The Northeast Pacific is an important net sink of atmospheric CO₂. However, substantial natural variability modulates the long-term increase of seawater partial pressure of CO₂ (pCO₂), potentially influencing the magnitude of the CO₂ sink. In addition to the seasonal cycle, the Pacific Decadal Oscillation (PDO) is known to play a role in driving a large fraction of the non-seasonal variability in the region. Yet, the magnitude of this natural variability, especially periods of high surface dissolved inorganic carbon (DIC), are not well constrained. Here we quantify the seasonal and non-seasonal variability in DIC and pCO₂ using observations from the Line P program, the longest marine carbonate system time-series transect in the NE Pacific (1990-2019), as well as an ensemble of historical simulations with an Earth system model (EC-Earth-CC). Preliminary results show that the mean amplitude of the DIC seasonal cycle is similar across our NE Pacific transect (23-30 µmol kg^-1) and decreases with depth to less than 5 µmol kg^-1 at 60 to 70 m. In contrast, the non-seasonal variability remains approximately constant with depth, ranging between 10 – 20 µmol kg^-1. We quantify the role of the PDO in driving this residual non-seasonal variability, and analyse the contrasting impact of temperature and DIC changes in controlling surface pCO₂ during opposite phases of the PDO.

How to cite: Franco, A. C., Ianson, D., Bernardello, R., and Monahan, A. H.: Seasonal and interannual inorganic carbon dynamics in the Northeast Pacific, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16428, https://doi.org/10.5194/egusphere-egu24-16428, 2024.

Planktic foraminifera are calcifying marine protists living in the upper water column of the world’s oceans. As they grow, the composition of their calcite shells is influenced by their local environment. Thus, by analysing the geochemical signature of their fossil shells, a record of past changes in temperature, pH and salinity of the seawater can be reconstructed. Globigerina bulloides is a spinose planktonic foraminifera species that tolerates sub-tropical to sub-polar conditions and is present in high concentrations in the southern Norwegian Sea. It is of ecological interest as it encroaches into previously ‘cold water’ territories due to climate warming and Nordic Seas “Atlantification”. The relative abundance and shell geochemistry of fossil specimens of G. bulloides are also widely used for palaeoceanographic reconstructions in the region. Despite the widespread use of G. bulloides for these reconstructions, biological, metabolic, and behavioural observations are scarce, leaving large knowledge gaps regarding the impacts of these ‘vital effects’ on its calcification and preserved geochemical signature. One way to fill this gap is via the study of the species in controlled culture conditions, however to date, all reported culturing studies have been carried out in a temperate to warm water setting (>14^oC), and using sub-tropical specimens. This reduces the applicability of these studies to G. bulloides inhabiting the high latitudes.

We cultivated over 250 individual specimens of G. bulloides from the Norwegian Sea across a range of temperatures (6 - 13^o C), salinities (30.4 – 37.8), pHs (7.7 - 8.3) and carbonate ion concentrations (70 – 230 µmol/kg). Experimental conditions were chosen relative to ambient seawater at the collection site(s) and were intended to reflect a plausible range of past and future scenarios.

After several weeks in culture, we observed that G. bulloides was tolerant of environmental conditions well outside their natural range, with no significant difference in mortality or final size. This was corroborated by a high percentage of spine regrowth and/or maintenance (~65% for most treatments) after the first week. Many individuals thrived in culture, with some surviving up to three months. Two alternative strategies appeared to be employed; specimens opted either for rapid growth shortly followed by death, or for a prolonged lifespan with minimal size increase. Longer living specimens developed ectoplasmic structures on multiple occasions. Our observations suggest G. bulloides can exhibit considerable adaptability to shifting environmental conditions with implications to its tolerance to ongoing ocean changes.

How to cite: Sykes, F. E., Meilland, J., Westgård, A., Chalk, T. B., Chierici, M., Foster, G. L., and Ezat, M. M.: The response of planktic foraminifera Globigerina bulloides to changing environmental parameters through extensive culturing experiments, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17424, https://doi.org/10.5194/egusphere-egu24-17424, 2024.

Seasonal-interannual climate change is an important component of Earth’s climate system and has a significant impact on ecosystems and social systems. However, the scarcity of modern observational data limits our understanding of the seasonal-interannual climates in long-term timescales. The natural archives can provide information on climate change before the industrial period, but most of their temporal resolution is too low to capture the seasonal-interannual climate signals. Tridacna spp. is the largest marine bivalve, and its shell has the potential to trace climatic/environmental changes on a daily to interannual scale. In this study, we have investigated the daily growth pattern of modern Tridacna gigas shell (PL-1) from Palau based on laser scanning confocal microscopy images. The results showed that the growth pattern of PL-1 was affected by OLR primarily on the seasonal timescale. On the interannual timescale, the growth rate of the shell was modulated by ENSO through the changing OLR, SLH, and upwelling, which affect the effective solar radiation and nutrients accepted by Tridacna. The growth rate of Tridacna PL-1 is accelerated during the ENSO positive phase, and vice versa. This study indicates that the daily growth rate of Tridacna shells in equatorial regions may have the potential to reconstruct seasonal-interannual climate/environment variations.

How to cite: Wen, H.: Seasonal to interannual variations of daily growth rate of Tridacna shell from Palau Island, western Pacific, and their paleoclimatic implication, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7699, https://doi.org/10.5194/egusphere-egu24-7699, 2024.

