Can anaerobic oxidation of methane prevent seafloor gas escape in a warming climate?

Christian Stranne,Martin Jakobsson,Volker Brüchert,Matt O'Regan,Marcelo Ketzer

doi:10.5194/se-10-1541-2019

Christian Stranne, Martin Jakobsson + Show 3 more

Open Access

https://doi.org/10.5194/se-10-1541-2019

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Abstract

Abstract. Assessments of future climate-warming-induced seafloor methane (CH4) release rarely include anaerobic oxidation of methane (AOM) within the sediments. Considering that more than 90 % of the CH4 produced in ocean sediments today is consumed by AOM, this may result in substantial overestimations of future seafloor CH4 release. Here, we integrate a fully coupled AOM module with a numerical hydrate model to investigate under what conditions rapid release of CH4 can bypass AOM and result in significant fluxes to the ocean and atmosphere. We run a number of different model simulations for different permeabilities and maximum AOM rates. In all simulations, a future climate warming scenario is simulated by imposing a linear seafloor temperature increase of 3 ∘C over the first 100 years. The results presented in this study should be seen as a first step towards understanding AOM dynamics in relation to climate change and hydrate dissociation. Although the model is somewhat poorly constrained, our results indicate that vertical CH4 migration through hydraulic fractures can result in low AOM efficiencies. Fracture flow is the predicted mode of methane transport under warming-induced dissociation of hydrates on upper continental slopes. Therefore, in a future climate warming scenario, AOM might not significantly reduce methane release from marine sediments.

Highlights

The atmospheric concentration of CH4 increased by a factor of 2.5 since the pre-industrial era, and anthropogenic emissions account for 50 %–65 % of annual global CH4 emissions (Stocker et al, 2013)
The modelling results show that the total mass of CH4 consumed by anaerobic oxidation of methane (AOM) over time becomes a function of either (1) the supply of CH4 to the sulfate reduction zone (SRZ) – when the AOM capacity is so high that all the CH4 transported to the SRZ is consumed by AOM or (2) the imposed AOM capacity itself – when the capacity is so low that there is an oversupply of CH4 to the SRZ, which leads to CH4 escaping the seafloor
For values of AOMmax in between, on the order of 10−8 mol cm−3 d−1, the AOM efficiency is to a large extent controlled by fluid flow rates, which is in line with observations

Summary

Introduction

The atmospheric concentration of CH4 increased by a factor of 2.5 since the pre-industrial era, and anthropogenic emissions account for 50 %–65 % of annual global CH4 emissions (Stocker et al, 2013). Natural hydrate deposits are susceptible to destabilization via ocean warming (Archer et al, 2009; Kretschmer et al, 2015; Dickens et al, 1995). A warming climate can lead to destabilization of the part of the marine hydrate reservoir sensitive to temperature perturbations, potentially leading to CH4 transport from sediments to the oceans and atmosphere, where the CH4 becomes a positive feedback on climate warming. As a result, anthropogenicinduced destabilization of natural marine CH4 hydrate has been proposed as a climate warming mechanism that could exhibit threshold behaviour, implying that if climate warming continues, this feedback could cause an abrupt and irreversible transition into a warmer climate state (Stocker et al, 2013)

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