Hydrothermal versus microbial methane degassing pathways: case studies from two Italian shallow coastal systems

Methane (CH₄), even though orders of magnitude less abundant than carbon dioxide (CO₂) in the atmosphere, is today recognized as one of the most powerful greenhouse gases, being an even stronger absorber of Earth’s emitted thermal infrared radiation than CO₂. Atmospheric CH₄ concentrations are now more than 2.6 times above estimated pre-industrial equilibrium levels and such an increase is largely the result of anthropogenic emissions related to human activities. In order to verify potential emission reductions linked to the adoption of effective climate change mitigation strategies, a precise quantification of the global CH₄ budget is actually needed. According to the most recent modelling, these calculations are still subject to considerable uncertainties, the most important of which is due to the potential ocean natural emissions. In particular, the global marine CH₄ flux appears to be mainly driven by shallow marine coastal environments (depth <50 m), where gas released from the seafloor could escape to the atmosphere before oxidation. However, due to limited and sparse data, there are large uncertainties in quantifying the actual contribution of coastal areas to atmospheric CH₄. 

The MEFISTO project, combining classical physical, chemical, and molecular methods with innovative hydroacoustic approaches, aims to help close this knowledge gap by investigating the forcings favouring or preventing the release of CH₄ in the atmosphere from two Italian shallow coastal areas: a seepage zone located off the Marano and Grado lagoon (Gulf of Trieste, Northern Adriatic Sea - NAd), characterised by the intermittent release of microbial gases, and the persistently degassing hydrothermal vent area off the Panarea Island (Aeolian Archipelago, Southern Tyrrhenian Sea). Preliminary data collected during seasonal sampling campaigns revealed large differences in terms of gas composition and flux intensity between the two study sites. In particular, very low gas fluxes (1.1 x 10^-4 L/h) were detected in the NAd site, where seafloor depth varies between 14 and 21 meters below sea level (mbsl) and seeping gases are mainly composed of CH₄. Conversely, remarkably variable fluxes, ranging from 1.0 x 10^-1  to 2.2 x 10³ L/h, were observed in the Panarea site, where seafloor depth varies between 8 and 21 mbsl but leaking gases have an extremely high CO₂ content (about 98%). Our preliminary results suggest that neither the microbial nor the hydrothermal CH₄ is released into the atmosphere in the respective study areas. Low CH₄ fluxes combined with high hydrostatic loads and temperature seems to act as controlling factors in both sites, while tides appear to play a key role in particular in the NAd site. However, as methanotrophic bacteria can act as "methane filters" along the water column, ongoing metagenomic analyses on different environmental matrices will help us to assess the presence of these microorganisms, providing useful information about their role in regulating the CH₄ fluxes. This in turn will help to achieve the MEFISTO project’s goal of elucidating the complex interplay between physical and biological forcings that favour or prevent the release of CH₄ into the atmosphere from such environments.