A model of methane concentration profiles in the open ocean

Methane-bearing particulate matter formed in the upper ocean layer is allowed to settle and degrade, releasing methane into the water column as a source in one-dimensional advection-diffusion equations. Predicted carbon and methane particulate fluxes are in good agreement with sediment trap data, using parameters of expected magnitude and particulate methane production well within the mixed layer. This suggests a rapid pathway to the atmosphere and reduced effects on methane concentrations below. Vertical advection rates yielding a good fit between methane concentration calculations and data are larger than expected unless methane oxidation is included. This confirms the significance of methane oxidation in shaping open-ocean methane concentration profiles in spite of turnover times of decades. Predictions of the isotopic composition of dissolved methane δ13 C with the one-dimensional model are more difficult, although trends in measured vertical profiles can be reproduced. While this work does not shed light on the purported mechanism of methane generation in the upper ocean, it shows that methane of particulate origin is sufficient to explain observed open-ocean methane concentrations

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 (δ13C), 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‰ to values exceeding -5‰ 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