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Recent studies investigating future warming scenarios have shown that the ocean carbon sink will weaken over the coming century due to ocean warming and changes in oceanic circulation. However, significant uncertainties remain regarding the magnitude of the oceanic carbon cycle response to warming. Here, we investigate the Southern Ocean's (SO, south of 40∘ S) carbon cycle response to warmer conditions, as simulated under last interglacial boundary conditions (LIG, 129–115 ka). We find a ∼150 % increase in carbon dioxide (CO2) outgassing over the SO at the LIG compared to pre-industrial conditions (PI), due to a 0.5 ∘C increase in SO sea surface temperatures. This is partly compensated for by an equatorward shift of the Southern Hemisphere (SH) westerlies and weaker Antarctic Bottom Water formation, which both lead to an increase in dissolved inorganic carbon (DIC) in the deep ocean at the LIG compared to PI. These deep-ocean DIC changes arise from increased deep- and bottom-water residence times and higher remineralization rates due to higher temperatures. While our LIG simulation features a large reduction in SO sea ice compared to the PI, we find that changes in sea ice extent exert a minor control on the marine carbon cycle. The projected future strengthening and poleward shift of the SH westerlies coupled to warmer conditions at the surface of the SO should thus weaken the capacity of the SO to absorb anthropogenic CO2 over the coming century.

Recent paleo reconstructions suggest that increased carbon storage in the Southern Ocean during glacial periods contributed to low glacial atmospheric carbon dioxide concentration (pCO2). However, quantifying its contribution in three-dimensional ocean general circulation models (OGCMs) has proven challenging. Here, we show that OGCM simulation with sedimentary process considering enhanced Southern Ocean salinity stratification and iron fertilization from glaciogenic dust during glacial periods improves model-data agreement of glacial deep water with isotopically light carbon, low oxygen, and old radiocarbon ages. The glacial simulation shows a 77-ppm reduction of atmospheric pCO2, which closely matches the paleo record. The Southern Ocean salinity stratification and the iron fertilization from glaciogenic dust amplified the carbonate sedimentary feedback, which caused most of the increased carbon storage in the deep ocean and played an important role in pCO2 reduction. The model-data agreement of Southern Ocean properties is crucial for simulating glacial changes in the ocean carbon cycle., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The climate variability induced by the El Nino-Southern Oscillation (ENSO) cycle drives significant changes in the physical state of the tropical Western Pacific, which has important impacts on the upper ocean carbon cycle. During 2015–2016, a super El Nino event occurred in the equatorial Pacific. Suspended particulate matter (SPM) data and related environmental observations in the tropical Western Pacific were obtained during two cruises in Dec. 2014 and 2015, which coincided with the early and peak stages of this super El Nino event. Compared with the marine environments in the tropical Western Pacific in Dec. 2014, an obviously enhanced upwelling occurred in the Mindanao Dome region; the nitrate concentration in the euphotic zone almost tripled; and the size, mass concentration, and volume concentration of SPM obviously increased in Dec. 2015. The enhanced upwelling in the Mindanao Dome region carried cold but eutrophic water upward from the deep ocean to shallow depths, even into the euphotic zone, which disrupted the previously N-limited conditions and induced a remarkable increase in phytoplankton blooms in the euphotic zone. These results reveal the mechanism of how nutrient-limited ecosystems in the tropical Western Pacific respond to super El Nino events. In the context of the ENSO cycle, if predicted changes in biogenic particles occur, the proportion of carbon storage in the tropical Western Pacific is estimated to be increased by more than 52%, ultimately affecting the regional and possibly even global carbon cycle. This paper highlights the prospect for long-term prediction of the impact of a super El Nino event on the global carbon cycle and has profound implications for understanding El Nino events.

Quantifying the local marine reservoir age (ΔR) and its change over time is critical for precise radiocarbon calibration of marine samples and for the study of the ocean carbon cycle. ΔR values are scarce for the African coast facing the Indian Ocean, and the few values available were obtained from pre-bomb shells collected during the 19th century. Here, the ΔR value for calibrated year 1110 ± 25 (1σ) CE was reconstructed from radiocarbon dating and Bayesian analysis of marine and terrestrial materials coexisting in a tsunami deposit discovered in Pangani Bay (Tanzania, western Indian Ocean coast). The reconstructed ΔR of –8 ± 40 (1σ, n = 3) is similar to pre-bomb regional estimates and provides new information to investigate regional ΔR change over time. The Bayesian analysis of the dated samples revises the age of the tsunami event found in Pangani Bay to 1064–1157 cal CE (95.4% confidence level) or 1110 ± 25 (1σ) cal CE, about one century younger compared to the previous estimate. Our results indicate that the new ΔR value and the proposed calibration approach can be used to refine existing chronologies in the region, with implications for paleo-environmental reconstructions and archaeological studies of Early Swahili societies.

Recent studies indicate that aluminum (Al) could play an important role in the ocean carbon cycle by increasing phytoplankton carbon fixation and reducing organic carbon decomposition. However, how Al may influence the decomposition of organic carbon has not yet been explicitly examined. Here we report the effects of Al on carbon fixation by marine diatoms and their subsequent decomposition. By using radiocarbon as a tracer, the carbon fixation and decomposition of three model marine diatoms were examined in Aquil* media at different concentrations (0, 40, 200, and 2000 nM) of dissolved Al. Addition of Al enhanced net carbon fixation by the diatoms in the declining growth phase (by 9%–29% for Thalassiosira pseudonana, 15%–20% for T. oceanica, 15%–23% for T. weissflogii). Under axenic conditions the decomposition rates (d⁻¹) of the diatom-produced particulate organic carbon (POC) significantly decreased (by 21%–57% for T. pseudonana, 0%–41% for T. oceanica, 29%–58% for T. weissflogii) in the Al-enriched treatments. In the presence of bacteria, the decomposition rates of T. weissflogii-produced POC were still 37%–38% lower in Al-enriched treatments compared to the control. Significant increases in cell size, cellular carbon content (pmol C/cell) and cellular carbon density (pmol C/μm³) of T. weissflogii were also observed in the Al-enriched treatments compared to the control. The Al-related increase in net carbon fixation and cell size, and the decrease in POC decomposition rate may facilitate carbon export to ocean depths. The study provides new evidence for the iron–aluminum hypothesis, which suggests that Al could increase phytoplankton uptake of atmospheric CO₂ and influence climate change.