Motivated by the goal to increase our knowledge of the impact of hydrothermal iron (Fe) nanoparticles on ocean chemistry and to explore their unique catalytic capabilities, we sampled suspended and dissolved matter in the water column above the Rainbow (36°-33°N) hydrothermal vent field at the Mid-Atlantic Ridge. Innovative sampling techniques were used to constrain the (trans)formation of hydrothermal iron-based nanoparticles. Instead of filtration of plume particles, freezing, and later resuspension, which is commonly used to separate particles from their surrounding solution and preserve them¹, we immediately drop cast small amounts of the fluid on transmission electron microscopy (TEM) grids and plunge-freeze them, resulting in vitrification of dissolved compounds and preservation of containing nanoparticles. Using an array of (micro)spectroscopic techniques, TEM, and a machine learning approach, we can characterize the Fe nanoparticles and unravel their fate in the ocean biogeochemical cycle.

Initial results show that the new sampling approach allows us to successfully collect Fe colloids with minimal artifacts – specifically avoiding aggregation of various suspended phases during filtration, which can result in spurious spatial correlations. The hydrothermal plume samples collected closest to an active vent show crystalline spherical Fe-nanoparticles that predominantly consist of poorly-ordered Fe-oxyhydroxide and are in parts enriched in P, S, Ni, and/or Cu. Using the machine learning model SIGMA² further allows us to explore the distribution of distinct Fe phases and reveals the local occurrence of reduced Fe as chalcopyrite and pyrite. On the outside, the Fe-nanoparticles are covered with an amorphous phase enriched in Mg, Cl, ± P, and S. Amorphous silica clusters are omnipresent and often co-occur with the Fe colloids. Notably, our results do not show associations of Fe with (organic) carbon.

These observations suggest that a higher local concentration of P within the Fe-colloids is potentially a crucial component affecting the Fe-nanoparticle's properties and environmental fate. Furthermore, this shows that C-rich phases do not significantly affect, at least in the early stages, the particles at the Rainbow vent field, contrasting previous studies, which suggest that organic compounds play a key role in stabilizing and transporting hydrothermal Fe^1,3. While Si is abundant in the hydrothermal fluid and often interacts with Fe precipitates similar to P, we show spatial decoupling suggestive of a distinct precipitation mechanism. Neither in the hydrothermal plume away from the active vent nor in the sediment did we observe much transformation of the poorly-ordered Fe-colloids, suggesting that these were stable early on. However, we do observe an enrichment in organic compounds associated with the Fe-colloids further up in the buoyant plume.

Our research presents the first indications that during the early formation of hydrothermal Fe colloids, the properties of the Fe-based nanoparticle and, subsequently, the environmental fate and impact are more likely affected by P and Si than by organic carbon compounds.

1. Toner, B. M., et al. Acc. Chem. Res. 49, 128–137 (2016).

2. Tung, P., et al. Geochem., Geophys., Geosystems 24, (2023).

3. Bennett, S. A. et al. Deep Sea Res. Part : Oceanogr. Res. Pap. 58, 922–931 (2011).

How to cite: Ternieten, L., Preiner, M., Kraal, P., and Plümper, O.: The early fate of hydrothermal Fe nano-colloids at the Rainbow Hydrothermal Vent Field, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-4689, https://doi.org/10.5194/egusphere-egu24-4689, 2024.

Seafloor cold seepage systems are environments known for their high levels of arsenic, a prevalent and toxic metalloid in nature. However, the biogeochemical mechanisms of arsenic enrichment remain poorly understood. In this study, sediment cores recovered from the active, inactive, and reference areas of the Hama cold seep in the South China Sea were examined using geochemical and metagenomic analyses. Geochemical data revealed evident enrichment of total arsenic, sulfide-bound arsenic, and organic arsenic in the seep site sediments compared to the reference area. Metagenomic analyses identified genes involved in arsenic oxidation (aoxA and arxA) primarily present in the upper zone, while arsenic reduction genes (arrA) were predominantly detected in the deeper zone of the sediment cores in the active area. Moreover, 25 metagenome-Assembled Genomes possessing arsenic reduction genes were affiliated with the Desulfobacterota, possibly syntrophic partners of the Anaerobic Methane Oxidizing Archaea. Furthermore, a strong linear correlation was observed between the dissolved arsenic and DIC (R²=0.64, p<0.01) and δ¹³C_DIC values (R²=0.86, p<0.01) in the active area. These results suggest a synergistic relationship between arsenate reduction and methane oxidation, possibly indicating the occurrence of AsR-AOM, within the sediment column of the active seep site. Combining this with the Fe and Mn geochemistry, we propose that arsenic was efficiently transported from the seawater column to the sediment column through rapid and continuous redox cycling of Mn and Fe, then stored in the sediment columns by HS^-, which is produced by bacterially mediated sulfate reduction. In summary, these findings indicate that cold seepage systems function as crucial sink for arsenic in the deep ocean, which would create arsenic-deficient seawater environments during the large-scale methane release events in earth histroy.

How to cite: Li, J., Dong, X., and Peng, X.: Seafloor Cold Seeps as Crucial Mediators of Arsenic Enrichment in Deep Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2725, https://doi.org/10.5194/egusphere-egu24-2725, 2024.

Typhoons can significantly change marine biogeochemical processes. The organic matter (OM) decomposition (carbon source) plays an important role in biogeochemical process in coastal waters after typhoons. However, more field investigations are needed to quantify the contributions of particulate organic matter (POM) and dissolved organic matter (DOM) to OM decomposition triggered by typhoons. To address this issue, the stable isotopes of particulate OM (POM) and the spectral properties of dissolved OM (DOM) were investigated before and after Typhoon "Barija" in Zhanjiang Bay, northwestern South China Sea. High-salinity seawater intruded from the lower bay to the upper bay due to the external forces of clockwise wind stress, thereby forming an ocean front in the middle bay during the typhoon. The POM decomposition induced by the typhoon in the upper bay (inventory 72%) was substantially higher than in the lower bay (inventory 5%) due to the barrier effect of ocean front in the middle bay. However, the decomposition removed only 1–4% DOM in the upper bay, and a net addition of DOM occurred in the lower bay due to phytoplankton growth and POM decomposition. More importantly, although the quantity of DOM is much larger than that of POM in the water, the inventory of POM in the upper bay removed by typhoon-induced decomposition (20.19 g m^-2) is much higher than that of DOM (16.08 g m^-2). Overall, our study suggests that POM decomposition is more critical than DOM decomposition after typhoons, mainly controlled by the strong ocean front and vertical mixing.

