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OS3.1

EDI
Response of ocean microbes and biogeochemical cycles to past, present and future climate change

Climate induced alterations to key microbial processes, such as net primary production and nitrogen fixation, act alongside changes to the biogeochemical cycling of oxygen and nutrients to affect marine ecosystem structure and function, as well as the ocean carbon cycle. These climate changes operate over a variety of spatiotemporal scales. Today, stratification, warming and acidification are driving global changes in microbial biogeochemistry to affect ocean health. These large-scale, long-term changes are accompanied by the short-term emergence of extremes at the regional scale. And in the past, climate change over glacial-interglacial cycles and even recent multi-decadal variability offer clues to understand how sensitive ocean microbes and biogeochemical cycles are to changes in the climate.
This session seeks submissions, from both observations and modelling efforts, that address the impact of past, present (i.e. historical) and future climate change (including variability) on key microbially-mediated flows. Investigations focussing on net primary production, nitrogen fixation, anaerobic processes in low oxygen zones, and the local to global cycling of nutrients and oxygen are welcome. We also welcome studies that investigate the cascading effects for marine ecosystems to modulate biodiversity and ecosystem services.

Convener: Alessandro Tagliabue | Co-conveners: Ivy FrengerECSECS, Pearse Buchanan, Christopher Somes, Francois Fripiat
Presentations
| Thu, 26 May, 08:30–11:05 (CEST)
 
Room 1.15/16

Thu, 26 May, 08:30–10:00

Chairpersons: Alessandro Tagliabue, Francois Fripiat, Pearse Buchanan

08:30–08:35
Introduction

08:35–08:45
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EGU22-9136
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ECS
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solicited
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Virtual presentation
Noelle Held et al.

Cyanobacteria of the genus Trichodesmium provide about 80 Tg of fixed nitrogen to the surface ocean per year and contribute to marine biogeochemistry, including the sequestration of carbon dioxide and mobilization of particulate iron. Trichodesmium fixes nitrogen in the daylight, despite the incompatibility of the nitrogenase enzyme with oxygen produced during photosynthesis. While the mechanisms protecting nitrogenase remain unclear, all proposed strategies require considerable resource investment. Here we describe a crucial benefit of daytime nitrogen fixation in Trichodesmium spp. that may counteract these costs. We analysed diel proteomes of cultured and field populations of Trichodesmium in comparison with the marine diazotroph Crocosphaera watsonii WH8501, which fixes nitrogen at night. Trichodesmium’s proteome is extraordinarily dynamic and demonstrates simultaneous photosynthesis and nitrogen fixation, resulting in balanced particulate organic carbon and particulate organic nitrogen production. Unlike Crocosphaera, which produces large quantities of glycogen as an energy store for nitrogenase, proteomic evidence is consistent with the idea that Trichodesmium reduces the need to produce glycogen by supplying energy directly to nitrogenase via soluble ferredoxin charged by the photosynthesis protein PsaC. This minimizes ballast associated with glycogen, reducing cell density and decreasing sinking velocity, thus supporting Trichodesmium’s niche as a buoyant, high-light-adapted colony forming cyanobacterium. To occupy its niche of simultaneous nitrogen fixation and photosynthesis, Trichodesmium appears to be a conspicuous consumer of iron, and has therefore developed unique iron-acquisition strategies, including the use of iron-rich dust. Particle capture by buoyant Trichodesmium colonies may therefore increase the residence time and degradation of mineral iron in the euphotic zone. These findings describe how cellular biochemistry defines and reinforces the ecological and biogeochemical function of these keystone marine diazotrophs, particularly their role as a microbial link in the nitrogen, carbon, and iron cycles. 

How to cite: Held, N., Waterbury, J., Webb, E., Kellogg, R., McIlvin, M., Jakuba, M., Valois, F., Moran, D., Sutherland, K., and Saito, M.: Why does Trichodesmium fix nitrogen during the day? Special biochemistry linking biogeochemical cycles., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9136, https://doi.org/10.5194/egusphere-egu22-9136, 2022.

08:45–08:50
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EGU22-10967
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ECS
Romain Darnajoux et al.

Biological nitrogen fixation (BNF) is a critical process for the N budget and productivity of marine ecosystems. Nitrogen-fixing organisms typically turn off BNF when less metabolically costly N sources, like ammonium (NH4+), are available. Yet, several studies have reported BNF in benthic marine sediments despite high porewater NH4+ concentrations (10-1,500 µM). These activities were generally linked to anaerobic sulfate-reducing bacteria (SRB) and fermenting firmicutes.

To better understand the regulation and importance of benthic marine BNF, we evaluate the sensitivity of BNF to NH4+ in benthic diazotrophs using incubations of increasing complexity. We conduct our experiment with cultures of model anaerobic diazotrophs (sulfate-reducer Desulfovibrio vulgaris var. Hildenborough, fermenter Clostridium pasteurianum strain W5), sulfate-reducing sediment enrichment cultures, and slurry incubations of sediments from three Northeastern salt marshes (USA).

All our samples demonstrate high sensitivity to external NH4+. BNF is inhibited by NH4+ beyond an apparent threshold [NH4+] of 2 µM in liquid cultures and 9 µM in sediment slurries. Consistent with other studies, we find SRB-like nitrogenase (nifH) gene and transcripts are prevalent in sediments. We compare our inhibition threshold value with a survey of porewater NH4+ data from diverse sediments, suggesting the confinement of benthic BNF to surficial sediments.

