, the total ocean uptake flux including the anthropogenic CO2 is estimated to be -2.0+/- 1.0 Pg-C y-1 in 2000.

An understanding of microbial interactions in first-year sea ice on Arctic shelves is essential for identifying potential responses of the Arctic Ocean carbon cycle to changing sea-ice conditions. This study assessed dissolved and particulate organic carbon (DOC, POC), exopolymeric substances (EPS), chlorophyll a, bacteria and protists, in a seasonal (24 February to 20 June 2004) investigation of first-year sea ice and associated surface waters on the Mackenzie Shelf. The dynamics of and relationships between different sea-ice carbon pools were investigated for the periods prior to, during and following the sea-ice-algal bloom, under high and low snow cover. A predominantly heterotrophic sea-ice community was observed prior to the ice-algal bloom under high snow cover only. However, the heterotrophic community persisted throughout the study with bacteria accounting for, on average, 44% of the non-diatom particulate carbon biomass overall the study period. There was an extensive accumulation of sea-ice organic carbon following the onset of the ice-algal bloom, with diatoms driving seasonal and spatial trends in particulate sea-ice biomass. DOC and EPS were also significant sea-ice carbon contributors such that sea-ice DOC concentrations were higher than, or equivalent to, sea-ice-algal carbon concentrations prior to and following the algal bloom, respectively. Sea-ice-algal carbon, DOC and EPS-carbon concentrations were significantly interrelated under high and low snow cover during the algal bloom (r values ≥ 0.74, p < 0.01). These relationships suggest that algae are primarily responsible for the large pools of DOC and EPS-carbon and that similar stressors and/or processes could be involved in regulating their release. This study demonstrates that DOC can play a major role in organic carbon cycling on Arctic shelves.

The principal goal in early missions of satellite-borne visible spectral radiometry (ocean colour) was to create synoptic fields of phytoplankton biomass indexed as concentration of chlorophyll-a. In the context of climate change, a major application of the results has been in the modelling of primary production and the ocean carbon cycle. It is now recognised that a partition of the marine autotrophic pool into a suite of phytoplankton functional types, each type having a characteristic role in the biogeochemical cycle of the ocean, would increase our understanding of the role of phytoplankton in the global carbon cycle. At the same time, new methods have been emerging that use visible spectral radiometry to map some of the phytoplankton functional types. Here, we assess the state of the art, and suggest paths for future work. © 2008 Elsevier Inc. All rights reserved.

The Canadian Model of Ocean Carbon (CMOC) has been developed as part of a global coupled climate carbon model. In a stand-alone integration to preindustrial equilibrium, the model ecosystem and global ocean carbon cycle are in general agreement with estimates based on observations. CMOC reproduces global mean estimates and spatial distributions of various indicators of the strength of the biological pump; the spatial distribution of the air–sea exchange of CO 2 is consistent with present-day estimates. Agreement with the observed distribution of alkalinity is good, consistent with recent estimates of the mean rain ratio that are lower than historic estimates, and with calcification occurring primarily in the lower latitudes. With anthropogenic emissions and climate forcing from a 1850–2000 climate model simulation, anthropogenic CO 2 accumulates at a similar rate and with a similar spatial distribution as estimated from observations. A hypothetical scenario for complete elimination of iron limitation generates maximal rates of uptake of atmospheric CO 2 of less than 1 PgC y −1 , or about 11% of 2004 industrial emissions. Even a ‘perfect’ future of sustained fertilization would have a minor impact on atmospheric CO 2 growth. In the long term, the onset of fertilization causes the ocean to take up an additional 77 PgC after several thousand years, compared with about 84 PgC thought to have occurred during the transition into the last glacial maximum due to iron fertilization associated with increased dust deposition.

