The large fossil methane (CH4) discharged from the destroyed Deepwater Horizon rig in the Gulf of Mexico (between 2-100 × 106 moles of CH4 per day) provides a unique opportunity to determine the impact of methane on dissolved oxygen concentrations and assess its burden on the atmosphere. Without considering ebullition effects, preliminary calculations indicate that a 10% reduction of dissolved oxygen can be achieved in the low oxygen zones of the Gulf of Mexico in 4 - 190 days at the estimated methane fluxes.
With funding from this NSF Rapid Response Research (RAPID) award, researchers from Texas A&M University in collaboration with a scientist from Yale University, will analyze water column samples for methane concentrations and stable isotopes, as well as measure the air-sea flux of methane, ethane, propane, carbon dioxide and del13C-CO2. In addition, dissolved oxygen measurements, methane oxidation rates, and the effect of microbial community structure on methane, oil, and oxygen concentrations will be assessed. Results will be used to test the following hypotheses: (1) Significant and quantifiable amounts of methane released rapidly from naturally decomposing oceanic clathrate hydrates will be both dissolved in the water column and emitted to the atmosphere, and (2) The oxidation and ebullition of methane in the water column will significantly contribute to the low oxygen zones in the northern Gulf of Mexico. Even though geochemical data has shown significant clathrate decomposition in the past, little, if any, information is available of whether methane released during these events entered the atmosphere or was retained in the ocean. This deepwater anthropogenic spill of oil and methane into the Gulf of Mexico can be used as an analog for a natural rapidly decomposing clathrate hydrate and provide much needed information on the fate of methane released during these past events.
The broader impacts of this proposal include the involvement of two graduates and one undergraduate student, as well as yield insights into the impact of the massive fossil methane released to the Gulf of Mexico by the Deepwater Horizon rig.
The Deepwater Horizon (DWH) hydrocarbon spill in the Gulf of Mexico was a tragic disaster on both human and environmental levels. Of the hundreds of hydrocarbons emitted from the DWH incident, methane (CH4) was significantly the most abundant. With measurements of air-sea flux, airplane over-flights, reservoir fluid samples, and water column investigations by ourselves and others, the release rate of DWH CH4 to the atmosphere was determined to be negligible (e.g. 1, 2); instead, CH4 was retained dissolved and suspended in neutrally bouyant "plume" or "intrusion" layers approximately 1000m below the sea surface. Since the largest natural global reservoir of CH4, a potent greenhouse gas, resides below the seafloor, and significant geological evidence exists suggesting this reservoir can participate in global climate change, this disaster established a natural laboratory to investigate the microbial response to large water column perturbations of CH4. Our initial measurements of CH4 oxidation were conducted in June 2010 onboard the R/V Cape Hatteras and displayed a few CH4 samples with high specific rates (k ≈ 0.1 day-1). However the majority of samples displayed slow rates (k ≈ 0.001 – 0.0001 day-1) similar to typical deep ocean values (3). Additionally, the microbial community structure was dominated by organisms known to consume oil not CH4 (3). This, and the fact that typical rates of aerobic CH4 oxidation in the deep ocean are relatively slow, led to our hypothesis that DWH CH4 would persist in the Gulf of Mexico for years. Our next three expeditions to the DWH disaster site spanned the end of August through the beginning of October 2010. Onboard the NOAA Ship Pisces (R 226), we identified the chemical fingerprints of the DWH plume namely hydrocarbon fluorescence, dissolved oxygen loss, and dioctyl sodium sulfosuccinate (DOSS; a key ingredient of the dispersant added at the wellhead that was shown to trace the plume) but surprisingly no CH4. We revised our hypothesis to DWH CH4 was completely respired by August 2010 and tested that hypothesis with 4 experiments. First, an exhaustive search for CH4 within and outside chemical indicators of the plume layers, as well as vertically above and below, produced concentrations that were mainly at or below background, with only a few samples slightly above background 1.4 ± 2.0 nM (maximum = 20.4 nM; n = 671). While limited pockets of CH4 potentially 2-5x above background may have existed at this time, our comprehensive search suggests that the ~100,000-fold higher levels observed in the immediate aftermath of the DWH event had largely been removed from the system. The second test of our hypothesis was to determine whether the observed oxygen drawdown was consistent with the hypothesized oxygen requirement for a respiration sink of CH4. The resulting total DO anomaly was sufficient to account for the respiration of all CH4, as well as most of the additional hydrocarbons, released from the DWH event. A third test of our hypothesis was to investigate if the plume water was enriched in populations of methanotrophic bacteria relative to samples collected earlier in the spill, which it was. Significant proportions of the bacteria that were identified had previously been linked to CH4 oxidation through stable isotope probing experiments. A fourth test of our hypothesis was to model the specific CH4 oxidation rates (a.k.a. the pseudo first-order CH4 oxidation rate constants) that occurred between June and September 2010 and compare then to values measured previously. The maximum modeled rate constant (k ≈ 0.2 day-1) was only slightly higher than previously measured values and this slight difference is expected given the much larger initial CH4 concentration. The model also predicts an exponential increase in rate constants in the middle of June 2010, similar to our observations as the more labile substrate ethane was consumed (3, 4). The results of these tests suggest a bloom of methanotrophic bacteria that experienced all stages of microbial growth, limited ultimately by the availability of CH4. References 1. Ryerson et al., Proceedings of the National Academy of Sciences, (2012). 2. S. A. Yvon-Lewis, L. Hu, J. D. Kessler, Geophysical Research Letters 38, (2011). 3. D. L. Valentine et al., Science 330, 208 (2010). 4. J. D. Kessler et al., Science 331, 312 (2011).
