Anaerobic Oxidation of Methane Beneath Tropical Peat
Abstract
We present evidence of anaerobic oxidation of methane (AOM) coupled to sulfate reduction in a tropical peat deposit on the island of Borneo. The process occurs at the bottom of a five-meter thick peat layer, at the interface with an underlying deposit of marine clay, where sulfate diffuses upward into the peat and mixes with methane produced within the peat. A diverse set of microbiological and geochemical data suggest that AOM is coupled with sulfate reduction: (1) A strong sulfate gradient drives diffusion of sulfate upward into the peat while advection transports methane downward through the peat towards the clay; (2) The abundance of ANME-1 Archaea increases sharply at the bottom of the peat; (3) Incubation experiments with radiocarbon labeled methane demonstrate AOM throughout the peat profile with the largest rates at the base of the peat; (4) Lipid biomarkers are consistent with AOM, although they do not have the carbon-13 depletion found in marine sediments where AOM occurs.
In addition to the methane sink from AOM, we quantify other methane fluxes from the peat, including lateral advection, upward diffusion and ebullition. To determine these fluxes, we apply a reactive transport model to interpret a set of porewater concentrations of methane, dissolved organic carbon and their isotopic signatures. We find that although rates of AOM are low, they may exert an important control on methane emission to the atmosphere. This is because AOM consumes methane at the bottom of the peat where bubbles may form. Methane only reaches the critical bubbling partial pressure at the base of the peat, so methane consumption there may diminish or even prevent ebullition, the dominant pathway by which methane can avoid aerobic oxidation and escape to the atmosphere.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.B43F..12H
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES