AS4.13

Analysis of new high frequency dimethylsulfide (DMS) measurements indicates a latitudinal dependence to the patterns of small-scale variability; this points to previously unrecognised drivers of DMS spatial variability. DMS makes a significant contribution to natural marine aerosol. The amount and distribution of preindustrial DMS emissions is important for constraining the influence of anthropogenic aerosol on climate. The impact of variations in seawater DMS concentration on climatological (Lana et al. 2011) flux uncertainty is as large as the choice of gas transfer velocity parameterization. Improving understanding of the spatial variability of seawater DMS will help improve climatological flux estimates. High frequency data enables an assessment of the spatial variability lengthscale of DMS. We use 35 high frequency observational datasets, including measurements from the GSSDD (Global Surface Seawater DMS Database), NAAMES (North Atlantic Aerosol and Marine Ecosystem Study), and SCALE (Southern oCean SeAsonaL Experiment), to assess the variability lengthscale of DMS globally, and in all ocean basins at different stages of the seasonal cycle. We interpret our results within the context of ancillary physical and biogeochemical measurements, which may be potential drivers of the regional variability patterns of DMS concentrations.

How to cite: Manville, G., Halloran, P., Bell, T., Mulcahy, J., Mahajan, A., Hulswar, S., Simo, R., and Gali, M.: What can we learn about small-scale spatial variability of surface ocean dimethylsulfide (DMS) concentrations from high frequency novel measurements?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7667, https://doi.org/10.5194/egusphere-egu21-7667, 2021.

An updated estimation of the bottom-up global surface seawater dimethyl sulphide (DMS) climatology, DMS-Rev3, is the third of its kind and includes five significant changes from the last climatology, ‘L11’ (Lana et al., 2011) that was released about a decade ago. The first change is the inclusion of new observations that have become available over the last decade, i.e., the total number of observations included in DMS- Rev3 are 865,109 as compared to 47,313 data points used in the last estimation (~1728% increase in raw data). The second was significant improvements in data handling, processing, filtering, to avoid bias due to different observation frequencies. Thirdly, we incorporated the dynamic seasonal changes observed in the ocean biogeochemical provinces and their variable geographic boundaries. Fourth change was refinements in the interpolation algorithm used to fill up the missing data. And finally, an upgraded smoothing algorithm based on observed DMS variability length scales (VLS) which helped reproduce a more realistic distribution of the DMS concentration data. The results show that DMS-Rev3 estimates the global annual mean DMS concentration at 2.34 nM, 4% lower than the current bottom-up ‘L11’ climatology. However, significant regional differences of more than 100% are observed. The largest changes are observed in high concentration regions such as the polar oceans, although oceanic regions which were under-sampled in the past also show large differences. DMS-Rev3 reduces the previously observed patchiness in high productivity regions.

How to cite: Hulswar, S., Manville, G., Simo, R., Gali, M., Bell, T. G., Halloran, P., Lana, A., and Mahajan, A. S.: Third Revision of the Bottom-up Global Surface Seawater Dimethyl Sulphide Climatology (DMS-Rev3), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4652, https://doi.org/10.5194/egusphere-egu21-4652, 2021.

Surface ocean dimethylsulfide (DMS) was measured during four shipboard field campaigns conducted during the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES).  Variations in surface seawater DMS are discussed in relation to biological and physical observations. The interplay of biomass and physics influences DMS concentrations at regional/seasonal scales and at smaller spatial and shorter temporal scales. Observations are compared with the best-available climatological predictions of seawater DMS, including output from an empirical algorithm and a neural network model. The input terms common to the algorithm and neural network approaches are biological (chlorophyll) and physical (mixed layer depth, photosynthetically active radiation, seawater temperature). DMS concentrations tend to be under-predicted and the episodic occurrence of higher DMS concentrations is poorly predicted. The choice of climatological seawater DMS product makes a substantial impact on the estimated DMS flux into the North Atlantic atmosphere. These results suggest that additional input terms are needed to improve the predictive capability of current state-of-the-art approaches to estimating seawater DMS.