Variations in the timing to onset BNF inhibition, NH4+ uptake rate, and sediment composition and biophysics could affect measurements of the apparent sensitivity of benthic BNF to NH4+. We propose a simple model based on NH4+ transporter affinity as a fundamental mechanistic constraint on NH4+ control of BNF to improve biogeochemical models of N cycling.

How to cite: Darnajoux, R., Reji, L., Zhang, X., Luxem, K., and Zhang, X.: Ammonium sensitivity of biological nitrogen fixation by anaerobic diazotrophs in cultures and benthic marine sediments., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10967, https://doi.org/10.5194/egusphere-egu22-10967, 2022.

08:50–08:55
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EGU22-7615
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ECS
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Virtual presentation
Na Li et al.

Nitrogen (N) is one of the crucial limiting nutrients for phytoplankton growth in the ocean. The main source of bioavailable N to the ocean is N2-fixing diazotrophs in the surface layer. Since the global coverage of N2 fixation observations is sparse on temporal and spatial scales, the fundamental processes and mechanisms controlling N2 fixation are not well understood nor constrained. We implement benthic denitrification in the optimality-based plankton ecosystem model (OPEM), which is incorporated into an Earth System Model of intermediate complexity (UVic-ESCM 2.9). Benthic denitrification occurs mostly in coastal upwelling regions and shallow continental shelves, and affects significantly the marine fixed-N budget since it is the largest N-loss process in the global ocean. We carry out model calibration and parameter selection based on observations of Chl, NO3-, PO43-, O2 and N*=NO3--16PO43-. Compared to considering water-column denitrification in suboxic zones as the only N-loss process in the ocean, our new model version simulates a more realistic distribution and higher global rates of N2 fixation, which are supported by independent rate measurements. The optimized cellular N:P ratios of diazotrophs in the model version with benthic denitrification better correspond to independent culture estimates, and result in a closer reproduction of the particulate N:P ratios. Our model results indicate that benthic denitrification plays an important role shaping patterns, rates and even physiological aspects of N2 fixation throughout the global ocean and should be accounted for when understanding and predicting changes to N2 fixation. 

How to cite: Li, N., Somes, C., Landolfi, A., Chien, C.-T., Pahlow, M., and Oschlies, A.: Global impact of benthic denitrification on diazotroph physiology and N2 fixation rates, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7615, https://doi.org/10.5194/egusphere-egu22-7615, 2022.

08:55–09:00
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EGU22-7176
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Virtual presentation
Angela Landolfi et al.

The factors that control the distribution of marine diazotrophs and their ability to fix N₂ are not fully elucidated. We discuss insights that can be gained from the emerging picture of a wide geographical distribution of marine diazotrophs and provide a critical assessment of environmental (bottom-up) versus trophic (top-down) controls. Expanding a simplified theoretical framework, we find that selective mortality on non-fixing phytoplankton is identified as a critical process that can broaden the ability of diazotrophs to compete for resources in top-down controlled systems and explain an expanded ecological niche for diazotrophs. Our simplified analysis predicts a larger importance of top-down control on competition patterns as resource levels increase. As selective mortality can control the faster growing phytoplankton, coexistence of the slower growing diazotrophs can be established. However, these predictions require corroboration by experimental and field data, together with the identification of specific traits of organisms and associated trade-offs related to selective top-down control. Elucidation of these factors could greatly improve our predictive capability for patterns and rates of marine N2fixation.

 

How to cite: Landolfi, A., Prowe, A. E. F., Pahlow, M., Somes, C. J., Chien, C.-T., Schartau, M., Koeve, W., and Oschlies, A.: Top-down controls on the ecological niche of marine N2 fixers, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7176, https://doi.org/10.5194/egusphere-egu22-7176, 2022.

09:00–09:05
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EGU22-2930
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Virtual presentation
Boris Sauterey and Ben Ward

The stoichiometric coupling of carbon to limiting nutrients in marine phytoplankton determines that of the main biogeochemical cycles through the process of biomass production by the phytoplankton. While clear links between phytoplankton stoichiometry and environmental drivers have been identified, the nature and direction of these links, as well as the underlying physiological and ecological mechanisms, remain uncertain. Here we compare the predictions of a well-constrained mechanistic model of plankton ecophysiology to multiple observational data sets to investigate the specific case of the C:N phytoplankton stoichiometry in the North-Atlantic. We show that N availability and temperature emerge as the main drivers of phytoplankton stoichiometry. The biological mechanisms involved however vary depending on the spatiotemporal scale and region considered, leading to opposite predictions regarding the evolution of phytoplankton primary productivity in response to environmental changes. At low to intermediate latitudes phytoplankton stoichiometry is predominantly driven by ecoevolutionary shifts in the functional composition of the phytoplankton communities while phytoplankton stoichiometric plasticity in response to dropping temperatures and increased grazing pressure dominates at higher latitudes. Those results shine a new light on what currently influences the circulation of elements through marine ecosystems but also have great implications regarding the evolution of oceans’ primary productivity and of the main biogeochemical cycles under a regime of climate change.

How to cite: Sauterey, B. and Ward, B.: Environmental control of marine phytoplankton C:N stoichiometry in the North Atlantic Ocean today and under climate change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2930, https://doi.org/10.5194/egusphere-egu22-2930, 2022.

09:05–09:09
Discussion

09:09–09:14
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EGU22-1846
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ECS
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On-site presentation
Gesa Schulz et al.

Nitrous oxide (N2O) is a greenhouse gas contributing to global warming. Estuaries are a potential source for N2O.  We aimed to identify seasonal and spatial variations of N2O production and emission along the Elbe estuary in Germany.

