Probable patterns of gas flow and hydrate accretion at the base of the hydrate stability zone

Marine gas hydrate is the largest carbon reservoir in the global organic carbon cycle, but there is limited knowledge of how hydrate is accreted in space and time. Three-dimensional seismic imaging of the dipping base of the deep-water marine gas hydrate from offshore Mauritania reveals extraordinary patterns of vertical chimneys and connected teardrop-shaped trails of both high and low seismic reflection amplitudes. The high-amplitude trails are interpreted as being caused by the downward transition from hydrate- to free gas-bearing sediments. Their teardrop form shows that gas emanating from the chimneys flowed updip along the base of the hydrate. The geometrically similar, lower-amplitude trails are possibly earlier flows that may have already converted to hydrate. For this area we propose a model of intermittent flow of gas to the base of the hydrate. Active flows were blocked updip by earlier, probably hydrate-clogged chimneys, and may have been laterally confined by flows that had already converted to hydrate that were in their path. The process of hydrate formation reduces sediment permeability and may suppress subsequent gas flows, resulting in the emergence of patterns of gas flow and hydrate accretion.