Orals: Thu, 1 May | Room 1.85/86
Nitrous oxide (N2O) is a potent greenhouse gas, with the Eastern Tropical Pacific (ETP) being a hotspot of N2O emissions due to high N2O production in the oxygen minimum zones (OMZs). However, N2O emissions in this region remain poorly constrained due to (i) temporal variability, which is hypothesized to be largely driven by the El Niño-Southern Oscillation (ENSO), and (ii) limited process understanding. To address these shortcomings and improve the quantification of N2O emissions and ENSO-driven variability in the ETP, we run a regional ocean model on a telescopic grid (~4km), spanning the entire Pacific Ocean, from 1979 to 2019. The model includes a biogeochemical model and a novel nitrogen module (NitrOMZ), which explicitly resolves the N2O production via incomplete denitrification and ammonium oxidation and accounts for the different oxygen inhibition thresholds of these biological N2O production pathways. We find that 1 Tg N of N2O is emitted annually in the ETP, and that N2O emissions deviate up to ±0.18 Tg N y-1 from the mean during ENSO events across the entire ETP, with La Niña increasing N2O emissions and El Niño decreasing them. Most of the ENSO-driven N2O emission anomalies can be attributed to variability in incomplete denitrification in the oxyclines of the oxygen minimum zones. Compensatory effects among gross N2O production, consumption, and transport reduce both the total N2O emissions and their interannual variability by an order of magnitude. Our results alleviate previously raised concerns that La Niña events may substantially amplify N2O emissions. Such compensatory mechanisms might also reduce N2O emissions in other OMZs and mitigate the impact of climate change on N2O emissions, provided that compensatory mechanisms remain effective in the future.
How to cite: Härri, J., McCoy, D., Vogt, M., Bianchi, D., and Gruber, N.: Compensatory Mechanisms Reduce ENSO-driven Nitrous Oxide Emission Variability in the Eastern Tropical Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15858, https://doi.org/10.5194/egusphere-egu25-15858, 2025.
Traditional gas transfer velocity formulations for air-sea CO2 fluxes scale solely with wind speed, ignoring wave activity, including wave breaking and bubble-mediated transfers that enhance the rate of gas exchange. Here, we incorporate a wind-wave dependent gas transfer velocity formulation into an ocean general circulation model to quantify the effects of wave-induced spatiotemporal variability on CO2 fluxes and ocean carbon storage. Our results reveal that wave activity introduces a hemispheric asymmetry in ocean carbon storage, with gains in the southern hemisphere, where wave activity is robust year-round, and losses in the northern hemisphere, where continental sheltering reduces carbon uptake. Compared to a traditional wind-dependent formulation, incorporating wave activity yields a modest global increase in ocean carbon storage of 4.3 PgC over 1959-2018 (~4%), but on average, enhances the CO2 gas transfer velocity and flux variability by 5-30% on high-frequency and seasonal timescales in the extratropics and up to 200-300% during storms (>15 m s-1 wind speed). The magnitude of fluxes from wave activity is comparable to expected marine carbon dioxide removal (mCDR) efforts. This underscores the need to incorporate wind-wave variability into modeled fluxes to distinguish natural variability from anthropogenic impacts and ensure accurate mCDR verification and monitoring.
How to cite: Rustogi, P., Resplandy, L., Liao, E., Reichl, B., and Deike, L.: The influence of wave-induced variability on ocean carbon uptake, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1242, https://doi.org/10.5194/egusphere-egu25-1242, 2025.
The Southern Ocean plays a critical role in modulating excess atmospheric carbon dioxide, accounting for roughly 40% of global ocean anthropogenic CO2 uptake since industrialisation. Given its significance in the global carbon cycle, understanding the Southern Ocean carbon sink is important but studies show high uncertainties in the magnitude and evolution of this carbon sink. The Southern Ocean is a remote and challenging region to measure, and the resulting sparsity of observational data is the main cause of uncertainty in air-sea carbon flux in the region. Long term, high-temporal-frequency data sets especially are rare for the Southern Ocean, but these can give valuable insights into the carbon cycle processes occurring in the region.
This work presents ten years of high-temporal-frequency in situ atmospheric carbon dioxide mixing ratios measured from two coastal Antarctic research stations; Halley, operated by the British Antarctic Survey, and the German research station, Neumayer. The coastal location of these stations means they are ideally placed to explore air-sea CO2 exchange over the Southern Ocean.
Both the Halley and Neumayer records show short-term fluctuations in CO2 mixing ratios during austral summer, with over ~0.5 ppm decreases in CO2 sometimes observed over the course of a day - about one fifth of the average annual growth rate (~2.4 ppm per year-1 for this 10-year record). Analysis of air mass trajectories reveal that these fluctuations in CO2 occur when the sampled air has spent considerable time in contact with the Southern Ocean, suggesting CO2 uptake has occurred, leading to the reduced CO2 mixing ratios observed.
We present an in-depth analysis of the drivers of the short-term variability observed during austral summer, including the role of mixing height, sea-ice coverage, wind speed and biology. Observational data represent an important tool with which to tease out key factors determining Southern Ocean CO2 uptake, and thus in assessing how uptake may evolve in the future.
How to cite: Squires, F., Jones, A., Phillips, T., Juranyi, Z., Weller, R., and France, J.: Evidence of seasonal carbon dioxide uptake by the Southern Ocean from a 10-year record of atmospheric carbon dioxide, observed from coastal Antarctica. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13484, https://doi.org/10.5194/egusphere-egu25-13484, 2025.
In polar regions, aquatic viruses play a pivotal role in shaping microbial communities yet face significant challenges such as low host availability and harsh environmental conditions. During the Synoptic Arctic Survey 2021 aboard the icebreaker Oden (Snoeijs-Leijonmalm, 2022), we investigated viral diversity, survival mechanisms, and host interactions in the Central Arctic's surface microlayer (SML), the uppermost millimeter of the ocean, and compared them with ~60 cm depth from the ocean and a melt pond. This study addresses the knowledge gap surrounding near-atmosphere aquatic ecosystems, highlighting the SML as a critical platform for viral adaptation and dispersal in one of Earth's most extreme environments. Our study uncovered 1154 viral operational taxonomic units (vOTUs) >10 kb in size, two-thirds of which were predicted bacteriophages (viruses that infect bacteria). Flavobacteriales were identified as key hosts, with one dominant melt pond vOTU linked to a Flavobacterium sp. isolate. Melt pond viral communities displayed lower diversity compared to open water, indicating selective pressures in these transient systems. We found that 17.2% of vOTUs carried 87 unique auxiliary metabolic genes (AMGs) involved in pathways such as amino acid, glycan polymer, and porphyrin metabolism, supporting host survival under extreme conditions. Notably, 16 vOTUs encoded glycerol-3-phosphate cytidylyltransferase (tagD), which may function in cryoprotection. While lytic phages could not be found via plaque assays, prophage induction experiments using the bacterial isolate Leeuwenhoekiella aequorea Arc30 and mitomycin C revealed active phages with siphovirus morphology and minimal protein similarity to known phages. Our findings also highlight the SML’s role in viral dispersal, as vOTU abundance correlated with spread across the Arctic via the boundary layer. These sophisticated viral strategies emphasize their ability to thrive in remote, inhospitable, and host-limited environments (Rahlff et al., 2024). These discoveries underscore the importance of viruses in Arctic ecosystem dynamics, influencing microbial communities, and in the broader context, nutrient cycling, gas exchange and resilience to climate change.
