Periodic Reporting for period 2 - SEAGAS (Multi-disciplinary Comparison of Fluid Venting from Gas Hydrate Systems on the Mediterranean and Brazilian Continental Margins over Glacial-Interglacial Timescales)
Período documentado: 2018-04-30 hasta 2019-04-29
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 experience 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 collaborations of lasting impact relevant to the offshore environment and climate change.
The SEAGAS project achieved its objectives through planned activities, but also through actions to address challenges that arose while taking opportunities for innovation. The primary challenge to the work programme lay in the scheduling of a joint acquisition campaign, which required a separate application to the French oceanographic fleet. Exceeding contingency plans, two ship-time proposals were submitted and given priority scheduling, but neither could be scheduled during the project, even after a 1 year suspension to extend its end date. These French-Brazilian campaigns based on SEAGAS hypotheses will nonetheless take place after the end of project, to both the Nile and Amazon margins, ensuring its long-term impact. Deferment of the campaigns was accommodated by expanding other research activities, to incorporate an additional modelling method, a second Brazilian case study, and a broader collaborative network, all of which contributed to key results on gas hydrate dynamics. Numerical modeling of the gas hydrate stability zone was extended to inverse methods, resulting in quantitative 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 include a second gas hydrate province, the Amazon fan, taking advantage of confidential datasets that became available to the outgoing host after the submission of the SEAGAS project. The Amazon case study area brought multiple benefits, allowing collaborative multi-disciplinary analyses and interpretations of sample data from fluid vents that satisified the research and training activities intended for the joint acquisition campaign, and resulting in the first discoveries of gas hydrates, of seafloor fluid vents and of mud volcanoes on the equatorial Brazilian margin. Together with work on the Rio Grande cone, results from the Brazilian case study areas generated hypotheses on gas hydrate dynamics in relation to fluid migration and venting that were successfully applied to the Nile margin. These activities led to the identification in all three study areas of discrepancies between the predicted and observed base of the gas hydrate stability zone (GHSZ), indicative of spatial variations in subsurface temperatures linked to heat transfer by fluid flux. In effect, the base of the GHSZ is inferred to be ‘elevated’ by upward fluid flow over broad areas, which may include narrower seafloor vents. This is particularly clear on the Amazon fan, where the upward flux of gas-rich fluids from depth is spatially associated with subjacent thrust-faults, the activity of which is inferred to drive changes in the stability of gas hydrates and of fluid venting through time. On the Rio Grande cone and Nile fan, spatial variations in the depth of the base of the GHSZ and the distribution of fluid vents can also be interpreted in terms of changes in the rates and/or styles of fluid flux. These results were both tested and disseminated in presentations at national and international conferences in Brazil, the USA and Europe. The project also involved a range of activities to promote the MSCA, the researcher acted as a Marie Curie Ambassador via a range of public engagements in Brazil, including seminars and interviews, and collaboration with EURAXESS in the founding of a Brazil Chapter of the Marie Curie Alumni Association (MCAA).
The SEAGAS project achieved its objectives, but interestingly did not obtain the expected results regarding the dynamics of gas hydrate systems. Rather than insights into their response to climate-driven (external) changes in changes in ocean temperatures and depths, the project provided compelling evidence of their relation subsurface fluid flow within (internal to) sedimentary depocentres on passive continental margins. The project resulted in the first comparison of gas hydrate systems on different passive continental margins, within three major deep-sea fans, those of the Nile and the Amazon (the world’s two longest rivers) and the Rio Grande cone. All of these passive margin depocentres are undergoing gravitational collapse above deep shale detachments, and it was recognised that upward fluid flow through syn-sedimentary extensional and compressional structures is a primary control on gas hydrate dynamics and fluid venting. Heat transfer by upward fluid flux is ‘elevating’ the base of the gas hydrate stability zone over broad areas, and locally in association with seafloor features that are venting gas to the oceans (pockmarks, mud volcanoes). These findings have led to a model of ‘bottom-up’ hydrate dynamics, in which changes in fluid flux may drive recurrent reductions in the stability of relatively concentrated gas hydrates from below, reducing the strength of subsurface sediments with implications for the generation of giant submarine landslides in deep-sea fans on passive continental margins. This model will be tested following the project during follow-on acquisition campaigns to the Nile and Amazon margins, which will engage French and Brazilian researchers in joint activities that will contribute to the lasting impact of the SEAGAS project on Euro-Brazilian research cooperation in a field of strategic interest to the European Research Area.