A reduction of the carbonate-carbon to organic-carbon export rain ratio during glacial times has been advanced to explain the glacial–interglacial atmospheric CO 2 variations. This hypothesis is tested and implications for the dynamics of sedimentary carbonate preservation and dissolution are explored with a multi-box model ( mbm ) of the ocean carbon cycle, fully coupled to a new transient early diagenesis model (called medusa ). A peak reduction of the rain ratio by 40% at the Last Glacial Maximum (LGM) was found to produce a net atmospheric p CO 2 reduction of about 40 ppm. Changing shelf carbonate accumulation rates and continental weathering inputs produced a 55–60 ppm reduction. The combination of the two mechanisms generates a p CO 2 change of 90–95 ppm, which compares well with the observed data. However, the resulting model sedimentary record does not conform to actual sedimentary records. The changes related to continental shelf processes and variable weathering flux depress the calcite saturation horizon (CSH) by about 1 km at the LGM; if rain ratio variations are also considered, that depression increases by another km. In addition to this large amplitude for the CSH, possibly due to the adopted box-model approach, the changing rain ratio also leads to transition zone changes in the model sedimentary record that are opposite in phase with data-based reconstructions. Realistic changes in the aragonite fraction of the carbonate rain were found to have only a minimal impact on atmospheric p CO 2 . Finally, chemical erosion of deep-sea sediment was shown to reduce the amplitude of variation of the sedimentary CCD by about 10–20%. It may provide a mechanism to improve the model-data agreement.

International audience; The mismatch between the 100 and 400 k.y. components of Pleistocene climate and the relative power of those terms from the eccentricity of the Earth's orbit remains a challenge to the Milankovitch hypothesis. Coccolithophores have the potential to respond to parameters of orbital forcing other than insolation, and, as a critical component of the ocean carbon cycle, can act to modify the climate response. The first-direct comparison of coccolith fraction Sr/Ca, alkenone abundance and automated coccolithophore counts, shows that CF Sr/Ca is largely driven by changing production of bloom species, with unusually high Sr/Ca ratios. The periods of high CF Sr/Ca and high bloom production mark periods of high global coccolithophore production, which correlate inversely with the low amplitude 100 and higher amplitude 400 k.y. eccentricity orbital frequency similar to 400 k.y. cycles of coccolithophore bloom production correspond to periods of enhanced carbonate accumulation in some parts of the ocean, deep ocean dissolution in others, positive shifts in global ocean delta(13)C, and acmes of Gephyrocapsa caribbeanica and Emiliania huxleyi. The link between production of coccolithophore blooms and eccentricity may be due to orbital control of silica leakage from the Southern Ocean, to the orbitally defined inverse correlation between insolation and growing season length and the asymptotic growth response to these parameters, or to changes in nutrient input from weathering. During the Pleistocene, the eccentricity induced coccolithophore acmes have no apparent influence on atmospheric carbon dioxide (pCO(2)) due to the shift towards small bloom coccolithophores, or to coupling with increased diatom productivity, or the ballast effect of the calcium carbonate rain, such that Pleistocene climate has no significant variance at the largest amplitude eccentricity forcing of 400 k.y. Coccolithophores and their influence on the carbon cycle may act as a filter between the incident orbital forcing and resultant climate. (c) 2006 Elsevier B.V. All rights reserved.

The ability to predict phytoplankton growth rates under light and nutrient limitation is fundamental to modeling the ocean carbon cycle. Equally fundamental is the ability to predict chlorophyll:carbon ratios, since satellite-based chlorophyll estimates are one of the few data sets to which model output can be compared globally. Because the Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694] model addresses both desiderata, it has become the model of choice for representing photosynthesis in a wide range of ecosystem models. The Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694] model follows previous models in positing that maximum photosynthetic rate can be reached only when nitrogen cell quota (nitrogen:carbon ratio) reaches a fixed maximum value q N , max C . Empirically, this assumption is contradicted by the extremely thorough data set of Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of Thalassiosira fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] and by other studies: maximum growth rate does not seem to require maximum nitrogen cell quota. To the extent that existing models do not reflect this key characteristic, they may fail to yield reliable predictions of chlorophyll:carbon ratios as functions of nitrogen:carbon ratios. They also may fail to reflect differences in growth rates of competing phytoplankton species, an essential feature of state-of-the-art ecosystem models used in biogeochemistry simulations. In the present paper I replace the nitrogen limitation function of previous models by one that does not require maximum nitrogen cell quota to produce maximum photosynthetic rate. This new nitrogen-limitation function permits derivation of a steady-state optimality-based relationship between chlorophyll:carbon ratios and nitrogen:carbon ratios; the predictions of this new model are shown to be at least as good as predictions based on the “chlorophyll a synthesis regulation term” of Geider et al. [Geider, R.J., MacIntyre, H.L., Kana, T.M., 1996. A dynamic model of photoadaptation in phytoplankton. Limnology and Oceanography 41, 1–15; Geider, R.J., MacIntyre, H.L., Kana, T.M., 1998. A dynamic regulatory model of phytoplanktonic acclimation to light, nutrients, and temperature. Limnology and Oceanography 43, 679–694]. The Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of T. fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] data suggest that the relationship between chlorophyll:carbon ratio and nitrogen cell quota is independent of nitrogen source (nitrate vs. ammonium) for nitrogen-limited cells. Finally, a full set of parameters for the Laws and Bannister [Laws, E.A., Bannister, T.T., 1980. Nutrient- and light-limited growth of T. fluviatilis in continuous culture, with implications for phytoplankton growth in the ocean. Limnology and Oceanography 25, 457–473] data set is estimated and used to predict chlorophyll:carbon and nitrogen:carbon ratios as functions of growth rate. This improved conceptualization of nitrogen:carbon and chlorophyll:carbon relationships in photosynthesis should provide a robust theoretical underpinning for a new generation of models of multiple-nutrient limitation.