References:
Rahlff, J., Westmeijer, G., Weissenbach, J., Antson, A., & Holmfeldt, K. (2024). Surface microlayer-mediated virome dissemination in the Central Arctic. Microbiome, 12(1), 218. https://doi.org/10.1186/s40168-024-01902-0
Snoeijs-Leijonmalm, P. (2022). Expedition Report SWEDARCTIC Synoptic Arctic Survey 2021 with icebreaker Oden. In: Swedish Polar Research Secretariat.
How to cite: Rahlff, J., Westmeijer, G., Weissenbach, J., Antson, A., and Holmfeldt, K.: Surface microlayer ecosystems as platforms for viral adaptation and dispersal in the Central Arctic, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1665, https://doi.org/10.5194/egusphere-egu25-1665, 2025.
Increasing atmospheric CO₂ concentrations drives ocean acidification, potentially leading to substantial impacts on marine ecosystems and altering marine nutrient dynamics. Phosphorus (P) availability is a key limiting factor for primary productivity in the oceans. Atmospheric particles, such as wildfire ash, supply the oceans with substantial amounts of nutrients such as P. The solubility of P from aerosol particles, especially from wildfire ash, plays a critical role in oceanic nutrient cycles and may significantly impact the biological carbon pump, a key mechanism for atmospheric CO₂ regulation.
As ocean acidification continues and wildfires are projected to increase in intensity and severity with climate change, understanding how changes in seawater pH influence P release from wildfire ash is essential. This study aims to investigate the effect of past, present, and future seawater pH levels on P solubility from different wildfire ash under controlled laboratory conditions. Specifically, the study aims to examine how elevated CO₂ levels, leading to lower pH (ocean acidification), impact the availability of P in wildfire ash compared to lower CO₂ levels.
Using artificial seawater and ash samples derived from Mediterranean and agricultural vegetation, this research will analyze P release patterns under a range of CO₂ concentrations, encompassing current levels, future projections, and historical baselines.
Preliminary results demonstrated a significant dependence of P release from wildfire ash on pCO₂ concentrations and its influence on the pH. Elevated CO₂ levels of the projected future and of ancient atmosphere enhanced P solubility in both Mediterranean vegetation and agricultural vegetation treatments while reduced levels of the preindustrial and pre-Holocene periods decreased P solubility. These findings are anticipated to shed light on the role of wildfire ash in marine nutrient dynamics and its broader impact on ocean productivity and the global carbon cycle, especially in regions experiencing increasing wildfire activity.
These initial findings lay the groundwork for continued research, where I will investigate the cultivation of microalgae under controlled laboratory conditions at varying atmospheric CO2 concentrations. The research will focus on understanding how P release from wildfire ash, influenced by different CO2 levels, impacts the growth rate of phytoplankton. The experiments will assess the role of wildfire ash as a potential P source for phytoplankton grown in P-depleted water.
How to cite: Naiman, N., Gross, A., and Antler, G.: The Effect of Ocean Acidification on Phosphorus Solubility from Wildfire Ash and its Role in Enhancing the Biological Carbon Pump, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5059, https://doi.org/10.5194/egusphere-egu25-5059, 2025.
Oceanic nitrogen deposition influences marine ecosystem eutrophication and the global carbon cycle. Its future global spatiotemporal features still remain unclear driven by changing anthropogenic emissions. Furthermore, existing studies reported air quality and climate benefits of ambitious nitrogen emission reductions, while consequent impacts for global marine ecosystems through atmospheric nitrogen deposition are unexplored. Here we utilize the global atmospheric chemistry transport model GEOS-Chem to evaluate changes in global oceanic nitrogen deposition between 2015 and 2050 under three CMIP6 SSP-RCP emission scenarios and its responses to multiple levels of NH3 and NOx emission reductions. We find that global oceanic nitrogen deposition is projected to change by −24%-+6% between 2015-2050, with a substantially increasing share contributed by NHx-N across all scenarios. Coastal regions respond much more drastically to nitrogen emission reductions than open ocean areas. Ocean carbon sink related to nitrogen-contributed marine primary productivity is projected to decrease from 290 Tg C in 2015 to 222 Tg C (-23%) in SSP1-RCP2.6 scenario in 2050, posing challenges to climate mitigation and affecting global carbon budget. Our findings highlight nitrogen management and the overlooked climate mitigation impacts on marine ecosystems through atmospheric nitrogen deposition and call for increasing attention for holistic assessments of nitrogen management impacts on air, terrestrial and ocean systems.
How to cite: Deng, J., Guo, Y., Zhang, L., Lu, N., Ye, X., Zhao, Y., Xu, J., and Wang, X.: Global Oceanic Nitrogen Deposition under Future Emission Pathways and Responses to Nitrogen Emission Reductions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15135, https://doi.org/10.5194/egusphere-egu25-15135, 2025.
NH₃ drives nutrient cycling in the surface ocean and contributes to new particle formation in the marine atmospheric boundary layer. Surface ocean NH₃/Ammonium(NH₄⁺) is a vital component of the recycled nutrient pool, and NH₃ air-sea fluxes influence its redistribution. There are significant uncertainties in global NH₃ flux estimates due to a lack of concurrent air-sea measurements and ambiguity surrounding NH₃ sources. Southern Ocean, a major driver of global climate, is experiencing rapid warming, altering the exchange of climate-relevant aerosols and precursor gases such as NH₃. Models systematically underpredict cloud droplet number concentrations and aerosol production in this region, a bias that arises from poorly captured aerosol precursor sources and lack of detailed microphysical cloud processes. We present atmospheric and seawater NH₃ measurements, along with NH₃ air-sea flux estimates, across the Southern Ocean during November and December 2024. Our study focuses on 1) identifying key NH₃ sources and sinks in the marine polar environment, and 2) quantifying how NH₃ fluxes vary across distinct emission hotspots. Preliminary observations show penguin colonies and volcanic activity drive distinct, localised NH₃ emission hotspots. The open ocean is generally thought to be a source of NH₃, but our data show that the open waters of the Southern Ocean is a sink of NH₃. By quantifying these fluxes, we reveal the variability across NH₃ source/sink regions and their potential to influence regional ocean-atmosphere biogeochemical processes.
