Active methane venting observed at giant pockmarks along the U.S. mid-Atlantic shelf break

Earth and Planetary Sciences Letters (Impact Factor: 4.35). 03/2008; 267(1-2):341-352. DOI: 10.1016/j.epsl.2007.11.053
Source: OAI

ABSTRACT 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.

1 Bookmark
  • Source
    Ciencias Marinas 06/2013; 39(2):119-135. · 0.62 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We examine the linkage between the sediment geochemical milieu and the process of carbonate degradation over a wide range of continental shelf and slope sediments using molluscan shells deployed for 13 years by the Shelf and Slope Experimental Taphonomy Initiative (SSETI). Geochemical characterization of the environment of preservation included the breadth of the pore-water carbonate undersaturation window, a depth-integrated carbonate dissolution index, the depth of minimum pore-water saturation, diffusive fluxes of oxygen and calcium, average sulfate and chloride concentration in the upper 5 cm, and the carbonate and organic carbon fractions in the same sedimentary horizon. Taphonomic indices included the maximum degree of dissolution; average dissolution; the incidences of chalkiness, pitting, deep dissolution, and a soft shell surface; the maximum degree of discoloration; the incidences of fading, gray-to-black discoloration, brown discoloration, and orange discoloration; the presence of pyrite; and edge rounding. Geochemical variables characterize the extent of most taphonomic processes with high three-variable multiple regression coefficients (R2 > 0.85). Dissolution was most intense at petroleum seeps where enhanced sediment respiration fueled by petroleum carbon and oxidation of reduced species (e.g., H2S) resulted in acute pore-water carbonate undersaturation near the sediment-water interface and high diffusive oxygen flux. In contrast, discoloration occurred as often or more commonly in shelf and slope sediments that were not subject to seep influence. The tendency for correlations between many taphonomic metrics, including those relating to dissolution, pyritization, and discoloration, and the breadths of the calcite/aragonite undersaturation windows, Ωcalcite, and oxygen flux emphasize the importance of near-surface geochemical conditions relating to organic carbon decomposition in determining the degree and type of carbonate degradation occuring at SSETI sites.
    Palaios 08/2012; 27(8):571-584. · 1.79 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    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; · 2.20 Impact Factor

Full-text (2 Sources)

Available from
Jun 6, 2014