How to cite: Lu, X.: Using stable isotopes and spectral properties to quantify the contributions of particulate organic matter and dissolved organic matter to organic matter decomposition triggered by typhoons, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1273, https://doi.org/10.5194/egusphere-egu24-1273, 2024.

Typhoons are extreme whether events that can not only affect marine dynamics, but also change marine biogeochemistry, which greatly impact on the marine eco-environment and climate. Recently, we reported that decomposition (as a carbon source) of organic matter (OM) is the dominant process in coastal waters after typhoons, which is contrary to phytoplankton blooms (as a carbon sink) in previous studies. The decomposition directly affects the global carbon and nitrogen cycles, and the efficiency of the biological pump and global climate change. Yet, the mechanisms of OM decomposition after typhoons and the question of whether the decomposition is mainly of particulate organic matter (POM) or dissolved organic matter are still unclear. To address these issues, more than ten typhoons with different intensity, moving speed, and path of the typhoon were chosen, and physicochemical parameters and multiple isotopes in the coastal waters were measured before and after typhoons. The results showed that not all typhoons can trigger phytoplankton blooms in the oceans, which mainly depends on the supply of nutrients after typhoons. However, a positive apparent oxygen utilization value occurred in the coastal waters, suggesting that decomposition of OM was the dominant biogeochemical process regardless of whether phytoplankton blooms occurred after the typhoon. Despite the overall larger DOM in the water column, the amount of POM removed by typhoon-induced decomposition is much greater than that of DOM. Our study suggests that typhoon-induced decomposition might be dominated by POM, which is not conducive to the storage of OM in sediments. It means that the capacity of sediments as a carbon sink will be weakened under global warming (increasing typhoon events).

How to cite: Chen, F. and Lao, Q.: Characteristics and Mechanisms of Typhoon-Induced Decomposition of Organic Matter and Its Implication for Climate Change, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1951, https://doi.org/10.5194/egusphere-egu24-1951, 2024.

The Adriatic Sea is home to diverse marine ecosystems, showing rich biodiversity and distinctive ecological dynamics. Its complex coastal habitats and waters support a variety of species, playing a crucial role in the region's ecology and economy. Understanding the impact of ongoing climate changes on this delicate environment is vital for the basin's future preservation. To address this, we developed a comprehensive biogeochemical model for the entire basin, featuring a horizontal resolution of approximately 2 km and 120 vertical levels. This model is driven by atmospheric, hydrological, and ocean circulation projections from 1992 to 2050, downscaled from one Med-CORDEX model under the RCP8.5 emission scenario, developed within the AdriaClim project. The projected changes between 1992-2011 and 2031-2050 were evaluated in distinct trophic ecoregions identified by means of a k-medoid classification. The results reveal a strong oligotrophication tendency, particularly pronounced in the northern estuarine area. This trend can be largely attributed to a significant decrease in river discharge projected by our modelling system for the rivers of the Po Plain. This scenario of unproductive resources, ongoing warming, salinization, and acidification poses concerns about the long-term resilience of the Northern Adriatic food web structure, adapted to thrive in high trophic conditions. Our study provides the stakeholders with insights into how potential long-term decreases in Northern Adriatic river regimes might impact the marine ecosystem and its future goods and services.

How to cite: Mentaschi, L., Lovato, T., Butenschön, M., Alessandri, J., Aragao, L., Verri, G., Guerra, R., Coppini, G., and Pinardi, N.: Projected extreme oligotrophication of the marine ecosystems of the Adriatic Sea, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11752, https://doi.org/10.5194/egusphere-egu24-11752, 2024.

Net primary production (NPP) by marine phytoplankton transfers organic matter and energy to higher tropic levels, supporting ocean food webs, as well as enhancing ocean carbon sequestration. Conversely, ocean acidification is a consequence of anthropogenic carbon uptake and alongside wide-ranging ecosystem impacts, reduces the capacity of the ocean to absorb future anthropogenic emissions. Multi-model projections of NPP (including those of CMIP exercises) have typically had very high associated uncertainty, with model uncertainty generally exceeding scenario uncertainty and limited confidence in even the sign of twenty-first century change. In contrast, projections of acidification have often been portrayed as having almost no associated uncertainty for a given emissions scenario. Here I will reassess these divergent characterizations. Can we say anything about projected changes in ocean NPP with confidence? And do we have any novel insights into projected ocean acidification? Efforts to constrain model projections of NPP have been challenging, with a dramatic increase in global projection uncertainty in CMIP6. Past efforts, which used the observable sensitivity of NPP to ENSO variability to constrain the multi-model NPP response to climate change, have been shown to have their limitations. Notably, parameterizations of marine diazotrophy and phytoplankton iron requirements can both limit the applicability of such emergent constraints. Nonetheless, at regional scales there is often broad agreement across multi-model NPP projections. With respect to acidification, the apparent lack of projection uncertainty is often a result of focusing on the annual mean global surface ocean. Numerous recent advances have been made in understanding regional and subsurface acidification as well as characterizing how the temporal variability of the ocean carbonate system is likely to respond. Particularly notable is the Arctic Ocean, where the amplitude and phasing of the seasonal cycle of CO₂ partial pressure are projected to modify in response to both the geochemical and radiative impacts of anthropogenic emissions.