How to cite: Bell, T., Porter, J., Wang, W.-L., Lawler, M., Hoyle, F., Chase, A., Boss, E., Karp-Boss, L., Kramer, S., Siegel, D., Graff, J., Behrenfeld, M., and Saltzman, E.: Predictability of seawater DMS during the North Atlantic Aerosol and Marine Ecosystem Study (NAAMES), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10126, https://doi.org/10.5194/egusphere-egu21-10126, 2021.

Volatile organic compounds (VOCs) are ubiquitous in the atmosphere and are important for atmospheric chemistry. Large uncertainties remain in the role of the ocean in the atmospheric VOC budget because of poorly constrained marine sources and sinks. There are very few direct measurements of air-sea VOC fluxes near the coast, where natural marine emissions could influence coastal air quality (i.e. ozone (O₃), aerosols) and terrestrial gaseous emissions could be taken up by the coastal seas.

To address this, we present air–sea fluxes of acetone, acetaldehyde and dimethylsulfide (DMS) at the coastal Penlee Point Atmospheric Observatory (PPAO) in the South-West UK during the spring (Apr-May 2018). Fluxes are quantified simultaneously by eddy covariance (EC) using a proton transfer reaction quadrupole mass spectrometer. Comparisons are made between two wind sectors representative of different air-water exchange regimes: the open water sector facing the North Atlantic Ocean and the fetch-limited Plymouth Sound fed by two estuaries.

Mean EC (± 1 standard error) fluxes of acetone, acetaldehyde and DMS from the open-water wind sector were ‑8.01±0.77, ‑1.55±1.44 and 4.67±0.56 μmol m^-2 d^-1 respectively (- sign indicates air-to-sea deposition). These measurements are generally comparable (same order of magnitude) to previous measurements in the Eastern North Atlantic Ocean at the same latitude. In comparison, the terrestrially influenced Plymouth Sound wind sector showed respective fluxes of -12.93±1.37, -4.45±1.73 and 1.75±0.80 μmol m^-2 d^-1. The greater deposition fluxes of acetaldehyde and acetone within the Plymouth Sound were largely driven by higher atmospheric concentrations from the terrestrial wind sector. The reduced DMS emission from the Plymouth Sound was caused by a combination of lower wind speed and likely lower dissolved concentrations as a result of the freshwater estuarine influence (i.e. dilution).

In addition, we measured the near surface seawater concentrations of acetone, acetaldehyde, DMS and isoprene from a marine station 6 km offshore. Comparisons are made between EC fluxes from the open water and diffusive VOC fluxes calculated with a two-layer (TL) model of gas transfer using air/water concentrations. The calculated TL fluxes are largely consistent with our direct measurements in the directions and magnitudes of fluxes. Generally, the TL model predicted acetone and acetaldehyde fluxes that were ~12–33 % higher (greater deposition) than the EC measurements. This could be due to sea surface processes that produce these carbonyl compounds that were not accounted for by the TL technique.

How to cite: Phillips, D., Hopkins, F., Bell, T., Wohl, C., Reeves, C., Nightingale, P., Liss, P., and Yang, M.: Air-sea exchange of acetone, acetaldehyde, DMS and isoprene at a UK coastal site., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6326, https://doi.org/10.5194/egusphere-egu21-6326, 2021.

Desert dust as a source of iron and other micronutrients is recognized to fertilize oceans, but little attention has been paid to dust as a source of iodine. Empirical observations find iodate on dust measured during ship cruises downwind of the Sahara desert. However, it remains unclear whether iodine in dust is the result of marine iodine uptake on dust during transport in the marine boundary layer, or whether such iodine accumulates over geological time scales, and is emitted together with dust. Significant enhancements of iodine have been observed in Sahara dust events in form of methyl iodide (CH₃I) and iodine monoxide (IO) radicals, but atmospheric models currently do not consider dust as a source of iodine. Furthermore, dust plumes are often accompanied by significant ozone loss, which is commonly attributed to reactive uptake of O₃ and other odd oxygen species (i.e., N₂O₅, HNO₃) on dust surfaces. However, laboratory experiments struggle to reproduce the large reactive uptake coefficients needed to explain field observations, and do not consider iodine chemistry. We present evidence that dust induced "mini ozone holes" in the remote (Southern Hemisphere) lower free troposphere west of South America (TORERO field campaign) are largely the result of gas-phase iodine chemistry in otherwise unpolluted (low NO_x) dust layers that originate from the Atacama and Sechura Deserts. Ozone concentrations inside these elevated dust layers are often 10-20 ppb, and as low as 3 ppb, and influence entrainment of low ozone air from aloft into the marine boundary layer. Ozone depletion is found to be widespread, extending up to 6km altitude, and thousands of kilometers along the coast. Elevated IO radical concentrations inside decoupled dust layers are higher than in the marine boundary layer, and serve as a source of iodine, and vigorous ozone sink following entrainment to the marine boundary layer. The implications for our perception of iodine sources, surface air quality, oxidative capacity, and climate are briefly discussed.

