Geological, geochemical, and microbiological heterogeneity of the seafloor around methane vents in the Eel River Basin, offshore California

Marine methane vents and cold seeps are common features along continental margins worldwide, serving as localized sites for methane release and colonization by microbial and chemosynthetic megafaunal communities. The Eel River Basin (ERB), located on the continental slope off Northern California, contains active methane vents and seep-associated chemosynthetic biological communities (CBC) on the crests of anticlines in ∼520-m water depth. Seep-related features on the seafloor have a patchy distribution and include active bubbling vents, chemosynthetic clam beds, and sulfide-oxidizing bacterial mats. Methane sources supplying local seeps are heterogeneous on all spatial scales and support a large and diverse microbial assemblage involved in the anaerobic oxidation of methane (AOM). To develop a comprehensive understanding of the complex biological, geochemical and physical processes associated with, and influencing seafloor methane seepage, a multidisciplinary approach is required. Here we present an integrative, multidisciplinary study that illustrates the diverse processes associated with seafloor methane seepage within the Eel River Basin and the complex interactions defining the geochemistry, mineralogy and microbiology within this environment

Distributions of microbial activities in deep subseafloor sediments

Diverse microbial communities and numerous energy-yielding activities occur in deeply buried sediments of the eastern Pacific Ocean. Distributions of metabolic activities often deviate from the standard model. Rates of activities, cell concentrations, and populations of cultured bacteria vary consistently from one subseafloor environment to another. Net rates of major activities principally rely on electron acceptors and electron donors from the photosynthetic surface world. At open-ocean sites, nitrate and oxygen are supplied to the deepest sedimentary communities through the underlying basaltic aquifer. In turn, these sedimentary communities may supply dissolved electron donors and nutrients to the underlying crustal biosphere

Formation of carbonate concretions in surface sediments of two mud mounds offshore Costa Rica: A stable isotope study

The surface sediments of two mud mounds (‘‘Mound 11’’ and ‘‘Mound 12’’) offshore southwest Costa Rica contain abundant authigenic carbonate concretions dominated by high-Mg calcite (14–20 mol-% MgCO3). Pore fluid geochemical profiles (sulfate, sulfide, methane, alkalinity, Ca and Mg) indicate recent carbonate precipitation within the zone of anaerobic oxidation of methane (AOM) at variable depths. The current location of the authigenic carbonate concretions is, however, not related to the present location of the AOM zone, suggesting mineral precipitation under past geochemical conditions as well as changes in the flow rates of upward migrating fluids. Stable oxygen and carbon isotope analysis of authigenic carbonate concretions yielded d18Ocarbonate values ranging between 34.0 and 37.7 % Vienna standard mean ocean water (VSMOW) and d13Ccarbonate values from -52.2 to -14.2 % Vienna Pee Dee belemnite (VPDB). Assuming that no temperature changes occurred during mineral formation, the authigenic carbonate concretions have been formed at in situ temperature of 4–5 °C. The d18Ocarbonate values suggest mineral formation from seawater-derived pore fluid (d18Oporefluid = 0 % VSMOW) for Mound 12 carbonate concretions but also the presence of an emanating diagenetic fluid (d18Oporefluid &5 %) in Mound 11. A positive correlation between d13Ccarbonate and d18Ocarbonate is observed, indicating the admixing of two different sources of dissolved carbon and oxygen in the sediments of the two mounds. The carbon of these sources are (1) marine bicarbonate (d13Cporefluid &0 %) and (2) bicarbonate which formed during the AOM (d13Cporefluid &-70 %). Furthermore, the d18Oporefluid composition, with values up to ?4.7 % Vienna standard mean ocean water (VSMOW), is interpreted to be affected by the presence of emanating, freshened and boronenriched fluids. Earlier, it has been shown that the origin of 18O-enriched fluids are deep diagenetic processes as it was indicated by the presence of methane with thermogenic signature (d13CCH4 = -38 %). A combination of present geochemical data with geophysical observations indicates that Mounds 11 and 12 represent a single fluid system interconnected by deep-seated fault(s)