Nearly every nation has signed the UNFCC Paris Agreement, committing to mitigate anthropogenic carbon emissions so as to limit the global mean temperature increase above pre-industrial levels to well below 2 ∘C, and ideally to no more than 1.5 ∘C. A consequence of emission mitigation that has received limited attention is a reduced efficiency of the ocean carbon sink. Historically, the roughly exponential increase in atmospheric CO2 has resulted in a proportional increase in anthropogenic carbon uptake by the ocean. We define growth of the ocean carbon sink exactly proportional to the atmospheric growth rate to be 100 % efficient. Using a model hierarchy consisting of a common reduced-form ocean carbon cycle model and the Community Earth System Model (CESM), we assess the mechanisms of future change in the efficiency of the ocean carbon sink under three emission scenarios: aggressive mitigation (1.5 ∘C), intermediate mitigation (RCP4.5), and high emissions (RCP8.5). The reduced-form ocean carbon cycle model is tuned to emulate the global-mean behavior of the CESM and then allows for mechanistic decomposition. With intermediate or no mitigation (RCP4.5, RCP8.5), changes in efficiency through 2080 are almost entirely the result of future reductions in the carbonate buffer capacity of the ocean. Under the 1.5 ∘C scenario, the dominant driver of efficiency decline is the ocean's reduced ability to transport anthropogenic carbon from surface to depth. As the global-mean upper-ocean gradient of anthropogenic carbon reverses sign, carbon can be re-entrained in surface waters where it slows further removal from the atmosphere. Reducing uncertainty in ocean circulation is critical to better understanding the transport of anthropogenic carbon from surface to depth and to improving quantification of its role in the future ocean carbon sink.

With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)., The spectrophotometric methodology for carbonate ion determination in seawater was first published in 2008 and has been continuously evolving in terms of reagents and formulations. Although being fast, relatively simple, affordable, and potentially easy to implement in different platforms and facilities for discrete and autonomous observations, its use is not widespread in the ocean acidification community. This study uses a merged overdetermined CO2 system data set (carbonate ion, pH, and alkalinity) obtained from 2009 to 2020 to assess the differences among the five current approaches of the methodology through an internal consistency analysis and discussing the sources of uncertainty. Overall, the results show that none of the approaches meet the climate goal (± 1 % standard uncertainty) for ocean acidification studies for the whole carbonate ion content range in this study but usually fulfill the weather goal (± 10 % standard uncertainty). The inconsistencies observed among approaches compromise the consistency of data sets among regions and through time, highlighting the need for a validated standard operating procedure for spectrophotometric carbonate ion measurements as already available for the other measurable CO2 variables., E.F.G. was supported by a Personal Técnico de Apoyo contract (PTA2016-12441-I) and N.M.F. was supported by a Juan de la Cierva postdoctoral contract (FJCI2015-24394), both from the Spanish Ministry of Science, Innovation and Universities and GAIN Grupo de Referencia Competitiva IN607A 2018/2 from Xunta de Galicia. M.I.G.-I. was supported by NOAA’s Ocean Acidification Program (OAP) via Award No. NA17OAR0170332, and by NERC’s CUSTARD (Carbon Uptake and Seasonal Traits of Antarctic Remineralisation Depths) project NE/P021263/1. A.E.R.H. was supported via the 2018 NF-POGO Shipboard Fellowship. F.F.P. was supported by the BOCATS2 (PID2019-104279GB-C21/AEI/10.13039/501100011033) project funded by MCIN/AEI/10.13039/501100011033 and contributing to WATER:iOS CSIC PTI. M.A. was supported by IEO RADIALES, RADPROF, and MEDSHIP18 programs. The MEDWAVES cruise was funded under the ATLAS project (Grant Agreement No. 678760). The RADPROF (2020) cruise was funded under the INTERREG Atlantic Area iFADO project.

As key parts of land-sea transition zones, estuary ecosystems play a very important role in the ocean carbon cycle processes. The sources, degradation, and preservation of dissolved organic matter (DOM) in estuaries have long been the subject of intense study. To examine the aforementioned issues, this study examined three-dimensional fluorescence spectroscopy and ultraviolet-visible absorption spectroscopy to determine the spatial distribution and sources of DOM in the pore water of three sedimentary cores from the Pearl River Estuary (S1, S2 and S3, with increasing salinity). Using the parallel factor analysis (PARAFAC) method to analyze the three-dimensional fluorescence spectrum data, five fluorescent components were obtained—three humic-like components (C1, C3, and C4), and two protein-like components (C2 and C5). C2 exhibited a significant positive correlation with the sediment microbial deoxyribose nucleic acid (DNA) concentration ( R 2=0.69, P R 2=0.40, P S R), which indicates that DOM transitioned from low-molecular-weight protein-like components in the surface sediment to high-molecular-weight humic-like components in the subsurface. This study provides valuable information for understanding the pore water size/reactivity (PWSR) model of DOM and its biochemical processes occurring in estuary sediments.

We show an annual overview of the sea-air CO2 exchanges and primary drivers in the Gerlache Strait, a hotspot for climate change that is ecologically important in the northern Antarctic Peninsula. In autumn and winter, episodic upwelling events increase the remineralized carbon in the sea surface, leading the region to act as a moderate or strong CO2 source to the atmosphere of up to 40 mmol m–2 day–1. During summer and late spring, photosynthesis decreases the CO2 partial pressure in the surface seawater, enhancing ocean CO2 uptake, which reaches values higher than − 40 mmol m–2 day–1. Thus, autumn/winter CO2 outgassing is nearly balanced by an only 4-month period of intense ocean CO2 ingassing during summer/spring. Hence, the estimated annual net sea-air CO2 flux from 2002 to 2017 was 1.24 ± 4.33 mmol m–2 day–1, opposing the common CO2 sink behaviour observed in other coastal regions around Antarctica. The main drivers of changes in the surface CO2 system in this region were total dissolved inorganic carbon and total alkalinity, revealing dominant influences of both physical and biological processes. These findings demonstrate the importance of Antarctica coastal zones as summer carbon sinks and emphasize the need to better understand local/regional seasonal sensitivity to the net CO2 flux effect on the Southern Ocean carbon cycle, especially considering the impacts caused by climate change.

The Australian Community Climate and Earth System Simulator (ACCESS) has been extended to include land and ocean carbon cycle components to form an Earth System Model (ESM). The current version, ACCESS-ESM1.5, has been mainly developed to enable Australia to participate in the Coupled Model Intercomparison Project Phase 6 (CMIP6) with an ESM version. Here we describe the model components and changes to the previous version, ACCESS-ESM1. We use the 500-year pre-industrial control run to highlight the stability of the physical climate and the carbon cycle. The long spin-up, negligible drift in temperature and small pre-industrial net carbon fluxes (0.02 and 0.08 PgC year−1 for land and ocean respectively) highlight the suitability of ACCESS-ESM1.5 to explore modes of variability in the climate system and coupling to the carbon cycle. The physical climate and carbon cycle for the present day have been evaluated using the CMIP6 historical simulation by comparing against observations and ACCESS-ESM1. Although there is generally little change in the climate simulation from the earlier model, many aspects of the carbon simulation are improved. An assessment of the climate response to CO2 forcing indicates that ACCESS-ESM1.5 has an equilibrium climate sensitivity of 3.87°C.