How to cite: Volkamer, R., Koenig, T., Apel, E., Bresch, J., Cuevas, C., Dix, B., Eloranta, E., Fernandez, R., Hall, S., Hornbrook, R., Pierce, B., Reeves, M., Saiz-Lopez, A., Spuhler, S., and Ullman, K.: Mini ozone holes due to dust release of iodine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13841, https://doi.org/10.5194/egusphere-egu21-13841, 2021.

Atmospheric aerosols supply nutrients and other substances to the ocean and may influence net primary productivity and carbon uptake. The most pronounced influence of atmospheric deposition (AD) on marine biogeochemical processes occurs in oligotrophic seas and it is expected to increase in the future scenarios of a warmer atmosphere with increased atmospheric emissions and deposition rates. The first direct contact of atmospheric substances with seawater occurs at the sea-surface microlayer (SML), a 1-1000 µm top seawater layer placed between the atmosphere and the sea. AD of specific organic substances affects the bioavailability and toxicity of marine trace metals by changing their speciation.

We studied Cu - organic matter interactions in samples of the SML and underlying water (ULW, 0.5 m depth) collected during the period of retrieval of sea surface oligotrophic conditions (February-July 2019) at the coastal central Adriatic sites (Martinska and Jadrija, Šibenik archipelago). During the sampling period, specific intensive atmospheric events in the area such as open field biomass burning (20.2.2019, 2.3.2019, 13.6.2019), pollen (2.4.2019, 17.4.2019), and Saharan dust (22.-23.4.2019) have been identified. We applied the electrochemical method of differential pulse voltammetry (DPV), square-wave voltammetry (SWV) and chronopotentiometric stripping (CPS) to determine the complexation capacity of Cu (CuCC), reduced sulfur species (RSS), and proteinaceous compounds, respectively. CuCC was determined according to the Ružić-van den Berg linearization model with the assumption of Cu : ligand = 1 : 1. Containing functional groups with S, N, and/or O, RSS, and proteinaceous compounds were followed due to their very high affinity toward Cu binding.

CuCC concentrations ranged from 23-654 nM, with corresponding apparent stability constants log K_app 7.2-10.0. The highest CuCC values ​​were determined in the SML samples from March and April 2019 at both stations: 654 nM (2.4.2019, Martinska), 336 nM, and 152 nM (2.4.2019 and 17.4.2019, Jadrija), 282 nM (6.3.2019, Jadrija). In those samples, the highest concentrations of RSS (up to 24.6 µg/L equiv. of glutathione) and proteinaceous compounds (up to 19.7 µg/L equiv. of bovine serum albumin) were also detected. Furthermore, selected SML samples were also enriched for CuCC by a factor of : 5.4 (2.4.2019, Martinska), 5.5 (6.3.2019, Jadrija), 5.3 (2.4.2019, Jadrija), and 2.1 (17.4.2019, Jadrija) relative to the corresponding values obtained for ULW samples.

The assessment of specific atmospheric sources and the nature of the enrichments taking place within the SML will be discussed. For example, intensive pollen deposition in April had the most pronounced impact on the concentration of sea surface proteinaceous compounds, indirectly increasing the CuCC in the SML at the coastal middle Adriatic sites.

Acknowledgment

This research was financed by the Croatian Science Foundation project BiREADI (IP-2018-01-3105).

How to cite: Strmečki Kos, S., Dešpoja, I., and Bakija Alempijević, S.: Impact of specific atmospheric depositions on Cu-organic matter interaction in the sea-surface microlayer of the middle Adriatic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4052, https://doi.org/10.5194/egusphere-egu21-4052, 2021.

