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High-resolution mapping with an autonomous underwater vehicle (AUV) of a section of the San Clemente fault, offshore Southern California, reveals the largest documented cold-seep-associated barite deposits discovered to date. Although barite deposits along this fault, north of the mapped area, have been observed and sampled before in submersible dives, this study reveals massive newly found outcrops. The high-resolution surveys resolve their small-scale morphology, their large geographical extent and the structural controls on their emplacement. Detailed bathymetry (1 m × 1 m × 0.25 m grid resolution) of a ∼12 km2 area, ranging in water depths between 962 and 1,300 m and intersected by the fault, exhibits quasi-circular mounds of 10−30 m planar dimensions rising up to 11 m above the surrounding seafloor on a 30−45 m high and at least 1,100 m long ridge, and along truncated strata, but not along the main fault strand. Observations from a remotely operated vehicle (ROV) show that the mounds consist of steep sided dark-varnished blocks of barite. Active barite precipitation occurs as white friable spires emerging from the older deposits and as white porous precipitates filling fissures. Upward thinning spires and microbial mat occurrences atop the spires are consistent with aggradational growth due to precipitation from upward flowing solutions. Live Lamellibrachia tubeworms are found in association with the fissure-filling barite precipitates. Mapping surveys were also conducted along a short section of the San Clemente fault south of the Navy Fan. In addition, a 9 km2 area along the San Diego Trough Fault south of the United States-Mexico border was mapped and visited in an ROV dive. In the three mapped regions sub-bottom chirp profiles indicate that often barite precipitation occurs where strata, truncated and uplifted by the fault, has thin or no sediment drape, allowing for Ba-rich solutions, that may have ascended through the main fault zone, to flow along bedding planes and mix with seawater sulfate at the seafloor interface. Despite the massive scale of the barite mounds, they are not resolvable in surface ship multibeam-generated bathymetry (25 m grid resolution). As only a few areas have been mapped at the high-resolution employed in this study, the full extent of these deposits along the San Clemente Fault and other faults remains unknown.

Despite years of research into microbial activity at diffuse flow hydrothermal vents, the extent of microbial niche diversity in these settings is not known. To better understand the relationship between microbial activity and the associated physical and geochemical conditions, we obtained co-registered metatranscriptomic and geochemical data from a variety of different fluid regimes within the ASHES vent field on the Juan de Fuca Ridge. Microbial activity in the majority of the cool and warm fluids sampled was dominated by a population of Gammaproteobacteria (likely sulfur oxidizers) that appear to thrive in a variety of chemically distinct fluids. Only the warmest, most hydrothermally-influenced flows were dominated by active populations of canonically vent-endemic Epsilonproteobacteria. These data suggest that the Gammaproteobacteria collected during this study may be generalists, capable of thriving over a broader range of geochemical conditions than the Epsilonproteobacteria. Notably, the apparent metabolic activity of the Gammaproteobacteria—particularly carbon fixation—in the seawater found between discrete fluid flows (the intra-field water) suggests that this area within the Axial caldera is a highly productive, and previously overlooked, habitat. By extension, our findings suggest that analogous, diffuse flow fields may be similarly productive and thus constitute a very important and underappreciated aspect of deep-sea biogeochemical cycling that is occurring at the global scale.