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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    ABSTRACT: Autonomous Underwater Vehicles (AUVs) have a wide range of applications in marine geoscience, and are increasingly being used in the scientific, military, commercial, and policy sectors. Their ability to operate autonomously of a host vessel makes them well suited to exploration of extreme environments, from the World’s deepest hydrothermal vents to beneath polar ice sheets. They have revolutionized our ability to image the seafloor, providing higher resolution seafloor mapping data than can be achieved from surface vessels, particularly in deep water. This contribution focuses on the major advances in marine geoscience that have resulted from AUV data. The primary applications are i) submarine volcanism and hydrothermal vent studies, ii) mapping and monitoring of low-temperature fluid escape features and chemosynthetic ecosystems, iii) benthic habitat mapping in shallow- and deep-water environments, and iv) mapping of seafloor morphological features (e.g. bedforms generated beneath ice or sediment-gravity flows). A series of new datasets are presented that highlight the growing versatility of AUVs for marine geoscience studies, including i) multi-frequency acoustic imaging of trawling impacts on deep-water coral mounds, iii) collection of high-resolution seafloor photomosaics at abyssal depths, and iii) velocity measurements of active submarine density flows. Future developments in AUV technology of potential relevance to marine geoscience include new vehicles with enhanced hovering, long endurance, extreme depth, or rapid response capabilities, while development of new sensors will further expand the range of geochemical parameters that can be measured.
    Marine Geology 06/2014; 352. DOI:10.1016/j.margeo.2014.03.012 · 2.71 Impact Factor
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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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    ABSTRACT: High-resolution topographic mapping of Norwegian deep-water Lophelia coral reefs and their immediate surrounding seafloor has disclosed striking associations with small (<5 m diameter) ‘unit’ pockmarks. A total of four study areas with Lophelia reefs and unit pockmarks are here described and discussed. At the large Fauna reef, which spans 500 m in length and 100 m in width (25 m in height), there is a field of 184 unit pockmarks occurring on its suspected upstream side. Three other, intermediate-sized Morvin reefs are associated with small fields of unit pockmarks situated upstream of live Lophelia colonies. For two of the latter locations, published data exist for geochemical and microbial analyses of sediment and water samples. Results indicate that these unit pockmarks are sources of light dissolved hydrocarbons for the local water mass, together with nutrient-rich pore waters. It is suggested that the ‘fertilized’ seawater flows with the prevailing bottom current and feeds directly into the live portion of the Lophelia reefs. With an estimated growth rate of ~1 cm per year for the Morvin Lophelia corals, it would take between 1,000 and 2,000 years for the reefs to colonize the closest unit pockmarks, currently occurring 10–20 m from their leading (live) edges.
    Geo-Marine Letters 12/2012; 32(5-6). DOI:10.1007/s00367-012-0284-0 · 2.12 Impact Factor
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    • "The flux of methane from the sediments is prescribed as M vent = 1.2 z D slope , where D slope is the total depth of the model domain (1050 m), and 1.2 mol CH 4 m −2 d −1 or 28 g CH 4 m −2 yr −1 is the observed flux of methane from the sediments on the slope; that is, at z = D slope [Reeburgh, 2007]. Methane is released from the sediments uniformly, which is consistent with measurements of methane in the bottom boundary layer that indicate it is uniformly distributed over the shelf [Newman et al., 2008]. [22] The oxidation rate constant for methane in the model is faster than an observed methane lifetime of 1.5 years in a coastal ocean with active venting [Valentine et al., 2001]. "
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    ABSTRACT: On western margins of ocean basins, such as the eastern continental shelf of the United States, rates of biological productivity are higher than in the open ocean, in spite of the mean downwelling circulation. We use a nonhydrostatic, three-dimensional, process study ocean model with idealized shelf-slope geometry, wind forcing, and tracers to explore the interplay between the circulation and the biogeochemistry of the shelf and slope; the pathways that can transport nutrients from the deep ocean and from the sediments to the surface ocean euphotic zone. Cross-shelf exchange between the open and coastal ocean is regulated by a shelf break front that separates light waters on the shelf from denser waters on the slope. The wind direction and strength influence both the position and slope of the isopycnals at the front, which become more vertical in response to northerly winds and flatten in response to southerly winds. When the wind direction oscillates between northerly and southerly, it pumps nutrient and gas-rich bottom boundary layer water up to the sea surface. Nutrients tend to accumulate in the benthic boundary layer during southerly winds and are pumped to the surface during periods of northerly winds. Stratification of the water column in summertime reduces the shelf break pump by dampening the effect of the winds on the movement of the front. When extrapolated over the northeast coast of the United States, the nutrients supplied by the shelf break pump from the open ocean to the coastal ocean are three times the estimated nitrogen delivered to the shelf from estuaries.
    Journal of Geophysical Research Atmospheres 06/2011; 116(C6). DOI:10.1029/2010JC006365 · 3.43 Impact Factor
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