In this study, we evaluate the performance of the Nanjing University of Information Science and Technology (NUIST) Earth System Model version 3 (hereafter NESM v3) in simulating the marine biogeochemical cycle and carbon dioxide ( CO2 ) uptake. Compared with observations, the NESM v3 reproduces the large-scale patterns of biogeochemical fields reasonably well in the upper ocean, including nutrients, alkalinity, dissolved inorganic, chlorophyll, and net primary production. Some discrepancies between model simulations and observations are identified and the possible causes are investigated. In the upper ocean, the simulated biases in biogeochemical fields are mainly associated with shortcomings in the simulated ocean circulation. Weak upwelling in the Indian Ocean suppresses the nutrient entrainment to the upper ocean, thus reducing biological activities and resulting in an underestimation of net primary production and the chlorophyll concentration. In the Pacific and the Southern Ocean, nutrients are overestimated as a result of strong iron limitation and excessive vertical mixing. Alkalinity is also overestimated in high-latitude oceans due to excessive convective mixing. The major discrepancy in biogeochemical fields is that the model overestimates nutrients, alkalinity, and dissolved inorganic carbon in the deep North Pacific, which is caused by the excessive deep ocean remineralization. The model reasonably reproduces present-day oceanic CO2 uptake. Model-simulated cumulative oceanic CO2 uptake is 149 PgC between 1850 and 2016, which compares well with data-based estimates of 150±20 PgC. In the 1 % yr −1 CO2 increase (1pt CO2 ) experiment, the diagnosed carbon-climate ( γ = - 7.9 PgC K −1 ) and carbon-concentration sensitivity parameters ( β=0.88 PgC ppm −1 ) in the NESM v3 are comparable with those in Coupled Model Intercomparison Project phase 5 (CMIP5) models ( β : 0.69 to 0.91 PgC ppm −1 ; γ : −2.4 to −12.1 PgC K −1 ). The nonlinear interaction between carbon-concentration and carbon-climate sensitivity in the NESM v3 accounts for 10.3 % of the total carbon uptake, which is within the range of CMIP5 model results (3.6 %–10.6 %). Overall, the NESM v3 can be employed as a useful modeling tool to investigate large-scale interactions between the ocean carbon cycle and climate change.

Here, we investigate the seasonal variability in the dissolved inorganic carbon (DIC) cycle in the Northwest Pacific using a high-resolution biogeochemical and carbon model coupled with an operational ocean model. Results show that the contribution to DIC from air–sea CO2exchange is generally offset by vertical mixing at the surface at all latitudes, with some seasonal variation. Biological processes in subarctic regions are evident at the surface, whereas in the subtropical region these processes take place within the euphotic layer and then DIC consumption deepens southward with latitude. Such latitudinal differences in biological processes lead to marked horizontal and vertical contrasts in the distribution of DIC, with modulation by horizontal and vertical advection–diffusion processes.

The ocean stores 60 times as much carbon as the atmosphere, and the ocean carbon cycle has a critical role in controlling atmospheric CO2 concentration. The concept of ocean carbon pump is widely used for describing the ocean carbon cycle, but the term “ocean carbon pump” is not necessarily strictly defined and has been differently referred in previous studies. Here, using three dimensional distributions of dissolved inorganic carbon concentration, total alkalinity, phosphate, and salinity, four types of ocean carbon pump (organic matter, calcium carbonate, gas exchange, and freshwater flux pumps) were formulated. Although previously proposed decomposition emphasizes the enrichment in the deep ocean, my decomposition focuses on surface depletion which directly affects air-sea CO2 exchanges. Based on this decomposition, vector diagram for quantifying the individual roles of the pumps in controlling the ocean surface pCO2, which is in balance with atmospheric CO2 concentration, was demonstrated in this study. The method was applied to the observational climatology, and the contributions of the four carbon pump components to atmospheric CO2 were visualized in a single figure (the vector diagram); each carbon pump component was represented by one vector, and its contribution to CO2 concentration was measured from the difference in the contour values between the beginning and end of the vector. The same analysis was also applied to the CMIP5 earth system model simulations. All the models reproduced the same level of atmospheric CO2 concentration as the observation; however, the contributions from the four carbon pumps varied. The vector diagram was shown to quantify the differences in the contributions from the pumps between the models and against the observation. This study demonstrated that the proposed vector diagram analysis is a useful tool for quantifying the individual contributions of the ocean carbon pumps to atmospheric CO2 concentration and is helpful for evaluating the reproducibility of ocean carbon cycle models.

The element carbon plays a central role in climate and life on Earth. It is capable of moving among the geosphere, cryosphere, atmosphere, biosphere and hydrosphere. This flow of carbon is referred to as the Earth's carbon cycle. It is also intimately linked to the cycling of other elements and compounds. The ocean plays a fundamental role in Earth's carbon cycle, helping to regulate atmospheric CO2 concentration. The ocean biological carbon pump (OBCP), defined as a set of processes that transfer organic carbon from the surface to the deep ocean, is at the heart of the ocean carbon cycle. Monitoring the OBCP is critical to understanding how the Earth's carbon cycle is changing. At present, satellite remote sensing is the only tool available for viewing the entire surface ocean at high temporal and spatial scales. In this paper, we review methods for monitoring the OBCP with a focus on satellites. We begin by providing an overview of the OBCP, defining and describing the pools of carbon in the ocean, and the processes controlling fluxes of carbon between the pools, from the surface to the deep ocean, and among ocean, land and atmosphere. We then examine how field measurements, from ship and autonomous platforms, complement satellite observations, provide validation points for satellite products and lead to a more complete view of the OBCP than would be possible from satellite observations alone. A thorough analysis is then provided on methods used for monitoring the OBCP from satellite platforms, covering current capabilities, concepts and gaps, and the requirement for uncertainties in satellite products. We finish by discussing the potential for producing a satellite-based carbon budget for the oceans, the advantages of integrating satellite-based observations with ecosystem models and field measurements, and future opportunities in space, all with a view towards bringing satellite observations into the limelight of ocean carbon research.

Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these is that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighbouring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. We identify the onset of the surface phytoplankton growth as concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic carbon derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favours carbon uptake by phytoplankton lasting a few days. Simultaneously, the air–sea net heat flux switches from negative to positive, linked to changes in the latent air–sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the surface phytoplankton bloom. We conclude that decreases in the mixing- and mixed-layer depths lead to the onset of the phytoplankton growth due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 d between 2016 and 2019 as between 38 and 191 mmol C m−2. These results have important implications, as biological processes play a major role in the seasonal evolution of surface pCO2 and thereby the rate of reduction in atmospheric CO2 by exchange at the air–sea interface.

