Methane Occurrence and Quantification in a Very Shallow Water Environment: A Multidisciplinary Approach

Abstract

Methane (CH 4 ) is a powerful greenhouse gas and, over a 20-year period, the global-warming potential of 1 tonne of atmospheric CH 4 is similar to ca. 85 tonnes of CO 2 (Hartmann et al., 2013). Global CH 4 emissions have increased nearly 10% over the past two decades (Schiermeier, 2020) and its atmospheric concentration is now more than 2.5 times above pre-industrial levels, approaching 1.9 ppm. CH 4 has a short lifetime in the atmosphere, ca. 10 years (Prather et al., 2012), and hence a stabilization/reduction of emissions will lead in a few decades to a stabilization/reduction of its atmospheric concentration and therefore its radiative forcing (Saunois et al., 2020). The most important source of uncertainty in the global CH 4 budget is attributable to natural emissions (Saunois et al., 2020), and particularly to geological seepage. Indeed, once considered a minor natural CH 4 source globally, geological degassing is today recognized as a major contributor to atmospheric CH 4 (Etiope et al., 2019). A challenging aspect on the quantification of present-day emissions is that seepage sites may be more widespread and abundant than expected, especially in the marine environment (Thornton et al., 2020). It is estimated that 30-45 Tg of CH 4 per year are released from marine seeps to the atmosphere (Etiope & Ciccioli, 2009;