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Abstract: We report the results of an experiment in which we measured 222Rn (15,000 observations), CH4 (40,000 observations), and associated variables in seawater nearly continuously at a coastal site in the Gulf of Mexico for almost two years. Significant correlations between 222Rn and CH4 imply that they are derived from a common source, most likely groundwater. However, we were unable to explain the overall tracer variability as a single function of groundwater table height, temperature, tidal range, and wind speed, indicating multiple, overlapping controls on SGD dynamics at this site. Methane and radon concentrations may vary 2-fold in a given well in the subterranean estuary over tidal time scales, demonstrating the complexity of determining SGD endmember concentrations and suggesting that unaccounted for temporal changes in groundwater may explain some of the patterns observed in seawater. Surprisingly, the variability of 222Rn and CH4 in seawater over short (e.g., hourly) time scales was generally comparable to or even more pronounced than fluctuations over much longer (e.g., monthly) scales. While high tracer concentrations usually occurred during low tide and low tracer concentrations during high tide, this pattern was occasionally inverted or absent indicating that no single model can be used to describe the entire data set. We also describe a sequence of events in which SGD tracers were depleted in coastal waters during storms and regenerated afterwards. We found no increase in radon activities immediately after the largest storm (75 mm rainfall) perhaps because of the short residence times of groundwater in contrast to the ingrowth time of radon. Marine controls appeared to be the most important SGD drivers with only minor influence relating to the shallow and deep aquifers. This implies that seasonal investigations of SGD tracers in the coastal ocean may be masked by short-term variability. [Copyright &y& Elsevier]

Porewater flow at cold seeps controls methane oxidation and carbonate precipitation rates, and also plays an important role in gas hydrate dynamics. We constrain the porewater flow rates in the upper sediment of Gulf of Mexico cold seeps by fitting measured 226Ra and Cl− profiles. The approach exploits their natural increase with depth through shallow sediments at non-brine (226Ra) and brine (Cl−) seeps. Vertical flow velocities at non-brine and brine seeps are found to be similar in magnitude, typically on the order of 0.1 m y−1. The accuracy of the flow estimates is assessed and shown to depend substantially on uncertainties in both concentrations and sediment tortuosity. Model simulations further reveal that the natural heterogeneity of sediment permeability can cause considerable lateral variability in vertical flow at the scale of a sediment core, which implies that the coring location and core size can have a substantial effect on the estimated porewater velocities. The formation of gas hydrates and calcium carbonates reduce sediment permeability, and decrease the spatial variability in flow over time, as the formation rates are higher along preferential flow paths. This analysis of the effect of spatiotemporal variability on chemical profiles improves the assessment of uncertainties in the estimated magnitude of flow, and is important for the evaluation of benthic-pelagic coupling at seeps. [ABSTRACT FROM AUTHOR]

In late July 2013, the Hercules 265 drilling rig in the Northern Gulf of Mexico experienced a catastrophic loss of control. Large quantities of natural gas spewed into the environment for ~2 days before the well self-sealed through down-hole collapse below the seafloor. Ecosystem Impacts of Oil and Gas Inputs to the Gulf (ECOGIG) and collaborating Gulf of Mexico Research Initiative (GoMRI) consortia mounted a rapid response cruise to characterize the waters around the Hercules 265 rig, beginning just 4 days after the blowout. Our analysis showed an immediate microbial response to the elevated concentrations of methane in the water column, as evidenced by the drawdown of oxygen to hypoxic conditions, the incorporation of methane-derived carbon into particles, and measurable rates of methane-assimilation and nitrogen-fixation. Additionally, radium isotope measurements allowed us to constrain the timescale of bottom water exposure to the influence of the rig. A second sampling by the Center for Integrated Modeling and Analysis of Gulf Ecosystems (C-IMAGE) consortium indicated that the ecosystem had returned to near pre-blowout conditions within one month. [ABSTRACT FROM AUTHOR]