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 H(2). Production of C(2) and C(3) 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.

Fig. 1.

Map showing the locations of studied ODP drill sites.

Fig. 2.

Concentrations of ethane (filled symbols) and propane (open symbols) in sediments at ODP Sites 1226–1231, Equatorial Pacific and Peru Margin, expressed in μmol per kg of dry sediment: Site 1226 (red squares), Site 1227 (orange circles), Site 1228 (blue circles), Site 1229 (green circles), Site 1230 (black circles), and Site 1231 (purple squares). Sediments were deposited under a wide range of environmental conditions. In most samples, the bulk of the ethane and propane was sorbed to the sediment, i.e., dissolved ethane and propane were below the detection limit.

Fig. 3.

Carbon-isotopic compositions of methane (δ₁, red symbols), ethane (δ₂, blue), and propane (δ₃, green) at ODP Sites 1226 (open circles), 1228 (open squares), 1229 (diamonds), 1230 (triangles), and 1231 (filled circles) [δ_n ≡ (¹³R_n/¹³R_vpdb) − 1, where ¹³R ≡ ¹³C/¹²C and vpdb designates the Vienna PeeDee Belemnite isotopic standard. Reported values of δ are customarily multiplied by 1,000 and expressed in permil units (‰).] Note that δ₁ reflects the isotopic compositions of mixtures of sorbed and dissolved methane with variable relative proportions (e.g., Site 1226, CH_4,aq. < CH_4,sol.; Site 1228: CH_4,aq. < CH_4,sol.; Site 1229: CH_4,aq. ≈ CH_4,sol.; Site 1230: CH_4,aq. > CH_4,sol.; Site 1231: CH_4,aq. ≪ CH_4,sol.), whereas δ₂ and δ₃ largely pertain to the sorbed fraction of the respective gases.

Fig. 4.

Conditions yielding ΔG = −15 kJ (thick, solid lines) and ΔG = 0 kJ (dotted lines) for ethanogenesis (reaction 1, blue), propanogenesis (reaction 2, red), and homoacetogenesis (black) as a function of concentrations of acetate and hydrogen, assuming conditions considered typical for marine sediments (e.g., ref. 1): T = 10°C, [DIC] = (10 mM), pH = 8, [ethane]_aq = 20 nM, [propane]_aq = 20 nM, P = 200 bar. The horizontal axis depicts the range of acetate concentrations observed in marine sediments.

Fig. 5.

Suggested reaction network and supporting geochemical evidence. (A) A schematic reaction network showing precursors and isotope effects related to ethane and propane. (B) Values of δ₂ versus δ₃ from samples that yielded both results, symbols are defined in C. Lines are δ₃ = 2δ₂/3 + x, with values of x specified. (C) Apparent linear relationship between δ₂ and log[acetate] for ODP Sites 1226, 1228–1231, and 997, Blake Ridge, suggesting that acetate is a precursor of ethane. When necessary, acetate concentrations (1) were linearly interpolated between two data points to match the sampling interval of the closest hydrocarbon gas sample; error bars designate the acetate concentrations used for interpolation. No isotopic data for ethane was obtained for Site 1227. Data for ODP Site 997 are derived from refs. (δC₂) and ([acetate]).