The Mississippi Canyon 118 Gas Hydrate Site: A Complex Natural System

Abstract The easternmost gas hydrate site in Mississippi Canyon (MC) was discovered using the Johnson Sea-Link research submersible in 2002. Free gas (C1-C5 hydrocarbons and minor CO2) vents from the seafloor to the water column at ?890 m water depth where temperature is ?5.7° Celsius. Vent gas rapidly crystallizes as massive white fracture-fillings of gas hydrate in mud at the seafloor. The Structure II gas hydrate is relatively deficient in methane (70.0%), relatively rich in ethane (7.5%) and propane (15.9%). The site is characterized by crater-like depressions and mounds of authigenic carbonate rock over an area of ?1 km2. The authigenic carbonate rock is the result of episodic microbial hydrocarbon oxidation. Chemosynthetic communities include bacterial mats (Beggiatoa) with tube worms, mussels, and bivalves. The MC 118 site is a unique seafloor laboratory to address important questions concerning the processes that result in crystallization of gas hydrate in fractures, microbially-driven precipitation of enormous volumes of authigenic carbonate rock, and the development of complex chemosynthetic communities in an extreme environment for life. Introduction Gas hydrate is a crystalline energy mineral that occurs globally along deep continental margins at high pressure and low temperatures (1). The Mississippi Canyon (MC) 118 gas hydrate site (Fig. 1; 28.852295 N and -88.491950 W) was first discovered during dives of the Johnson Sea Link (JSL) research submersible late in 2002. Maximum water depth at the site during measured during dives of the Johnson Sea-Link (JSL) research submersible is ?890 meters and measured seafloor temperature is ?5.7 °C. Hydrocarbon gases vent at the site because of fault migration-conduits that are related to an isolated salt body at relatively shallow depth in the sediment section. The present paper synthesizes the preliminary results of multidisciplinary investigations at the MC 118 gas hydrate site (Fig. 1). Initial questions include the source of gas that vents to the water column and its relation to gas hydrate outcrops. In addition, it appears that hydrocarbon-driven microbial processes have greatly modified the seafloor over time (Fig. 2). The MC 118 site is characterized by enormous volumes of carbonate rock that strongly modifies the seafloor over an area of ?1 km2. The carbonate rock is the result of complex consortia of microbes which drive carbon and sulfur cycles in an isolated lightless environment. Complex chemosynthetic communities have developed in association with gas vents, gas hydrate, and authigenic carbonate rock. It is remarkable that microbes have so strongly affected the seafloor because of abundant hydrocarbons. The main objective of this paper is to begin the process of understanding how the isolated MC 118 site developed from episodic venting of hydrocarbons over a considerable span of time. The site is a valuable natural laboratory that warrants further research by means of a seafloor observatory because so many questions remain unanswered.