Between 2015 and 2021, we performed nine research cruises along the Elbe estuary. Most of the cruises took place in growing seasons (April – September), while one cruise was conducted in winter (early March). We continuously measured the dissolved N2O dry mole fraction 2 m below the surface using a laser-based analyzer coupled with an equilibrator. Based on these profiles, we calculated N2O concentration, saturation and emissions.

During all cruises, the Elbe estuary was supersaturated in N2O. Highest N2O concentration occurred in the Hamburg port region, a hotspot of N2O production by nitrification in the water column and denitrification in the sediments. The maximum concentration in this region was 158 nmol L‑1 in March 2021. Nitrification in the maximum turbidity zone (MTZ) produced a second local N2O maximum.  Average N2O emissions were 0.19 Gg a‑1(0.52 Mg d-1­) during the growing season. The N2O emission was highest in winter with 0.64 Gg a-1 (1.76 Mg d-1).

During growing seasons emissions were strongly correlated with pH (R2 = 0.73) and suspended particulate matter concentration (R2 = 0.55). A trend toward higher N2O saturations and emissions during cruises in summer is evident. We presume that N2O saturation and emission were likely driven by temperature-dependent turnover processes in high turbidity areas of the Elbe estuary, such as nitrification and denitrification.  

However, the maximum N2O concentrations in winter (March 2021) cannot be explained that way, because water temperature was low. N2O production may be driven by the dissolved inorganic nutrient (DIN) load, which is more than doubled in comparison to all other cruises. Two other possible explanations come to mind: First, N2O production in this case may be less sensitive to water temperature, possibly due to sedimentary sources. Second, a sink for N2O in the water column may exist, which is more active during higher temperatures. These two scenarios may both apply and might interact over the course of the year.  

Overall, seasonality affects N2O production in the Hamburg port region more than in the maximum turbidity zone. In late spring/summer, N2O production is driven mainly by enhanced microbial productivity. High N2O concentrations in colder seasons may result from high DIN concentration, but further research on the controls on N2O production, and possibly consumption, is clearly needed.

How to cite: Schulz, G., Sanders, T., and Dähnke, K.: Spatial and seasonal variation of dissolved nitrous oxide along the Elbe estuary, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1846, https://doi.org/10.5194/egusphere-egu22-1846, 2022.

09:14–09:19
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EGU22-11261
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Virtual presentation
Marta Borecka et al.

Denitrification and anammox are the main nitrogen (N) removal pathways in seawaters. Both processes are important in regions, such as the Baltic Sea, which receive high nutrient loads, that enhance primary production and eutrophication. The Baltic Sea is also characterized by a strong vertical salinity gradient and the presence of a permanent halocline hampering mixing in the water column and ventilation of the deep water layers. Rare events of deep water renewal, together with high oxygen consumption, lead to suboxic and anoxic conditions in the Baltic Sea, which are favorable for denitrification and anammox – processes for which the end product is a non-reactive N2. In seawater, the concentration of dissolved gases is controlled by biological and physical processes. The latter can be traced by measuring inert gases such as argon (Ar). Hence, the N2/Ar ratio can be used to separate physical and biological effects influencing N2 fields. This approach may suit especially to the stratified water bodies, where deep waters are separated from the surface water layer influenced by the gas exchange with the atmosphere.

The study aimed at investigating the potential use of the supersaturation ratio – ΔN2 as a tracer of denitrification and anammox processes in the water column of the Baltic Proper. The ΔN2 ratio was derived as an anomaly from the N2/Ar ratio in seawater being at equilibrium with the atmosphere. The used technique was Membrane Inlet Mass Spectrometry, which allows performing high-precision measurements of dissolved N2 and Ar in water (masses 28 and 40 were detected, respectively). Seawater samples were collected between 2017 and 2021 from nineteen stations, including Gdańsk, Gotland, and Bornholm Deeps.

The ΔN2 indicated N2 accumulation in the oxygen minimum zones below the halocline with the highest values found​​ in the bottom layers. This can be explained by both denitrification and possibly anammox in the water column and with N2 release from sediments. ΔN2 values ranged from 1.0 to 32.6 µmol L-1. In autumn 2021 a significant difference in ΔN2 (p = 0.0008) between the studied sites was observed. For example on station located in Gotland Deep ΔN2 values were in the range from 17.6 to 32.6 µmol L-1, while on station located in the Central Baltic Proper the maximum was 6.1 µmol L-1. The seasonal ΔN2 changes (autumn, spring, and winter) were investigated for two stations located in the Gdańsk Deep and indicated statistically significant variability (p=0.0077) with the highest ΔN2 observed in winter. Additionally, ΔN2 was negatively correlated with nitrate (R2=0.5469) and oxygen (R2=0.6382), positively with phosphate (R2=0.4382) and ammonium (R2=0.2898), while no clear dependency was observed for nitrite (R2=0.0388).

The presented study was the first attempt performed on such a large scale in the Baltic Proper. It demonstrates a high potential in the use of supersaturation ratio for identification of the active sites for denitrification and anammox processes.

 

The results were obtained within the framework of the statutory activities of the Institute of Oceanology Polish Academy of Sciences and the research project: 2019/34/E/ST10/00217 funded by the Polish National Science Centre.

How to cite: Borecka, M., Winogradow, A., Koziorowska-Makuch, K., Makuch, P., Diak, M., Kuliński, K., Pempkowiak, J., and Szymczycha, B.: Investigation of the potential use of the supersaturation ratio of N2 (ΔN2) for the estimation of the seasonal and spatial variability of denitrification and anammox in the water column of the Baltic Proper, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11261, https://doi.org/10.5194/egusphere-egu22-11261, 2022.