A decade-long Synthesis and Modeling Project (SMP) was conducted as the final element of the US Joint Global Ocean Flux Study (JGOFS). The SMP goal was to synthesize knowledge gained from field studies into a set of models that reflect our current understanding of the oceanic carbon cycle. Specific, innovative aspects of the project included the close partnership among scientists conducting field, laboratory, remote sensing, and numerical research and the strong emphasis on data management and web-based, public release of models and data products. Several recurrent science themes arose across the SMP effort including: the development of a new generation of ocean ecosystem and biogeochemistry models that include iron limitation, flexible elemental composition, size structure, geochemical functional groups, and particle composition; the application of inverse models and data assimilation techniques to marine food-web data; the creation of whole-ocean synthesis products from the JGOFS global CO 2 survey and other studies; and the analysis and modeling of ecosystem and biogeochemical responses to climate and CO 2 system perturbations on time-scales ranging from seasonal and inter-annual variability to anthropogenic climate warming and longer.

A lower-trophic marine ecosystem model that takes into account both the grazing food web and the microbial food web has been developed to investigate the ocean carbon cycle. The ecosystem model was coupled to an oceanic general circulation model and a simulation was performed to examine the temporal and spatial distribution of primary production in the world ocean. Numerical results revealed that the total amount of annual net primary production reaches nearly 61.2GtC, showing fair agreement with the recent estimates based on the satellite image analysis. The annual flux of particulate organic carbon toward the subsurface layer, viz., the export flux, was evaluated as 5.5GtC. The model results reproduced the general tendency that the regions of low latitude are characterized by high primary productivity as well as low export flux, and suggested a dominant role for the microbial food web in the oceanic carbon cycle.

We present a simplified method for solving the local equilibrium carbonate chemistry in numerical ocean biogeochemistry models. Compared to the methods typically used, the scheme is fast, efficient and compact. The accuracy of the solution is dictated by the number of species retained in the expression for alkalinity and there is almost no computational penalty for retaining minor contributions. We demonstrate that this scheme accurately reproduces the results of the commonly used method in the context of a three-dimensional global ocean carbon cycle model. Using this model we also show that neglecting the regional variations in surface dissolved inorganic phosphorus and silicic acid concentrations can lead to significant systematic bias in regional estimates of air–sea carbon fluxes using such models.

As part of the 2002 Western Arctic Shelf–Basin Interactions (SBI) project, spatio-temporal variability of dissolved inorganic carbon (DIC) was employed to determine rates of net community production (NCP) for the Chukchi and western Beaufort Sea shelf and slope, and Canada Basin of the Arctic Ocean. Seasonal and spatial distributions of DIC were characterized for all water masses (e.g., mixed layer, halocline waters, Atlantic layer, and deep Arctic Ocean) of the Chukchi Sea region during field investigations in spring (5 May–15 June 2002) and summer (15 July–25 August 2002). Between these periods, high rates of phytoplankton production resulted in large drawdown of inorganic nutrients and DIC in the Polar Mixed Layer (PML) and in the shallow depths of the Upper Halocline Layer (UHL). The highest rates of NCP (?1000–2850 mg C m?2 d?1) occurred on the shelf in the Barrow Canyon region of the Chukchi Sea and east of Barrow in the western Beaufort Sea. A total NCP rate of 8.9–17.8×1012 g for the growing season was estimated for the eastern Chukchi Sea shelf and slope region. Very low inorganic nutrient concentrations and low rates of NCP (?