Air-sea carbon dioxide (CO₂) flux is often indirectly estimated by the bulk method using the in-situ air-sea difference in CO₂ fugacity and a wind speed dependent parameterisation of the gas transfer velocity (K). In the summer, sea-ice melt in the Arctic Ocean generates strong shallow stratification with significant gradients in temperature, salinity, dissolved inorganic carbon (DIC) and alkalinity (TA), and thus a near-surface CO₂ fugacity  (fCO_2w) gradient. This gradient can cause an error in bulk air-sea CO₂ flux estimates when the fCO_2w is measured by the ship’s underway system at ~5 m depth. Direct air-sea CO₂ flux measurement by eddy covariance (EC) is free from the impact of shallow stratification because the EC CO₂ flux does not rely on a fCO_2w measurement. In this study, we use summertime EC flux measurements from the Arctic Ocean to back-calculate the sea surface fCO_2w and temperature and compare them with the underway measurements. We show that the EC air-sea CO₂ flux agrees well with the bulk flux in areas less likely to be influenced by ice melt (salinity > 32). However, in regions with salinity less than 32, the underway fCO_2w is higher than the EC estimate of surface fCO_2w and thus the bulk estimate of ocean CO₂ uptake is underestimated. The fCO_2w difference can be partly explained by the surface to sub-surface temperature difference. The EC estimate of surface temperature is lower than the sub-surface water temperature and this difference is wind speed-dependent. Upper-ocean salinity gradients from CTD profiles suggest likely difference in DIC and TA concentrations between the surface and sub-surface water. These DIC and TA gradients likely explain much of the near-surface fCO_2w gradient. Accelerating summertime loss of sea ice results in additional meltwater, which enhances near-surface stratification and increases the uncertainty of bulk air-sea CO₂ flux estimates in polar regions.

How to cite: Dong, Y., Bakker, D., Bell, T., Liss, P., Brown, I., Kitidis, V., and Yang, M.: The impact of shallow stratification on air-sea CO2 flux in the summer Arctic Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-715, https://doi.org/10.5194/egusphere-egu21-715, 2021.

Biotic and abiotic processes that form, alter, transport, and remineralize particulate organic carbon, silicon, calcium carbonate, and other minor and trace chemical species in the water column are central to the ocean’s ecological and biogeochemical functioning and of fundamental importance to the ocean carbon cycle. Sinking particulate matter is the major vehicle for exporting carbon from the sea surface to the deep sea. During its transit towards the sea floor, most particulate organic carbon (POC) is returned to inorganic form and redistributed in the water column. This redistribution determines the surface concentration of dissolved CO₂, and hence the rate at which the ocean can absorb CO₂ from the atmosphere. The ability to predict quantitatively the depth profile of remineralization is therefore critical to deciphering the response of the global carbon cycle to natural and human-induced changes.

Aiming to investigate the significant biogeochemical and ecological features and provide new insights on the sources and cycles of sinking particulate matter, a mooring line of five sediment traps was deployed from 2006 to 2015 (with some gap periods) at 5 successive water column depths (700, 1200, 2000, 3200 and 4300 m) in the SE Ionian Sea, northeastern Mediterranean (‘NESTOR’ site). We have examined the long-term records of downward fluxes for Corg, N_tot, δ¹³Corg and δ¹⁵N_tot, along with the associated ballast minerals (opal, lithogenics and CaCO₃), lipid biomarkers, Chl-a and PP rates, phytoplankton composition, nutrient dynamics and atmospheric deposition.  

The satellite-derived seasonal and interannual variability of phytoplankton metrics (biomass and phenology) and atmospheric deposition (meteorology and air masses origin) was examined for the period of the sediment trap experiment. Regarding the atmospheric deposition, synergistic opportunities using Earth Observation satellite lidar and radiometer systems are proposed (e.g. Cloud‐Aerosol Lidar with Orthogonal Polarization - CALIOP, Moderate Resolution Imaging Spectroradiometer - MODIS), aiming towards a four‐dimensional exploitation of atmospheric aerosol loading (e.g. Dust Optical Depth) in the study area.

