Carbonate system analysis of two Mediterranean areas characterised by methane (CH4) emissions

Methane (CH4) is one of the most powerful greenhouse gases released into the atmosphere by natural and human activities, absorbing thermal infrared radiation emitted by the Earth ≈ 30 times more than CO2. CH4 concentrations in the atmosphere today are more than 2.6 times above estimated pre-industrial equilibrium levels, and this increase is largely the result of anthropogenic emissions associated with human activities. Coastal areas are a major contributor to natural CH4 emissions (about 7 TgCH4 year-1), with fluxes varying greatly depending on the environmental characteristics of the sites (depth, oxygen content, temperature, seagrass and seagrass cover, and more). In the presence of oxygen, the CH4 released from the sediments is easily oxidised to CO2 by the methanotrophic bacteria and archaea present in the water column. However, methane can also be oxidised under anoxic conditions in the presence of sulphates by methanotrophic archaea and sulphate-reducing bacteria. Little information is available on methane fluxes in particularly shallow environments (<50 m), where methane is released in large quantities from the seafloor for natural reasons (biological or geomorphological) and can escape to the atmosphere before oxidation due to the shallow depth. The MEFISTO project aims to combine classical physical, chemical and molecular methods with innovative hydroacoustic approaches to investigate the factors that favour or prevent the release of CH4 to the atmosphere from two shallow Italian coastal areas: a seepage zone off the lagoon of Marano and Grado (TR, Gulf of Trieste, Northern Adriatic Sea - NAd), characterised by the intermittent release of gases of biogenic origins, and the persistently degassing hydrothermal vent area off the island of Panarea (PANA, Aeolian Archipelago, Southern Tyrrhenian Sea). An important part of the activities planned within the MEFISTO project included the study of the carbonate system (derived by CO2Sys from total alkalinity - TA and pHT25°C data) at the different sites sampled. This presentation will report on the methodology used for sample preparation and analysis and discuss the preliminary results of the first three MEFISTO sampling campaigns (two campaigns in PANA - 03/24 and 09/24 and one for TR - 07/24). The PANA sites showed a decrease in both pHT25°C and TA (7.987 ± 0.054 and 2550.64 ± 12.84 μmol kgsw-1, respectively) at the surface/reference sites compared to thePage 23 of 27 hydrothermal vent-influenced sites (pHT25°C 6.428 ± 0.164 and TA 2371.60 ± 52.55 μmol kgsw-1). At the same time, dissolved inorganic carbon (DIC) increased from 2237.23 ± 25.83 μmol kgsw-1 to 2992.62 ± 204.21 μmol kgsw-1 at the sites surrounding the vents. Depth was the main parameter influencing the carbonate system at the TR sites (bottom depth ranged between 13-20 m), indicating a decrease in pHT25°C and TA from the surface (respectively 8.222 ± 0.019 and 2735.60 ± 16.54 μmol kgsw-1) to the seafloor (respectively 7.891 ± 0.068 and 2610.89 ± 10.22 μmol kgsw-1) and an opposite pattern for DIC (surface 2270.89 ± 42.33 μmol kgsw-1; seafloor and 2363.23 ± 37.16 μmol kgsw-1). A lower pHT25°C (95%CI: 7.859 ± 0.037) and a higher DIC (95%CI: 2381.20 ± 13.25 μmol kgsw-1) characterised the bottom water layer of the TR sites interested by CH4 releases, if compared with the control station (no gas emissions, pHT25°C 7.984 and DIC 2309.4 μmol kgsw-1). These preliminary results indicate that, despite the different characteristics between PANA and TR sites, common trends could be detected between the different stations, mainly related to pH (decrease) and DIC (increase) in the waters around the sites with CH4 emissions (either hydrothermal or biogenic origin). These data will be integrated with innovative acoustics and ongoing metagenomics analyses that will shed more light on the methane cycle of these two sites assessed within the MEFISTO project.