Active Methane Venting Observed at Giant Pockmarks Along the U.S. Mid-Atlantic Shelf Break

Earth and Planetary Sciences Letters (Impact Factor: 4.73). 03/2008; 267(1-2):341-352. DOI: 10.1016/j.epsl.2007.11.053
Source: OAI


Detailed near-bottom investigation of a series of giant, kilometer scale, elongate pockmarks along the edge of the mid-Atlantic continental shelf confirms that methane is actively venting at the site. Dissolved methane concentrations, which were measured with a commercially available methane sensor (METS) designed by Franatech GmbH mounted on an Autonomous Underwater Vehicle (AUV), are as high as 100 nM. These values are well above expected background levels (1–4 nM) for the open ocean. Sediment pore water geochemistry gives further evidence of methane advection through the seafloor. Isotopically light carbon in the dissolved methane samples indicates a primarily biogenic source. The spatial distribution of the near-bottom methane anomalies (concentrations above open ocean background), combined with water column salinity and temperature vertical profiles, indicate that methane-rich water is not present across the entire width of the pockmarks, but is laterally restricted to their edges. We suggest that venting is primarily along the top of the pockmark walls with some advection and dispersion due to local currents. The highest methane concentrations observed with the METS sensor occur at a small, circular pockmark at the southern end of the study area. This observation is compatible with a scenario where the larger, elongate pockmarks evolve through coalescing smaller pockmarks.

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    • "Since then, chemical sensors on AUVs deployed for marine geoscience purposes have principally been used when searching for active hydrothermal plumes in the water column (e.g. Yoerger et al., 2007a; German et al., 2008b; Connelly et al., 2010) or for detecting active methane venting from pockmarks (Newman et al., 2008). However, new drivers such as CCS are leading to new sensors being developed, e.g. to monitor CO 2 leakage from subsurface CCS reservoirs. "
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    • "Pockmark craters in the seafloor were first discovered by Lew King and Brian MacLean in sedimentary basins off Nova Scotia (King and MacLean 1970) and, subsequently, in most seas and many lakes worldwide (e.g. van Weering et al. 1978; Newton et al. 1980; Hovland 1981; Solheim and Elverhøi 1985; Hovland and Judd 1988; Rise et al. 1999; Judd and Hovland 2007; Newman et al. 2007; Chand et al. 2008; Cathles et al. 2010; Plaza-Faverola et al. 2010). They occur in many sizes, ranging from normal (5 to 200 m in diameter, reaching 15 m deep), to giant (e.g. "
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