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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - SEAGAS (Multi-disciplinary Comparison of Fluid Venting from Gas Hydrate Systems on the Mediterranean and Brazilian Continental Margins over Glacial-Interglacial Timescales)

Teaser

Summary

Gas hydrates are ice-like compounds that form within near-seabed sediments on continental margins, concentrating greenhouse gases (mainly methane) within what is thought to be the largest reservoir of carbon on Earth. The stability of these frozen sub-oceanic accumulations is sensitive to changes in sea level and water temperatures over time, but also to the poorly understood dynamics of chimney-like structures observed to vent fluids, including gas, to the oceans. An understanding of fluid venting from hydrate systems over time is of broad scientific and societal relevance, due to impacts on global carbon budgets, regional geohazard assessments and potential energy resources. SEAGAS is the first project to propose a comparative analysis of these phenomena on ocean margin settings that undergo different forms of glacial-interglacial climate forcing: the open South Atlantic offshore Brazil, and the semi-enclosed Mediterranean Sea. The overall scientific objective is to obtain an improved understanding of fluid venting from deep-sea gas hydrate provinces, using numerical modeling of gas hydrate stability to guide interpretations of multi-disciplinary datasets (marine geophysical, geological, geochemical, geothermal) available through host institutions in Brazil and in France, as well as data to be acquired during a joint oceanographic campaign. The associated training objective is for the researcher to diversify his competencies, through the acquisition of new skills in marine geochemical methods and concepts at a state-of-the-art laboratory in Brazil during the two year outgoing phase, followed by a return year in France. The strategic objective is to build and consolidate research links between Brazilian and European research groups so as to ensure further trans-Atlantic cooperations of lasting impact relevant to the offshore environment and climate change.

Work performed

During the two year outgoing phase in Brazil, clear progress was made towards all project objectives. Research and training activities were undertaken within four main work packages correlated to the specific objectives of the project : numerical modeling of gas hydrate stability (WP1), its integration with multi-disciplinary datasets available for case studies of gas hydrate provinces on the Brazilian margin (WP2), and on the Nile margin (WP4), and acquisition of data during a joint oceanographic campaign (WP3). The campaign was a calculated risk within the work programme, as it was contingent on a separate application for ship-time. Two such proposals were submitted to annual calls of the French oceanographic fleet, for campaigns to the Nile and Amazon margins, and both were accepted for priority scheduling within the first year of the project. Despite their success, it was not possible to have either scheduled during the project, even after extending its end date to April 2020 through a one year suspension (as per contingency plans). The Nile campaign has been scheduled for September 2020, while the Amazon campaign is programable until at least 2021. Deferment of the campaign was accommodated by expanding the work programme to incorporate a second modelling method, a third study area and a wider collaborative network. Numerical modeling of the gas hydrate stability zone was extended from forward to inverse methods, resulting in well-constrained estimates of subsurface temperature variations linked to heat and fluid flux. The Brazilian margin case study was enlarged from the original study area, the Rio Grande cone, to a second gas hydrate province, the Amazon fan, taking advantage of confidential datasets that became available through the outgoing host and research groups in Rio de Janeiro after the submission of the project. Collaborative multi-disciplinary analyses and interpretations of sample data from fluid vents on the Amazon fan satisified the research and training activities intended for the acquistion campaign and, together with work on the Rio Grande cone, generated working hypotheses on gas hydrate dynamics in relation to fluid migration and venting that were successfully applied to the Nile margin case study. The main results from these activities involve the identification in all three study areas of discrepancies between the predicted and observed base of the gas hydrate stability zone, which can be linked to spatial variations in subsurface temperatures consistent with heat transfer by fluid flux. In effect, the base of the gas hydrate stability zone is inferred to be â€˜elevatedâ€™ by upward fluid flow over broad areas, which may include narrower seafloor vents. This is the case on the Amazon fan, where the upward flux of gas-rich fluids from depth is found to be closely linked to subjacent thrust-faults, the activity of which may therefore drive changes in the stability and concentration of gas hydrates and of fluid venting through time. On the Rio Grande cone and Nile fan, the occurrence of fluid vents can also be related to spatial variations in the rates and/or styles of fluid flux. These results have been presented at several national and international conferences, and in contributions to two publications on the Brazilian margin.

Final results

The SEAGAS projectâ€™s comparison of gas hydrate systems on different continental margins is generating research results that are advancing our understanding of their dynamics in relation to fluid migration and seafloor venting. The original idea of the project was to use known differences in climate forcing on the two margins to investigate changes in gas hydrate stability driven from above, by glacial-interglacial changes in ocean depths and temperatures. Instead, the comparison of three gas hydrate provinces is pointing to the importance of stability changes driven from below, by fluid migration influenced by the internal dynamics of continental margin depocentres. These aspects will be more fully explored during the comparison of the margins in last year of the project (WP5). They will be further explored following the project during an acquisition campaign to the Nile margin, and a subsequent campaign to the Amazon margin, which will engage French and Brazilian researchers in joint activities to explore and refine hypotheses generated by the project. The campaigns will contribute to the lasting impact of the project on Euro-Brazilian research cooperation in a field of strategic interest to the European Research Area.