Potential methane production in sediments from the Cascadia Margin, IODP Expedition 311

In many cases methane in natural gas hydrates was interpreted to have been produced through microbial methanogenesis. Some fundamental questions, for example, where microbial methane was produced or how to form the hydrates, still remain unsolved. Expedition 311 of Integrated Ocean Drilling Program (IODP) investigated gas hydrate in the accretionary prism of the Cascadia margin. We participated the Expedition 311 and obtained core samples from 5 drilling sites. The four sites transected across the Cascadia Margin and represented different stages in the evolution of gas hydrate across the margin from the earliest occurrence on the westernmost, first-accreted ridge (U1326) to its final stage at the eastward limit of gas-hydrate occurrence on the margin in shallow water (U1326). The fifth site (U1328) was at a nearby cold vent with active fluid and gas flow. We conducted culture experiments of the sediment cores under near in-situ temperature and tracer experiments using 14-C labeled bicarbonate and acetate in order to estimate potential methane production in the region and to elucidate which methanogenesis pathways were employed for methane production. Results of the culture experiments showed methane production would have occurred in the samples from gas hydrate zone and the deeper sediments below the BSRs. Sediments from near surface sediments did not show methane production. At the site of U1328, where gas hydrate was observed from surface to 50-m below sediment, culture experiments showed no methane production in the shallower sediments, but some methane production was detected in the deeper sediments. Results of the tracer experiments showed both carbonate reduction and acetate fermentation pathway would be employed for methane production. We detected activity of methanogenesis in the sediments from the shallower sediments, as well as the samples from gas hydrate zone and the deeper sediments. These results would provide an important clue for constraining formation model for gas hydrate.