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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.

Asphalt volcanoes and lava-like flows of solidified asphalt on the seafloor (Figure 1) were first discovered and described by MacDonald et al. [2004]. The flows covered more than one square kilometer of a dissected salt dome at abyssal depths (∼3000 m) in the southern Gulf of Mexico. “Chapopote” (93°26′W, 21°54′N) was one of two asphalt volcanoes they discovered. MacDonald et al. determined that the apparently fresh asphalt must initially have flowed in a hot state, and subsequently chilled, contracted, and solidified, much in the same way as normal lava does on the surface of the Earth. The two asphalt-volcanoes discovered occur at the apex of salt domes that pierce through the seafloor. These “piercement salt domes,” known as the Campeche Knolls, are pertinent features of the deep Campeche Sedimentary Basin, which has a sediment thickness of about 10 km. According to conventional theory [Vendeville and Jackson, 1992], piercement salt domes represent “salt diapirs” that have risen up, due partly to density contrasts between salt and clay/sand from the “mother salt” located between 7 and 10 km below seafloor. A salt diapir is a vertical body of sub-surface salt, which is most often circular in cross section, is one to several kilometers in diameter, and can be 8–10 km high.

Chemosynthetic cold-seep vestimentiferan tubeworms and vesicomyid clams inhabiting oceanic basaltic rock have been discovered on the Gorda Escarpment sector of the Mendocino transform fault 73 km west of Cape Mendocino, northern California. The sparse cold-seep animals are "biomarkers" that identify zones of focused fluid venting from a methane gas horizon seismically imaged as a bottom-simulating reflector (BSR) within sediments on the southern flank of the escarpment. This is the first example of a methane-based cold-seep community on exposed oceanic basement rock within an area dominated by transform tectonics. This discovery extends the range of known environments in which the subsurface flow and venting of methane-rich fluids are linked both with geological activity and chemosynthetic communities. Observations by remotely operated vehicle of the distribution of the animals, sediment, basaltic talus, and basement outcrop delineate a large slump headscarp that channels subsurface fluid. Seismic surveys of the southern flank of the Mendocino transform fault (the Vizcaino block) define a BSR hosted in the thick sedimentary sequence that projects to the wall of the Gorda Escarpment at the same depth as the chemosynthetic community. The well-defined BSR in the marine sediment of the Vizcaino block results from an accumulation of methane gas possibly capped by methane hydrate. The isotopic composition of Mg-calcite found along the headscarp (δ 1 3 C = -65‰; δ 1 8 O = 4.8‰) is consistent with fluids derived from dissociated methane hydrate. We propose that the tectonic uplift along this transform margin has resulted in the lateral, northward movement of methane from the Vizcaino sedimentary sequence to the east-trending wall of the north-facing Gorda Escarpment.