A mechanism of lateral transport of remineralized carbon from the subsurface Weddell Sea into the abyssal world oceans is presented and its impact is quantified. In the Weddell Sea interior, full remineralization of the export production occurs at shallow depths. This shallow, CO2-charged water stands in isopycnal contact with the abyssal world ocean waters to the north of the subpolar Weddell Sea. Via isopycnal water transport, remineralized CO2 is transferred and sequestered in the deep sea. The amount involved is 1.9×1013 g C yr−1, which is equal to at least 6% of the presently estimated world-wide natural CO2 sequestration in the abyssal oceans. It thus constitutes an important component of the lower limb of the global oceanic carbon cycle. There may be more regions like the Weddell Sea, where this mechanism could be active. It is likely to play a significant role on the glacial–interglacial time scale. During a glacial period, reduced CO2 transport via the CIW would tend to increase the atmospheric partial pressure of CO2 as opposed to the generally decreasing pCO2 trend.

The eventual evasion of injected CO2 to the atmosphere is one consideration when assessing deep-sea disposal of CO2 as a potential response option to climate change concerns. Evasion estimated using an ocean carbon cycle model is compared to long-term trajectories for future CO2 emissions, including illustrative cases leading to stabilization of CO2 concentration at various levels. Modeled residence time for CO2 injected into the deep ocean exceeds the 100-year time-scale usually considered in scenarios for future emissions, and the potential impacts of climate change. Illustrative cases leading monotonically to constant CO2 concentration have been highlighted by the Intergovernmental Panel on Climate Change to give guidance on possible timing of emission reductions that may be required to stabilize greenhouse gas concentrations at various levels. For stabilization cases considered, significant modeled evasion does not occur until long after CO2 emissions have reached a maximum and begun to decline. Illustrative cases can also lead to a maximum in CO2 concentration followed by a decline to slowly decreasing concentrations. In such cases, future injection of emissions into the deep ocean leads to lower maximum CO2 concentration, with less effect on concentration later on in time.

We present new uranium-series and radiocarbon measurements for deep-sea corals from the Southern Ocean. These data are used to reconstruct ventilation ages, both at present and at the end of the last glacial period approximately 16 500 years ago. We apply an improved two-component mixing approach to correct uranium-series dates for contaminant thorium and protactinium present in oxide coatings. Calculated seawater radiocarbon values for contemporary samples decrease with depth in the water column and agree with direct seawater radiocarbon measurements for this area. This indicates that deep-sea corals can accurately record seawater radiocarbon distributions. Two of three glacial samples experienced open-system uranium-series systematics, however, a third sample from the Drake Passage yields concordant thorium and protactinium dates as well as seawater values for initial 234U/238U. This coral yields a ventilation age that is approximately 20–40% greater than modern values for its location. This increase is consistent with published deep-sea coral and calibrated planktonic–benthic foraminifera radiocarbon data, suggesting that the glacial oceans as a whole may have been substantially less ventilated, presumably due to decreased formation of North Atlantic Deep Water. An overall decrease in oceanic mixing rates could have contributed to lower dissolved carbon in surface ocean water and lower atmospheric pCO2 during the past glacial period.

During the late Eocene–early Oligocene period of rapid climate change (across Oxygen isotope shift Oi-1) productivity showed major changes in the region around Australia (DSDP Site 592 off New Zealand, ODP Site 763 off NW Australia). We estimated paleoproductivity at these sites using benthic foraminiferal accumulation rates, as well as accumulation rates of siliceous and other calcareous microfossils, and carbon isotope data. In the Eocene productivity was generally higher at Site 763, where it reached 4 maxima, but dropped at about 34.3 Ma. At both sites productivity increased strongly at Oi-1 (as recognized in the oxygen isotopic record at the sites), and fluctuted during the early Oligocene at higher levels than in the Eocene. We attribute the difference in Eocene productivity to differences in oceanographic settings: Site 592 was probably under a N–S flowing western boundary current of the Pacific gyre, whereas Site 763 showed periodic high productivity as a result of local upwelling. This upwelling occurred when a warm, N–S flowing current (proto-Leeuwin current) replaced the cold, S–N flowing eastern boundary current of the Indian Ocean gyre. Upwelling ended at the opening of the Tasman Sea. The earliest Oligocene increase in productivity occurred at many other locations in the Southern Oceans, and is accompanied by a strong increase in carbon isotopic values, indicating increased burial of organic matter on a global scale. This productivity increase as well as the increased burial indicates that the oceanic carbon cycle may have been part of the climate change in the earliest Oligocene.

The field data collected as part of the international Joint Global Ocean Flux Study (JGOFS) provide an unprecedented view of marine biogeochemistry and the ocean carbon cycle. Following the completion of a series of regional process studies, a global CO2 survey, and a decade of sampling at two open-ocean timeseries, US JGOFS initiated in 1997 a final research phase, the Synthesis and Modeling Project (SMP). The objective of the US JGOFS SMP is to ‘‘synthesize knowledge gained from the US JGOFS and related studies into a set of models that reflect our current understanding of the oceanic carbon cycle’’. Here we present an overview of the SMP and highlight the early scientific results from the project. r 2001 Elsevier Science Ltd. All rights reserved.

Understanding the relationship between Earth's climate and the oceanic carbon cycle requires an understanding of the time-variations of CO2 in the ocean, it's exchange with the atmosphere, and the rate of uptake of anthropogenic CO2 by the ocean. Since 1988, hydrographic and biogeochemical data have been collected at the Bermuda Atlantic Time-Series Study (BATS) site in the Sargasso Sea, located in the North Atlantic subtropical gyre. With over a decade of oceanographic data, interannual trends of CO2 species and air–sea exchange of CO2 at BATS can be examined. Between 1988 and 1998, surface seawater total carbon dioxide (TCO2) and salinity normalized TCO2 (nTCO2) increased at a rate of 2.2±6.9 and 1.6±5.8 μmol kg−1 yr−1, respectively. During the same period, the partial pressure of CO2 (pCO2) of seawater increased at a rate of 1.4±10.7 μatm yr−1, similar to the rate of increase in atmospheric pCO2 (∼1.3 μatm yr−1). The increase in seawater TCO2 and pCO2 can be attributed to a combination of uptake of anthropogenic CO2 from the atmosphere and interannual changes in hydrographic properties of the subtropical gyre. Underlying interannual trends were examined by determining how hydrographic and biogeochemical anomalies, or deviations from the mean state, vary over time. Significant correlations existed between anomalies of temperature, salinity, integrated primary production, mixed-layer depth, TCO2, salinity normalized TCO2 (nTCO2), and alkalinity. For example, cold temperature anomalies (up to −0.5°C) in 1992 and 1995 were associated with increased mixed-layer depth, higher rates of integrated primary production (

In simulation studies of the ocean's role in the global carbon cycle, predicting the depth-distribution for remineralization of particulate organic carbon (POC) is of particular importance. Following Sarmiento et al. (Global Biogeochemical Cycles 7 (1993) 417), most simulation models have the power-law curve of Martin et al. (Deep-Sea Research 34 (1987) 267) for this purpose. The Martin et al. curve is an empirical fit to data, most of which is from shallow floating sediment traps. Using such a fit implies that all the information necessary for prediction is contained in the carbon flux itself, so that the organic-carbon flux F OC ( z ) at any depth z can be predicted from the flux of organic carbon F OC ( z 0 ) at some near-surface depth z 0 . Here, we challenge this basic premise, arguing that fluxes of ballast minerals (silicate and carbonate biominerals, and dust) determine deep-water POC fluxes, so that a mechanism-based model of POC flux must simultaneously predict fluxes of both POC and ballast minerals. This assertion is based on the empirical observation that POC fluxes are tightly linked quantitatively to fluxes of ballast minerals in the deep ocean. Here, we develop a model structure that incorporates this observation, and fit this model to US JGOFS EqPac data. This model structure, plus the preliminary parameter estimates we have obtained, can be used to explore the implications of our model in studies of the ocean carbon cycle.

The stable carbon isotopic composition of particulate organic matter in the ocean, δ 13 C POC , shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ 13 C POC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ 13 C POC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ 13 C POC are much greater than in δ 13 C DIC , suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ 13 C POC . Understanding the factors that control isotope variability is a prerequisite when applying δ 13 C POC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ 13 C POC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO 2 in seawater and the organic material produced by algae, e P , is a function of the ambient CO 2 concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, e P , depends on the CO 2 concentration in seawater and on the (instantaneous) growth rates, μ i , of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ 13 C POC distribution. The interhemispheric asymmetry of δ 13 C POC can mostly be attributed to the interhemispheric asymmetry of CO 2 concentration in the water. However, the strong seasonal variations of δ 13 C POC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.