Our findings are crucial for improving the representation of clouds and aerosols in climate models, offering deeper insight into poorly understood aerosol-cloud interactions in this region. Improving these mechanisms will help reduce persistent Southern Ocean biases in model simulations of surface radiation and sea surface temperature and enhance our capacity to model regional and global climate.
How to cite: Louw, S., Bell, T., Browse, J., Woodward, M., and Yang, M.: Air-sea ammonia fluxes in the Southern Ocean: Quantifying sources and sinks from surface waters to penguins. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4242, https://doi.org/10.5194/egusphere-egu25-4242, 2025.
Sea spray aerosol (SSA), produced by bubble bursting at the ocean's surface, plays a critical role in climate regulation and atmospheric chemistry. It also provides a unique microenvironment for gas-to-particle partitioning and aqueous-phase reactions. Understanding these processes requires a detailed examination of the physicochemical properties and the transformations of SSA during atmospheric aging.
Hence, we designed a comprehensive experimental setup comprising a sea spray simulation tank for generating SSA, a chemical ionization mass spectrometer (CIMS) for analyzing molecular-level composition, an oxidation flow reactor (PAM) for simulating atmospheric oxidation, and a differential mobility particle counter (DMPS) for determining particle size distribution. We deployed this setup in May 2022 during the AGENA campaign on Graciosa Island in the Azores, Portugal, a remote marine site. We collected surface ocean water samples from the Atlantic, and generated SSA using a plunging jet. We used DMPS and CIMS to analyze physicochemical properties of SSA present in the tank headspace, and also collected filter samples for offline CIMS analysis.
Our results revealed significant particle formation in the PAM chamber at an aging period equivalent to 3–3.5 days in the atmosphere. Notably, the increase was primarily restricted to particles below 100 nm, suggesting that new particle formation dominated over condensation in the PAM environment, likely due to high oxidant concentrations. This observation also indicates the presence of numerous volatile organic compounds (VOCs) in the nascent SSA, which may have condensed onto pre-existing particles in natural settings. Further analysis of the VOCs using CIMS showed that nascent SSA contained compounds with longer carbon chains (1–16 carbons) and higher oxidation states, indicating low volatility. In contrast, gases exiting the PAM chamber exhibited shorter carbon chains (1–10 carbons) and lower oxidation levels, suggesting condensation of oxidation products onto newly formed particles within the reactor. Additionally, we identified oxidation products of dimethyl sulfide (DMS), such as dimethyl sulfoxide (DMSO) and methanesulfonic acid (MSA), in both nascent and aged samples. Intriguingly, nascent SSA also exhibited strong signals for fluorinated compounds, including hydrofluoric acid, likely formed from protonation of fluoride ions (F⁻) and other fluoride-containing salts like MgF⁺, CaF⁺, and NaF⁺ found in sea salt. These findings provide valuable insights into the molecular composition and dynamic behaviour of SSA, with implications for understanding its role in atmospheric processes and climate.
How to cite: Aggarwal, S., Garmash, O., Zinke, J., Kilgour, D., Wang, J., Bertram, T., Thornton, J., Salter, M., Zieger, P., and Mohr, C.: Physicochemical properties of nascent versus aged sea spray aerosol – A study from the eastern North Atlantic Ocean , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12549, https://doi.org/10.5194/egusphere-egu25-12549, 2025.
We present continuous measurements of volatile organic compounds (VOCs) and their fluxes in the marine atmospheric boundary layer using proton‐transfer‐reaction time‐of‐flight mass spectrometry (PTR-TOF-MS) coupled with a sonic anemometer for direct eddy covariance measurements at the Utö Atmospheric and Marine Research Station in the Baltic Sea. The measurements, conducted from July to September 2024, identified over 200 distinct masses corresponding to a diverse array of volatile compounds, representing a comprehensive characterization of marine VOC composition. Our experimental setup combines VOC mixing ratio and flux measurements with concurrent monitoring of physical and biogeochemical parameters, providing a unique dataset for understanding air-sea gas exchange processes. Preliminary principal component analysis reveals strong correlations between VOC mixing ratio variability and key parameters including water-side pCO2, dissolved oxygen concentration, and air temperature, suggesting complex biogeochemical controls on VOC emissions. The high temporal resolution and sensitivity of the PTR-TOF-MS, combined with direct flux measurements, enables detailed investigation of both abundant and trace VOC species, their diurnal patterns, and their response to varying environmental conditions. This comprehensive dataset will provide valuable insights into the complexity of VOC emissions in marine environments and their coupling with biological and physical processes in the Baltic Sea region.
How to cite: Foroughan, M., Holst, T., Laakso, L., Hellén, H., Seppälä, J., Kraft, K., Stenbäck, K., Aurela, M., and Rinnan, R.: Characterizing marine atmospheric VOC diversity and fluxes using PTR-TOF-MS measurements in the Baltic Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15375, https://doi.org/10.5194/egusphere-egu25-15375, 2025.
Ocean-emitted dimethyl sulfide (DMS) is a major source of climate-cooling aerosols. However, most of the marine biogenic sulfur cycling is not routed to DMS but to methanethiol (MeSH), another volatile whose reactivity has hitherto hampered measurements. Therefore, the global emissions and climate impact of MeSH remain unexplored. We compiled a database of seawater MeSH concentrations, identified their statistical predictors, and produced monthly fields of global marine MeSH emissions adding to DMS emissions. Implemented into a global chemistry-climate model, MeSH emissions increase the sulfate aerosol burden by 30 to 70% over the Southern Ocean and enhance the aerosol cooling effect while depleting atmospheric oxidants and increasing DMS lifetime and transport. Accounting for MeSH emissions reduces the radiative bias of current climate models in this climatically relevant region.
How to cite: Villamayor, J., Wohl, C., Galí, M., Mahajan, A. S., Fernández, R. P., Cuevas, C. A., Bossolasco, A., Li, Q., Kettle, A. J., Williams, T., Sarda-Esteve, R., Gros, V., Simó, R., and Saiz-Lopez, A.: Marine emissions of methanethiol increase aerosol cooling in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17784, https://doi.org/10.5194/egusphere-egu25-17784, 2025.
A 25-year record of wet deposition has been collected and analysed for Fe(II), soluble iron (DSRFe) and total Iron (Fe) at Finokalia station on Crete island in the East Mediterranean from 1997 to 2022. A significant temporal increase in rain pH values is observed, mainly due to the reduction in sulfur concentrations. Regardless of the pH value of the rain, the Fe(II)/DSRFe ratio appeared to remain always above 50%, indicating that a significant amount of Fe(II), hence bioavailable iron, enters the sea surface via rain. However, Fe(II)/DSRFe ratio gradually decreases from 0.70 to 0.52 with increasing pH until pH 7.0, while from pH 7.0 and above it increases again, reaching an average value of about 0.67 at very basic pH levels. This is related to the general decrease in Fe solubility with increasing pH and the respective association of the forms in which Fe(II) and Fe(III) exist. It is therefore evident that the observed increase in pH in wet deposition affects the amount of dissolved iron deposited in the oceans, particularly Fe(II), that is directly bioavailable to the marine ecosystem, with consequent impacts on marine productivity.