09:19–09:24
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EGU22-4872
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Virtual presentation
Christopher Somes et al.

Nitrogen and iron are the key limiting nutrients throughout the majority of the global ocean. These nutrient systems have important source and/or sink processes that are highly sensitive to low oxygen thresholds. In this study, we use a global ocean biogeochemical model within an Earth system model of intermediate complexity to investigate anthropogenic controls on marine nitrogen and iron cycling under warming and atmospheric pollutant scenarios. We performed sensitivity simulations to isolate the individual and combined effects of the marine nitrogen and iron internal feedbacks, as well as the impact from increasing atmospheric pollutant deposition. Our model simulations demonstrate strong negative (stabilizing) feedbacks on marine productivity from both the marine nitrogen and iron cycles when feedbacks from only one individual nutrient cycle were considered at a time. However, when the full set of marine nitrogen-iron feedbacks were activated, enhanced iron sources from the atmosphere and sediments under anthropogenic scenarios were sufficient to stimulate additional N2 fixation by 16% globally, with much of it occurring near tropical oxygen minimum zones enhancing regional productivity there. These marine nitrogen-iron biogeochemical feedbacks driven by anthropogenic scenarios including atmospheric pollutant deposition were responsible for a projected 40% expansion in the volume of oxygen minimum zones by year 2100 in the model, whereas a sensitivity simulation with these feedbacks deactivated resulted in a 40% reduction. Our model study suggests that increasing marine nitrogen and iron sources in the Anthropocene can play an important role on future ocean biogeochemistry and productivity that significantly contribute to expanding oxygen minimum zones.

How to cite: Somes, C., Landolfi, A., and Oschlies, A.: Anthropogenic impacts on marine nitrogen and iron biogeochemical feedbacks and their contribution to expanding oxygen minimum zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4872, https://doi.org/10.5194/egusphere-egu22-4872, 2022.

09:24–09:29
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EGU22-68
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ECS
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Matvey Novikov et al.

Climate change and anthropogenic impact drastically affect the biogeochemical regime of the Black Sea, the contains the largest in the world volume of sulfidic water. The volume of the oxic layer of the sea depends on vertical mixing, that transports dissolved oxygen (DO) from the upper euphotic layer in the deeper layers and dissolved oxygen consumption for oxidation organic matter (OM). Changes in the Sea hydrodynamic properties due to warmer winters restricts renovation of the Black Sea Cold Intermediate Layer and therefore the DO flux to the deeper layers. The main goal of this study was to model upper 350 m with emphasis on redox layer.

In the study we use the benthic-pelagic biogeochemical BROM model combined with 2DBP model for vertical and horizontal transport via FABM. BROM combines a relatively simple ecosystem model with a detailed biogeochemical model considering interconnected transformations of chemical species (N, P, Si, C, O, S, Mn, Fe). OM dynamics include parameterizations of production (via photosynthesis and chemosynthesis) and decay via oxic mineralization, denitrification, metal reduction, sulfate reduction and methanogenesis.

Hydrophysical forcing (temperature, salinity and northward and eastward sea water velocity) was hourly data for year 2010 for a point with coordinates 43.5 °N. 37.75 °E from the E.U. Copernicus Marine Service Information “Copernicus”. The data is the result of a reanalysis calculation based on the NEMO hydrodynamic numerical model.

The 2DBP/BROM model was applied for the 350 m water column with bottom boundary positioned in sulfidic layer. A steady-state solution was reached after 50 calculation years. The results of calculations were compared with the data of field observations of the expedition on the RV "Knorr" in March 2003. The obtained vertical distributions of hydrochemical characteristics are consistent with the existing understanding of the hydrochemical structure of the Black Sea. The dissolved oxygen has the similar structure in the model and observations occupying the upper 70 m layer, its onset was positioned shallower than the appearance of hydrogen sulfide at appr. 80. In the limits of the redox layer there were reproduced maxima of nitrite, Mn(IV), Mn(III), Fe(III), elemental sulfur and phosphate minimum. Below the redox layer the model reproduced maxima of Mn(II) and Fe(II).

Calculated seasonal variability in the upper layer shows seasonality in development of phytoplankton and corresponding changes dissolved oxygen and nutrients. Organic matter distributions changes in accordance with seasonality of its production and destruction. At the same time, the redox layer remains practically unchanged during the year.

At the present the BROM model is applied for the Black Sea and satisfactory validated against the data of observations. The calculated seasonal dynamics of the biogeochemical properties of the Black Sea will be used as an initial condition for studying of effects of changing in mixing (considering modeling interannual changes with difference in hydrodynamical scenarios) and allochthonous OM delivery on the biogeochemical structure of the Sea.

This study was funded by the Ministry of Science and Higher Education of the Russian Federation, theme 13.2251.21.0008.

How to cite: Novikov, M., Berezina, A., Pakhomova, S., and Yakushev, E.: An application of biogeochemical model BROM with 1-D transport model for studying of the vertical biogeochemical structure of the Black Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-68, https://doi.org/10.5194/egusphere-egu22-68, 2022.

09:29–09:34
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EGU22-7896
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ECS
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On-site presentation
Hal Bradbury et al.

