MASS, ISOTOPIC, AND REDOX BALANCE CONSTRAINTS ON THE OXIDATION RATE OF ANCIENT SEDIMENTARY ORGANIC CARBON

The dominant source of long term oxidizing potential at the Earth’s surface is the synthesis and burial of organic carbon. Inventory and stable isotope mass balances can be use to estimate the organic carbon burial rate. A useful starting point is the carbonate sedimentation rate, estimated with less uncertainty than other fluxes in the carbon cycle. However the processes and rates by which sedimentary organic carbon (C_org) is decomposed in the geochemical cycle are less well understood. Biosynthesis and diagenesis result in inputs to sediments that are more reduced than the simple sugar stoichiometry (nCO₂ + nH₂O <=> C_nH_2nO_n + nO₂ ; O/C = 1) often used to represent photosynthesis (and oxidation). Ratios of O/C in marine organic matter are ≈ 0.35, while kerogens typically have O/C ≤ 0.15. Further decarboxylation, catagenetic and hydrolytic reactions lead to the production of strongly reduced hydrocarbons and methane. Erosion and seepage return these reduced compounds to the surface. The oxidation rate of mature kerogens is slow relative to erosion and transport rates such that an important but still poorly defined fraction of eroded kerogen is re-sedimented, particularly in systems with rapid erosion rates [1]. The oxidation of CH₄ in the surface environment is rapid and complete. Additional redox sinks include the oxidation of reduced Fe and S in both the continental and oceanic crust. Combined mass, isotopic and redox budgets can be used to constrain the global scale oxidation rate of ancient C_org and the growth of the long term reduced carbon reservoir through time. Recent estimates of geological CH₄ sources [2] and crustal Fe and S oxidation [3] imply that only 20 to 60% of eroded kerogen is oxidized. The rest must be preserved to supply the long-term oxidation potential needed to oxidize crustal Fe and S, and CH₄ fluxes. Under low O₂ conditions the fraction of kerogen remineralized must be significantly less because kerogen oxidation using sulfate as an e^-acceptor is metabolically unfavorable [4].

[1] Blair & Aller (2012) Ann Rev Mar Sci 4, 401-423. [2] Etiope et al. (2011) Planet. Space Sci. 59, 182-195. [3] Lecuyer & Ricard (1999) Earth Planet Sci Lett 165, 196-211; Bach & Edwards (2003) Geochim Cosmochim Acta 67, 3871-3887. [4] LaRowe & Van Cappellen (2011) Geochim Cosmochim Acta 75, 2030-2042.