How to cite: Kanakidou, M., Tsagkaraki, M., and Mihalopoulos, N.: A 25-year record of atmospheric deposition of iron speciation in the East Mediterranean: The impact of pH, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12999, https://doi.org/10.5194/egusphere-egu25-12999, 2025.
Wildfires contribute significantly to biomass burning. The deposition of ash from wildfires into surface ocean waters is a source of iron (Fe), namely pyrogenic Fe, and may enhance primary production in Fe-limited domains. However, due to the low solubility of Fe and the operational definition of its dissolved fraction, a portion of the dissolved Fe (DFe) released during ash dissolution may reprecipitate as authigenic inorganic colloids. This process can lead to an overestimation of the bioavailable pyrogenic DFe. To remain in a soluble form, Fe must be complexed with organic ligands capable of undergoing biochemical processes such as bacterial degradation, direct uptake, or photoreduction, leading to potentially bioavailable forms of DFe. Among the diverse range of iron-binding ligands, humic-type ligands (LFeHS) are important. LFeHS are ubiquitous in seawater, soluble, and may lead Fe to a bioavailable form. LFeHS are ubiquitous in seawater, soluble, and keep Fe in a bioavailable form. Here we present results from dissolution experiments. Ash samples collected in 2009 after wildfire events in the Spanish Mediterranean region were put in contact with non-euxinic, filtered Mediterranean surface seawater in a 7-day batch experiment. Four deposition fluxes were tested. The concentrations of DFe, fluorescent dissolved organic matter (FDOM), LFeHS, and the amount of Fe complexed by humic-type ligands were measured. Our results indicate that ash dissolution induces an increase in LFeHS, proportional to the ash concentration in the experimental medium. FDOM measurements confirm a time-dependent increase in humic-type material of terrestrial origin. Additionally, the observed increase in protein-like FDOM (C4) suggests that ash deposition enhances the modification of dissolved organic matter by bacteria. Using a simple kinetic model, we determined the dissolution rate constant for the tested ash. This constant can be incorporated into global oceanic models such as PISCES or REcoM to improve predictions of pyrogenic Fe bioavailability and its impacts on marine ecosystems.
How to cite: Dulaquais, G., Bressac, M., Ortega-Retuerta, E., Uher, E., Marie, B., and Nault, N.: Impact of wildfires ash deposition on iron binding humic substances concentrations in surface waters: Results from a dissolution experiment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1376, https://doi.org/10.5194/egusphere-egu25-1376, 2025.
The ocean’s surface is covered by the sea-surface microlayer (SML), a distinct boundary layer that plays a critical role in mediating the air-sea exchange of atmospheric trace gases. The oxidation of unsaturated organic material enriched in the SML by ozone is a significant but poorly quantified abiotic mechanism leading to the emission of volatile organic compounds (VOCs) into the marine boundary layer. The properties of these VOCs make them efficient precursors for secondary organic aerosol formation which can alter the oxidative capacity of the atmosphere.
In this laboratory study, axenic cultures of the model marine diatom Phaeodactylum tricornutum and its coculture with Yoonia bacteria were selected as biologically and chemically relevant proxies for the SML. Ozone-enriched air was passed over the culture medium in a heterogenous flow reactor, and the emitted gas-phase VOCs were monitored using high resolution proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). Experiments were conducted on the cultures in both their exponential and stationary growth phases with nonanal, the C5H8H+ peak, and the C6H10H+ peak being identified as major product ions. Ozonolysis-mediated abiotic VOC emissions were greater from cultures in exponential phase compared to stationary phase. Additionally, emissions from the P. tricornutum axenic monoculture were higher than from the P. tricornutum-Yoonia coculture indicating consumption of precursor compounds by the bacteria. The addition of iodide, a well-known reactant with ozone, to axenic P. tricornutum cultures in the exponential phase was associated with a reduction in the VOC emissions. This research provides a deeper insight into the interactions between iodide and organics during ozone uptake to the SML, and the impact of these competing processes on marine atmospheric chemistry.
How to cite: Stapleton, C., Fenselau, R., Padaki, V., Lyp, A., Halsey, K., Carpenter, L., and Bertram, T.: Investigating Volatile Organic Compound Emissions from Ozonolysis of Phytoplankton Cultures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8419, https://doi.org/10.5194/egusphere-egu25-8419, 2025.
Carbohydrates, amino acids, and lipids are significant contributors to organic carbon in the marine environment, playing key roles in ocean-atmosphere interactions. To investigate their sea-to-air transfer, enrichment in the sea surface microlayer (SML), and potential transformations during atmospheric transport, we conducted field studies in the tropical Atlantic Ocean at the Cape Verde Atmospheric Observatory. This study links measurements of these compounds in surface seawater, including the SML, with their presence and composition in submicron aerosol particles.
The study found moderate enrichment of lipids and carbohydrates in the SML, while amino acids exhibited higher enrichment, despite their relatively lower surface activity. In aerosol particles, lipids were markedly more enriched compared to amino acids and carbohydrates, likely due to their surface-active and lipophilic nature.
Detailed molecular analyses revealed shifts in the relative abundance of organic compounds during atmospheric transport, particularly for amino acids, suggesting in situ atmospheric transformations via biotic or abiotic processes. On average, 49% of aerosol OC was attributable to specific compound groups, with lipids accounting for the largest fraction. Amines, oxalic acid, and carbonyls contributed around 6%, while carbohydrates and amino acids each represented less than 1% of the total aerosol OC. Notably, carbohydrate-like compounds likely reside in glycolipids within the lipid fraction, underscoring the complexity of organic matter in marine aerosols.
These findings advance our understanding of the processes governing organic carbon transfer from the ocean to the atmosphere, including the roles of the SML and atmospheric processing. This knowledge is crucial for refining models of marine aerosols and their impact on atmospheric chemistry and climate.
The study contributes to the international SOLAS program.
Ref: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Amino acids, carbohydrates, and lipids in the tropical oligotrophic Atlantic Ocean: sea-to-air transfer and atmospheric in situ formation, Atmos. Chem. Phys., 23, 6571–6590, https://doi.org/10.5194/acp-23-6571-2023, 2023.
How to cite: van Pinxteren, M., Zeppenfeld, S., Fomba, K. W., Triesch, N., Frka, S., and Herrmann, H.: Organic Compounds in the Tropical Oligotrophic Atlantic Ocean: Insights into Sea-to-Air Transfer and Atmospheric Transformations , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1323, https://doi.org/10.5194/egusphere-egu25-1323, 2025.
Due to its position at the air-sea interface, the sea-surface microlayer (SML) modulates the exchange of gases, including the deposition of ozone to the ocean. While ozone deposition to the ocean is a large sink of ozone from the troposphere, the processes involved are not well understood. Previous work has focussed on seawater iodide as a driver of ozone uptake to the ocean, however the SML contains a complex mixture of organic material, which could also impact ozone uptake. The contribution of this organic material to ozone uptake remains particularly unclear.
During this project, ozone uptake to seawater was measured by eddy covariance from coastal towers near Penlee Point (Plymouth, UK) and Tudor Hill (Bermuda), and at sea aboard the RV Atlantic Explorer, operating at and around the Bermuda Atlantic Time-series Study site in the Sargasso Sea. Additionally, the chemical component of ozone uptake to seawater was measured using a flow reactor during a trans-Atlantic cruise. This suite of observations has been combined to investigate the driving forces of oceanic ozone uptake. We present data that demonstrate that iodide was not a strong predictor of ozone uptake, despite its fast chemical reaction with ozone and the ubiquitous presence of iodide in the surface ocean.
Organic compounds in the SML are of interest to this work because some organic compounds have ozone-reactive functional groups. An example of this is carbon-carbon double bonds, present in some oceanic fatty acids. By increasing chemical reactivity, organic material can therefore augment ozone uptake to the ocean. The contribution of chemical reactions between ozone and organic material to ozone uptake was investigated using the kinetic multilayer model of surface and bulk chemistry (KM-SUB). A simplified system of a monolayer of an unsaturated fatty acid (oleic acid) over seawater was modelled and demonstrated that a monolayer of ozone-reactive surfactants on the ocean surface could contribute substantially more to ozone uptake, compared to environmental levels of aqueous iodide.
This work indicates that the commonly applied iodide-based parameterisation for ozone uptake to seawater may not accurately represent the chemical processes involved in ozone deposition to the sea surface. This has implications not only for predicted spatial and temporal variations in the magnitude of ozone deposition, but also for the chemical profile of oxidised gases emitted from the sea surface to the remote marine troposphere.
How to cite: Brown, L., Loades, D., Stapleton, C., Drysdale, W., Jones, M., Chance, R., Lakey, P., Shiraiwa, M., Yang, M.-X., Bell, T., Brooks, I., Peters, A., Johnson, R., Lethaby, P., Quack, B., and Carpenter, L.: Chemical Drivers of Oceanic Ozone Uptake – Iodide vs Surfactants, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10303, https://doi.org/10.5194/egusphere-egu25-10303, 2025.
Dimethylsulfide (DMS) is the main natural source of atmospheric sulfur and plays a critical role in marine aerosol formation (Mansour et al., 2020b; Mansour et al., 2020a; O'Dowd et al., 2004). It influences cloud radiative forcing, with feedback on regional and global climate (Charlson et al., 1987; Mansour et al., 2022). Despite its importance, the accurate representation of biogenic sulfur emissions in climate models remains a challenge (Mansour et al., 2023; Mansour et al., 2024a). We employed machine learning (ML) based approaches to characterize seawater DMS concentrations, sea-to-air DMS emission flux (FDMS), as well as the atmospheric concentrations of marine biogenic methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO42–). This study focuses on the Mediterranean Sea, a warm, oligotrophic marine basin and a climate change hotspot with rapidly increasing temperatures.
In our methodology, a set of ML models (Mansour et al., 2024b) is trained and evaluated using nested cross-validation, forced by high-resolution satellite data (chlorophyll-a, sea surface temperature, photosynthetically available radiation) and Mediterranean physical reanalysis (mixed layer depth and seawater salinity) datasets, combined with in situ DMS measurements. The optimized model generates daily gridded fields of DMS and FDMS at mesoscale resolution (0.083° × 0.083°, ~9 km) spanning 23 years (1998–2020). These high-resolution FDMS estimates align with observational data of MSA and nss-SO42–, secondary aerosol products from DMS oxidation, collected at the Lampedusa monitoring site in the central Mediterranean (Becagli et al., 2013). Compared to existing coarse-resolution global DMS datasets, the reconstructed FDMS fields capture seasonal patterns of biogenic sulfur with much greater accuracy across the Mediterranean Sea.
Furthermore, the FDMS outputs are integrated with high-resolution atmospheric datasets from the Copernicus European Regional Reanalysis (CERRA) to predict atmospheric concentrations of MSA and nss-SO42–. The ML models produce daily time-series predictions over the same 23-year period, achieving finer temporal and spatial coverage than observational datasets alone.
This analysis demonstrates the potential of ML techniques to enhance the estimation of seawater DMS fluxes and associated sulfur aerosol concentrations, achieving outstanding predictive performance. The spatiotemporal dynamics of these variables over the 23 years are analysed to elucidate mesoscale oceanographic variability and its influence on sulfur cycling. Ongoing analyses of long-term trends and interannual variability aim to identify the main drivers of these patterns, with results to be presented and discussed in detail.
Funding:
This work was funded by the European Commission’s EU Horizon 2020 Framework program, project FORCeS (grant no. 821205), and the European Union’s Horizon, project CleanCloud (Grant No. 101137639).
References:
Becagli, et al. (2013), Atmospheric Environment, 79, 681-688, 10.1016/j.atmosenv.2013.07.032.
Charlson, et al. (1987), Nature, 326, 655-661, 10.1038/326655a0.
Mansour, et al. (2023), Science of The Total Environment, 871, 10.1016/j.scitotenv.2023.162123.
Mansour, et al. (2024a), npj Climate and Atmospheric Science, 7, 10.1038/s41612-024-00830-y.
Mansour, et al. (2022), Journal of Geophysical Research-Atmospheres, 127, 10.1029/2021jd036355.
Mansour, et al. (2024b), Earth System Science Data, 16, 2717–2740, 10.5194/essd-16-2717-2024.
Mansour, et al. (2020a), Atmospheric Research, 237, 10.1016/j.atmosres.2019.104837.
Mansour, et al. (2020b), Journal of Geophysical Research-Atmospheres, 125, 10.1029/2019jd032246.
O'Dowd, et al. (2004), Nature, 431, 676-680, 10.1038/nature02959.
How to cite: Rinaldi, M., Decesari, S., Paglione, M., Becagli, S., and Mansour, K.: Advancing predictions of Dimethylsulfide emissions and biogenic sulfur aerosol in the Mediterranean region via machine learning, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17040, https://doi.org/10.5194/egusphere-egu25-17040, 2025.
At EGU2024 we presented initial laboratory results from bubbling a simulated pre-industrial atmosphere through samples of freshwater and seawater across a range of temperatures, making comparison with literature values for the CO2/water partition equilibrium as determined at a higher partial pressures of the gas as reviewed by Carroll et al 1991.