Reconstructing the oxygen content at the base of the ocean provides insight into ocean circulation, carbon storage in the deep ocean and hence, the global carbon cycle. The microbial breakdown of organic carbon within marine sediment through aerobic respiration consumes oxygen in the pore fluid and releases dissolved inorganic carbon. The offset in the carbon isotopic composition of epifaunal and infaunal foraminifera is related to the respiration of the organic carbon and can be used to reconstruct the oxygen content of the bottom water. Previous work has provided a data-derived calibration which is valid for oxygen reconstructions between 55–235 micromolar. In this study, we apply a biogeochemical reactive transport model (RTM) to extend and update the calibration, which allows for the reconstruction of oxygen concentrations up to ~400 micromolar. Using the RTM and new data from the Iberian Margin, we also demonstrate that the calibration between the carbon isotope gradient and bottom water oxygen concentrations must account for the coupled changes in all aspects of the carbon system due to the respiration of organic carbon. We apply the improved calibration to reconstruct the changes in oxygen content in the North Atlantic over the past 1.4 Myr.

How to cite: Bradbury, H., Thomas, N., and Hodell, D.: Revisiting the relationship between the pore water carbon isotope gradient and bottom water oxygen concentrations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7896, https://doi.org/10.5194/egusphere-egu22-7896, 2022.

09:34–09:37
Discussion

09:37–09:42
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EGU22-3562
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Highlight
Peter Landschützer et al.

The ocean absorbs around a quarter of the annual man-made CO2 emissions, with the Southern Ocean being responsible for the lion-share of this uptake. Despite the disproportional role of the Southern Ocean in taking up anthropogenic CO2, it still remains one of the most sparsely observed ocean regions. While autonomous measurement devices have started to fill this void, high quality shipboard measurements remain limited. One fleet with the potential to fill this gap has thus far received little attention: sailboats. There is growing willingness among skippers to help science, providing a opportunity to collect valuable measurements of the sea surface partial pressure of CO2 (pCO2) during round-the-world racing events. Using the latest membrane sensor technology, we have thus far – together with professional racing teams - collected high frequency measurements of the sea surface pCO2 from nearly all ocean basins and most notably in remote southern hemisphere ocean regions where no shipboard pCO2 data were collected in the past 70 years (based on the SOCAT database). Our results highlight the potential for equipping sail yachts as a low-cost solution to fill data gaps and provide a new constraint for high resolution model studies.

How to cite: Landschützer, P., Tanhua, T., and Behncke, J.: Round-the-globe racing events to fill the pCO2 data void in the Southern Ocean , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3562, https://doi.org/10.5194/egusphere-egu22-3562, 2022.

09:42–09:47
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EGU22-2322
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Highlight
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Virtual presentation
Wilken-Jon von Appen et al.

The ocean moderates the world’s climate through absorption of heat and carbon, but how much carbon the ocean will continue to absorb remains unknown. The North Atlantic Ocean west (Baffin Bay/Labrador Sea) and east (Fram Strait/Greenland Sea) of Greenland features the most intense absorption of anthropogenic carbon globally; the biological carbon pump (BCP) contributes substantially. As Arctic sea-ice melts, the BCP changes, impacting global climate and other critical ocean attributes (e.g. biodiversity). Full understanding requires year-round observations across a range of ice conditions. Here we present such observations: autonomously collected Eulerian continuous 24-month time-series in Fram Strait. We show that, compared to ice-unaffected conditions, sea-ice derived meltwater stratification slows the BCP by 4 months, a shift from an export to a retention system, with measurable impacts on benthic communities. This has implications for ecosystem dynamics in the future warmer Arctic where the seasonal ice zone is expected to expand. (Published in Nature Communications, December 2021, https://www.nature.com/articles/s41467-021-26943-z )

How to cite: von Appen, W.-J., Waite, A., and Boetius, A. and the FRAM team: Sea-ice derived meltwater stratification slows thebiological carbon pump: results from continuousobservations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2322, https://doi.org/10.5194/egusphere-egu22-2322, 2022.

09:47–09:52
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EGU22-9409
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ECS
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Virtual presentation
Thibauld M. Béjard et al.

Ocean Acidification (OA) is considered a major threat and is projected to impact all areas of global ocean, therefore understanding its ecological impacts remains a priority for science and management. The Mediterranean Sea is considered a highly vulnerable region, so we analyzed material coming from Planier sediment trap in order to characterize the seasonal variability of weight and calcification of planktic foraminifera. This sediment trap is located in the Gulf of Lions (GoL), in the northwestern part of the Mediterranean Sea, one of the few non-oligotrophic regions in the Mediterranean (high productivity period from January to May). We performed planktic foraminifera picking focusing on 3 different species: Globigerina bulloides, Neogloboquadrina incompta and Globorotalia truncatulinoides. A mean of 13 to 27 specimens per sample were picked. These foraminifera samples were then cleaned with the ultrasonication in methanol technique and then weighted using a Sartorius ME5 balance (precision= 0.001mg) in the micropaleontology laboratory of the University of Salamanca. A total of 126 samples and 2077 individuals were weighted. SBW (Sieve Based Weight) results showed that traditional used sieved size fractions do not provide enough control on the effect of morphometric parameters on the weight/calcification data, highlighting the need of a size-normalization. Area and diameter measurements were carried using a Nikon SMZ18 and a DS-Fi3 through the NIS Elements. MBW (Measured Based Weights) results showed that both of these parameters (area and diameter) have no influence on MBW values, indicating these values are good index for calcification intensity. Seasonal MBW variations differ according to the species: G.bulloides showed a maximum MBW values during the high productivity period, N.incompta reached its maximum values slightly after the high productivity period while G.truncatulinoides displayed a maximum calcification value during the low productivity period. Finally, we compared these results with “Optimum Growth Conditions” (Chlorophyll-a and species relative abundance) data.

