Seasonal variations of D/H and ¹³C/¹²C ratios of microbial methane in surface sediments

Abstract

Application of stable isotope ratios (δD and δ¹³C) to the study of the global CH₄ cycles^1–3 requires isotopic characterization of the chief atmospheric CH₄ sources. Shallow aquatic sediments are postulated to be an important atmospheric CH₄ source⁴; however, the stable isotope composition of CH₄ produced in these systems is not well defined. Further, the relative importance of the various processes that influence CH₄ δ values is not generally predictable and probably varies from site to site. We found appreciable seasonal and spatial (probably depth-dependent) variation in the δD and δ¹³C of gas-phase microbial CH₄ produced in the marine sediments of Cape Lookout Bight (CLB), North Carolina⁵. In light of seasonal and depth-dependent variations of CH₄ production from acetate^6,7 and acetate concentrations⁸ in CLB sediments, our observations provide indirect evidence that the CLB CH₄ δ variation reflects variation in the relative contributions of the main CH₄ precursors (acetate and CO₂/H₂)^9,10.δD-CH₄ and δ¹³C-CH₄ are inversely correlated, indicating that microbial consumption^11,12 is not responsible for the δ variation. Owing to its marine nature, CLB is not strictly representative of wetlands in general; however, some of the key microbial processes and environmental characteristics that appear largely responsible for the observed CLB CH₄ δ variation are probably similar in typical temperate and high-latitude wetlands.