Biological formation of ethane and propane in the deep marine subsurface

Abstract

Concentrations and isotopic compositions of ethane and propane in cold, deeply buried sediments from the southeastern Pacific are best explained by microbial production of these gases in situ. Reduction of acetate to ethane provides one feasible mechanism. Propane is enriched in 13 C relative to ethane. The amount is consistent with derivation of the third C from inorganic carbon dissolved in sedimentary pore waters. At typical sedimentary conditions, the reactions yield free energy sufficient for growth. Relationships with competing processes are governed mainly by the abundance of H2. Production of C2 and C3 hydrocarbons in this way provides a sink for acetate and hydrogen but upsets the general belief that hydrocarbons larger than methane derive only from thermal degradation of fossil organic material.ethanogenesis ͉ hydrocarbon gases ͉ marine sediments ͉ propanogenesis ͉ stable carbon isotopes L eg 201 of the Ocean Drilling Program was dedicated to the study of microbial life in deeply buried marine sediments (1, 2). Cores were obtained from open-ocean sites in the Equatorial Pacific, where sediments deposited 40 million years ago are underlain by seafloor basalts through which oxygenated seawater is flowing, and from the Peruvian Margin, where drilling penetrated sediments up to 15 million years old (Fig. 1). Temperatures in sediments ranged from 2°C to 25°C. All sites are isolated from reservoirs of fossil hydrocarbons. At both openocean and ocean-margin sites, treatment of sediments with strong base released ethane and propane (Fig. 2). When the treatment was repeated with fresh sediment and isotopically labeled water (␦D ϭ ϩ4000‰), no excess deuterium appeared in the ethane or propane. Therefore, we conclude that the hydrocarbons were strongly sorbed, indigenous constituents of the sediment and did not derive from a chemical reaction between the strong base and an organic substrate.Earlier reports describe sediments offshore Peru (3) and Spitsbergen (4), from which similar mixtures of hydrocarbons could be released by treatment with hot solutions of phosphoric acid. In each case, the carbon-isotopic compositions and abundance ratios (C 1 ͞C 2ϩ ) led to reluctant suggestions that the gases must be of thermogenic origin and thus have migrated into the unconsolidated seafloor sediments: ''the [postulated] migration of C 2ϩ hydrocarbons. . . is somehow related to these fluids [brines that might have flowed from one basin to another]'' (3); and ''. . . elevated seepages [of thermogenic hydrocarbons] occurred irregularly but are not currently active. . . it remains speculative whether the detected hydrocarbon anomalies are related to reservoirs and͞or active source rocks'' (4). No mechanism for sorbing the putatively migrated hydrocarbons more strongly than indigenous microbial products has been offered.Ethane and propane with similar isotopic characteristics and abundance ratios have recently been reported in Cretaceous marine shales in the Western Canadian sedimentary basin (5). Previous work has also pointe...

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“…The planned drilling specifically addresses fundamental questions identified in the IODP Science Plan (https://www.iodp.org/about-iodp/iodp-science-plan-2013-2023) regarding Biosphere Frontiers, Earth Connections, and Earth in Motion: The processes controlling fluid flow and a deep biosphere are intimately linked, and over the past years many studies have concentrated on understanding sedimentary or volcanic systems with high-temperature discharge or on areas of diffuse flow in volcanic units (D'Hondt et al, 2002(D'Hondt et al, , 2004Shipboard Scientific Party, 2003;Expedition 301 Scientists, 2005;Nakagawa et al, 2006;Lipp et al, 2008;Parkes et al, 2005;Biddle et al, 2006;Hinrichs et al, 2006;Inagaki et al, 2006;Huber et al, 2002Huber et al, , 2003. In contrast, the spatial scale of lithologic variability between mafic and ultramafic seafloor at slow-spreading ridges, the implications for fluid flow paths and fluxes, and the subsurface ecosystems supported by these systems remain almost completely unconstrained.…”

Uncultured archaea in deep marine subsurface sediments: have we caught them all?

“…Stable carbon isotopic signatures of archaeal phospholipids and archaeal cell biomass from ODP Sites 1227 and 1230 showed that buried organic carbon, not methane, is the primary carbon source for archaeal communities at methane-sulfate interface sediments that yielded mostly DSAG/MBG-B archaeal clones . Assimilation of buried organic carbon and lack of assimilation of methane-derived carbon would be consistent with three types of metabolism: heterotrophic fermentation of buried biomass; assimilation of organic carbon sources while performing methane oxidation solely for energy generation ; and-a speculative possibility-microbial ethanogenesis by acetate reduction, a recently proposed, thermodynamically feasible and microbiologically mediated process in deep subsurface sediments (Hinrichs et al, 2006).…”

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

“…Recent geochemical studies on marine sediments indicate that considerable amounts of methane can be sorbed to clay minerals [6,7]. Previous studies show that smectite is generally the most abundant mineral in gas-hydrate-bearing marine sediments [8,9].…”