Sources of methane for marine gas hydrate: inferences from a comparison of observations and numerical models

The supply of methane for marine gas hydrate is commonly attributed to local conversion of organic material within the zone of stability or to migration of methane-bearing fluids from a deeper source area. However, the relative importance of these methane sources is not well known. We use numerical models for the formation of gas hydrate to assess the influence of methane supply on observable features of hydrate occurrences. Model predictions for the pore fluid chlorinity and the volumes of hydrate and free gas are compared with observations from the Blake Ridge and Cascadia Margin. We find that the observations from the Blake Ridge can be explained with a deep methane source if a saturated fluid rises with a velocity of 0.26 mm yr−1. An equally good fit to the observations is obtained by combining in situ production of methane with an upward fluid velocity of 0.08 mm yr−1, which is compatible with estimates of fluid velocity at Site 997. Observations from the Cascadia Margin can be explained with a deep methane source if the upward fluid velocity is 0.42 mm yr−1. The predicted volume of hydrate (∼2% of the pore space) is much smaller than some previous estimates and a layer of free gas (∼1% of the pore space) develops below the stability zone. Allowing for in situ production at the Cascadia Margin lowers the upward fluid velocity (0.35 mm yr−1) needed to explain the pore water chlorinity.