Auswahl der wissenschaftlichen Literatur zum Thema „Oceanic Suess effect“

Abstract. The stable carbon isotopic composition (δ13C) is an important variable to study the ocean carbon cycle across different timescales. We include a new representation of the stable carbon isotope 13C into the HAMburg Ocean Carbon Cycle model (HAMOCC), the ocean biogeochemical component of the Max Planck Institute Earth System Model (MPI-ESM). 13C 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: ϵpPopp varies with surface dissolved CO2 concentration (Popp et al., 1989), while ϵpLaws additionally depends on local phytoplankton growth rates (Laws et al., 1995). When compared to observations of δ13C of dissolved inorganic carbon (DIC), both parameterisations yield similar performance. However, with regard to δ13C in particulate organic carbon (POC) ϵpPopp shows a considerably improved performance compared to ϵpLaws. This is because ϵpLaws produces too strong a preference for 12C, resulting in δ13CPOC that is too low in our model. The model also well reproduces the global oceanic anthropogenic CO2 sink and the oceanic 13C Suess effect, i.e. the intrusion and distribution of the isotopically light anthropogenic CO2 in the ocean. The satisfactory model performance of the present-day oceanic δ13C distribution using ϵpPopp and of the anthropogenic CO2 uptake allows us to further investigate the potential sources of uncertainty of the Eide et al. (2017a) approach for estimating the oceanic 13C Suess effect. Eide et al. (2017a) derived the first global oceanic 13C 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. (2017a) approach to the model data we are able to investigate in detail potential sources of underestimation of the 13C Suess effect. Based on our model we find underestimations of the 13C 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. (2017a) approach: the spatially uniform preformed component of δ13CDIC 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 δ13CPOC over the industrial period. The new 13C 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.

Abstract. We describe the design and evaluation of a large ensemble of coupled climate-carbon cycle simulations with the Earth-system model of intermediate complexity GENIE. This ensemble has been designed for application to a range of carbon cycle questions including utilizing carbon isotope (δ13C) proxy records to help constrain the state at the last glacial. Here we evaluate the ensemble by applying it to a transient experiment over the recent industrial era (1858 to 2008 AD). We employ singular vector decomposition and principal component emulation to investigate the spatial modes of ensemble-variability of oceanic dissolved inorganic carbon (DIC) δ13C, considering both the spun-up pre-industrial state and the transient change due to the 13C Suess Effect. These analyses allow us to separate the natural and anthropogenic controls on the δ13CDIC distribution. We apply the same dimensionally reduced emulation techniques to consider the drivers of the spatial uncertainty in anthropogenic DIC. We show that the sources of uncertainty governing the uptake of anthropogenic δ13CDIC and DIC are quite distinct. Uncertainty in anthropogenic δ13C uptake is dominated by uncertainties in air-sea gas exchange, which explains 63% of modelled variance. This mode of variability is absent from the ensemble variability in CO2 uptake, which is rather driven by uncertainties in ocean parameters that control mixing of intermediate and surface waters. Although the need to account for air-sea gas exchange is well known, these results suggest that, to leading order, uncertainties in the 13C Suess effect and anthropogenic CO2 ocean-uptake are governed by different processes. This illustrates the difficulties in reconstructing one from the other and furthermore highlights the need for improved spatial coverage of both δ13CDIC and DIC observations to better constrain the ocean sink of anthropogenic CO2.

Abstract. Carbon isotopes in the ocean are frequently used as paleoclimate proxies and as present-day geochemical ocean tracers. In order to allow a more direct comparison of climate model results with this large and currently underutilized data set, we added a carbon isotope module to the ocean model of the Community Earth System Model (CESM), containing the cycling of the stable isotope 13C and the radioactive isotope 14C. We implemented the 14C tracer in two ways: in the "abiotic" case, the 14C tracer is only subject to air–sea gas exchange, physical transport, and radioactive decay, while in the "biotic" version, the 14C additionally follows the 13C tracer through all biogeochemical and ecological processes. Thus, the abiotic 14C tracer can be run without the ecosystem module, requiring significantly fewer computational resources. The carbon isotope module calculates the carbon isotopic fractionation during gas exchange, photosynthesis, and calcium carbonate formation, while any subsequent biological process such as remineralization as well as any external inputs are assumed to occur without fractionation. Given the uncertainty associated with the biological fractionation during photosynthesis, we implemented and tested three parameterizations of different complexity. Compared to present-day observations, the model is able to simulate the oceanic 14C bomb uptake and the 13C Suess effect reasonably well compared to observations and other model studies. At the same time, the carbon isotopes reveal biases in the physical model, for example, too sluggish ventilation of the deep Pacific Ocean.

