MICROBIALLY MEDIATED REACTIONS IN RECENTLY SUBDUCTED SEDIMENTS: FEASIBLE REACTIONS AND IMPLICATIONS

The dark sub-seafloor sustains slow, steady, and widespread microbial activity. At convergent plate margins, sediment subducts and encounters increasing temperature and pressure conditions, with diagenetic and early metamorphic reactions altering the geochemical composition of interstitial fluids and solid phases. Depending on the temperature and geochemistry of sediments and ambient formation fluids, microbial assemblages may take advantage of different metabolic reactions for survival. Given the wealth of information assembled by the Ocean Drilling Program (ODP) at convergent margins, we constrain environmental variables in recently subducted sediments and consider the range of habitable niches in those sediments in terms of thermodynamic feasibility. We model the habitability of near trench settings for subduction zones that capture the global range in crustal age and observed heat flow: Nankai (22 Ma crust, 130 mW/m² heat flow; Yamano et al. 2003 and Shipboard Scientific Party 2000), Cascadia (8Ma, 140 mW/m²; Westbrook et al. 1994), Izu-Bonin (132 Ma, ~15 mW/m²; Stern et al. 2003), and Costa Rica (25 Ma crust, 30 mW/m²; Shipboard Scientific Party 2003). Data used here include sub-seafloor temperatures, pH, NH₄⁺, HCO₃^-, SO₄^2-, aqueous methane, acetic acid, H_2(aq), and O_2(aq) contents. We quantify the Gibbs Free Energy of reaction (ΔG_r) (i.e., bioenergetic yield) associated with microbially mediated reactions under specific conditions. ΔG_r <0 for exergonic, energy-yielding reactions, and is a necessary condition for any metabolic reaction to take place.  Reactions under consideration include: (1) CO_2(aq) + 4H_2(aq) = CH_4(aq) + 2H₂O_(l); (2) CH₃COO^- + H₂O_(l) = CH_4(aq), + HCO₃^-; (3) CH_4(aq) +SO₄^2- + Ca²⁺ = CaCO_3(aq) + H₂S_(aq) + H₂O_(l); and (4) 3CH_4(aq) + 6H₂O_(l) + 8H⁺ + 4N_2(aq) = 3CO_2(aq) + 8NH₄⁺. We show that reactions (1) and (4) are thermodynamically favored for warm subduction settings and bioenergetic yield increases with temperature, while reactions (2) and (3) are typically not favored, except at lower temperatures. We are able thus to contrast the habitability of selected sites and to relate results to existing data on microbial activity in convergent margin settings. We also address the implications of methane cycling in recently subducted sediments for the ocean reservoir.