Volumetric Mapping of Methane Concentrations at the Bush Hill Hydrocarbon Seep, Gulf of Mexico

Abstract

The role of methane as a green-house gas is widely recognized and has sparked considerable efforts to quantify the contribution from natural methane sources including submarine seeps. A variety of techniques and approaches have been directed at quantifying methane fluxes from seeps from just below the sediment water interface all the way to the ocean atmosphere interface. However, there have been no systematic efforts to characterize the amount and distribution of dissolved methane around seeps. This is critical to understanding the fate of methane released from seeps and its role in the submarine environment. Here we summarize the findings of two field studies of the Bush Hill mud volcano (540 m water depth) located in the Gulf of Mexico. The studies were carried out using buoyancy driven gliders equipped with methane sensors for near real time in situ detection. One glider was equipped with an Acoustic Doppler Current Profiler (ADCP) for simultaneous measurement of currents and methane concentrations. Elevated methane concentrations in the water column were measured as far away as 2 km from the seep source and to a height of about 100 m above the seep. Maximum observed concentrations were ∼400 nM near the seep source and decreased away steadily in all directions from the source. Weak and variable currents result in nearly radially symmetric dispersal of methane from the source. The persistent presence of significant methane concentrations in the water column points to a persistent methane seepage at the seafloor, that has implications for helping stabilize exposed methane hydrates. Elevated methane concentrations in the water column, at considerable distances away from seeps potentially support a much larger methane-promoted biological system than is widely appreciated.

Fate of Methane Released From a Destroyed Oil Platform in the Gulf of Mexico

“…In the two erosion craters, the ROV sonar survey showed that bubbles were released from several individual vents (Figure 7). Similar venting has been reported from natural seeps (De Beukelaer et al, 2003;Johansen et al, 2014;Sahling et al, 2016;Johansen et al, 2020;Meurer et al, 2021). Under such circumstances, bubble density would be locally variable.…”

“…Hu et al (2012), who collected pumped water samples from the surface interface, failed to confirm this result at Bush Hill and a second ~1,000 m seep. Meurer et al (2021), sampling with MET sensors deployed on gliders over Bush Hill, measured methane concentrations of up to 0.4 μM/L, well below the observations of Solomon et al (2009). Notably, results from the work of Yvon-Lewis et al (2011), using techniques to similar to those used by Hu et al (2012) and Ryerson et al (2011), using airborne measurements, suggested that oil reaching the surface (~3,000 m 3 /d) from the Deepwater Horizon oil spill, which leaked from 4,500 m depth, was a negligible source of methane to air.…”

Physical Controls on the Creation and Persistence of Natural Marine-Seepage Slicks

“…The model can be used to predict assuming oil droplets only, oil-coated gas bubbles only, or both droplets and bubbles. For example, ADCP profiles of the water column above the Bush Hill seep, north-central Gulf of Mexico (Meurer et al, 2021), can be used to model the expected OSO locations for the time period represented by the data (Figure 9A, 9B). The spread of model OSOs uses the bounding ADCP profiles for each day and allows a 35% correlated, relative standard deviation for the Monte Carlo simulations.…”

Reply on RC2

“…However, the Pro Oceanus Mini Pro CO2 sensor used at the time did not withstand the pressure changes imposed by glider missions. The Franatech METS CH4 sensor has been integrated into Alseamar SeaExplorer and Teledyne Slocum gliders and successfully used to generate concentration maps of a methane seep in a semi-quantitative way (Meurer et al, 2021).…”