Two changes have been made. Our 2024 results were based on a temperature range of 0.1’C to 16.5’C, and following valued discussion with Raphael Hebert we have brought that range closer to the global average ocean temperature range since the 1940s hockey-stick, i.e. 15’C to 16.5’C. At the same time, in addressing whether last year’s `paradox’ and `slow-release’ were artefacts of laboratory simulation, we test whether changes in CO2 fraction as measured in the headspace have a reciprocal effect in the liquid phase, measured by a continuous-reading conductivity probe in each flask.
Two recent papers are of relevance within this temperature range. Firstly the Universities of Exeter and Plymouth, UK, report transects in the Atlantic Ocean and note that temperature gradients near the ocean surface will affect the proportion of atmospheric CO2 taken into solution (D Ford et al `Enhanced ocean CO2 uptake due to near-surface temperature gradients’, Nature Geoscience (Sept 2024). They conclude that `accounting for near-surface temperature gradients would increase estimates of global ocean CO2 uptake.’ In parallel the University of East Anglia, UK, finds ‘that process-based models underestimate the amplitude of the decadal variability in the ocean CO2 sink, but that observation-based products on average overestimate the decadal trend in the 2010s’. (N Mayot et al `Constraining the trend in the ocean CO2 sink during 2000–2022’ Nature Communications, September 2024)
We understand from Raphael Hebert (pers. comm.) that the Alfred Wegener Institute, Germany, is also investigating this issue using a different approach, hence our interest in confirming the partition constant at relevant partial pressures, as a fourth contribution.
How to cite: Durham, B. and Pfrang, C.: Laboratory simulation of ocean-atmosphere CO2 exchange, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7196, https://doi.org/10.5194/egusphere-egu25-7196, 2025.
The equatorial Pacific serves as the largest oceanic source of CO2. The contrasting ocean environment in the eastern (i.e., upwelling) and western (i.e., warm pool) regions makes it difficult to fully characterize the CO2 dynamics with limited in situ observations. In this study, we addressed this challenge using monthly surface partial pressure of CO2 (pCO2sw) and air–sea CO2 fluxes (FCO2) data products reconstructed from satellite and reanalysis data at spatial resolution of 1°×1° in the period of 1982–2021. We found that, during the very strong El Niño events (1997/1998, 2015/2016), both pCO2sw and FCO2 showed significant decrease of 41–58 μatm and 0.5–0.8 mol m-2 yr-1 in the eastern equatorial Pacific, yet remained at normal levels in the western equatorial Pacific. In contrast, during the very strong La Niña events (1999/2000, 2007/2008, and 2010/2011), both pCO2sw and FCO2 showed strong increase of 40–48 μatm and 1.0–1.4 mol m-2 yr-1 in the western equatorial Pacific, yet with little change in the eastern equatorial Pacific. In the past 40 years, pCO2sw in the eastern equatorial Pacific was increasing at a higher rate (2.32–2.51 μatm yr-1) than that in the western equatorial Pacific (1.75 μatm yr-1), resulting in an accelerating CO2 outgassing (at rate of 0.03 mol m-2 yr-2) in the eastern equatorial Pacific. We comprehensively analyzed the potential effects of different factors such as sea surface temperature, sea surface wind speed, and ΔpCO2 in driving CO2 fluxes in the equatorial Pacific, and found that ΔpCO2 had the highest correlation (R ≥ 0.80, at p ≤ 0.05), highlighting the importance of accurate estimates of pCO2sw from satellites.
How to cite: Chen, S.: Accelerating CO2 outgassing in the equatorial Pacific from sat-ellite remote sensing, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5360, https://doi.org/10.5194/egusphere-egu25-5360, 2025.
The open ocean plays a critical role in mitigating climate change by absorbing approximately 25% of annual anthropogenic carbon dioxide (CO₂). In contrast, Eastern Boundary Upwelling Systems (EBUSs) are net sources of CO2, primarily due to the high concentrations of dissolved inorganic carbon (DIC) from upwelled waters. However, the carbon dynamics in EBUSs exhibit significant variability, both temporally and spatially, with differences between systems. This study focuses on two Pacific Ocean EBUSs with distinct physical characteristics: the upwelling systems off Peru and off Baja California, where the relative contribution of Ekman transport and pumping, and geostrophic compensation to upwelling differ. Based on seasonal simulations of a regional biogeochemical model configured for the two regions, we characterise the seasonal variability of CO₂ fugacity (FCO₂) in these systems, and identify the processes driving this variability through a Taylor expansion of the flux formulation. Our results show that FCO₂ is highly dynamic and exhibits notable spatial variability. The processes influencing FCO₂ seasonality differ between subregions. Off Peru, the primary drivers of FCO₂ seasonal variability are: the oceanic partial pressure of CO₂ (pCO₂), primarily influenced by changes in DIC, and alongshore winds (Ekman transport). Similarly, off Baja California, changes in pCO₂ are the dominant contributor to the FCO₂ seasonality, with DIC and sea surface temperature (SST) also playing significant roles. This comparative analysis deepens our understanding of how large-scale climate processes shape FCO₂ dynamics, offering valuable insights for interpreting future changes in CO2 fluxes within EBUSs.
How to cite: Bahamondes Dominguez, A., Dewitte, B., Montes, I., Garçon, V., Aguilera, V., Barranco, L., and Hammond, M.: Understanding the drivers of the air-sea CO2 flux seasonal variability in the upwelling systems off Peru and Baja California, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13680, https://doi.org/10.5194/egusphere-egu25-13680, 2025.
Estimates of atmospheric CO2 uptake by the Arctic Ocean over recent decades from multiple methods indicate accelerating regional carbon uptake (Yasunaka et al. 2024). This trend is attributed to such factors as regional climate-change impacts and associated sea-ice loss. Yasunaka et al. (2024) also note a significant range of uncertainty among the various model and data analysis methods that were employed to derive regional Arctic Ocean air-sea fluxes (e.g., from surface ocean pCO2 products, ocean biogeochemical models, and atmospheric inversions). This highlights a need for more robust flux estimation methods involving expanded observational networks and improved modelling tools to enable more accurate quantification of regional fluxes and an improved prediction capability to estimate future changes in oceanic CO2 uptake in the rapidly evolving Arctic.
In this analysis we employ the GEOSChem-Local Ensemble Transform Kalman Filter inverse analysis system (Chen et al. 2021) to develop sets of Observing System Sampling Experiments (OSSEs) that assess alternative atmospheric CO2 sampling strategies and observational network extensions towards improved estimates of Arctic Ocean air-sea CO2 fluxes. We assess the performance of individual sampling strategies using a range of metrics applied to the atmospheric inversions; these include regional CO2 flux error reductions and model concentration biases at sampling sites.
How to cite: Suntharalingam, P., Ghosh, J., and Chen, Z.: Estimation of Arctic Air-Sea CO2 Fluxes by Inverse Methods: Use of OSSEs to Assess Atmospheric Sampling Strategies , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7519, https://doi.org/10.5194/egusphere-egu25-7519, 2025.
