Modeling Fluid Flow in a Low Flux Methane Hydrate Province: A Multifaceted Geophysical Approach

Although the Blake Ridge Diapir cold seep (ODP Site 996) has long been recognized as site where localized, anomalously high fluid flux occurs, previous flux estimates based on geochemical data are too low to reconcile with the observation of seafloor gas emission and the survival of a chemosynthetic community. In this study, we describe models of both the background fluid flux (away from the diapir) and fluid flux along the vertical fluid conduit located directly above the salt intrusion and constrain the results using high-resolution multichannel seismic (MCS) images, coincident heat flow data, and pore water geochemistry. Analysis of modeling results shows that the background fluid flux at Blake Ridge Diapir in the far-field is dominated by a conductive thermal regime and low fluid flux, similar to the outer Blake Ridge hydrate province, and that dissolution of the salt diapir is not an important process controlling the thickness of the hydrate stability zone. However, along the vertical fluid conduit directly beneath the seep, our model estimates significantly greater fluid flux than previously suggested. Specifically, we infer fluid flux of 40 to 1700 m/ky in this chimney, at least 100 times larger than published estimates and consistent with fluxes that sustain chemosynthetic communities in other gas hydrate provinces. Though the chimney appears as a single sediment disruption event in the MCS data, new three-dimensional seismic images from chirp data reveal with remarkable clarity the structural complexity of shallow gas chimney fluid-conduits . The shallow 3D images demonstrate the limits of 2D fluid flow modeling by revealing how even in apparently simple systems, fluid flow may occur in complex, three-dimensional plumbing systems.