How to cite: Kwiatkowski, L., Bopp, L., Torres, O., Orr, J., and Tagliabue, A.: Reevaluating progress and uncertainties associated with projections of ocean net primary production and acidification , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-7894, https://doi.org/10.5194/egusphere-egu24-7894, 2024.

Coral reefs are currently under threat due to climate change and ocean acidification. However, future atmospheric CO₂ levels, climate change and associated impacts on coral reefs remain uncertain. Critically, corals not only respond to atmospheric and climatic conditions but modify them. The calcification of corals modifies the concentration of dissolved inorganic carbon and total alkalinity in the upper ocean, impacting air-sea gas exchange, atmospheric CO₂ concentrations, and ultimately climate. These feedbacks between atmospheric conditions and coral biogeochemistry can only be accounted for with a coupled coral-carbon-climate model.

To simulate coral-mediated climate-carbon interactions, we have implemented a coral reef calcification module into the iLOVECLIM Earth system model of intermediate complexity. We then performed an ensemble of 210 parameter perturbation simulations to derive carbonate production parameter values that optimise the simulated distribution of coral reefs and associated carbonate production rates. The tuned model simulates the presence of coral reefs and regional-to-global carbonate production values in good agreement with data-based estimates. We have used this new coupled model to project future changes in coral reef carbonate production. The use of a computationally efficient intermediate complexity model allows us to cover a large range of possible futures that encompass different emissions scenarios (SSPs), climate sensitivities (hence different levels of warming) as well as the possibility of coral reefs adapting to higher SSTs which would reduce the risk of bleaching. We found a high sensitivity of the simulations to the ability of corals to adapt to thermal changes and to climate sensitivity, with the possibility of 20 to 100% coral extinction in scenario SSP1-2.6 depending on these parameters. This highlights the importance of improving the constraints on these factors in models and observations.

How to cite: Bouttes, N., Kwiatkowski, L., Bougeot, E., Berger, M., Brovkin, V., and Munhoven, G.: Future evolution of coral reef carbonate production from a global climate-coral reef coupled model, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-14377, https://doi.org/10.5194/egusphere-egu24-14377, 2024.

Human-induced climate change is expected to have a significant impact on primary production in the world's oceans, affecting marine ecosystems and the biological carbon pump. The sixth phase of the Coupled-Models Intercomparison Project (CMIP6) provides a common tool for investigating this impact. However, the extent and direction of the impact can vary widely among these models, partly due to the diverse representations of oceanic biogeochemistry. A comprehensive analysis of the biogeochemistry component in each model is therefore crucial for understanding the origins of diverse responses to similar environmental forcings. In this context, we focused on primary production projections in the subtropical North Atlantic Ocean under the RCP5-8.5 scenario in eight CMIP6 models. To ensure comparability across models with different ocean dynamics, a subtropical region was constructed in the observations using satellite and in-situ measurements from 2000 to 2020. A classification procedure based on a Self-Organizing Maps algorithm was applied to the entire North Atlantic Ocean, and the resulting classification was then used to identify the closest subtropical region in each model. While the eight regions obtained were similar in terms of location and physical boundaries, their surfaces varied from 19.1 to 24.1 million km². Among the eight models, three projected an increase in primary production in the region (up to +19.5 g-C/m2/yr), while the remaining five predicted a decrease (down to -33.4 g-C/m2/yr), despite a consistent decrease of nitrate concentrations across models. Nitrogen fixation, a crucial source of nitrogen in the area, emerged as a key differentiating factor among these models. Three of them (IPSL-CM6A-LR, CanESM5-CanOE, CanESM5) featured an implicit representation of diazotrophy, with no effective control from other nutrients. This led to a substantial increase in nitrogen fixation under the influence of rising temperatures. In IPSL-CM6A-LR and CanESM5-CanOE, the heightened nitrogen fixation sustained ammonium concentrations, enabling an increase in primary production with NH4 serving as an alternative nitrogen source. However, in CanESM5, where only one nitrogen pool was represented, the increase in nitrogen fixation was insufficient to offset the nitrogen decrease, resulting in a decline in primary production. In the models with an explicit representation of diazotrophs (MPI-ESM1-2-LR, CESM2, CESM2-WACCM), primary production was limited by declining phosphate concentrations, leading to a decrease in both nitrogen fixation and primary production. Finally, in models without any representation of nitrogen fixation, primary production decreased when nutrients were initially limiting (UKESM1-0-LL) and increased if they were not (ACCESS-ESM1-5). In conclusion, biogeochemical models with a more realistic representation of diazotrophy consistently projected a decline in primary production in the subtropical North Atlantic Ocean throughout the 21st century. The increases observed in some models were attributed to the absence or inadequate representation of interactions between phosphate, ammonium, and diazotrophy, making such scenarios unlikely. Consequently, an explicit representation of diazotrophs seems necessary for projecting the primary production response to climate change in the subtropical North Atlantic Ocean.

How to cite: Doléac, S., Lévy, M., and Bopp, L.: Nitrogen fixation as a key diverging factor of primary production projections in the subtropical North Atlantic Ocean in CMIP6 models, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9341, https://doi.org/10.5194/egusphere-egu24-9341, 2024.

Dissolved organic carbon (DOC) is a substantial pool of bioreactive carbon in the ocean, comparable in quantity to the atmospheric inorganic carbon reservoir. DOC plays an important role in the marine carbon cycle; its contribution to total organic carbon export corresponds to about 20%. Current modeling studies suggest a broad range of DOC surface concentration estimates, and the response of DOC concentration and export to climate change is unclear and has not yet been described in an Earth System Model. To address this knowledge gap, we make use of the ocean biogeochemistry and ecosystem model COBALTv2. We analyze DOC dynamics and export under the present and future climate conditions within the high-emission scenario. The COBALTv2 model, coupled to the GFDL’s ESM2M Earth System Model, enables us to trace DOC from its primary sources, including phytoplankton activity and grazer-prey dynamics, to its sinks such as bacterial remineralization. We also account for the physical processes such as advection that influences DOC distribution. Preliminary findings suggest that the current distribution of DOC in the ocean may undergo significant changes, contingent upon the biological sources and sinks that are sensitive to ocean temperature increases. The relative contributions of different phytoplankton groups to DOC production are expected to shift across various ocean regions, along with the magnitude of heterotrophic respiration, which is the predominant DOC sink. This study contributes to understanding and forecasting of potential shifts in oceanic DOC dynamics under current and future climate conditions.