Atmospheric inverse analyses use optimization methods to calculate surface CO2 fluxes using atmospheric transport models in combination with observed gradients in atmospheric CO2 concentration. In our present study we present an inverse estimate of Arctic Ocean air-sea CO2 fluxes using the GEOSChem–LETKF system; this system has previously been used to derive estimates of regional North Atlantic CO2 fluxes (Chen et al. 2021). Our analysis reports on estimates of Arctic Ocean fluxes and assesses patterns of spatial and inter-annual variability. Our results indicate significant spatial variability of air-sea CO2 fluxes in the different regional seas of the Arctic Ocean. The western Arctic Ocean predominantly act as a sink region for atmospheric CO2. However, the eastern Arctic Ocean act more as a source of CO2 . We also present results of sensitivity analyses conducted to assess the impact of alternate ocean prior flux specifications.
How to cite: Ghosh, J., Suntharalingam, P., and Chen, Z.: Evaluation of Arctic Ocean surface carbon fluxes from Atmospheric Inverse Analysis , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5221, https://doi.org/10.5194/egusphere-egu25-5221, 2025.
The ocean is generally thought to be a small source of atmospheric methane. However, the contribution of the Southern Ocean remains poorly quantified due to its remoteness and lack of measurements. In this study we investigate sea-air methane fluxes in the Southern Ocean measured by two different methods, bulk flux and eddy-covariance, to better understand the region's role in global methane emissions. We focus on both on-shelf and off-shelf areas, including regions where methane seeps from the seabed into the water column, using several years of ship-based measurements.
Our results show that coastal and on-shelf regions of the Southern Ocean, including areas with known seabed seeps, act as small sources of methane to the atmosphere. This is possibly driven by methane produced at the seabed reaching the surface or inputs from terrestrial sources, such as subglacial discharge. We also find possible indications of increased methane release from coastal areas compared to previous studies. Given the potential for increased methane release from these regions in the future under a warming climate, our findings emphasise the importance of ongoing monitoring in the Southern Ocean to quantify its contribution to the global methane cycle and track any changes over time.
Open ocean sea-air methane flux measurements in the Scotia and Weddell Seas during consecutive Antarctic summers revealed a source and a sink of methane depending on the method used (bulk flux or eddy-covariance). As these measurements techniques were not deployed simultaneously, a dedicated measurement campaign is necessary to collect parallel data and better understand whether the observed differences reflect measurement technique variability or potential changes in the Southern Ocean system.
How to cite: Workman, E., Jones, A., Fisher, R., France, J., Linse, K., Yang, M.-X., Bell, T., Delille, B., Squires, F., and Dong, Y.: Evaluating Methane Emissions and Sea-Air Fluxes in the Southern Ocean, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10513, https://doi.org/10.5194/egusphere-egu25-10513, 2025.
Ocean is a net source of atmospheric methane (CH4), but there are still large uncertainties in the estimations of global oceanic CH4 emission due to sparse data coverage. In this study, we investigated the spatial distribution and influencing factors of CH4 in the Western North Pacific (WNP) during two cruises in 2021 and 2022. High-resolution continuous underway measurements showed that surface CH4 concentrations ranged from 1.95 to 3.92 nM, indicating an obvious spatial gradient with a gradual increase from the south to the north due to the influence of water mixing and primary productivity. Vertically, subsurface CH4 maxima were ubiquitously observed due to in situ production through multiple pathways including MPn degradation and phytoplankton production. Surface water was oversaturated with respect to the atmospheric CH4 with the air-sea CH4fluxes in the tropical Western Pacific (1.28 ± 1.12 μmol/m2/d) higher than those in the Kuroshio Extension region (2021: 0.49 ± 0.89 μmol/m2/d; 2022: 0.37 ± 0.53 μmol/m2/d). Overall CH4 emission from the Western North Pacific is 0.08 Tg/yr, accounting for 13% of the total emission from the open ocean.
How to cite: Zhang, G., Wang, H., and Zhang, Z.: Methane distribution, production, and emission in the Western North Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1770, https://doi.org/10.5194/egusphere-egu25-1770, 2025.
Nitrous oxide (N2O) distribution and dynamics in high latitude fjords are relatively unknown. Surface water N2O concentrations were measured in six fjords located in Sweden, Iceland, and Greenland, which represent highly diverse environmental conditions in terms of oxygen, eutrophication and climate. This study provides one of the few high spatial resolution observations of N2O sea-air fluxes currently available in fjords. The two Icelandic fjords showed highest emissions (97.6±10.5 μg N2O m⁻² day⁻¹), likely driven by aquaculture-induced nutrient enrichment and not fully oxygenated subsurface waters. The three Swedish fjords, characterized by inputs from nutrient-rich rivers and by poor water circulation, exhibited relatively high N2O emissions averaging 19.9±19.3 μg N2O m⁻² day⁻¹, with subsurface water anoxia enhancing emissions in By Fjord (64.4±24.0 µg N2O m⁻² day⁻¹). In contrast, the Greenland fjord displayed net N2O uptake (–8.3±7.8 μg N2O m⁻² day⁻¹), likely due to glacier meltwater dilution. Each fjord appeared to be influenced by distinct N2O drivers, including temperature, salinity, chlorophyll, and pH, but no single, unifying driver was found across all fjords. As a preliminary global upscaling effort, we integrated our measured fluxes from six fjords with literature data from thirteen additional fjords. We estimate that global fjords emit 7.9±1.7 Gg N2O yr⁻¹, accounting for 2–13% of global coastal ecosystem emissions and do not significantly offset (3.5%) CO₂ sequestration in fjords. These findings underscore the role of fjords in greenhouse gas dynamics and highlight the need for further spatial and seasonal studies to refine global N2O emissions from coastal ecosystems.
How to cite: Politi, T., Yau, Y. Y. Y., Santos, I., Cabral, A., Cheung, H. L. S., Majtényi-Hill, C., Ulfsbo, A., Wåhlin, A., and Bonaglia, S.: North Atlantic fjords are minor sources of nitrous oxide to the atmosphere, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11404, https://doi.org/10.5194/egusphere-egu25-11404, 2025.
Estuaries are potential sources for the important greenhouse gas nitrous oxide (N2O). Estuaries are among the most complex ecosystems in the world with biogeochemical processes occurring on a range of spatial and temporal scales, depending on geomorphology, tides, and discharge patterns. Due to the high spatiotemporal variability and limited data availability, N2O emissions from estuaries are associated with significant uncertainty, presenting a big challenge for the global N2O emission estimates and budgeting of coastal regions.
This study presents N2O measurements from three temperate German estuaries discharging into the North Sea: Ems, Weser and Elbe, which are all heavily affected by anthropogenic impacts. During a cruise in September 2024, N2O dry mole fractions were measured continuously using an analyzer based on off-axis integrated cavity output (Picarro G2508) absorption spectroscopy coupled with an equilibrator system. For calibration and quality control, distinct water samples were taken in 30-min intervals and preserved for later GC analysis. Based on these measurements, we calculated N2O concentrations and fluxes.
