Notes on the modeling of methane in aging hydrothermal plumes

Marine hydrothermal vent fields represent a unique environment for the study of aerobic microbial methane oxidation because of high methane concentrations and limited spatial and temporal scales. Earlier data collected in lateral plumes at the Endeavour Segment of the Juan de Fuca Ridge, including methane concentration, methane oxidation rate and stable carbon isotopic composition (delta(13) C), are carefully interpreted with a suite of simple analytical models. Methane oxidation is defined with a rate constant k as a first order process with respect to both substrate and methanotroph concentration. This elementary formalism coupled with simplified representations of advection and diffusion through the lateral plume is sufficient to reproduce salient features of the data: maximum methane turnover times of about a week 2 km from the vent field location and stable carbon isotopic enrichment from -47 parts per thousand to values exceeding -5 parts per thousand over a distance of 15 km. Results suggest that k is of order 10(-8) (nM-s)(-1) at local conditions and that methane oxidizing bacteria hold about 12 fg of carbon per cell.