Research offers a new perspective on the stability of frozen water-gas compounds in the deep ocean
Gas hydrates are ice-like solids composed of gas molecules (mainly methane) in water-ice cages, stable at high pressure and low temperature. In nature, they are found along ocean margins in water depths greater than about 500 m, forming the world’s largest reserve of organic carbon. Many questions remain about what controls the stability over time of these frozen gas deposits. “An improved understanding of the controls on gas hydrate stability is important to address their contributions to global climate change, regional seafloor stability and national energy strategies,” notes Daniel Praeg, coordinator of the SEAGAS project that received funding under the Marie Skłodowska-Curie programme. SEAGAS is the first project to undertake a comparative analysis of gas hydrate occurrences on ocean margins that have experienced different climate forcing factors: the Mediterranean Sea and the South Atlantic Ocean. The project used predictive modelling of gas hydrate stability to guide observations using geophysical, geological, geochemical and geothermal datasets available to the French and Brazilian project partners. The results provide new insights into gas hydrate dynamics in relation to fluid discharge from the seafloor.
Gas hydrate stability in relation to upward fluid flow
SEAGAS yielded compelling evidence that the stability of gas hydrates is influenced by upward fluid flow within thick sediment deposits on continental margins. The project examined gas hydrate provinces within three major depocentres: the deep-sea fans of the Nile and Amazon rivers, and the Rio Grande cone offshore southern Brazil. All of these sedimentary depocentres are undergoing gravitational collapse above deep shale detachments, resulting in syn-sedimentary tectonism. “We found that upward fluid flow through tectonic structures has a pronounced effect on gas hydrate stability and fluid venting in all three depocentres. Heat transfer ‘elevates’ the base of the stability zone over wide areas, while rapid hydrate formation in response to the enhanced supply of gas can account for the formation of chimney-like fluid vents. Our findings led us to propose a model of ‘bottom-up’ hydrate dynamics, in which increases in upward fluid flow (e.g. during fault movements) lead to reductions in the stability of hydrate-rich sediments from below. In turn, this reduces the strength of subsurface sediments, which could trigger the generation of giant submarine landslides,” explains Praeg. A particularly relevant example is that of the Amazon fan, where the upward flux of gas-rich fluids is spatially associated with subjacent thrust-faults. “The proposed model of ‘bottom-up’ dynamics contrasts with the conventional ‘top-down’ perspective that associates gas hydrate stability with climate-driven changes in sea level and ocean temperatures,” adds Praeg.
Next course of action
SEAGAS results were disseminated in presentations at conferences in Europe, Brazil and the United States and in a variety of publications, including articles in peer-reviewed scientific journals. The project also resulted in successful proposals for three French-led oceanographic campaigns to the Amazon and Nile margins, which will take place from 2021. These initiatives will engage European and Brazilian researchers in testing the model proposed by SEAGAS, while promoting further research cooperation in a field of strategic interest to the European research area.