Particulate organic carbon (POC) plays a vital role in the ocean carbon cycle and is related to many important ocean biogeochemical processes. This study investigates the annual and seasonal variability of POC and its relationship with sea surface temperature (SST), chlorophyll-a (Chl-a), and wind vector in the northern Bay of Bengal (BoB) of Bangladesh and validates satellite data with in situ measurement data. Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite level-3 data of POC, Chl-a, and SST were used in this study. Wind vector data were obtained from WindSat Polarimetric Radiometer satellite. Annual average of 18-year (2002–2019) surface POC concentration in the northern BoB ranged from 126.83 mg m−3 to 180.58 mg m−3 (average of 153.84±11.17 mg m−3) with a declining rate of -1.12 mg m−3 year−1. The POC was highest (181.80±22.34 mg m−³) during the northeast-monsoon (December–February) and lowest (136.56±36.24 mg m−³) in the pre-monsoon (March-May). Statistically significant (F3,202 =18.09; p

State-of-the-art carbon cycle prediction systems are initialized from reconstruction simulations in which state variables of the climate system are assimilated. While currently only the physical state variables are assimilated, biogeochemical state variables adjust to the state acquired through this assimilation indirectly instead of being assimilated themselves. In the absence of comprehensive biogeochemical reanalysis products, such approach is pragmatic. Here we evaluate a potential advantage of having perfect carbon cycle observational products to be used for direct carbon cycle reconstruction. Within an idealized perfect-model framework, we define 50 years of a control simulation under pre-industrial CO2 levels as our target representing observations. We nudge variables from this target onto arbitrary initial conditions 150 years later mimicking an assimilation simulation generating initial conditions for hindcast experiments of prediction systems. We investigate the tracking performance, i.e. bias, correlation and root-mean-square-error between the reconstruction and the target, when nudging an increasing set of atmospheric, oceanic and terrestrial variables with a focus on the global carbon cycle explaining variations in atmospheric CO2. We compare indirect versus direct carbon cycle reconstruction against a resampled threshold representing internal variability. Afterwards, we use these reconstructions to initialize ensembles to assess how well the target can be predicted after reconstruction. Interested in the ability to reconstruct global atmospheric CO2, we focus on the global carbon cycle reconstruction and predictive skill. We find that indirect carbon cycle reconstruction through physical fields reproduces the target variations on a global and regional scale much better than the resampled threshold. While reproducing the large scale variations, nudging introduces systematic regional biases in the physical state variables, on which biogeochemical cycles react very sensitively. Global annual surface oceanic pCO2 initial conditions are indirectly reconstructed with an anomaly correlation coefficient (ACC) of 0.8 and debiased root mean square error (RMSE) of 0.3 ppm. Direct reconstruction slightly improves initial conditions in ACC by +0.1 and debiased RMSE by −0.1 ppm. Indirect reconstruction of global terrestrial carbon cycle initial conditions for vegetation carbon pools track the target by ACC of 0.5 and debiased RMSE of 0.5 PgC. Direct reconstruction brings negligible improvements for air-land CO2 flux. Global atmospheric CO2 is indirectly tracked by ACC of 0.8 and debiased RMSE of 0.4 ppm. Direct reconstruction of the marine and terrestrial carbon cycles improves ACC by 0.1 and debiased RMSE by −0.1 ppm. We find improvements in global carbon cycle predictive skill from direct reconstruction compared to indirect reconstruction. After correcting for mean bias, indirect and direct reconstruction both predict the target similarly well and only moderately worse than perfect initialization after the first lead year. Our perfect-model study shows that indirect carbon cycle reconstruction yields satisfying initial conditions for global CO2 flux and atmospheric CO2. Direct carbon cycle reconstruction adds little improvements in the global carbon cycle, because imperfect reconstruction of the physical climate state impedes better biogeochemical reconstruction. These minor improvements in initial conditions yield little improvement in initialized perfect-model predictive skill. We label these minor improvements due to direct carbon cycle reconstruction trivial, as mean bias reduction yields similar improvements. As reconstruction biases in real-world prediction systems are even stronger, our results add confidence to the current practice of indirect reconstruction in carbon cycle prediction systems.

We assess the performance of an Isotope Ratio Infrared Spectrometer (IRIS) to measure carbon (δ13C) and oxygen (δ18O) isotope ratios in atmospheric carbon dioxide (CO2) and report observations from a 26 day field deployment trial at Baring Head, New Zealand, NIWA's atmospheric observatory for Southern Ocean baseline air. Our study describes an operational method to improve the performance in comparison to previous publications on this analytical technique. By using a calibration technique that reflected the principle of identical treatment of sample and reference gases, we achieved a reproducibility of 0.07 ‰ for δ13C-CO2 and 0.06 ‰ for δ18O-CO2 over multiple days. This performance is within the "extended compatibility goal" of 0.1 ‰ for both δ13C-CO2 and δ18O-CO2, which was recommended by the World Meteorological Organisation (WMO) for studies of regional or urban CO2 fluxes. One goal of this study was to assess the capabilities and limitations of the IRIS analyser to resolve δ13C-CO2 and δ18O-CO2 variations under field conditions. Therefore, we selected multiple events within the 26 day record for Keeling Plot Analysis. This resolved the isotopic composition of end members with an uncertainty of ≤ 1 ‰ when the magnitude of CO2 signals is larger than 10 ppm. The uncertainty of the Keeling Plot Analysis strongly increased for smaller CO2 events (2–7 ppm), where the instrument performance is the limiting factor and may only allow for the distinction between very different end members, such as the role of terrestrial versus oceanic carbon cycle processes. Further improvement in measurement performance is desirable to meet the WMO "network compatibility goal" of 0.01 ‰ for δ13C-CO2 and 0.05 ‰ for δ18O-CO2, which is needed to resolve the small variability that is typical for background air observatories such as Baring Head.

Since preindustrial times, atmospheric CO2 content increased continuously, leading to global warming through the greenhouse effect. Oceanic carbon sequestration mitigates global warming; on the other hand, oceanic CO2 uptake would reduce seawater pH, which is termed ocean acidification. We perform Earth system model simulations to assess oceanic CO2 uptake, surface temperature, and acidification for Zhejiang offshore, one of the most vulnerable areas to marine disasters. In the last 40 years, atmospheric CO2 concentration increased by 71 ppm, and sea surface temperature (SST) in Zhejiang offshore increased at a rate of 0.16°C/10a. Cumulative oceanic CO2 uptake in Zhejiang offshore is 0.3 Pg C, resulting in an increase of 20% in sea surface hydrogen ion concentration, and the acidification rate becomes faster in the last decade. During 2020–2040, under four RCP scenarios, SST in Zhejiang offshore increases by 0.3–0.5°C, whereas cumulative ocean carbon sequestration is 0.150–0.165 Pg C. Relative to RCP2.6, the decrease of surface pH in Zhejiang offshore is doubled under RCP8.5. Furthermore, simulated results show that the relationship between CO2 scenario and oceanic carbon cycle is nonlinear, which hints that deeper reduction of anthropogenic CO2 emission may be needed if we aim to mitigate ocean acidification in Zhejiang offshore under a higher CO2 concentration scenario. Our study quantifies the variation characteristics of oceanic climate and carbon cycle fields in Zhejiang offshore, and provides new insight into the responses of oceanic carbon cycle and the climate system to oceanic carbon sequestration.