How to cite: Béjard, T. M., Rigual-Hernández, A. S., Sierro, F. J., and Pérez-Tarruella, J.: Planktic foraminifera seasonal calcification variations in the northwestern Mediterranean Sea, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9409, https://doi.org/10.5194/egusphere-egu22-9409, 2022.

09:52–09:57
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EGU22-3621
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ECS
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On-site presentation
Anna Cutmore et al.

Distinctive organic-rich sapropel layers found in the Eastern Mediterranean are associated with stratification-linked reduction in ventilation and enhanced primary productivity resulting from freshwater outflow during precession minima (Rossignol-Strick, 1983; Rohling and Hilgen, 2007). These layers can therefore provide insight into the consequences of periods of reduced oxygen contents which are predicted for future oceans under global warming.

Along the south coast of Sicily, over four kilometres of exposed marine Pliocene sediments can be found as continuous cliff outcrops which cover the lower part of the Pliocene “Trubi” formation (Brolsma, 1978). These sediments show a quadripartite cycle of white limestone, grey marl, white limestone, beige marl (Brolsma, 1978). The grey marl sapropel layers are characterized by enhanced organic carbon content, lower d18O of planktic foraminifera, and low Ti/Al, which coincide with with a minimum in the precession index, whereby perihelion occurs in the N. Hemisphere summer; the beige marl layers coincide with a maximum in the precession index and are characterised by reduced organic carbon, higher d18O of planktic foraminifera and enhanced Ti/Al (Nijenhuis, 1999).

This study focuses on Pliocene Trubi sediments from two locations of this outcrop: Lido Rossello (LR) and Punta di Maiata (PM), located ~2 km apart. Both records span three Pliocene precession-forced climate cycles (4.605 – 4.685 Ma) which includes three grey marl sapropel layers (31, 30 and 29). Our research studies lipid biomarkers in order to explore changes in biogeochemical cycling over these three Pliocene climate sequences. Our records demonstrate the presence of both isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs), including those associated exclusively with ammonia oxidising archaea (Sinninghe Damsté et al., 2002) as well as terrestrial derived branched GDGTs. Beyond enhancing our understanding of in-situ biogeochemical cycling, these biomarkers will allow us to further reconstruct regional sea surface temperatures (TEX86 ratio; Schouten et al., 2002), and the relative input of terrestrial organic matter in marine sediments (BIT Index; Hopmans et al., 2004). We will also examine the presence of a range of other biomarker lipids, including heterocyst glycolipids (HGs) in order to examine the role of N2 fixation by cyanobacteria in stimulating primary productivity.

 

References:

Brolsma, 1978. Utrecht Micropaleontological Bulletins. 18, 1-160; Hopmans et al., 2004. Earth & Planetary Science Letters. 224, 107-116; Nijenhuis, 1999. Thesis. Universiteit Utrecht; Rossignol-Strick, 1983. Nature. 304, 46-49; Rohling and Hilgen, 2007. Netherlands Journal of Geosciences. 70, 253-264; Schouten et al., 2002. Earth & Planetary Science Letters. 204, 265-274; Sinninghe Damsté et al., 2002. Journal of Lipid Research. 43, 1641-1651

How to cite: Cutmore, A., Bale, N., Hopmans, E., Schouten, S., and Lourens, L.: The origin of unusual Pliocene sapropel and diatomite layers: A case study for future climate projections, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3621, https://doi.org/10.5194/egusphere-egu22-3621, 2022.

09:57–10:00
Discussion

Thu, 26 May, 10:20–11:50

Chairpersons: Ivy Frenger, Christopher Somes

10:20–10:30
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EGU22-8687
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solicited
Laurent Bopp et al.

Ocean net primary production (NPP) consists of CO2 fixation by marine phytoplankton and hence supports most marine food webs, fisheries and ocean carbon sequestration. Recent Earth System Model (ESM) projections of NPP changes under global warming scenarios, performed as part of the 6th phase of Coupled Model Intercomparison Project (CMIP6), show large uncertainty both in the magnitude and spatial distribution of NPP, which may have consequences for assessing ecosystem impacts and ocean carbon uptake. NPP uncertainty has increased since the previous intercomparion project (CMIP5), and likely does not even capture the full range of possible outcomes due to the general simplicity of ecosystem parameterizations employed in ESMs and the failure to account for non-climate drivers. Here, we exploit the full set of ESM projections from CMIP6, documenting NPP uncertainties and identifying certain physical and biogeochemical mechanisms that give rise to these uncertainties. We then use different versions of the IPSL ESM to explore (1) the specific role of N-fixation by diazotrophs in the upper ocean and (2) the influence of coupling to higher trophic levels in shaping the response of NPP, marine ecosystems and biogeochemistry to anthropogenic climate change. We show that the response of  N-fixation to global warming is a key driver of NPP projection uncertainties in the coming decades, even determining the sign of the global NPP response.  Despite contrasting projections of future NPP, all our model versions simulate similar and significant reductions in planktonic biomass. This suggests that plankton biomass may be a more robust indicator than NPP of the potential impact of anthropogenic climate change on marine ecosystems across models. In a second step, we show that an explicit coupling to higher trophic levels modifies the response of lower trophic levels (plankton) and shifts the ecosystem equilibrium, but seems to have limited influence on 21st century anthropogenic carbon uptake under the RCP8.5 high emissions scenario. These results provide new insights regarding the expectations for trophic amplification of climate impacts through the marine food chain and regarding the necessity to explicitly represent marine animals in Earth System Models.

 

 

How to cite: Bopp, L., Aumont, O., Kwiatkowski, L., Le Mezo, P., Maury, O., Séférian, R., and Tagliabue, A.: Projecting net primary production in a sea of uncertainty: next steps and why should we care?, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8687, https://doi.org/10.5194/egusphere-egu22-8687, 2022.

