Coupled models of free methane gas and anaerobic oxidation of methane : from core to regional scales. Geologica Ultraiectina (339)

Abstract

Methane is a potent greenhouse gas that is produced in marine sediments containing high amounts of degrading organic carbon. It is therefore not surprising that marine sediments contain vast amounts of methane (500-5000 gigatons) present in dissolved (aqueous), free gas (gaseous), and solid (clathrate) forms. Nevertheless, very little methane ultimately escapes into the atmosphere (circa 10 - 15 Tg/yr, or 2% of the total methane emissions into the atmosphere) due, for the most part, to the anaerobic oxidation of methane (AOM), a microbially-mediated process whereby sulfate is used as a terminal electron acceptor. In this thesis, an analysis of the global significance of AOM in marine sediments from the perspective of both current and documented modeling studies is used to compare depth-integrated AOM rates across different marine environments, while also providing important correlations between AOM rates, the depth where AOM takes place (the sulfate-methane transition zone), and the advective velocity of the porewater. Furthermore, reactive transport models were developed in order to further understand the methane cycle in shallow marine sediments, specifically with respect to methane production, the formation of a free methane gas phase, and methane consumption. The models were applied to porewater data from Baltic Sea sediments, in locations where low oxygen concentrations and high organic matter depositional fluxes were conducive to methane formation. Model results reveal that (1) downward-diffusing sulfate is an important oxidizer for the upwards-diffusing methane through AOM; (2) the location of the SMTZ will become shallower with increasing methane fluxes from below (and increasing depth-integrated AOM rates); (3) trapped methane gas is subject to dissolution from the seasonally-changing methane saturation concentration, a process which may enhance (double) the local AOM rates; (4) millennial timescales of high organic matter deposition are required for free methane gas to form in the sediment; (4) diagnostic indicators such as the free gas depth and the thickness of organic-rich mud (the layer where methane production takes place) can be used to estimate regional rates of methane production and consumption in the sediment; and (5) using the free gas depth as an indicator for depth-integrated AOM rates, 45 megamoles of yearly methane consumption were estimated in gassy sediments of the Danish Belt Seas