How to cite: Flanjak, L., Wienkers, A., and Laufkötter, C.: Dissolved organic carbon dynamics in a changing ocean: A COBALTv2 Earth System Model analysis, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-5943, https://doi.org/10.5194/egusphere-egu24-5943, 2024.

Understanding how climate variability and climate change affects marine ecosystem dynamics and its cascading implications for the carbon cycle is a “known-unknown” that was highlighted in the past four Assessment reports of the IPCC. We present results from a novel set of global ocean biogeochemistry model branches which were designed to explore the role of marine ecosystem structure for carbon dynamics both globally and regionally, with a focus on the Southern Ocean.

PlankTOM12 is a global ocean biogeochemistry model based on the representation of marine microorganisms grouped into twelve Plankton Functional Types (PFTs) as a function of their importance for the carbon cycle. PlankTOM12 uniquely represents explicitly heterotrophic bacteria/archaea, six types of phytoplankton , and five types of zooplankton . We build three distinct branches of PlankTOM12, with identical ecosystem framework and identical physical environment, but each branch with its own set of ecosystem parameters allowing different ecosystem formations. Branch 1 (called GCB) is the historical branch that  underpinned much prior research on the carbon cycle using this model and contributed to the Global Carbon Budget 2023. Branch 2 (ECO) is optimised to reproduce the observed mean, seasonal cycle, and interhemispheric distribution of surface chlorophyll-a (Chla). Branch 3 (CO2) is optimised to reproduce the observed mean and seasonality of the partial pressure of surface ocean carbon dioxide (pCO2). Even though the parameterisations are optimised globally, many of the substantial differences between the three branches occur in the Southern Ocean. In particular, it was not possible to reproduce a good mean and seasonality for both Chla and pCO2 simultaneously in the Southern Ocean. Strikingly, each of the three PlankTOM12 model branches offers a different perspective on marine ecosystem dynamics. The branches differ most distinctly in the relative fraction of biomass that is distributed among PFTs: the GCB branch distributes most of its biomass in small phytoplankton PFTs and large zooplankton PFTs, the ECO branch distributes its biomass relatively evenly among PFTs, and the CO2 branch is intermediate with most biomass in the small phytoplankton PFTs and large zooplankton PFTs, but also substantial biomass in the medium-sized PFTs for both phyto- and zooplankton. We show how the differences in these ecosystem structures transfer through to differences in carbon dynamics, including primary and secondary production, sinking fluxes of organic carbon, calcium carbonate, and silica, and how they propagate to carbon export to the deep ocean and export efficiency. We present the response of the three branches to recent climate change and variability using hindcast simulations over 1948-2022, and discuss model evaluation based on available biogeochemistry and ecosystem observational data. Finally, we suggest future applications and questions which may be best addressed by each model branch.

How to cite: Wright, R., Guest, J., Jarníková, T., Racault, M.-F., Buitenhuis, E., Mayot, N., Townsend, P., and Le Quéré, C.: Community structure matters: diverse ecosystem structure shapes carbon dynamics in a model system., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-16901, https://doi.org/10.5194/egusphere-egu24-16901, 2024.

Understanding the relative importance of top-down vs. bottom up controls for setting plankton populations is an open challenge in marine ecology. Recent work has focused on the sharp spatial decline in Prochlorococcus biomass when moving northward in the North Pacific transition zone. This work has argued against bottom up controls like temperature and for top down mechanisms like apparent competition or viruses setting the location of the collapse. However, as temperature and light modify the underlying rates in the system we would expect them to play some role in the temporal dynamics. Here, we seek to unify bottom up and top down controls to make predictions about the seasonal progression of the ‘Propocalypse’ and the associated predators of Prochlorococcus. Observations suggest that the Propocalypse occurs further poleward in the summer and about 10 degrees further south in the winter. Here, we use models to confirm that ecological interactions allow for the existence of the transition, and that the meridional movement is determined by growth rate changes due to light and temperature in the spring, and by mixed layer depth changes during the fall and early winter. We go on to seek a diversity-mortality relation for Prochlorococcus connecting viral mortality to the seasonal motion of the Propocalypse. 

How to cite: Follett, C., Duckworth, B., and Dutkiewicz, S.: Do Temporal Diversity Dynamics Reconcile Viral and Grazer Based Theories for the Propocalypse?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-21938, https://doi.org/10.5194/egusphere-egu24-21938, 2024.