10:30–10:35
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EGU22-1642
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ECS
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Daan Boot et al.

To avoid tipping points in the Earth system, it is important to keep warming of our planet to a maximum of 1.5 to 2°C. To be able to make policy for this goal, it is important to know what our carbon budget is for the coming decades. Unfortunately, the Earth system is a complicated system with multiple feedbacks, which make it difficult to assess this budget. One of the feedbacks is between atmospheric CO2 concentration and the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is an important component of the global ocean circulation and plays a role in regulating the climate of the Northern Hemisphere. Simulations with earth system models project that the AMOC strength will decrease in the future. Changes in the AMOC influence the distribution of tracers such as heat, salt, nutrients and carbon in the ocean. These tracers all affect the marine carbon cycle by, for example, influencing the solubility of CO2, and biological production in the surface ocean, and thus the air-sea gas exchange of CO2. Therefore, changes in the AMOC may be relevant for the maximum emission levels. In this presentation, we discuss the relation between the AMOC and the air-sea CO2 exchange in the Community Earth System Model v2 (CESM2). By using results of CESM2 simulations, accompanied by the results of a box model, the Simple Carbon Project Model v1.0 (SCP-M), we find that the AMOC-CO2 feedback is positive, i.e. higher atmospheric CO2 concentrations result in a weaker AMOC, which leads to less CO2 uptake by the ocean. The mechanisms behind this feedback, related to changes in the solubility, soft tissue pump and phytoplankton composition, will be presented as well as the impact of this feedback on atmospheric CO2 concentration.

How to cite: Boot, D., Von Der Heydt, A., and Dijkstra, H.: Effect of ocean carbon cycle feedbacks on the air-sea gas exchange of CO2 in CESM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1642, https://doi.org/10.5194/egusphere-egu22-1642, 2022.

10:35–10:40
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EGU22-13378
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Virtual presentation
Jamie Wilson et al.

A fraction of the carbon fixed in the surface ocean by phytoplankton is isolated away from the atmosphere in the ocean interior with the respiration of sinking detritus (Particles of Organic Carbon: POC) - a process known as the "Biological Carbon Pump'' (BCP). The BCP sequesters ~1700 Pg of dissolved inorganic carbon (DIC) in the ocean beyond the concentration expected solely with physio-chemical drivers, effectively lowering the base-line atmospheric CO2 concentration by ~150-250 ppm. The components that make up the BCP (export production, sinking and remineralisation of POC, ocean ventilation timescales) are all expected to change in response to a changing climate but there is currently low confidence in how these changes will influence the magnitude and direction of the ocean carbon feedback.

Here we quantify the predicted historical and future changes in the Biological Carbon Pump in the latest CMIP6 projections as fully as possible. We find that all models consistently predict that the BCP will accumulate carbon in the ocean interior by 2100, i.e., acting as a sink for atmospheric CO2, albeit contributing only a small fraction (~10%) of the net carbon sink. The accumulation of carbon along with a concurrent decrease in globally integrated export production at 100m is associated with warming-driven stratification. In contrast there is significant disagreement in both the magnitude and direction of global mean trends and spatial patterns of transfer efficiency of POC at 1000m. This uncertainty arises because of the range of processes resolved across the biogeochemical models that influence the sinking and remineralisation rate of POC such as: temperature and oxygen-dependent remineralisation, ballasting, and dependence of sinking velocities on cell size. We demonstrate that these changes in transfer efficiency could likely determine the larger long-term impact of the BCP on atmospheric CO2 beyond 2100. Our results have wider implications for the biogeochemical cycling of nutrients and oxygen as well as implications for future impacts on twilight zone ecology.

How to cite: Wilson, J., Andrews, O., and Katavouta, A.: Future trends and uncertainties in the Biological Carbon Pump predicted by CMIP6 models, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13378, https://doi.org/10.5194/egusphere-egu22-13378, 2022.

10:40–10:45
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EGU22-10868
Keith Rodgers et al.

The low-latitude ocean regions spanning 30°S-30°N are thought to account for more than 50% of the global export production. However, previous analyses of paleo-proxy records and modeling studies strongly suggest contradictory evidence as to whether low latitude nutrient cycling and export production is locally or non-locally controlled. Here we address this question through the new application of observational (PACIFICA) and modeling (NEMO-PISCES) tools and show that low latitude recycling of nutrients within the thermocline overturning structures is largely responsible for sustaining low latitude export production (60%) for the mean state, with only second-order controls from the injection of new (preformed) nutrients from the Southern (16%) and northern (9%) oceans.  The implications for understanding controls on long-term changes under sustained anthropogenic climate perturbations is investigated using CMIP6 Earth system models under idealized 4xCO2 forcing, where significant reductions in low-latitude export production and net primary production over 30°S-30°N are investigated.

How to cite: Rodgers, K., Aumont, O., Toyama, K., Resplandy, L., Ishii, M., Lindsay, K., Yamaguchi, R., Sasano, D., and Nakano, T.: A weak role for Southern Ocean nutrient drawdown in low latitude marine export production, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10868, https://doi.org/10.5194/egusphere-egu22-10868, 2022.

10:45–10:48
Discussion

10:48–10:53
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EGU22-7709
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ECS
Damien Couespel et al.

