The Deepwater Horizon event resulted in suspension of oil in the Gulf of Mexico water column because the leakage occurred at great depth. The distribution and fate of other abundant hydrocarbon constituents, such as natural gases, are also important in determining the impact of the leakage but are not yet well understood. From 11 to 21 June 2010, we investigated dissolved hydrocarbon gases at depth using chemical and isotopic surveys and on-site biodegradation studies. Propane and ethane were the primary drivers of microbial respiration, accounting for up to 70% of the observed oxygen depletion in fresh plumes. Propane and ethane trapped in the deep water may therefore promote rapid hydrocarbon respiration by low-diversity bacterial blooms, priming bacterial populations for degradation of other hydrocarbons in the aging plume.

We report a new method for methane oxidation rate measurements that uses 10 3 -10 5 times less 14 C-CH 4 than existing measurements by taking advantage of the high sensitivity of accelerator mass spectrometry. Methane oxidation in the marine environment is a microbial process of global importance because it prevents methane released from underlying reservoirs from reaching the ocean and atmosphere. Rate measurements provide a crucial tool for assessing the efficacy of this process across a range of environments, but the current methods use high amounts of radioactive elements ( 3 H-or 14 C-CH 4 ), tend to increase methane concentrations in a sample markedly over in situ levels, and are limited by strict health and safety regulations. The low-level method presented here uses levels of 14 C-CH 4 that are below transportation regulations, produce samples that do not require treatment as radioactive waste, and allow for tracer level rate measurements in low methane environments. Moreover, the low-level method lays the analytical foundation for a below-regulation rate measurement that could be used broadly and in-situ. Parallel rate measurements with the low-level 14 C-CH 4 and existing 3 H-CH 4 methods are generally consistent with a correlation coefficient of 0.77. However, the low-level method in most cases yields slower rates than the 3 H method possibly due to temperature, priming, and detection limit effects.*Corresponding author: E-mail: mpack@uci.edu AcknowledgmentsWe thank the officers and crew of the R/V Atlantis for their support at sea,

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We report methane (CH 4 ) concentration and methane oxidation (MO x ) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low-CH 4 waters and a depth interval (~200-760 m) of CH 4 supersaturation that is located within a regional oxygen minimum zone (OMZ). MO x rate measurements were made in parallel using tracer-based methods with low-level 14 C-CH 4 (LL ). Priming and background effects associated with the 3 H-CH 4 tracer and LL 14 C filtering effects are implicated as the cause of the systematic difference. The MO x rates reported here include some of the slowest rates measured in the ocean to date, are the first rates for the ETNP region, and show zones of slow CH 4 turnover within the OMZ that may permit CH 4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH 4 and oxygen concentrations, suggesting that their combined availability regulates MO x rates in the region. Depth-integrated MO x rates provide an upper limit on the magnitude of regional CH 4 sources and demonstrate the importance of water column MO x , even at slow rates, as a sink for CH 4 that limits the ocean-atmosphere CH 4 flux in the ETNP region.

Methane concentrations and turnover rates were measured throughout the water column at nine stations in the Santa Monica Basin (SMB), at one station in the San Pedro Basin (SPB), and at one station in the Santa Catalina Basin (SCtB) in July 2007 and September 2009. Methane concentrations were elevated throughout the water column, with subsurface (4-15 nmol L 21 ), midwater (7.5-100 nmol L 21 ), and bottom-water (5-242 nmol L 21 ) maxima. The SMB water column was divided into four depth-dependent regimes, based on temperature-salinity relationships and patterns in methane concentration and fractional turnover rate. We propose that distinct physical controls on methane dynamics distinguish these regimes. In the upper water column, methanotrophic activity appears to be controlled by borderland-scale, seasonal fluctuations in circulation. The midwaters represent a transition regime between northern-sourced shallow waters and southern-sourced deep waters, wherein methanotrophic activity decreases with depth to its minimum at 500 m, coinciding with the midwater methane maxima. Below 500 m, methanotrophic activity increases to the sill depth of 737 m, coincident with increasingly restricted circulation of water. Bottom waters are restricted from through-basin circulation. These waters were oxygen depleted, but methane input sustained an active psychrotolerant methanotrophic community and had some of the fastest aerobic methane oxidation rates yet reported in the marine environment.