Serpentinisation, fluid flow, hydrogen and methane production at magma-poor margins: an example from the West Iberia margin

当今地球系统科学的一个关键挑战是量化地壳、海洋和大气之间由于构造运动而交换的元素通量。这项工作的第一步集中在估算大洋中脊热液环流过程中的元素通量。然而，这些研究忽略了水岩反应的热力学和动力学，这对这些通量有很大的影响。此外，大陆边缘伸展期间的元素通量实际上仍未被探索。大陆边缘通常位于现在海洋的两侧，因此占据的海底面积与洋中脊相似或更大。本项目的重点是估算岩浆-贫岩浆边缘活跃伸展岩石圈内热液循环过程中蛇纹岩生成的H2和CH4通量的数量。这些边缘在全球H2和CH4循环中的重要性不仅在于它们产生的数量，还在于它们拥有比海洋中脊更低温度（< 200°C）的蛇纹岩热液系统，在那里化学合成群落可以茁壮成长。本项目汇集了伸展期构造变形、热液循环和水岩反应热力学三个建模组件的专业知识，将它们耦合起来评估H2和CH4通量。我们的结果将以目前在大陆边缘可用的最好（如果不是最好的话）的一组观测结果为基准：在岩浆匮乏的西伊比利亚边缘进行的三维多通道广角地震调查和三次iODP探险。该边缘的热液环流发生于约1.12 M年前的伸展期，因此其模式约束较差。为了解决这个问题，我们打算给出一个与合理的热液流动模式和数据相容的H2和CH4通量范围。这些估计将被外推到其他不太为人所知的岩浆贫乏边缘，以评估它们在全球H2和CH4生产中的重要性，并评估它们承载海底化学合成生命的潜力，以及未来其对全球碳循环的影响。这是一个具有挑战性的项目，将在地球科学的关键领域产生专门知识，以便量化地壳和海洋之间的元素通量，并加强我们对地球系统反馈的理解。

A key challenge in Earth system science today is to quantify the fluxes of elements that are exchanged between crust, ocean and atmosphere as a result of tectonics. First steps towards this effort have focused on estimating element fluxes during hydrothermal circulation at mid-ocean ridges. These studies, however, have ignored the thermodynamics and kinetics of water-rock reactions, which greatly affect those fluxes. In addition, element fluxes during extension at continental margins remain practically unexplored. Continental margins are often found on both sides of present oceans, hence occupy an area of the sea-floor that is similar to or larger than that of mid-ocean ridges.This project focuses on estimating the amount of serpentinization-derived H2 and CH4 fluxes produced during hydrothermal circulation within actively extending lithosphere at magma-poor margins. The importance of these margins in the global cycle of H2 and CH4 resides not only on the amount they produce, but also on the fact that they host serpentinite-based hydrothermal systems of lower temperature ( < 200°C) than those at mid-ocean ridges, where chemosynthetic communities can thrive. This project gathers expertise in three modelling components, tectonic deformation during extension, hydrothermal circulation and thermodynamics of water-rock reactions, which will be coupled to evaluate H2 and CH4 fluxes. Our results will be benchmarked against one of the best, if not the best, set of observations currently available at a continental margin: a 3D multichannel and wide-angle seismic survey and three iODP expeditions at the magma-poor West Iberia margin. Hydrothermal circulation at this margin occurred during extension ~112 M yrs ago, hence its pattern is poorly constrained. To circumvent this problem we intend to give a range of H2 and CH4 fluxes that are compatible with plausible hydrothermal flow patterns and the data. These estimates will be extrapolated to other less well-known magma-poor margins in order to assess their importance in the global production of H2 and CH4, and evaluate their potential for hosting sub-seafloor chemosynthetic life and, in the future, its consequences for the global carbon cycle. This a challenging project that will produce know-how in key areas of Earth sciences in order to quantify elements fluxes between crust and ocean and enhance our understanding of the Earth system feedbacks.