Isotope fractionation and mixing in methane plumes from the Logatchev hydrothermal field

Abstract

As methane is consumed in the deep sea, its 13C/12C ratio progressively increases because of kinetic isotope fractionation. Many submarine hydrothermal vents emit methane with carbon isotope ratios that are higher than those of background methane in the surrounding ocean. Since the latter exists at low concentrations, mixing of background methane with vent fluid tends to decrease the 13C/12C ratio as concentration decreases, opposite to the trend produced by consumption. We investigated CH4 concentration and δ13C together with δ3He in plumes from the Logatchev hydrothermal field (LHF) located at 14°45′N, 45°W, which generates relatively heavy methane (δ13C ≈ −13‰) by serpentinization of ultramafic rock. The measured methane and δ3He were well correlated at high concentrations, indicating a CH4/3He ratio of 1 × 108 in the vent fluids. These tracer distributions were also simulated with an advection‐diffusion model in which methane consumption only occurs above a certain threshold concentration. We utilized δ3He to calculate the methane remaining in solution after oxidation, f, and the deviation of δ13C from the value expected from mixing alone, Δδ13C. Both in the model and in the data, the entire set of Δδ13C values are not correlated with log f, which is due to continuous oxidation within the plume while mixing with background seawater. A linear relationship, however, is found in the model for methane at concentrations sufficiently above background, and many of the samples with elevated CH4 north of LHF exhibit a linear trend of Δδ13C versus log f as well. From this trend, the kinetic isotope fractionation factor in the LHF plumes appears to be about 1.015. This value is somewhat higher than found in some other deep‐sea studies, but it is lower than found in laboratory incubation experiments.