Abstract. Carbon isotopes in the ocean are frequently used as paleo climate proxies and as present-day geochemical ocean tracers. In order to allow a more direct comparison of climate model results with this large and currently underutilized dataset, we added a carbon isotope module to the ocean model of the Community Earth System Model (CESM), containing the cycling of the stable isotope 13C and the radioactive isotope 14C. We implemented the 14C tracer in two ways: in the "abiotic" case, the 14C tracer is only subject to air–sea gas exchange, physical transport, and radioactive decay, while in the "biotic" version, the 14C additionally follows the 13C tracer through all biogeochemical and ecological processes. Thus, the abiotic 14C tracer can be run without the ecosystem module, requiring significantly less computational resources. The carbon isotope module calculates the carbon isotopic fractionation during gas exchange, photosynthesis, and calcium carbonate formation, while any subsequent biological process such as remineralization as well as any external inputs are assumed to occur without fractionation. Given the uncertainty associated with the biological fractionation during photosynthesis, we implemented and tested three parameterizations of different complexity. Compared to present-day observations, the model is able to simulate the oceanic 14C bomb uptake and the 13C Suess effect reasonably well compared to observations and other model studies. At the same time, the carbon isotopes reveal biases in the physical model, for example a too sluggish ventilation of the deep Pacific Ocean.

The isotopic composition of inorganic carbon in otoliths (δ13Coto) can be a useful tracer of metabolic rates and a method to study ecophysiology in wild fish. We evaluated environmental and physiological sources of δ13Coto variation in Icelandic and Northeast Arctic (NEA) cod (Gadus morhua) over the years 1914–2013. Individual annual growth increments of otoliths formed at age 3 and 8 were micromilled and measured by isotope-ratio mass spectrometry. Simultaneously, all annual increment widths of the otoliths were measured providing a proxy of fish somatic growth. We hypothesized that changes in the physiological state of the organism, reflected by the isotopic composition of otoliths, can affect the growth rate. Using univariate and multivariate mixed-effects models we estimated conditional correlations between carbon isotopic composition and growth of fish at different levels (within individuals, between individuals, and between years), controlling for intrinsic and extrinsic effects on both otolith measurements. δ13Coto was correlated with growth within individuals and between years, which was attributed to the intrinsic effects (fish age or total length). There was no significant correlation between δ13Coto and growth between individuals, which suggests that caution is needed when interpreting δ13Coto signals. We found a significant decrease in δ13Coto through the century which was explained by the oceanic Suess effect-admixture of isotopically light carbon from fossil fuel. We calculated the proportion of the respired carbon in otolith carbonate (Cresp) using carbon isotopic composition in diet and dissolved inorganic carbon of the seawater. This approach allowed us to correct the values for each stock in relation to these two environmental baselines. Cresp was on average 0.275 and 0.295 in Icelandic and NEA stock, respectively. Our results provide an insight into the physiological basis for differences in growth characteristics between these two cod stocks, and how that may vary over time.

