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

Asphalt volcanism and chemosynthetic life in the Campeche Knolls, Gulf of Mexico

In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico

(Table 1) Hydrocarbon gas composition of sediment and gas hydrate collected in video-guided grabs from Chapopote

In the Campeche Knolls, in the southern Gulf of Mexico, lava-like flows of solidified asphalt cover more than 1 square kilometer of the rim of a dissected salt dome at a depth of 3000 meters below sea level. Chemosynthetic tubeworms and bivalves colonize the sea floor near the asphalt, which chilled and contracted after discharge. The site also includes oil seeps, gas hydrate deposits, locally anoxic sediments, and slabs of authigenic carbonate. Asphalt volcanism creates a habitat for chemosynthetic life that may be widespread at great depth in the Gulf of Mexico

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 chimneys in the Mediterranean Sea linked to deep-water oxygen depletion

Marine sediments at ocean margins vent substantial amounts of methane1, 2. Microbial oxidation of the methane released can trigger the precipitation of carbonate within sediments and support a broad diversity of seafloor ecosystems3, 4. The factors controlling microbial activity and carbonate precipitation associated with the seepage of submarine fluid over geological time remain poorly constrained. Here, we characterize the petrology and geochemistry of rocks sampled from metre-size build-ups of methane-derived carbonate chimneys located at the Amon mud volcano on the Nile deep-sea fan. We find that these carbonates comprise porous structures composed of aggregated spherules of aragonite, and closely resemble microbial carbonate reefs forming at present in the anoxic bottom waters of the Black Sea5. Using U-series dating, we show that the Amon carbonate build-ups formed between 12 and 7 thousand years ago, contemporaneous with the deposition of organic-rich sediments in the eastern Mediterranean, the so-called sapropel layer S1. We propose that the onset of deep-water suboxic or anoxic conditions associated with sapropel formation resulted in the development of intense anaerobic microbial activity at the sea floor, and thus the formation of carbonate chimneys