Hydrocarbon migration pathway and methane budget for a Gulf of Mexico natural seep site: Green Canyon 600

Abstract Hydrocarbon seeps occur worldwide along continental margins and act as conduits for fluid discharge from the lithosphere to the overlying hydrosphere/atmosphere. The dynamics and rates of hydrocarbon release at cold seeps remain poorly constrained. Seepage enables a variety of processes that alter the seafloor morphology and affect the geochemistry of substrata, supporting diverse and important chemosynthetic communities. Here we merge complementary geochemical (oil fingerprinting), geophysical (deep seismic, subbottom, backscatter, multibeam) and video/imaging (Video Time Lapse Camera, DSV ALVIN video, ROV video, AUV photo surveys) data sets to constrain the pathways and magnitudes of methane fluxes from the source reservoir to the seafloor at a well-studied, prolific seep site, Green Canyon (GC) 600 in the Northern Gulf of Mexico. Oil samples from the reservoir, an active vent, and the sea-surface showed compositional similarities consistent with the plumbing system structure identified in seismic data. Spatial distribution of seep indicators such as bacterial mats, microbial communities, methane derived carbonates, and hydrate outcrops, were then used to quantify the total magnitude of methane potentially sequestered in the study domain. Using a systems approach, we combined published values for methane fluxes with data we collected across various scales and resolutions to compile a methane budget for GC600. The system considered for the methane budget is defined as the movement of methane from the salt ridge to the seafloor-water interface for each seep domain, Birthday Candles and Mega Plume. Total estimated input of methane was 2.8 × 10 8 – 2.2 × 10 9 mol/yr in the Birthday Candles domain, and 2.7 × 10 8 – 2.3 × 10 9 in the Mega Plume domain. The combined total output of the system ranged from 3.2 × 10 5 – 8.2 × 10 5 and 3.2 × 10 6 – 5.2 × 10 6 mol/yr respectively for Birthday Candles and Mega Plume domains, leaving a potential surplus (input minus output) of 2.6 × 10 8 – 2.3 × 10 9 mol/yr. Processes that could balance this budget include accumulation of gas hydrate and sediment free-gas, and the underestimated potential of biological sinks such as methane oxidation.