The influence of methane fluxes on the sulfate/methane interface in sediments from the Rio Grande Cone Gas Hydrate Province, southern Brazil

Luiz Frederico Rodrigues,Roberto Heemann,João Marcelo Ketzer,Adolpho Herbert Augustin,Rogerio Véscia Lourega,Adriano Viana,Gesiane Sbrissa,Dennis Miller,Antonio Fernando Menezes Freire,Sadoon Morad

doi:10.1590/2317-4889201720170027

Abstract

ABSTRACT: Much research has been published regarding the relation between major gas hydrate accumulations and the global carbon cycle. In this context, the determination of the sulfate/methane interface (SMI) depth is of primary importance in order to understand the dynamics of methane flux in the shallow section. This paper identifies the depth of the SMI in sediments based on sulfate and methane concentration profiles in cores recovered in the Rio Grande Cone Gas Hydrate Province, Pelotas Basin, southern Brazil. The shape of methane and sulfate concentration profiles in the sediments can be linked to the local methane flux rate as follows: (i) near linear, high upward-diffusing methane flux coupled with high sulfate diffusion from seawater; (ii) irregular, variable methane flux rates; and (iii) kink-type profile, which is indicative of variable rather than strictly high upward methane flux. The areas in which a high methane flux was identified are spatially associated with gas chimneys in sediments within pockmarks, whereas profiles with low methane flux are present in adjacent areas. These chimneys appear as acoustic blankings in seismic records and can therefore be mapped in subsurface. The wavy-like seismic reflection following the SMI coincides with the occurrence of authigenic carbonate nodules and concretions. In addition, high methane fluxes and the occurrence of concretions and nodules carbonates were correlated by stratigraphic position of the concretions bearing intervals and sulfate profiles.

Highlights

Methane in gas hydrates is generated by bacterial degradation of organic matter at shallow depths below seafloor and/or sourced from deep petroleum fields (Demirbas 2010, Gupta 2004)
Methane in deep-sea sediments is transported by advection and diffusion in pore water and, under favorable conditions of pressure, temperature and saturation, part of the methane fluxing through sediments forms methane hydrate within the pore space (Gupta 2004, Demirbas 2010).The consumption of methane owing to microbial activity may increase carbonate alkalinity, which can promote the precipitation of authigenic carbonates (Kennicutt et al.1988a, 1988b,Michaelis et al 2002, Sassen et al.1998, Matsumoto et al 2011)
The purpose of this paper was to identify the depth of the sulfate/ methane interface (SMI) in sediments based on sulfate and methane concentration profiles in cores obtained from Rio Grande Cone Gas Hydrate Province, Pelotas Basin, southern Brazil

Summary

Introduction

Methane in gas hydrates is generated by bacterial degradation of organic matter at shallow depths below seafloor and/or sourced from deep petroleum fields (Demirbas 2010, Gupta 2004). Anaerobic oxidation of methane (AOM) and the closely linked sulfate reduction (SR) are dominant processes in organic-rich sediments. AOM occurs at or close to the boundary between SR zone and the methanogenic zone, where methane diffuses upward. The boundary between these two zones is called sulfate/ methane interface (SMI).Upward fluxes of methane may affect the depth of SMI below the seafloor (Borowski et al 1996). AOM can be represented by the reaction ahead (Eq 1): CH4 + SO42- → HCO3- + HS- + H2O(1)

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