Improving Underwater Methane Flux Estimation through Passive Hydroacoustic Inversion and Geochemical Data in Shallow Coastal Systems

Methane (CH4) is recognised as one of the most powerful greenhouse gases but yet it represents a relevant energy resource. Therefore it is important to decipher the various sources that can provide inputs to the atmosphere (Saunois et al. 2016). To determine the actual need for future emission reductions, a precise quantification of the global CH4 budget is required, however, according to the most recent modelling, significant uncertainties still affect these calculations (Saunois et al., 2020). The most important source of uncertainty is attributable to natural emissions. While the open ocean CH4 emissions are relatively well constrained, the global marine flux appears to be mainly influenced by shallow near-shore environments (0-50 m b.s.l.), where CH4 released from the seafloor could escape to the atmosphere before oxidation (Weber et al., 2019). The factors that govern the magnitude of methane transfer through the water column to the atmosphere remain poorly understood and are highly site dependent, with water depth playing a critical role. Nevertheless, quantifying methane emissions from shallow coastal environments remains a challenge due to the complex thermo-fluid dynamics of bubble-mediated transport.
The present study, within the framework of the NRPP-PRIN project MEFISTO, focuses on advancing passive hydroacoustic techniques to improve gas flux detection and estimation at two distinct Mediterranean sites: the Panarea hydrothermal field ('hot seeps') and a seepage zone off the Marano and Grado lagoon, North Adriatic Sea ('cold seeps'), which exhibit contrasting degassing regimes. A central objective of the research is to enhance the Signal-to-Noise Ratio (SNR) in recorded acoustic data, which is often compromised by ambient coastal noise. We employ innovative acoustic inversion models based on the spectral analysis of bubble formation and detachment (pinch-off) events using a single hydrophone. By characterizing the unique acoustic signatures of individual bubbles and gas jets, and applying spectral denoising and an adaptive thresholding approach to detect non-overlapping individual bubbles, we aim to minimize the masking effects of the soundscape, allowing for a more precise reconstruction of the Bubble Size Distribution (BSD). These hydroacoustic flux estimates are integrated with and validated by water column geochemical investigations. This multidisciplinary approach allows us to track the fate of CH4 from the sediment-water interface to the surface, evaluating how different degassing regimes (hydrothermal vs. biogenic) and physical forcings influence the efficiency of gas transfer to the atmosphere. 
The acquired data revealed that different degassing styles are strongly influenced by natural forces driving the temporal evolution of degassing activity, particularly in  gentle or low flux emissions.
Preliminary results from four seasonal campaigns demonstrate that the synergy between acoustic monitoring and geochemical tracing significantly reduces uncertainties and provides new insights into  gas migration mechanisms through the use of non-invasive techniques, the temporal variability of emissions, and the fate of dissolved methane, ultimately contributing to a more refined marine methane budget for coastal systems.