Our main goals are to: i) develop a comprehensive knowledge of carbon fluxes and associated mineral ballast fluxes from the epipelagic to the mesopelagic and bathypelagic layers, ii) elucidate the mechanisms governing marine productivity and carbon export and sequestration to depth and iii) shed light on the impact of atmospheric forcing and deposition in respect to regional and large scale circulation patterns and climate variability and the prevailing oceanographic processes (internal variability).

Acknowledgments

We acknowledge support of this work by the Action ‘National Network on Climate Change and its Impacts – CLIMPACT’, funded by the Public Investment Program of Greece (GSRT, Ministry of Development and Investments).

How to cite: Gogou, A., Parinos, C., Stavrakakis, S., Proestakis, E., Kanakidou, M., Raitsos, D. E., Kontoyiannis, H., Velaoras, D., Christidi, A., Skampa, E., Triantaphyllou, M., Asimakopoulou, G., Civitarese, G., Krasakopoulou, E., Pavlidou, A., Souvermezoglou, E., Amiridis, V., Mihalopoulos, N., Karageorgis, A. P., and Lykousis, V.: Biogeochemical and ecological features of sinking particulate matter in the deep Ionian Sea (E. Mediterranean) during a 10-year time series study: impacts of atmospheric and oceanographic variabilities on carbon production and sequestration, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15201, https://doi.org/10.5194/egusphere-egu21-15201, 2021.

Sinking particles are a critical conduit for the export of organic material from surface waters to the deep ocean. Despite their importance in oceanic carbon cycling, little is known about the biotic composition and seasonal variability of sinking particles reaching abyssal depths. Herein, sinking particle flux data, collected in the deep Ierapetra Basin for a three-year period (June 2010 to June 2013), have been examined at the light of atmospheric and oceanographic parameters and main mass components (lithogenic, opal, carbonates, nitrogen, and organic carbon), stable isotopes of particulate organic carbon (POC) and source-specific lipid biomarkers. Our aim is to improve the current understanding of the dynamics of particle fluxes and the linkages between atmospheric dynamics and ocean biogeochemistry shaping the export of organic matter in the deep Eastern Mediterranean Sea (EMS). Overall, particle fluxes showed seasonality and interannual variability over the studied period. POC fluxes peaked in spring April-May 2012 (12.2 mg m⁻² d⁻¹) related with extreme atmospheric forcing. Summer export was approximately fourfold higher than mean wintertime, fall and springtime (except for the episodic event of spring 2012), fueling efficient organic carbon sequestration. Lipid biomarkers indicate a high relative contribution of natural and anthropogenic, marine- and land-derived POC during both spring (April-May) and summer (June-July) reaching the deep-sea floor. Moreover, our results highlight that both seasonal and episodic pulses are crucial for POC export, while the coupling of extreme weather events and atmospheric deposition can trigger the influx of both marine labile carbon and anthropogenic compounds to the deep Levantine Sea. Finally, the comparison of time series data of sinking particulate flux with the corresponding biogeochemical parameters data previously reported for surface sediment samples from the deep-sea shed light on the benthic-pelagic coupling in the study area. Thus, this study underscores that accounting the seasonal and episodic pulses of organic carbon into the deep sea is critical in modeling the depth and intensity of natural and anthropogenic POC sequestration, and for a better understanding of the global carbon cycle.

Acknowledgments

We acknowledge support of this work by the project ‘PANhellenic infrastructure for Atmospheric Composition and climatE change – PANACEA’ (MIS 5021516) which is implemented under the Action ‘Reinforcement of the Research and Innovation Infrastructure’, funded by the Operational Programme ‘Competitiveness, Entrepreneurship and Innovation’ (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

How to cite: Pedrosa-Pamies, R., Parinos, C., Sanchez-Vidal, A., Calafat, A., Canals, M., Velaoras, D., Mihalopoulos, N., Kanakidou, M., Lampadariou, N., and Gogou, A.: Atmospheric and oceanographic forcing impact on particle flux composition and carbon sequestration in the Eastern Mediterranean Sea: a three-year time-series study in the deep Ierapetra Basin, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15479, https://doi.org/10.5194/egusphere-egu21-15479, 2021.