We have conducted experiments with both laboratory cultures and natural plankton assemblages, together with basin-scale observations across the Atlantic Ocean, to investigate the interactive effects of temperature and nutrient supply on phytoplankton across multiple levels of biological organization, including molecules, cells, populations and communities. Laboratory data indicate that nutrient supply has a stronger effect than temperature on photosynthetic protein abundance, C:N stoichiometry, photosynthesis and growth. Due to changing resource allocation into photosynthetic machinery, the chlorophyll a (chl a) content of cells is strongly dependent on both temperature and nutrient availability, which has implications for the use of chl a concentration as a proxy for phytoplankton biomass in the ocean. Across the tropical and subtropical Atlantic, experimental nutrient enrichment consistently causes an increase in chl a concentration, picoeukaryote abundance and the contribution of small nanophytoplankton to total biomass, all of which take place irrespective of temperature. Light-harvesting capacity is synergistically stimulated by warming and nutrient addition in both picocyanobacteria and picoeukaryotes. The latitudinal variability in elemental composition of different phytoplankton groups, determined on single cells with X-ray microanalysis across the temperate, subtropical and tropical Atlantic, reveals the effect of changing temperature and nutrient supply on C:N:P stoichiometry. Our experimental and observational results suggest that while changes in nutrient supply have a stronger effect than temperature on growth, metabolic rates, community structure and elemental stoichiometry, the warming of the surface ocean may increase the ability of tropical phytoplankton assemblages to exploit events of enhanced nutrient availability. Across multiple levels of biological organization, nutrient limitation tends to reduce the effects of temperature on phytoplankton ecophysiology.

How to cite: Marañón, E., Fernández-González, C., Duhamel, S., and Segura, M.: Role of temperature and nutrients as drivers of resource allocation, elemental stoichiometry, metabolism and growth in marine phytoplankton, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11441, https://doi.org/10.5194/egusphere-egu24-11441, 2024.

The South Pacific Ocean stands out as a dynamical region with contrasted biogeochemical (BGC) regimes. Among others, it encompasses mesotrophic areas as well as the most oligotrophic waters of the global ocean. Within these regions, the existing thousands of islands can locally or regionally disrupt the oceanic circulation and, thereby, thus the nutrient availability in the upper sunlit layer and the phytoplankton growth. Yet, the phytoplankton seasonal dynamics in these contrasted BGC regions remain largely unknown and understood. Indeed, most existing studies on phytoplankton seasonal variability from observations are either dedicated to the global ocean or based on remotely sensed data due to a lack of in-situ observations in the water column, preventing the consideration of 3D processes.

Here we took advantage of in situ observations from 13 BGC-Argo profiling floats that have drifted from 2015 to 2023 in five subregions of the South Pacific Ocean: the Tasman and Coral Seas, the Fiji island region, the oligotrophic and equatorial mesotrophic areas. We used measurements of temperature, salinity, chlorophyll-a fluorescence (Chl), particulate backscattering at 700 nm (b_bp) used as a proxy of particulate organic carbon and Photosynthetically Active Radiation. The seasonal variations of Chl and b_bp vertical distributions are characterized among the subregions and physical and biogeochemical processes likely involved have been investigated. To do so, we considered isolume and nutricline depths, the Mixed Layer Depth (MLD) as well as the maximum Brunt-Vaissala depth as an indicator of the ocean stratification stability. The latter appears more suitable than the MLD when related to the phytoplankton seasonal dynamics. 

How to cite: Hermilly, T., Martinez, E., Uitz, J., Cornec, M., and Schmechtig, C.: Seasonal variability of the phytoplankton biomass and its underlying processes in contrasted regions of the South Pacific Ocean based on BioGeoChemical-Argo observations, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15355, https://doi.org/10.5194/egusphere-egu24-15355, 2024.

In the South Atlantic HNLC regions, productivity limitation is largely attributed to light in combination with the unavailability of micro-nutrients, mainly dissolved iron (dFe). Many of these regions fall within the dynamic marginal ice zone (MIZ), which are highly vulnerable to future change in climate where in the past decades Southern Ocean is absorbing more heat than the polar north. It is hypothesized that sea-ice formed in winter concentrates macro- and micro-nutrients and serves as their source, on melting, to surface waters in spring to support phytoplankton blooms, where dFe concentrations can be less than 0.1 nM. For the logistical difficulties in accessing these remote areas, especially in winters, and analytical challenges, sea ice as a source or sink of micro-nutrients in these remote regions has remained understudied and only ~78 dFe usable data is available for the entire Southern Ocean sea-ice.

Sea-ice cores were collected in winter and spring from South Atlantic MIZ and analyzed for dissolved and particulate iron (pFe) along with other ancillary data. Ice-core depth profiles of dFe show a typical C-shape with higher dFe concentrations at the top and bottom of the core. dFe profiles did not follow the salinity profiles, suggesting external input of Fe at the top and bottom of the core and not brine drainage. The average concentration of dFe (0.53 ± 0.64 nmol kg^-1; n = 34) in sea-ice remained consistently lower than pFe (3.82 ± 2.42 nmol kg^-1; n = 11) and there was considerable heterogeneity even in replicate cores collected from the same floe. The measured concentrations translate into a total iron flux (TFe) of 38.05 ± 27.49 mol y^-1 (n = 45) in South Atlantic MIZ. Both measured species show regularly lower concentrations than what has been measured from other regions of the Southern Ocean, resulting in the calculated flux from the studied MIZ being 30 times lower than what has been calculated from melting of near shelf ice. 

Experiments conducted in the laboratory show that majority of the dissolved iron accumulated in sea-ice is released within the first 10% of melting but with complete melting and mixing, resulting net dFe concentrations fall within the Fe-limiting conditions. pFe, although available at higher concentrations in sea-ice, had lower Fe/Al ratio compared to the typical crustal ratio. That is, a more refractory phase is released on melting of sea-ice, which is not ideal for uptake by phytoplankton. 

How to cite: Roychoudhury, A., Buchanan, K., and Samanta, S.: Marginal sea-ice is not a major source of iron to support spring blooms in the South Atlantic, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2748, https://doi.org/10.5194/egusphere-egu24-2748, 2024.

