Abstract
Hydrocarbon seeps, deep sea extreme environments where deeply sourced fluids discharge at the seabed, occur along continental margins across the globe. Energy-rich reduced substrates, namely hydrocarbons, support accelerated biogeochemical dynamics, creating unique geobiological habitats. Subseafloor geology dictates the surficial expression of seeps, generating hydrocarbon (gas and/or oil) seeps, brine seeps, and mud volcanoes. Biogeochemical processes across the redox spectrum are amplified at hydrocarbon seeps due to the abundance and diversity of reductant; anaerobic metabolism dominates within the sediment column since oxygen is consumed rapidly near the sediment surface. Microbial activity is constrained by electron acceptor availability, with rapid recycling required to support observed rates of hydrocarbon consumption. Geobiologic structures, from gas hydrate to solid asphalt to authigenic minerals, form as a result of hydrocarbon and associated fluid discharge. Animal-microbial associations and symbioses thrive at hydrocarbon seeps, generating diverse and dense deep sea oases that provide nutrition to mobile predators. ▪ Hydrocarbon seeps are abundant deep sea oases that support immense biodiversity and where specialization and adaptation create extraordinary lifestyles. ▪ Subseafloor geology shapes and defines the geochemical nature of fluid seepage and regulates the flux regime, which dictate the surface expression. ▪ High rates of anaerobic oxidation of methane require coupling to multiple processes and promote diversity in the anaerobic methanotroph microbial community. ▪ The recent discovery of novel phyla possessing hydrocarbon oxidation potential signals that aspects of seep biogeochemistry and geobiology remain to be discovered.
Highlights
Hydrocarbons range in complexity from the simplest form, methane, to the many and varying constituents of petroleum
Hydrocarbon seepage is a persistent feature of slope and shelf environments across the globe that impart unique geological, biogeochemical, and geobiological signatures on the seabed (Roberts et al 2006)
Underlying geology exerts a strong control on the surface expression of hydrocarbon seepage
Summary
Hydrocarbons range in complexity from the simplest form, methane, to the many and varying constituents of petroleum. Reports of seafloor hydrocarbon seeps occurred in the 1980s—first reported in the Gulf and off the coast of Oregon (Anderson et al 1983, Paull et al 1984, Suess et al 1985)—a few years after the discovery of hydrothermal vents in the Pacific (Ballard 1977). The roughly 914 natural seeps in the Gulf discharge between 2.53 and 9.48 × 104 m3 oil year−1 across the system, with the majority discharged in the western portion of the basin (MacDonald et al 2015) This systemwide natural input contrasts starkly with the input from the Deepwater Horizon oil well blowout (78 × 105 m3 oil at a daily rate of 105 m3 day−1) that was released from the damaged well. Relative to large accidents such as the Deepwater Horizon oil spill, about 1.1 × 105 m3 oil year−1 is released into North American waters by other anthropogenic activities (boats, runoff, etc.)
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