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

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. KEYWORDS: gas hydrate; methane flux; Rio Grande Cone; Brazil. RESUMO: Muitas pesquisas têm sido publicadas relacionando elevadas acumulações de hidrato de gás e o ciclo global do carbono. Nesse contexto, a determinação da profundidade da interface sulfato/metano (SMI) é importante para entender a dinâmica do fluxo de metano em locais rasos. Este artigo científico identifica a profundidade da SMI em sedimentos com base em perfis de concentração de sulfato e metano em testemunhos recuperados na Província do Hidrato de Gás do

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

“…In reality, microbial communities are dynamic and adapt not only to the supply of CH 4 from beneath but also to changes in salinity, temperature and sulfate fluxes (Michaelis et al, 2002;Nauhaus et al, 2007;Treude et al, 2003). Experimental studies show that, for instance, a temperature increase of only 2 • C can increase anaerobic organic matter degradation by 40 % (Roussel et al, 2015). In diffusive systems, the AOM process has been shown to operate at the thermodynamic limit for cell metabolism (Hoehler and Alperin, 1996), whereas advective systems apparently deliver CH 4 in amounts that allow for abundant cell growth and the development of thick biofilms capable of very high AOM rates (up to 10 −4 mol cm −3 d −1 (Boetius et al, 2000;Nauhaus et al, 2007;Treude et al, 2003).…”

Gas Seeps at the Edge of the Gas Hydrate Stability Zone on Brazil’s Continental Margin

“…The upper pockmark field lies within the estimated depth range of the MHSZ feather edge (510-760 m) and comprises a slope-parallel zone 20 km long by 3 km wide, widening to 6 km in the NW, including a central zone where pockmarks cover most of the seafloor (Figure 1). Sub-bottom profiles show acoustic blanking indicating free gas rising to seafloor through chimneylike features [47,53]. Methane concentration and sulphate profiles in pore waters obtained from piston cores samples in a cross section perpendicular to the pockmark field corroborate with the presence of shallow gas, notably focused at the isobath of 545 m [53], where there is a major concentration of pockmarks in the field [47].…”

“…Direct evidence of natural gas hydrate and gas seeps on the seafloor were found in the Rio Grande Cone [47] and in the Amazon deep-sea fan [48], in both areas in association with gas venting from the GHSZ. Living chemosynthesis-based communities in pockmarks in the Rio Grande Cone [51,52], in addition to acoustic disturbance caused by the presence of free gas at shallow depths (<10 m) below seafloor in sub-bottom profiles [53] are indicative of active methane seeps. Direct evidence of gas seepage was identified in the Amazon deep-sea fan by the presence of acoustic anomalies in the water column using multi-beam echo sounder backscatter data, which is also supported by the presence of remnants of chemosynthetic organisms at the seafloor [48].…”

Molecular and Isotopic Composition of Hydrate-Bound, Dissolved and Free Gases in the Amazon Deep-Sea Fan and Slope Sediments, Brazil

“…The direct recovery of gas hydrate was performed through sediment sampling by piston coring and the results showed that the gas was mainly composed of methane of biogenic origin [13]. Recently, other works described the methane-sulphate interface dynamics [16] and the origin of organic matter in hydrate-bearing sediments [17] in the Pelotas Basin region.…”