The annually integrated air–sea flux of CO 2 is governed by two quantities: the basic state of the ocean (e.g., the properties of the winter mixed layer) and the signal from the annual cycles. In this study, I focus on the role of the annual cycles in mixing, sea-surface temperature and biological production. By integrating these cycles, it is shown that the annually integrated flux of CO 2 can be written as F = α ( C T W − C Eq ), where F is the mean annual air–sea flux of CO 2 , α is an equilibrium rate constant (m/year), C T W is the total dissolved inorganic carbon concentration in the winter mixed layers and C Eq is the dynamic equilibrium concentration for C T W . In the formula, α and C Eq capture the influence of the annual cycles as well as the spatial variations in, for example, the alkalinity. I analyze data from the Ocean Weather Stations (OWS) in the North Atlantic and some results from the MIT biogeochemical ocean model to find adequate descriptions of α and C Eq . According to these model/data estimates, the influence of the annual cycles in sea-surface temperature and biological production on the dynamic equilibrium concentration is on the order of −5 to −20 and 0–30 μmol/kg, respectively. These numbers are not negligible showing that the annual cycles must be considered when analyzing the oceanic carbon cycle. The spatial distribution of the quantities C T W , α , and C Eq are estimated for the North Atlantic using data from the TTO/NAS expedition and a combination of model and data analyses. The air–sea flux is calculated according to the above formula, and the seawards flux is estimated to be 0.4 Gt C/year for the area north of 30°N.

In order to understand fluctuations in terrestrial and marine environments, a sedimentary core H3571 was investigated from the Hess Rise located in the mid-latitude North Pacific under the north westerly wind system. The δ 18 O , grain size and grain shape of type 1 quartz suggest that this quartz is of aerosol origin. Good correlation between Al and aerosol quartz in content and mass accumulation rate (MAR) indicates that the alumino silicate minerals in the sediments are mainly transported by wind. MAR of mineral aerosol (MARAerosol) varies from 156 to 732 mg cm−2 kyr−1 during the last 200 kyr. The MARAerosol maxima occur in oxygen isotope stage (OIS) 4 to latest OIS 5, middle OIS 6, moderate maxima occur in early OIS 1–2, late OIS 3 and middle OIS 3. These maxima are ascribed to reduced precipitation during the summer monsoon and to strengthened wind speed during the winter monsoon. The mean organic carbon/total nitrogen atomic ratio is 7.2, suggesting that the organic matter in the core H3571 is also mainly of marine origin. The MAR of organic carbon (MAROrganic) exhibits two prominent maxima in OIS 2 and 4 and relatively high values in middle and late OIS 6. Enhanced primary productivity is most likely to be responsible for the high burial rate of organic carbon in the sediments. Carbonate and phosphorus inputs with aerosol into sea surface have played a minor role through dissolution in diminishing PCO2 in surface water during glacial times. On the other hand, the aerosol silica supply to the ocean may have some potential to affect the burial of biogenic silica into sediments. The effect of mineral aerosol on the ocean carbon cycle would be greater in the coastal and hemipelagic regions of the Western Pacific and Equatorial Atlantic, as compared with that in the Hess Rise regime.

Fluxes of dissolved organic carbon (DOC) from marine sediments represent a poorly constrained component of the oceanic carbon cycle that may affect the concentration and composition of DOC in the ocean. Here we report the first in situ measurements of DOC fluxes from continental margin sediments (water depths ranging from 95 to 3,700 m), and compare these fluxes with measured benthic fluxes from 20 other coastal and continental margin sediments. With this combined data set data we have estimated that benthic DOC fluxes are less than ∼10% of sediment carbon oxidation rates, and that the integrated DOC flux from sediments in water depths less than 2,000 m is ∼180 Tg C/yr. These fluxes are roughly equivalent to the riverine DOC flux, and the organic carbon burial rate in marine sediments. Benthic DOC fluxes therefore represent an important net source of DOC to the oceans. We also note that: (1) benthic DOC fluxes represent a loss of organic carbon from sediments; (2) in many sediments these fluxes appear to be controlled by molecular diffusion (i.e., by pore water concentration gradients); (3) pore water DOC may be an important intermediate in sediment carbon burial and preservation. These observations therefore suggest a linkage between benthic DOC fluxes and sediment carbon preservation that may be mediated by pore water DOC concentrations and cycling. The magnitude and fate of DOC effluxing from marine sediments is thus important to understanding carbon cycles and budgets in the marine environment.