CO₂ emission-driven simulations are becoming a priority way of running Earth system model simulations of climate change as they directly link Earth system responses to climate policies. Also large ensemble simulations have been identified as a key tool to quantify effects of internal climate variability in the Earth system. We ran a large ensemble of emission-driven climate change projections in MPI-ESM with various CMIP6 emission scenarios. Our simulations produce a realistic temporal and spatial evolution of atmosphric CO₂, as well as ocean and land carbon sinks. An increased seasonal cycle in all carbon cycle compartments is projected in future simulations. Ongoing work focuses on discerning responses of the ocean and land carbon sinks to changing emissions. Thereby, we find that changes in atmospheric carbon are asymmetric to CO₂ emissions across various scenario pathways. Furthermore, temperature continues to increase after CO₂ emission mitigation. In our simulations, under negative emissions, the ocean and land shift from being sink to source after 2100. In particular, for the ocean carbon sink evolution, the regions which acted as a sink for anthropogenic CO₂ in the 20th century, remain sinks also during the 21st century. However, the major carbon uptake shifts from the North Atlantic to the Southern Ocean and much of the additional carbon is then stored in the south-west Pacific. In the 22nd century, it is mostly the North Atlantic that shifts towards less uptake of CO₂ and from which dissolved inorganic carbon is transported away, while the Barents Sea and WeddelSea keep taking up more CO₂ than during pre-industrial times, and parts of the ocean in the southern hemisphere keep accumulating carbon. Air-land CO₂ fluxes overlap among different scenarios emphasizing the substantial role of internal climate variability.

How to cite: Ilyina, T., Li, H., Ramme, L., and Li, C.: Unfolding responses in the global ocean and land carbon sinks and atmospheric CO2 to changing emissions with the large ensemble future projections with interactive carbon cycle., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19854, https://doi.org/10.5194/egusphere-egu24-19854, 2024.

For many ecosystems, biodiversity promotes productivity. Global biodiversity loss due to anthropogenic climate change has thus raised concerns about its resulting impact on productivity since ecosystem services like carbon sequestration are key in limiting climate change. Our oceans are especially important in this context, as they sequester more carbon than terrestrial ecosystems. Nonetheless, our understanding of how biodiversity relates to productivity in oceanic ecosystems is limited and hitherto undescribed for coccolithophores, a main ocean calcifier which plays a key role in the ocean carbon cycle. Here, by combining a new comprehensive coccolithophore abundance data set and species distribution models we illustrate that biodiversity is decoupled from standing stocks for coccolithophores. We show this is because the processes driving coccolithophore diversity do not influence coccolithophore standing stocks. Our results contribute new knowledge about the relationship between productivity for a key marine planktonic microorganism, highlighting that diversity loss due to anthropogenic carbon emission does not necessitate reduced ocean standing stocks and productivity. This result is important for climate models, suggesting that they do not need to capture our ocean’s full diversity to capture key biogeochemical processes such as calcification and carbon fixation.  

How to cite: de Vries, J., Monteiro, F., and Wolf, L.: Unexpected decoupling between biodiversity and standing stocks in marine calcifying phytoplankton , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-19449, https://doi.org/10.5194/egusphere-egu24-19449, 2024.

How to cite: Lim, H.-G., Dunne, J., Stock, C., Ginoux, P., John, J., and Krasting, J.: Oceanic and Atmospheric Drivers of Post-El-Niño Chlorophyll Rebound in the Equatorial Pacific , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-2739, https://doi.org/10.5194/egusphere-egu24-2739, 2024.

The ocean and land biosphere are the two major natural sinks of carbon at present and the ocean is projected to become the dominant sink on centennial timescales when anthropogenic carbon emissions become zero and temperatures stabilize, and. Despite the ocean’s importance for the carbon cycle and climate, uncertainties of the decadal variability of this carbon sink and the underlying drivers of this decadal variability remain uncertain. The main tools to assess the ocean carbon sink over the last decades are global observation-based pCO₂ products that extrapolate sparse pCO₂ observations in space and time and global ocean biogeochemical models forced with atmospheric reanalysis data. However, these tools (i) are limited in time over the last 3 to 7 decades, which hinders statistical analyses of the drivers, (ii) are all based on the same internal climate state, and (iii) cannot assess the robustness of the drivers in the future, especially when carbon emissions decline or cease entirely. Here, I use an ensemble of 12 Earth System Models (ESMs) from phase 6 of the Coupled Model Intercomparison Project (CMIP6) to understand drivers of decadal trends in the past, present and future ocean carbon sink. The simulations by these ESMs span a period of 251 years and include 4 different future Shared Socioeconomic Pathways, from low emissions and high mitigation to high emissions and low mitigation. Using this ensemble, I show that 80% of decadal trends in the multi-model mean ocean carbon sink can be explained by changes in decadal trends of atmospheric CO₂ as long as the ocean carbon sink remains smaller than 4.5 Pg C yr^-1. The remaining 20% are due to internal climate variability and ocean heat uptake, which results in a loss of carbon from the ocean. When the carbon sink exceeds 4.5 Pg C yr^-1, atmospheric CO₂ rises faster, climate change accelerates, and decadal trends in the ocean carbon sink are substantially smaller than estimated based on changes in atmospheric CO₂ trends. The breakdown of this relationship under high emission scenarios, also implies that the increase in the ocean carbon sink is effectively limited, even if the trend in atmospheric CO₂ continues to increase. This limit of decadal trends in the ocean carbon sink is here estimated to be 1 Pg C yr^-1 dec^-1.Previously proposed drivers, such as the atmospheric CO₂ or the growth rate of atmospheric CO₂ can explain trends in the ocean carbon sink for specific time periods, for example during exponential atmospheric CO2 growth, but fail when emissions start to decrease again. The robust relationship over a large Earth System Model ensemble also suggests that very large  positive and negative decadal trends of the ocean carbon sink by some pCO₂ products are highly unlikely, and that the change in the decadal trends of the ocean carbon sink in 2000 is likely substantially smaller than estimated by these pCO₂ products.