The deep ocean releases large amounts of old, pre-industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2 release is relevant to the global climate because its changes could alter atmospheric CO2 levels on long time scales, and also affects the present-day potential of the Southern Ocean to take up anthropogenic CO2. Here, year-round profiling float measurements show that this CO2 release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea-ice edge. This band of high CO2 subsurface water coincides with the outcropping of the 27.8 kg m(-3) isoneutral density surface that characterizes Indo-Pacific Deep Water (IPDW). It has a potential partial pressure of CO2 exceeding current atmospheric CO2 levels ( increment PCO2) by 175 +/- 32 mu atm. Ship-based measurements reveal that IPDW exhibits a distinct increment PCO2 maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas increment PCO2 decreases. Most of this vertical increment PCO2 decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2 outgassing from the high-carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2 fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.

Marine dissolved organic matter (DOM) holds ~660 billion metric tons of carbon, making it one of Earth’s major carbon reservoirs that is exchangeable with the atmosphere on annual to millennial time scales. The global ocean scale dynamics of the pool have become better illuminated over the past few decades, and those are very briefly described here. What is still far from understood is the dynamical control on this pool at the molecular level; in the case of this Special Issue, the role of microgels is poorly known. This manuscript provides the global context of a large pool of marine DOM upon which those missing insights can be built.

Respiration is a key process in the cycling of particulate matter and, therefore, an important control mechanism of carbon export to the ocean's interior. Most of the fixed carbon is lost in the upper ocean, and only a minor amount of organic material sustains life in the deep-sea. Conditions are particularly extreme in hadal trenches, and yet they host active biological communities. The source of organic carbon that supports them and the contribution of these communities to the ocean carbon cycle, however, remain uncertain. Here we report on size-fractionated depth profiles of plankton respiration assessed from the activity of the electron transport system in the Atacama Trench region, and provide estimates of the minimum carbon flux (FC) needed to sustain the respiratory requirements from the ocean surface to hadal waters of the trench and shallower nearby sites. Plankton < 100 μm contributed about 90% to total community respiration, whose magnitude was highly correlated with surface productivity. Remineralization rates were highest in the euphotic zone and declined sharply within intermediate oxygen-depleted waters, remaining fairly constant toward the bottom. Integrated respiration in ultra-deep waters (> 1000 m) was comparable to that found in upper layers, with 1.3 ± 0.4 mmol C m−2 d−1 being respired in the hadopelagic. The comparison between our FC models and estimates of sinking particle flux revealed a carbon imbalance through the mesopelagic that was paradoxically reduced at greater depths. We argue that large fast-sinking particles originated in the overlying surface ocean may effectively sustain the respiratory carbon demands in this ultra-deep marine environment.

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.

Understanding the dynamics and fate of methane (CH4) release from oceanic seepages on margins and shelves into the water column, and quantifying the budget of its total discharge at different spatial and temporal scales, currently represents a major scientific undertaking. Previous works on the fate of methane escaping from the seafloor underlined the challenge in both, estimating its concentration distribution and identifying gradients. In April 2019, the Envri Methane Cruise has been conducted onboard the R/V Mare Nigrum in the Western Black Sea to investigate two shallow methane seep sites at ∼120 m and ∼55 m water depth. Dissolved CH4 measurements were conducted with two continuous in-situ sensors: a membrane inlet laser spectrometer (MILS) and a commercial methane sensor (METS) from Franatech GmbH. Additionally, discrete water samples were collected from CTD-Rosette deployment and standard laboratory methane analysis was performed by gas chromatography coupled with either purge-and-trap or headspace techniques. The resulting vertical profiles (from both in situ and discrete water sample measurements) of dissolved methane concentration follow an expected exponential dissolution function at both sites. At the deeper site, high dissolved methane concentrations are detected up to ∼45 m from the seabed, while at the sea surface dissolved methane was in equilibrium with the atmospheric concentration. At the shallower site, sea surface CH4 concentrations were four times higher than the expected equilibrium value. Our results seem to support that methane may be transferred from the sea to the atmosphere, depending on local water depths. In accordance with previous studies, the shallower the water, the more likely is a sea-to-atmosphere transport of methane. High spatial resolution surface data also support this hypothesis. Well localized methane enriched waters were found near the surface at both sites, but their locations appear to be decoupled with the ones of the seafloor seepages. This highlights the need of better understanding the processes responsible for the transport and transformation of the dissolved methane in the water column, especially in stratified water masses like in the Black Sea.

The Canadian Earth System Model version 5 (CanESM5) is a global model developed to simulate historical climate change and variability, to make centennial-scale projections of future climate, and to produce initialized seasonal and decadal predictions. This paper describes the model components and their coupling, as well as various aspects of model development, including tuning, optimization, and a reproducibility strategy. We also document the stability of the model using a long control simulation, quantify the model's ability to reproduce large-scale features of the historical climate, and evaluate the response of the model to external forcing. CanESM5 is comprised of three-dimensional atmosphere (T63 spectral resolution equivalent roughly to 2.8∘) and ocean (nominally 1∘) general circulation models, a sea-ice model, a land surface scheme, and explicit land and ocean carbon cycle models. The model features relatively coarse resolution and high throughput, which facilitates the production of large ensembles. CanESM5 has a notably higher equilibrium climate sensitivity (5.6 K) than its predecessor, CanESM2 (3.7 K), which we briefly discuss, along with simulated changes over the historical period. CanESM5 simulations contribute to the Coupled Model Intercomparison Project phase 6 (CMIP6) and will be employed for climate science and service applications in Canada.

The attribution of anthropogenically forced trends in the climate system requires an understanding of when and how such signals emerge from natural variability. We applied time-of-emergence diagnostics to a large ensemble of an Earth system model, which provides both a conceptual framework for interpreting the detectability of anthropogenic impacts in the ocean carbon cycle and observational sampling strategies required to achieve detection. We found emergence timescales that ranged from less than a decade to more than a century, a consequence of the time lag between the chemical and radiative impacts of rising atmospheric CO2 on the ocean. Processes sensitive to carbonate chemical changes emerge rapidly, such as the impacts of acidification on the calcium carbonate pump (10 years for the globally integrated signal and 9–18 years for regionally integrated signals) and the invasion flux of anthropogenic CO2 into the ocean (14 years globally and 13–26 years regionally). Processes sensitive to the ocean’s physical state, such as the soft-tissue pump, which depends on nutrients supplied through circulation, emerge decades later (23 years globally and 27–85 years regionally). The components of the ocean carbon cycle will respond differently to climate change, with anthropogenic impacts first seen on processes sensitive to chemical changes—the calcium carbonate pump and oceanic uptake of CO2—with the soft-tissue pump (sensitive to the ocean’s physical state) emerging later.