The ocean plays an instrumental role in regulating the Earth’s climate through the buffering of the anthropogenic-induced excess carbon. Our capacity to predict long-term future oceanic carbon uptake depends on highly sophisticated numerical Earth system models, whose simulations of future climate have a wide inter-model dispersion. Inter-model spread in projections arises from three distinct sources: 1) internal variability of the climate system, 2) model uncertainty, and 3) scenario uncertainty. The spread related to (1) and (2) is even greater when predicting changes at regional scales. In order to elucidate the main origins of present and future internal variability and model uncertainty in oceanic carbon uptake, it is important to identify the uncertainty and sensitivity of the major underlying mechanisms in different ocean regions and across models. A limitation of this approach is the high costof computational and manpower required to systematically assess all mechanisms and identify processes that are important in a consistent way, especially across a large ensemble of model sets. Machine learning methods can be applied to simultaneously estimate the sensitivity of variable sets and explore them automatically across the ensemble of models. Here, we use the Kernel non-linear regression approach to reconstruct the inter-annual carbon uptake variability using monthly outputs of surface temperature, salinity, nutrient, dissolved inorganic carbon,alkalinity, atmospheric CO2 concentration, surface wind speed, and sea-ice cover. The exercise was performed on preindustrial, historical, and future scenario simulation outputs. The algorithm was optimized with a subset of ‘training’ data and evaluated with ‘test’ data. We applied bootstrapping method to delineate the main drivers for the projected inter-annual sea-air carbon fluxes variability in different ocean domains.

How to cite: Couespel, D., Tjiputra, J., and Johannsen, K.: Identifying the underlying mechanisms of present and future inter-annual variability of oceanic carbon uptake using a machine learning approach with CMIP6 simulations., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7709, https://doi.org/10.5194/egusphere-egu22-7709, 2022.

10:53–10:58
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EGU22-8669
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ECS
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On-site presentation
Alban Planchat et al.

The ocean carbonate pump influences the vertical gradient in ocean carbon content alongside the soft tissue pump. However unlike the soft tissue pump, the production of calcium carbonate, its export and subsequent dissolution are also the primary drivers of the vertical gradient in ocean alkalinity. Often overlooked, alkalinity is a key conservative tracer in the ocean, critical to the uptake of atmospheric carbon in surface waters and the extent of associated acidification. Within the context of model projections of future ocean carbon uptake and potential ecosystem impacts, the representation of the calcium carbonate cycle and alkalinity are a persistent uncertainty. Here we present an assessment, conducted alongside the international ocean biogeochemistry modelling community, reviewing trends in the representation of the calcium carbonate cycle and the associated biogeochemical tracer alkalinity, in the Earth system models involved in CMIP5 and CMIP6. Model representation of calcium carbonate production, sinking, dissolution and sedimentation is highly diverse. No model represents benthic calcification, and none of the CMIP5 and CMIP6 models have an explicit representation of pelagic planktonic calcifiers. Implicit pelagic calcification schemes are highly variable, with models typically representing calcite and not aragonite. In contrast, the representation of CaCO3 sinking and dissolution can be either implicit or explicit and variably includes sensitivity to the local seawater saturation state. Between CMIP5 and CMIP6 there is a clear improvement in the representation of both surface ocean alkalinity and its vertical gradient when compared to observations. This appears to be driven by an increase in the export of calcium carbonate in the CMIP6 ensemble however it is difficult to attribute this increase to specific model developments. Ongoing work is focussing on how the improved representation of ocean alkalinity in CMIP6 may affect model representation of the ocean CO2 system and projections of future ocean carbon uptake.

How to cite: Planchat, A., Bopp, L., and Kwiatkowski, L.: The representation of alkalinity and calcium carbonate cycling from CMIP5 to CMIP6 models and the potential influence on carbon cycle projections, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8669, https://doi.org/10.5194/egusphere-egu22-8669, 2022.

10:58–11:03
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EGU22-5421
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On-site presentation
Ralf Liebermann and Matthias Hofmann

The world's oceans have historically made a significant contribution in mitigating global warming by storing both large amounts of anthropogenic CO2 emissions and a significant portion of the heat generated by the enhanced greenhouse effect. However, precisely because of this buffering function, they are themselves subject to massive chemical and physical regime shifts that are suspected to continue long after anthropogenic CO2 emissions have ceased. For this reason, within the HORIZON2020-COMFORT project, we are studying the long-term effects that different scenarios of temporarily increasing atmospheric CO2 concentrations could have on marine biogeochemistry. To this end, we use CLIMBER3alpha+C, an Earth system model of intermediate complexity, to study the response of the ocean carbon cycle and associated nutrients during and after the period of elevated atmospheric pCO2 levels. Preliminary results show sustained changes in marine primary production, export of CaCO3, extent of hypoxic zones and production of dimethyl sulfide (DMS), with DMS acting as a condensation nucleus in cloud formation. This raises the possibility that the effects of elevated CO2 on the oceans will cause a change in both the Earth's radiative balance and the marine carbon pump long after atmospheric CO2 concentrations have returned to preindustrial levels.

 

 

Acknowledgements:

“This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 820989 (project COMFORT, Our common future ocean in the Earth system – quantifying coupled cycles of carbon, oxygen, and nutrients for determining and achieving safe operating spaces with respect to tipping points).”

 

Disclaimer:

“This [project/poster/presentation/etc.], reflects only the author’s/authors’ view; the European Commission and their executive agency are not responsible for any use that may be made of the information the work contains.”

 

How to cite: Liebermann, R. and Hofmann, M.: Century scale CO2 pulses could substantially alter marine primary production, CaCO3 export, oxygen concentrations and DMS emissions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5421, https://doi.org/10.5194/egusphere-egu22-5421, 2022.

11:03–11:05
Discussion