Serpentinization, redox conditions, and metabolic energy production within shallow portions of subduction zones: windows to carbon cycling and chemosynthetic life driven by modern and ancient serpentinite mud volcanism

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Serpentinite mud volcanoes are impressive seamounts at the Mariana forearc seafloor that expel fluids derived from the subducted Pacific plate. These fluids contain abundant serpentinization-derived geofuels (H2, CH4), which provide energy for chemosynthetic life within the mud volcanoes. It has been suggested that serpentinite mud volcanoes are potential sites where life could have originated on Earth. But it has to date not been quantified how much of the life-sustaining geofuels are being produced in the forearc mantle. Moreover, it is poorly constrained what the key biogeochemical processes are that result from the availability of geofuels. And did similar processes supporting chemosynthesis-based ecosystems as well take place in fossil serpentinite mud volcanoes?We propose a comprehensive study to understand the redox conditions in the shallow (<30 km) supra-subduction zone mantle. We will investigate serpentinites from the modern Mariana mud volcanoes as well as from fossil serpentinite mud volcanoes exposed in California and Oregon. Our combined empirical–modeling approach will yield estimates of O and S fugacities, from which concentrations of H2 and H2S of the interacting fluids can be calculated. We will also be able to assess the driving force for a large range of reactions of abiogenic organosynthesis.The source depths of the Mariana mud volcanoes are well known. We plan to reconstruct the source depths of the fossil serpentinite deposits by examining the peak metamorphic conditions, which metamafic clasts included in the serpentinites experienced during subduction. We will conduct petrological and bulk rock-/mineral-chemical work that, coupled with calculated pseudosections, provides reliable depth estimates.We further plan to investigate the ramifications of the serpentinization-related formation of metabolic energy for chemosynthetic life and the carbon cycle. Geochemical signatures of microbial activity preserved within secondary carbonates will be studied. Samples comprise carbonate chimneys from the Mariana mud volcanoes as well as carbonate veins within serpentinite clasts and fossil-rich seep carbonates associated with the serpentinite deposits at the US sites. Lipid biomarkers and compound-specific carbon stable isotope compositions, mineral chemistry, as well as kinetic and thermodynamic modeling will allow reconstructing key biogeochemical processes and the modes of carbonate formation.The proposed work is designed to further our understanding of redox conditions in subduction zones and its consequences for chemotrophic life and the deep carbon cycle in the oceanic lithosphere.