The natural and anthropogenic components of the radiocarbon measurements from seawater samples can be successfully separated by an improved method, which is based on a very well-defined relationship between natural radiocarbon and dissolved silica observed mainly during the GEOSECS survey for waters beneath 1000 m depth. This relationship is further reconfirmed by the 14 C measurements from large volume samples taken in the deep waters in the Pacific Ocean during the recent WOCE survey program. Analysis of upper ocean 14 C measurements made along 152°W, and north of 20°N, in the northeastern Pacific Ocean during the NOAA's CGC91 cruise, which is a part of the WOCE survey program, indicates that the bomb 14 C inventory in this part of the ocean has increased by 22% since the GEOSECS measurements made in 1974. This increase is consistent with the model prediction of 25% for the northern hemisphere ocean. Change of the surface water bomb Δ 14 C values during this period is insignificant. This feature is also consistent with the model simulation. Results of this new analysis will provide useful information of the temporal variations of bomb 14 C inventory in the ocean, in addition to the spatial distribution, which can be used as powerful constraints in calibrating the global ocean carbon cycle models, especially those based on three-dimensional ocean general circulation models, for estimating the uptake of CO2 by the ocean.

Ocean carbon cycle modeling is expected to play a key role in decisions concerning the purposeful sequestration of CO2 in the oceans. Modeling is probably the only practical way to know how much good sequestration is doing, or would do if implemented, since even with sequestration, atmospheric CO2 would be expected to continue to increase because of the release of non-sequestered CO2 and the return to the atmosphere of sequestered CO2. We have employed a carbon cycle model based on an ocean general circulation model to estimate the return of sequestered CO2 to the atmosphere for sequestration in the pre-industrial ocean at sites near Tokyo, San Francisco, New York and Miami. Significant differences in the effectiveness of sequestration are found. Off the East Coast of the USA, the atmospheric concentration due to the return of sequestered CO2 quickly rises to near the final equilibrium value, for all depths below about 800 m. Off the West Coast of the USA, the CO2 is effectively hidden from the atmosphere for several hundred years, for depths below 800 m. Then the atmospheric concentration rises to larger values than for sequestration off the East Coast. Sequestration off the coast of Japan at 800 m is similar to sequestration off the West Coast of the USA, but without the time delay. These predicted differences are understandable in terms of ocean circulation. Sequestration in the post-industrial ocean beginning in the year 2000 has been modeled also and sites near Tokyo and New York. This calculation includes additional CO2 from a representation of anthropogenic CO2 for the entire fossil fuel era. No-sequestered CO2 tends to flush the sequestered CO2 out of the ocean by reducing the concentration of the carbonate ion, with which CO2 reacts. Results are less favorable in the post-industrial ocean. The pre-industrial ocean model can help in the selection of location and depth, but the post-industrial ocean model is more realistic. “Thus human beings are now carrying out a large scale geophysical experiment of a kind that could not have happened in the past nor be reproduced in the future. Within a few centuries we are returning to the atmosphere and oceans the concentrated organic carbon stored in sedimentary rocks over hundreds of millions of years” Roger Revelle and Hans Suess.1

Pressure variations along an uneven permeable bottom, generated by the passage of gravity waves and by bottom currents, force water into the sediment below regions of high pressure, such as ripple troughs, and draw it out at regions of low pressure, such as ripple crests. The resulting pore water circulation brings organic matter and oxygen to the interior of the sediment, creates horizontal concentration gradients that can be as strong as the vertical gradients, and increases the flux of pore water constituents across the sediment-water interface. Here we review the present knowledge of pore water circulation and organic carbon mineralization in sandy continental shelf sediments. We argue that the circulation of water through the pores of the sediment enhances the mineralization rate of dissolved and particulate organic matter and that continental shelf sediments play a more important role in the oceanic carbon cycle than is generally thought.