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

The food source of hadal endemic fauna provides an insight into the carbon cycle in trenches and a biological adaptation to the impoverished and harsh trench environment. Here, we present the first Delta C-14 results of hadal amphipods from three trenches in the Pacific to define the organic matter source in these remote ecosystems. Amphipod muscle tissues contain a bomb C-14 signature (Delta C-14 from 10 +/- 2 parts per thousand to 65 +/- 2 parts per thousand), thereby revealing a dietary preference for labile and fresh organic matter derived from the surface water. Thus, the carbon cycle in the deepest ocean trench has a tight linkage with the surface ocean via the food chain. The bomb C-14 dating result suggests that hadal amphipods have a low tissue turnover rate and an unexpectedly long lifetime (>10 years), at more than 4 times higher than the common longevity (similar to 2 years) of amphipods in shallow waters. Plain Language Summary Hadal trenches (depth > 6,000 m) are the remotest and the least explored places on our planet, with characteristics of low temperatures, high pressure, limited food, and frequent geological activity. Knowing the source of organic matter for hadal life is key in understanding the ocean carbon cycle and the biological adaptation in trenches. We report the first C-14 results of hadal amphipods from three trenches in the West Pacific, including the deepest ocean trench, Mariana, which indicates that the organic matter for amphipods is mostly from the surface water. This tight linkage between the organic matter within the hadal fauna and in surface water points out that the anthropogenic pollution can reach the deepest ocean trench rapidly via the food chain. Moreover, the bomb C-14 dating method can be applied to hadal amphipods, thereby suggesting a low tissue turnover rate and an unexpectedly long lifetime, which might be a result of an adaptation to the impoverished and harsh environment.

Dissolved organic carbon, from marine biota excretions and decomposing detritus, is one of the main components of the carbon cycle in the ocean. In this study, an attempt was made to construct maps of the distribution and fluxes of DOC in the Arctic Ocean and the exchanges with the Pacific and Atlantic Oceans. Because of the limited data available a multiple linear regression technique was performed to identify significant relationships between DOC (2200 samples) and hydrologic parameters (temperature and salinity), as well as depth, horizon, latitude and offshore distance. Mapping of the DOC distribution and its fluxes was carried out at 38 horizons from 5 to 4150 m depth (resolution 1°×1°). Data on temperature, salinity and meridional and zonal components of current velocities were obtained from the Ocean Re-Analysis System 4 (ORAS4) database. All these parameters were averaged for the June–October period, the season of water sampling. The import of DOC in the Arctic Ocean is estimated to be 206 ± 17Tg C yr−1, and the export is 194 ± 23Tg C yr−1, so the import–export is balanced within the errors.

The settling of particulate carbon in seawater is a key component of the ocean carbon cycle. We deployed a set of sediment trap in the polynya of Prydz Bay from December 2010 to December 2011 to investigate the seasonal variations in particle fluxes. There was a clear seasonal variation in the particle fluxes, with maximum and minimum fluxes recorded during the summer and winter, respectively. The average total flux over the sampling period was 193.58 mg m−2 d−1, and the average fluxes of organic carbon (Corg), inorganic carbon (Cinorg), and biogenic silica (Sibio) were 721.78, 28.67, and 2382.80 μmol m−2 d−1, respectively. Sibio was the main contributor to the total mass flux, and strongly correlated with Corg. The high Sibio/Corg molar ratios (>1) suggest that Corg was transported to deep sea in association with Sibio. By comparing remote sensing data of sea ice and chlorophyll in the upper water column, we found that the dynamics of carbon fluxes were closely related to changes in sea ice. Algae in sea ice may have a key role in biological pump processes in early summer. Apart from the ice algae bloom period, variations in carbon fluxes generally corresponded with phytoplankton blooms in the upper water. The ballast effect controlled the particle settling velocity and the efficiency of the biological pump. Sea ice rafts initiated the first particle export event and enhanced the particle settling efficiency during melting period. As diatoms might become less dominant in the ice-free area, sea ice loss may cause the efficiency of the biological pump efficiency to decrease over the long term.

Natural mechanisms in the ocean, both physical and biological, concentrate carbon in the deep ocean, resulting in lower atmospheric carbon dioxide. The signals of these carbon pumps overlap to create the observed carbon distribution in the ocean, making the individual impact of each pump difficult to disentangle. Noble gases have the potential to directly quantify the physical carbon solubility pump and to indirectly improve estimates of the biological organic carbon pump. Noble gases are biologically inert, can be precisely measured, and span a range of physical properties. We present dissolved neon, argon, and krypton data spanning the Atlantic, Southern, Pacific, and Arctic Oceans. Comparisons between deep-ocean observations and models of varying complexity enable the rates of processes that control the carbon solubility pump to be quantified and thus provide an important metric for ocean model skill. Noble gases also provide a powerful means of assessing air–sea gas exchange parameterizations.

Dissolved organic carbon (DOC) release by seaweeds (marine macroalgae) is a critical component of the coastal ocean biogeochemical carbon cycle but is an aspect of seaweed carbon physiology that we know relatively little about. Seaweed-derived DOC is found throughout coastal ecosystems and supports multiple food web linkages. Here, we discuss the mechanisms of DOC release by seaweeds and group them into passive (leakage, requires no energy) and active release (exudation, requires energy) with particular focus on the photosynthetic "overflow" hypothesis. The release of DOC from seaweeds was first studied in the 1960s, but subsequent studies use a range of units hindering evaluation: we convert published values to a common unit (μmol C · g DW−1 · h−1) allowing comparisons between seaweed phyla, functional groups, biogeographic region, and an assessment of the environmental regulation of DOC production. The range of DOC release rates by seaweeds from each phylum under ambient environmental conditions was 0–266.44 μmol C · g DW−1 · h−1 (Chlorophyta), 0–89.92 μmol C · g DW−1 · h−1 (Ochrophyta), and 0–41.28 μmol C · g DW−1· h−1 (Rhodophyta). DOC release rates increased under environmental factors such as desiccation, high irradiance, non-optimal temperatures, altered salinity, and elevated dissolved carbon dioxide (CO2) concentrations. Importantly, DOC release was highest by seaweeds that were desiccated (

