Expedition 357 Scientific Prospectus: Atlantis Massif Serpentinization and Life

International Ocean Discovery Program (IODP) Expedition 357 will be implemented as a Mission Specific Platform (MSP) expedition that will address two exciting discoveries in mid-ocean-ridge research: off-axis, serpentinite-hosted hydrothermal activity exemplified by the Lost City hydrothermal field (LCHF) and the significance of tectono-magmatic processes in forming and exposing heterogeneous mafic and variably serpentinized ultramafic lithosphere that are key components of slow- and ultraslow-spreading ridges. Serpentinization is a fundamental process that controls rheology and geophysical properties of the oceanic lithosphere and has major consequences for heat flux, geochemical cycles, and microbial activity in a wide variety of environments. However, we currently have no constraints on the nature and distribution of microbial communities in ultramafic subsurface environments. Our planned drilling focuses on (1) exploring the extent and activity of the subsurface biosphere in young ultramafic and mafic seafloor; (2) quantifying the role of serpentinization in driving hydrothermal systems, in sustaining microbiological communities, and in the sequestration of carbon in ultramafic rocks; (3) assessing how abiotic and biotic processes change with aging of the lithosphere and with variations in rock type; and (4) characterizing tectono-magmatic processes that lead to lithospheric heterogeneities and the evolution of hydrothermal activity associated with detachment faulting. This expedition will be the first IODP expedition to utilize seafloor drill technology (MeBo and BGS Seafloor Rockdrill 2) to core a series of shallow (50–80 m) holes across Atlantis Massif—an oceanic core complex (30°N, Mid-Atlantic Ridge), where detachment faulting exposes mafic and ultramafic lithologies on the seafloor. We aim to recover in situ sequences of sediments, hydrothermal deposits/veins, and basement rocks that comprise a broad zone of detachment faulting across (1) a spreading-parallel (east–west) profile along the southern wall and at varying distances from the LCHF and (2) a ridge-parallel (north–south) profile into the center of the massif, where the dominant rock type changes from ultramafic to mafic. Drilling the east–west profile will allow us to evaluate how microbial communities evolve with variations in hydrothermal activity and with age of emplacement on the seafloor. We aim to compare microbial activity and diversity in areas of diffuse, H2-rich fluid flow and carbonate precipitation with communities in areas away from the active hydrothermal system and with variable substrates and crustal ages. By quantifying the extent and evolution of carbonate precipitation we will evaluate the potential for natural CO2 sequestration in serpentinizing peridotites. Drilling the north–south profile will allow us to evaluate the nature of the deep biosphere in varying lithologies and to assess the role of the differing rheologies of gabbros and serpentinized ultramafic rocks in localizing detachment faults. This expedition will also include engineering developments to sample bottom waters before and after drilling and to monitor methane, dissolved oxygen, redox, conductivity, temperature, and depth while drilling. In addition, seafloor operations will include deploying borehole plugs and swellable packers to seal the holes at high-priority sites after drilling to provide opportunities for future hydrogeological and microbiological experiments.