Determining the Fate of Methane Released from the Seafloor in Deep and Shallow Water Environments



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Marine gas seeps and accidental marine oil spills are sources of methane (CH_(4)) to the ocean, and potentially to the atmosphere, though the magnitude of the fluxes and dynamics of these systems are poorly defined. For example, the ultimate capacity of aerobic CH_(4) oxidation, a process converting CH_(4) to carbon dioxide (CO_(2)) and biomass in most ocean waters, is unknown. Deeper water environments may provide a longer conduit for CH_(4) to transit before atmosphere emission and thus a higher likelihood for an oxidative fate. Shallow water environments may provide a shorter conduit for CH_(4) to transit before being emitted to the atmosphere, however, these environments often have some of the highest rates of primary production causing pCO_(2) to be undersaturated. Thus the biochemical conversion of CH_(4) to CO_(2) may not enable this released carbon to be emitted to the atmosphere. To better constrain these variables in natural environments, studies of dissolved oxygen (DO) concentration, CH4 concentration and stable isotopic ratios were conducted at two contrasting sites: the Deepwater Horizon (DWH) oil spill in the Gulf of Mexico (GoM) and the natural seep field near Coal Oil Point (COP), CA.

The investigation of 1316 DO profiles measured from 11 May until 20 September 2010 revealed the spatial and temporal variability of bulk hydrocarbon respiration in these deep and intermediate plumes since DO is removed during hydrocarbon respiration. These analyses suggest that the general movement of these plumes was toward the southwest, and that a total mass of 0.18?0.05 Tg hydrocarbon in the plume layers was fully respired to CO_(2), and 0.10?0.08 Tg hydrocarbon was incorporated into biomass (i.e. conversion efficiency 0.36?0.11 mg biomass/mg hydrocarbon). A stable isotope model incorporating measurements of CH_(4) concentrations, CH_(4) oxidation rates, and current velocity was developed to determine CH_(4) oxidation rates, as well as the flow rate from the seafloor. This model was tested on 20 samples taken from 1 to 12 km from the wellhead from 11 June through 20 June 2010 during the DWH oil spill. Results suggest that the rate of CH4 oxidation ranged from 22 to 844 nM d^(-1) in mid-June 2010 and that the rate of flow from the Macondo well was 8.4?10^(7) moles d^(-1), both of which are in agreement with previous estimates determined independently.

High-resolution measurements of sea surface CH_(4) and CO_(2) concentrations and air-sea fluxes were conducted at the COP seep field. Results suggest that the diffusive air-sea fluxes of CH_(4) and CO_(2) were 0.18?0.19 mmol m^(-2) day^(-1) and -1.65?1.23 mmol m^(-2) day^(-1), respectively, and that the extent of microbial oxidation of CH_(4) was insufficient to change this shallow water environment from a sink of atmospheric CO_(2) to a source.

The seeps at COP released CH_(4) into waters at a rate that was an order of magnitude less than that from the DWH oil spill, and resulted a plume area that was also an order of magnitude less. In total, these results suggest that microbial oxidation provides the dominant sink of the released CH_(4) at both sites.