The ocean response to carbon emissions involves the combined effect of an increase in atmospheric CO2, acting to enhance the ocean carbon storage, and climate change, acting to decrease the ocean carbon storage. This ocean response can be characterised in terms of a carbon–concentration feedback and a carbon–climate feedback. The contribution from different ocean basins to these feedbacks on centennial timescales is explored using diagnostics of ocean carbonate chemistry, physical ventilation and biological processes in 11 CMIP6 Earth system models. To gain mechanistic insight, the dependence of these feedbacks on the Atlantic Meridional Overturning Circulation (AMOC) is also investigated in an idealised climate model and the CMIP6 models. For the carbon–concentration feedback, the Atlantic, Pacific and Southern oceans provide comparable contributions when estimated in terms of the volume-integrated carbon storage. This large contribution from the Atlantic Ocean relative to its size is due to strong local physical ventilation and an influx of carbon transported from the Southern Ocean. The Southern Ocean has large anthropogenic carbon uptake from the atmosphere, but its contribution to the carbon storage is relatively small due to large carbon transport to the other basins. For the carbon–climate feedback estimated in terms of carbon storage, the Atlantic and Arctic oceans provide the largest contributions relative to their size. In the Atlantic, this large contribution is primarily due to climate change acting to reduce the physical ventilation. In the Arctic, this large contribution is associated with a large warming per unit volume. The Southern Ocean provides a relatively small contribution to the carbon–climate feedback, due to competition between the climate effects of a decrease in solubility and physical ventilation and an increase in accumulation of regenerated carbon. The more poorly ventilated Indo-Pacific Ocean provides a small contribution to the carbon cycle feedbacks relative to its size. In the Atlantic Ocean, the carbon cycle feedbacks strongly depend on the AMOC strength and its weakening with warming. In the Arctic, there is a moderate correlation between the AMOC weakening and the carbon–climate feedback that is related to changes in carbonate chemistry. In the Pacific, Indian and Southern oceans, there is no clear correlation between the AMOC and the carbon cycle feedbacks, suggesting that other processes control the ocean ventilation and carbon storage there.

Dissolution of marine sediment is a key source of dissolved iron (Fe) that regulates the ocean carbon cycle. Currently, our prevailing understanding, encapsulated in ocean models, focuses on low-oxygen reductive supply mechanisms and neglects the emerging evidence from iron isotopes in seawater and sediment porewaters for additional nonreductive dissolution processes. Here, we combine measurements of Fe colloids and dissolved δ(56)Fe in shallow porewaters spanning the full depth of the South Atlantic Ocean to demonstrate that it is lithogenic colloid production that fuels sedimentary iron supply away from low-oxygen systems. Iron colloids are ubiquitous in these oxic ocean sediment porewaters and account for the lithogenic isotope signature of dissolved Fe (δ(56)Fe = +0.07 ± 0.07‰) within and between ocean basins. Isotope model experiments demonstrate that only lithogenic weathering in both oxic and nitrogenous zones, rather than precipitation or ligand complexation of reduced Fe species, can account for the production of these porewater Fe colloids. The broader covariance between colloidal Fe and organic carbon (OC) abundance suggests that sorption of OC may control the nanoscale stability of Fe minerals by inhibiting the loss of Fe(oxyhydr)oxides to more crystalline minerals in the sediment. Oxic ocean sediments can therefore generate a large exchangeable reservoir of organo-mineral Fe colloids at the sediment water interface (a “rusty source”) that dominates the benthic supply of dissolved Fe to the ocean interior, alongside reductive supply pathways from shallower continental margins.

Ocean biogeochemistry as part of the Earth system impacts the uptake of atmospheric CO2 and storage of carbon in the ocean. In the ICON-O (Icosahedral non-hydrostatic general circulation model) ocean model, ocean biogeochemistry is represented by the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013, Mauritsen et al. 2019, Maerz et al. 2020). Here, we present the results of an ongoing effort to tune HAMOCC (i.e. adapt parameters within the uncertainty range) to accommodate the ocean circulation simulated by ICON-O.The tuning of biogeochemical models, including HAMOCC, has previously been an iterative, and a rather random process combining expert knowledge and a suite of parameter testings. A documented, systematic procedure, describing how to tune these models is lacking. Therefore, while tuning HAMOCC in ICON-O, we aim at filling this gap by structuring the process and documenting the steps taken to tune a biogeochemistry model in a global general ocean circulation model.The ocean circulation has a large impact on the distribution of biogeochemical tracers, as biases in the circulation will, for example, impact the upwelling of nutrients or the CO2 exchange with the atmosphere. We investigate the impact of physical parameterization such as the Gent-McWilliam eddy parameterization and the vertical mixing scheme on the choice of HAMOCC tuning parameters. We then compare the spatial distribution of major state variables such as nutrients and alkalinity to observational data ( WOA; Garcia et al 2013, GLODAP; Key et al 2004) and evaluate the key tendencies such as CO2 surface fluxes and attenuation of particulate organic matter fluxes. Furthermore, we discuss the tuning steps, choices of the tuning parameters and their impact on the simulated biogeochemistry. The envisioned outcome of this work is a tuned ocean biogeochemistry component for the here used ICON-O model and a more generalized tuning procedure that can be applied to other models or HAMOCC in different model configurations (coupled runs, different resolution). Garcia, H. E., et al. 2014: World Ocean Atlas 2013, NOAA Atlas NESDIS 76, Volume4: Dissolved Inorganic Nutrients (phosphate, nitrate, silicate), 25pp.lyina, T., et al. 2013: Global ocean biogeochemistry model HAMOCC: Model architecture and performance as component of the MPI-Earth system model in different CMIP5 experimental realizations, J. Adv. Model. Earth Sy., 5, .Key, R., et al. 2004: A global ocean carbon climatology: Results from Global Data Analysis Project, Global Biogeochem. Cycles, 18, 4, https://doi.org/10.1029/2004GB002247.Maerz et al. 2020: Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean, Biogeosciences, 17, 7, https://doi.org/10.5194/bg-17-1765-2020.Mauritsen, T., et al. 2019: Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and Its Response to Increasing CO2, J. Adv. Model. Earth Sy., 11, https://doi.org/10.1029/2018MS001400.

