Methane oxidation in permeable sediments at hydrocarbon seeps in the Santa Barbara Channel, California

Abstract

Abstract.A shallow-water area in the Santa Barbara Channel, California, known collectively as the Coal Oil Point seep field, is one of the largest natural submarine hydrocarbon emission areas in the world. Both gas and oil are seeping constantly through a predominantly sandy seabed into the ocean. This study focused on the methanotrophic activity within the surface sediments (0-15 cm) of the permeable seabed in the so-called Brian Seep area at a water depth of ∼10 m. Detailed investigations of the sediment biogeochemistry of active gas vents indicated that it is driven by fast advective transport of water through the sands, resulting in a deep penetration of oxidants (oxygen, sulfate). Maxima of microbial methane consumption were found at the sediment-water interface and in deeper layers of the sediment, representing either aerobic or anaerobic oxidation of methane, respectively. Methane consumption was relatively low (0.6-8.7 mmol m −2 d −1 ) in comparison to gas hydratebearing fine-grained sediments on the continental shelf. The low rates and the observation of free gas migrating through permeable coastal sediments indicate that a substantial proportion of methane can escape the microbial methane filter in coastal sediments.

Bubble Transport Mechanism: Indications for a gas bubble-mediated inoculation of benthic methanotrophs into the water column

“…In this study, we confirmed the presence of ANME-2 and MOB in Rostocker Seep sediments, which is in accordance with findings by Treude and Ziebis (2010) for the nearby Brian Seep area. In their study the authors suggested that aerobic methane oxidation is active in the sediment surface layer and anaerobic methane oxidation in sediment layers deeper than 5 cm.…”

“…The presence of MOB within and below the depth of ANME is surprising, because those two groups should be mutually exclusive given their opposite relationship with oxygen: whereas ANME are inactivated permanently by oxygen (Treude et al, 2005b), MOB depend on oxygen for metabolism. As described by Treude and Ziebis (2010) for Brian Seep, gas bubble migration through permeable sediments can drive pore-water convection that discharges sediment pore water into the ocean (Dando et al, 2000;Haeckel et al, 2007). By continuity, discharged sediment fluid must be replaced, driving a water flow into the sediment.…”

“…Using biomolecular methods and biogeochemical techniques, the process of microbial anaerobic and aerobic CH 4 oxidation has been identified clearly in sediments and in the overlying water column of the Coal Oil Point seep field (e.g., Kinnaman et al, 2010;Mau et al, 2012;Treude and Ziebis, 2010). Seep gas composition varies between different regions of the Coal Oil Point seep field.…”

“…2.6 Quantification of MOB and ANME From each sediment sample, 0.5 cm 3 were fixed following the protocol of Treude and Ziebis (2010). In the home laboratory, fixed samples were diluted (1:3000) with phosphatebuffered saline (PBS) and treated by mild sonication for 25 s with a Sonotrode probe (Bandelin electronic, Type UW200) at an amplitude of 20% (40W), cycle at 15% (0.15 s).…”

“…The following probe/probe mixtures for ANME and aerobic MOB were used: (i) ANME 2-538 (Treude et al, 2007) specific for ANME-2 archaea (40% formamide, hybridization temperature of 46°C), (ii) a mixture (1:1:1) of M(α)450 specific for type II methanotrophs (Eller et al, 2001), and M(γ)84 and M(γ)705 (Eller et al, 2001) specific for type I methanotrophs (20% formamide, hybridization temperature of 46°C). ANME-2 probes were selected, because this type of ANME has been detected nearby to the sampling site in a previous study (Treude and Ziebis, 2010). MOB probes were pooled to ensure maximum detection of aerobic methanotrophs.…”