Preliminary results showed N2O oversaturation with distinct peaks observed along the salinity gradient of all three estuaries. The N2O concentration in the Weser estuary was nearly double the concentration recorded in the Ems and Elbe estuaries. The high variability in N2O concentration between the three estuaries indicated potential differences in dominating biological and biogeochemical processes that modulate N2O production in each estuary. We suspect that turbidity, organic matter quality and degradation, as well as nutrient availability are responsible for the observed differences between the estuaries, which all are heavily impacted by anthropogenic river alterations. Therefore, we aim to elucidate the impact of human alterations on N2O production and emissions in these temperate estuaries. Overall, our findings highlight the variability of N2O emissions depending on stream morphology and chemistry, emphasizing the urgent need for comprehensive measurement programs to ensure accurate emission estimates.
How to cite: Dähnke, K., Schulz, G., Rewrie, L., Macovei, V., Voynova, Y., Neumann, A., and Sanders, T.: Nitrous oxide from three temperate estuaries discharging in the North Sea: No estuary is like another , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12512, https://doi.org/10.5194/egusphere-egu25-12512, 2025.
Coastal ecosystems play a significant role in the cycling of greenhouse gases (GHGs), yet they remain understudied compared to open oceans and terrestrial systems. Here, we present measurements of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) concentrations from shallow coastal environments along the Swedish Baltic Sea coast and Auckland, New Zealand, highlighting the variability and drivers of GHG dynamics across diverse habitats.
In the Baltic Sea, we conducted measurements in April and September 2024, utilizing cavity ring-down spectroscopy coupled with a water equilibration system. Our focus was on shallow coastal bays in the wider Stockholm archipelago, including eutrophic and habitat-altered bays. These environments exhibited exceptionally high CH₄ concentrations in the surface water reaching up to 580 nmol L-1, suggesting the potential for significant CH₄ emissions. Notably, CH₄ concentrations below 200 nmol L-1 showed a negative correlation with N₂O, while CH₄ levels above 200 nmol L-1 revealed a distinct shift to a positive correlation with N₂O. We hypothesize that this transition reflects a change in oxygen availability, where hypoxic conditions (0.2< O2 < 2 mL L-1) favor CH₄ production and reoxygenation of euxinic sediments contributes to an additional late-summer N₂O peak. Furthermore, GHG concentrations in the surface seawater were associated with environmental parameters such as water retention time, vegetation coverage, total organic carbon content, turbidity, chlorophyll-a concentration, pH, and total phosphorus levels.
Expanding our investigation to coastal systems in the suburban regions of Auckland, New Zealand, in January 2025 we conducted a spatial survey across a range of coastal habitats, including tidal flats, mangroves and river estuaries. By linking the findings from the Baltic Sea with emerging insights from New Zealand’s coastal systems, we aim to better understand the influence of habitat type, redox conditions, and nutrient dynamics on GHG emissions in coastal zones globally.
Our comparative study underscores the need for integrated approaches to better understand GHG emissions in coastal zones, which are often subject to compounded anthropogenic pressures, such as excessive nutrient inputs and habitat alteration. These findings contribute to the broader understanding of coastal zones as dynamic interfaces in the global carbon and nitrogen cycles and the development of evidence-based policies.
How to cite: Zinke, J., Salter, M., Hermans, M., Fonseca Poza, A. A., Hansen, J., Kumblad, L., Rydin, E., Wikström, S. A., Norkko, A., Geilfus, N.-X., Villnäs, A., Thrush, S., Geibel, M., and Humborg, C.: Greenhouse Gas Dynamics in Coastal Ecosystems: Insights from the Baltic Sea and Auckland, New Zealand, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12341, https://doi.org/10.5194/egusphere-egu25-12341, 2025.
Posters virtual: Tue, 29 Apr, 14:00–15:45 | vPoster spot 5
EGU25-17950 | Posters virtual | VPS2
Some Recent Contributions from the Heidelberg Aeolotron to Understanding Air-Sea Gas ExchangeTue, 29 Apr, 14:00–15:45 (CEST) | vP5.31
The lack of knowledge about the parameters controlling the transfer velocity of the exchange of gases and volatile species across the air-sea interface besides the wind speed – such as the sea state (wave age), bubbles, and surfactants - hinders progress of a better estimate of fluxes for all relevant chemical species.
In 2021, a laboratory program was started at the large air-sea interaction facility, the Heidelberg Aeolotron. With four innovative key elements, most disadvantages of previous wind-wave tunnel experiments could be overcome:
-
Because of the infinite fetch of the annular facility, wind waves come into equilibrium with the wind that is more similar to the ocean compared to the linear facility.
-
The clean environment (walls coated with Teflon foil) facilitates experiments with surface films.
-
Two imaging techniques are used to measure transfer velocities locally and instantaneously. In this way, it is also possible to get direct insight into the mechanisms.
-
The whole fetch range and non-stationary conditions could be investigated.
An extensive measuring program finished in September 2024. In this talk, the focus is on some of the first results which are regarded to be the most important contributions concerning the conditions in the field:
-
The dependence of the transfer velocity on the fetch (wave age) seems to be only significant at lower wind speeds with an overshoot at young wind fields by more than a factor of two. This is an important contribution to the large variability of the gas transfer velocity at low wind speeds.
-
Once surface active materials, either soluble or insoluble suppress waves, gas transfer velocities are reduced to the same velocities and are governed by the same mechanisms. The measurements included insoluble films of hexadecanol and olive oil and the soluble surfactants TritonX-100 and Tergitol 15-S-12. At wind speeds larger than 8 m/s, wind waves cannot be suppressed by any films.
-
From a statistical analysis of the spatial-temporal patterns gained by both imaging techniques, it is possible to infer the Schmidt number exponent. This means that no longer multi-tracer experiments are required using tracers with a large difference in the diffusion coefficients.
-
At high wind speeds, breaking of the dominant waves does not play a dominant role. It is a rather fast surface renewal taking place all over the surface at scales of a few centimeters, which is associated with the smaller-scale wind wave field riding on the dominant wave.
-
Simplified forms of the two imaging techniques used in the Aeolotron seem to be suitable also for field measurements. A first experiment is planned for the BASS Baltic Sea cruise in June 2025.
-
It was possible to compare gas transfer measured with flux chambers in the Aeolotron with those gained at the free water surface using imaging thermography. The results clearly show that no useful measurements can be performed by flux chambers as soon as wind-induced effects are dominant, which is already the case at wind speeds as low as 2 m/s.
How to cite: Jähne, B., Krall, K., Hofmann, D., and Dong, Y.: Some Recent Contributions from the Heidelberg Aeolotron to Understanding Air-Sea Gas Exchange, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17950, https://doi.org/10.5194/egusphere-egu25-17950, 2025.