How to cite: Terhaar, J.: Decadal trends of the ocean carbon sink in Earth System Models in the past, present and future, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-6435, https://doi.org/10.5194/egusphere-egu24-6435, 2024.

   This study presents an analysis of the historical upper ocean (0-700m) dissolved oxygen (O₂) and heat content changes from a suite of  the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemistry simulations. The simulations include both forced ocean-only models (the Ocean Model Intercomparison Project, OMIP1 and OMIP2) and coupled historical simulations from Earth System Models (ESMs)  (CMIP6 Historical). Simulated changes in the O₂ inventory and ocean heat content (OHC) over the past five decades spatially and temporally diverge across the models. The multi-model mean and spread of the upper ocean global O₂ inventory trend for each of the simulations is 0.03 ± 0.39 × 10¹⁴ [mol/decade] for OMIP1,  -0.37 ± 0.15 × 10¹⁴ [mol/decade] for OMIP2, and -1.06 ± 0.68 × 10¹⁴ [mol/decade] for CMIP6 Historical simulations, respectively. The latest observational trend based on the World Ocean Database 2018 is -0.98 × 10¹⁴ [mol/decade],  in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends.  A comparison between OMIP1 and OMIP2 simulations suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across simulations in O₂ inventory and OHC changes. An additional comparison between OMIP and CMIP6 Historical simulations indicates that differences in background mean states due to differences in spin-up strategy and equilibrium states result in substantial differences in the climate change response of O₂. In this presentation, we will discuss gaps and gaps and uncertainties in both ocean biogeochemistry simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O₂ and changes in associated ocean biogeochemical cycles. This presentation is based on the recently published work (Takano et al., 2023).

Reference

Takano Y., Ilyina T., Tjiputra J., Eddebbar Y.A., Berthet S., Bopp L., Buitenhuis E., Butenschön M., Christian J.R., John  J.P., Gröger M., Hayashida H., Hieronymus J., Koenigk T., Krasting J.P., Long M.C., Lovato T., Nakano H., Palmieri J., Schwinger J., Séférian R., Suntharalingam P., Tatebe H., Tsujino H., Urakawa S., Watanabe M., and Yool A.: Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models, Front. Mar. Sci., 10:1139917, doi: 10.3389/fmars.2023.1139917.

How to cite: Takano, Y. and the Ocean Biogeochemistry Modelling Group: What are the challenges simulating historical ocean deoxygenation?, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-15731, https://doi.org/10.5194/egusphere-egu24-15731, 2024.

Projections for the future under high scenarios using state-of-the-art Earth System Models suggest a warmer climate for both the land and the ocean, and this leads to ocean acidification globally and ocean deoxygenation. Strengthened denitrification and anammox from Oxygen Minimum Zones (OMZs) or hypoxic coastal areas would possibly enhance nitrous oxide emissions which is one of the main greenhouse gases (GHGs). However, the global dynamics of nitrous oxide are largely uncertain. Considering current emission rates, revisiting past warm periods (e.g., the Paleocene-Eocene Thermal Maximum and the Mid-Miocene Climatic Optimum) would improve our understanding of the biogeochemical-climate feedback. One of the uncertainties in such studies is the paleogeographic boundary condition which is shown to have significant impacts on the global climate and the ocean circulations. However, to date, few studies have been able to explore how paleogeographic features could impact the ocean biogeochemical processes during past warm periods. We applied a suite of sensitivity simulations with modifications only to the topographic features based on three sets of paleogeographies for the mid-Miocene, namely the Getech, the Scotese, and the Robertsons. We found there are huge differences in the ocean circulation patterns, with the strength of the Atlantic Meridional Overturning Circulation (AMOC) ranging from 0Sv to over 12Sv. We will likely see apparent biogeochemical responses accordingly, especially in tropical upwelling regions and the North Atlantic. Results TBC.

How to cite: Li, Y., Farnsworth, A., and Valdes, P.: Paleogeographic impacts on the ocean biogeochemistry during the mid-Miocene, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11155, https://doi.org/10.5194/egusphere-egu24-11155, 2024.

The Cenozoic shift from a hothouse to icehouse provides a natural experiment to explore how a changing climate and macroevolutionary trends control marine pelagic carbonate production and burial. In the modern ocean, the key components of pelagic carbonate burial — planktic foraminifera and coccolithophores — contribute approximately evenly. However, in the past, coccolithophores dominated open ocean inorganic carbon burial. Exactly when and why this shift away from a coccolithophore dominated ooze occurred is unresolved. To this end, we reconstructed a 65Myr record of foraminifer to nannofossil ratios from sites covering the Pacific, Southern, Indian, and Atlantic Ocean. To better understand the climate and macroevolutionary controls on carbonate production, we move away from the commonly reported bulk changes and instead investigate the individual components of carbonate production:  foraminiferal and coccolithophore size, weight and abundance. We use a suite of methodologies to extract these data, including the novel application of imaging flow cytometry to rapidly and digitally reconstruct the fossil record of coccolithophore size and abundance. Our ratio data shows a shift towards calcareous zooplankton during the Neogene. Initial qualitative analysis reveals that coccolithophore size is relatively smaller in the modern part of the record, whilst automated microscopy shows that modern subtropical and tropical foraminiferal size is greater than recorded in the previous 65Myr. Foraminiferal size-normalised weight (SNW) is expected to be higher in the modern ocean than in the past due to its suggested carbonate system control (i.e. higher carbonate ion concentrations being conducive to heavier tests). However, SNW data from a high latitude site during the Palaeogene are similar to modern values for extant species – potentially implying something other than a carbonate system control on SNW.

How to cite: Barrett, R., Power, A., Halloran, P., and Schmidt, D.: Co-evolution of the oceans and inorganic carbon cycle, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11030, https://doi.org/10.5194/egusphere-egu24-11030, 2024.