The stable carbon isotopic composition (δ13 C) is an important variable to study the ocean carbon cycle across different timescales. We include a new representation of the stable carbon isotope 13 C into the HAMburg Ocean Carbon Cycle model (HAMOCC), the ocean biogeochemical component of the Max Planck Institute Earth System Model (MPI-ESM). 13 C is explicitly resolved for all oceanic carbon pools considered. We account for fractionation during air–sea gas exchange and for biological fractionation ϵp associated with photosynthetic carbon fixation during phytoplankton growth. We examine two ϵp parameterisations of different complexity: ϵ p Popp varies with surface dissolved CO 2 concentration ( Popp et al. , 1989 ) , while ϵ p Laws additionally depends on local phytoplankton growth rates ( Laws et al. , 1995 ) . When compared to observations of δ13 C of dissolved inorganic carbon (DIC), both parameterisations yield similar performance. However, with regard to δ13 C in particulate organic carbon (POC) ϵ p Popp shows a considerably improved performance compared to ϵ p Laws . This is because ϵ p Laws produces too strong a preference for 12 C, resulting in δ13 C POC that is too low in our model. The model also well reproduces the global oceanic anthropogenic CO 2 sink and the oceanic 13 C Suess effect, i.e. the intrusion and distribution of the isotopically light anthropogenic CO 2 in the ocean. The satisfactory model performance of the present-day oceanic δ13 C distribution using ϵ p Popp and of the anthropogenic CO 2 uptake allows us to further investigate the potential sources of uncertainty of the Eide et al. ( 2017 a ) approach for estimating the oceanic 13 C Suess effect. Eide et al. ( 2017 a ) derived the first global oceanic 13 C Suess effect estimate based on observations. They have noted a potential underestimation, but their approach does not provide any insight about the cause. By applying the Eide et al. ( 2017 a ) approach to the model data we are able to investigate in detail potential sources of underestimation of the 13 C Suess effect. Based on our model we find underestimations of the 13 C Suess effect at 200 m by 0.24 ‰ in the Indian Ocean, 0.21 ‰ in the North Pacific, 0.26 ‰ in the South Pacific, 0.1 ‰ in the North Atlantic and 0.14 ‰ in the South Atlantic. We attribute the major sources of underestimation to two assumptions in the Eide et al. ( 2017 a ) approach: the spatially uniform preformed component of δ13 C DIC in year 1940 and the neglect of processes that are not directly linked to the oceanic uptake and transport of chlorofluorocarbon-12 (CFC-12) such as the decrease in δ13 C POC over the industrial period. The new 13 C module in the ocean biogeochemical component of MPI-ESM shows satisfying performance. It is a useful tool to study the ocean carbon sink under the anthropogenic influences, and it will be applied to investigating variations of ocean carbon cycle in the past.

Since the industrial revolution 30% of the anthropogenic CO2 is absorbed by oceans, resulting in ocean acidification, which is a threat to calcifying algae. As a result, there has been profound interest in the study of calcifying algae, because of their important role in the global carbon cycle. The species studied, coccolithophore Emiliania huxleyi, is considered to be globally the single most dominant calcifying algae, which creates a unique exoskeleton from inorganic calcium carbonate platelets. The PIC (particulate inorganic carbon): POC (particulate organic carbon) ratio describes the relative amount of inorganic carbon in the algae and is a critical parameter in the ocean carbon cycle. In this research we explore the use of microfluidic single-cell impedance spectroscopy in the field of calcifying algae. Microfluidic impedance spectroscopy enables us to characterize single-cell electrical properties in a non-invasive and label-free way. We use the ratio of the impedance at high frequency vs. low frequency, known as opacity, to discriminate between calcified coccolithophores and coccolithophores with a calcite exoskeleton dissolved by acidification (decalcified). We have demonstrated that using opacity we can discriminate between calcified and decalcified coccolithophores with an accuracy of 94.1%. We have observed a correlation between the measured opacity and the cell height in the channel, which is supported by FEM simulations. The difference in cell density between calcified and decalcified cells can explain the difference in cell height and therefore the measured opacity.

The ocean response to carbon emissions involves a competition between the increase in atmospheric CO2 acting to enhance the ocean carbon storage, characterised by the carbon-concentration feedback, and climate change acting to decrease the ocean carbon storage, characterised by the carbon-climate feedback. The contribution from different ocean basins to the carbon cycle feedbacks and its control by the ocean carbonate chemistry, physical ventilation and biological processes is explored in diagnostics of 10 CMIP6 Earth system models. To gain mechanist insight, the dependence of these feedbacks to the Atlantic Meridional Overturning Circulation (AMOC) is also investigated in an idealised climate model and the CMIP6 models. The Atlantic, Pacific and Southern Oceans contribute equally to the carbon-concentration feedback, despite their different size. This large contribution from the Atlantic Ocean relative to its size is associated with an enhanced carbon storage in the ocean interior due to a strong local physical ventilation and an influx of carbon transported from the Southern Ocean. The Atlantic Ocean provides the largest contribution to the carbon-climate feedback relative to its size, which is primarily due to climate change acting to reduce the physical ventilation. The Southern Ocean provides a relatively small contribution to the carbon-climate feedback, due to a compensation between the climate effects of the combined decrease in solubility and physical ventilation, and the increase in accumulation of regenerated carbon in the ocean interior. In the Atlantic Ocean, the AMOC strength and its weakening with warming has a strong control on the carbon cycle feedbacks that leads to a moderate dependence of these feedbacks to AMOC on global scale. In the Pacific, Indian and Southern Oceans there is no clear correlation between AMOC and the carbon cycle feedbacks, suggesting that other processes control the ocean ventilation and carbon storage there.

The implications of climate change and other human perturbations on the oceanic carbon cycle are still associated with large uncertainties. Global-scale modelling studies are essential to investigate anthropogenic perturbations of oceanic carbon fluxes but, until now, they have not considered the impacts of temporal changes in riverine and atmospheric inputs of P and N on the marine net biological productivity (NPP) and air–sea CO2 exchange (FCO2). To address this, we perform a series of simulations using an enhanced version of the global ocean biogeochemistry model HAMOCC to isolate effects arising from (1) increasing atmospheric CO2 levels, (2) a changing physical climate and (3) alterations in inputs of terrigenous P and N on marine carbon cycling over the 1905–2010 period. Our simulations reveal that our first-order approximation of increased terrigenous nutrient inputs causes an enhancement of 2.15 Pg C year−1 of the global marine NPP, a relative increase of +5% over the simulation period. This increase completely compensates the simulated NPP decrease as a result of increased upper ocean stratification of −3% in relative terms. The coastal ocean undergoes a global relative increase of 14% in NPP arising largely from increased riverine inputs, with regional increases exceeding 100%, for instance on the shelves of the Bay of Bengal. The imprint of enhanced terrigenous nutrient inputs is also simulated further offshore, inducing a 1.75 Pg C year−1 (+4%) enhancement of the NPP in the open ocean. This finding implies that the perturbation of carbon fluxes through coastal eutrophication may extend further offshore than that was previously assumed. While increased nutrient inputs are the largest driver of change for the CO2 uptake at the regional scale and enhance the global coastal ocean CO2 uptake by 0.02 Pg C year−1, they only marginally affect the FCO2 of the open ocean over our study's timeline., SCOPUS: ar.j, info:eu-repo/semantics/published

Anthropogenic CO2 emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2 uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21st century climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2 (due to increasing air-sea CO2 partial pressure difference, dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2 uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2 emissions. The increase in CO2 uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2 uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominately dpCO2–driven and should be a focus of efforts to constrain projection uncertainty.