L’océan joue un rôle important dans le système climatique du fait des importants échanges de gaz carbonique avec l’atmosphère et du déplacement de ses échanges vers un puits océanique lors de l’Anthropocène. Les océans Atlantique Nord et Austral sont reconnus comme étant des acteurs majeurs de cette séquestration du carbone anthropique (Cant). En effet, ~25% du Cant pénètre dans les eaux de surface de l’Atlantique Nord et ~40% résident dans les eaux modales et intermédiaires de l’océan Austral. Il est clairement établi que le puits de carbone présente des variations dans le temps mais mal connues, rendant les prévisions climatiques difficiles. Il est donc recommandé de concentrer les efforts d’observations dans les régions où l’absorption de CO2 est élevée : les océans Atlantique Nord et Austral. Dans ce contexte, l’étude de la variabilité saisonnière, interannuelle à décennale des paramètres du système des carbonates dans ces deux régions est requise pour appréhender l’impact des changements actuels sur le cycle du carbone océanique. Basée sur des observations acquises dès le milieu des années 1990 et jusqu’en 2021 dans le cadre des programmes français SURATLANT et OISO, ces travaux de thèse visent à décrire l’évolution spatiale et temporelle des paramètres du système des carbonates (AT, CT, fCO2, pH et δ13CDIC) dans le gyre subpolaire nord Atlantique (NASPG) et le secteur Indien de l’océan Austral. Les processus physiques et biogéochimiques contrôlant l’évolution de la fCO2, de l’acidification des eaux et de l’effet Suess océanique, ont été étudiés en séparant le signal anthropique des signaux naturels. L’évolution de la fCO2 et du pH, sur l’ensemble de la période et dans ces deux régions, est en accord avec l’augmentation de CO2 atmosphérique et les tendances moyennes pour l’océan global. Toutefois, selon la saison, la zone sélectionnée ou sur de plus courtes périodes, les résultats peuvent être différents. L’augmentation du Cant a été identifié comme le driver contrôlant majoritairement les changements de fCO2 et pH observés, mais d’autres processus peuvent moduler ces tendances. Ainsi, le réchauffement (refroidissement) des eaux de surface accélère (limite) l’augmentation de la fCO2 et la diminution du pH. De plus, des tendances à l’augmentation de AT ont également été observées dans chacune des deux régions, ce qui a limité en partie l’acidification des eaux par rapport à l’augmentation du Cant. Cependant, les résultats suggèrent une certaine stabilité, voir une inversion de la tendance à l’augmentation de la fCO2 et de l’acidification autour de 2010, tant dans le NASPG que dans la zone antarctique de l’océan Indien Austral. Les observations de 13CDIC semblent confirmer cette analyse et permettent de mettre en avant un effet Suess différent entre les deux régions. Ce paramètre complémentaire a cependant été moins échantillonné et ne permet pas encore de valider les changements observés autour de 2010. Mon travail met en avant l’importance de maintenir des observations à long terme dans ces régions où l’absorption de CO2 atmosphérique est importante, afin de suivre l’évolution du carbone anthropique, de l’effet Suess océanique et de l’acidification des eaux de surface au cours des prochaines décennies
The ocean plays a very large role in the climate system due to the large exchange of carbon dioxide with the atmosphere and the recent shift of the exchanges towards a large oceanic sink of CO2 in the Anthropocene era. The North Atlantic and the Southern oceans are acknowledged to be major repositories of this anthropogenic carbon (Cant). Indeed, ~25% of the Cant penetrates through the surface waters of the North Atlantic and ~40% reside in the intermediate and mode waters of the Southern ocean. It has been established that this oceanic carbon sink presents a large time variability of seasonal to multidecadal times scales, but that is poorly known, resulting in large uncertainties in long term climate predictions. It has thus been recommended to focus observing efforts in the regions where the absorption of CO2 is large: the North Atlantic and the Southern oceans. In this frame, the study of the seasonal to decadal variability of the oceanic carbonate system is required to better understand the effects of current changes on the oceanic carbon cycle. I use data collected since the mid-1990s until 2021 within the framework of the two French surveys SURATLANT and OISO, in order to describe the spatial and temporal variability of parameters of the carbonate system (AT, CT, fCO2, pH and δ13CDIC) in the North Atlantic subpolar gyre (NASPG) as well as in the Indian sector of the Southern Ocean. I studied the physical and biogeochemical processes that control the evolution of fCO2, water acidification and the oceanic Suess effect, separating the anthropogenic induced changes from natural variability. The long-term evolution of fCO2 and pH during the period samples has a similar magnitude to the atmospheric CO2 increase and the overall surface ocean trends. Nonetheless, results can differ from this average view, depending on season, the particular region or specific periods. Cant increase has been identified as the prime driver controlling the observed changes in fCO2 and pH, but other processes modulate these tendencies. For instance, the warming (cooling) of the surface waters will increase (restrain) the increase of fCO2 and the decrease of pH. Furthermore, an increase of AT has been identified in both regions, which partially limit the increase of ocean acidification induced by Cant increase. Also, the data suggest that changes have been smaller since 2010, with even some reversal in the increase in fCO2 and ocean acidification, both in the NASPG than in the Antarctic region of the Southern Indian ocean. 13CDIC data seem to reinforce these conclusions and to identify a different Suess effect in the two regions. This additional parameter has nonetheless been less sampled and the current data do not allow to clearly identify the change since 2010. My work supports the need to continue the long-term observations in these key regions for anthropogenic CO2 export to the deep ocean, in order to better characterize the changes in anthropogenic carbon, the oceanic Suess effect, and the acidification of surface waters for the next decades

The ocean and the atmosphere exchange massive amounts of carbon dioxide (CO2). The pre-industrial influx from the ocean to the atmosphere was 70.6 Gt C yr –1 , while the flux in the opposite direction was 70 Gt C yr –1 ( IPCC 2007 ). Since the Industrial Revolution an anthropogenic flux has been superimposed on the natural flux. The concentration of CO2 in the atmosphere, which remained in the range of 172–300 parts per million by volume (ppmv) over the past 800 000 years ( Lüthi et al. 2008 ), has increased during the industrial era to reach 387 ppmv in 2009. The rate of increase was about 1.0% yr –1 in the 1990s and reached 3.4% yr –1 between 2000 and 2008 ( Le Quéré et al. 2009 ). Future levels of atmospheric CO2 mostly depend on socio-economic parameters, and may reach 1071 ppmv in the year 2100 ( Plattner et al. 2001 ), corresponding to a fourfold increase since 1750. As pointed out over 50 years ago, ‘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’ ( Revelle and Suess 1957 ). Anthropogenic CO2 has three fates. In the years 2000 to 2008, about 29% was absorbed by the terrestrial biosphere and 26% by the ocean, while the remaining 45% remained in the atmosphere ( Le Quéré et al. 2009 ). The accumulation of CO2 in the atmosphere increases the natural greenhouse effect and generates climate changes ( IPCC 2007 ). It is estimated that the surface waters of the oceans have taken up 118 Pg C, or about 25% of the carbon generated by human activities since 1800 ( Sabine et al. 2004 ). By taking CO2 away from the atmosphere, the oceanic and terrestrial sinks mitigate climatic changes. Should their efficiency decrease, more CO2 would remain in the atmosphere, generating larger climate perturbations. This book has four main groups of chapters.