Résumé
Water that flows through permeable ultramafic rocks produces high abundances of molecular hydrogen (H 2 ), methane (CH 4 ), and other small organic molecules. Such products can fuel life in the rocky subseafloor, be extracted for energy, and may have played a role in pre-biological chemical synthesis on early Earth or other planetary bodies. The International Ocean Discovery Program drilled a new 1268-m-deep borehole (U1601C) into serpentinized mantle with minor gabbroic rocks on the Atlantis Massif, ~800-m north of the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge). Measured temperatures of the disturbed borehole reached 91.3°C, and equilibrated temperatures of the deepest section are estimated to be between 110 -140°C. Water collected every ~5-m during drilling operations had H 2 concentrations that were regularly > 200 nM and spiked to > 10 µM at multiple depths. In these waters, carbon monoxide was only present in deeper, hotter sections, and potentially associated with gabbroic intrusions into the peridotite host. Open borehole fluids were sampled after drilling and samples recovered from the deepest portion contained elevated short-lived 222-Radon and strontium isotope ratios similar to Lost City fluids, pointing to the presence of in situ subseafloor formation waters that have equilibrated with the host rock. The deepest samples were actively degassing upon recovery and contained 740 ± 360 µM H 2 , 340 ± 36 µM CH 4 , and 25.5 µM ∑formate (= formate and formic acid). The shallowest fluids from the open borehole also contain micromolar H 2 and ∑formate concentrations, the presence of which cannot be attributed to the upward migration of the deeper, higher concentration fluids. We interpret these data as reflecting two distinct and interconnected regimes of fluid flow and composition. Deep waters that are channelized along faults, lithologic contacts, and other high permeability pathways host high H 2 and CH 4 concentrations plus micromolar ∑formate that closely mirror the chemistry and isotopic signatures of LCHF vent fluids. Pervasive fluid flow permeates the mesh texture and microfracture network of the serpentinized peridotite and sustains H 2 and ∑formate even in the shallowest subseafloor intervals at mild temperatures. These findings demonstrate that both focused and pervasive fluid flow contribute to the transport, and potentially the generation, of reduced volatiles and C1 compounds within the Atlantis Massif.