Measuring the interstitial methane and gas hydrate pool:Lessons from the Blake Ridge

The scientific community is rapidly becoming aware that the pool of methane within the upper kilometer of continental rise sediments is extremely large and potentially mobile. This methane occurs dissolved in the pore waters, trapped in gas hydrates, and/or as free gas bubbles within the sediment column. The dynamics of methane may play very important roles in sediment stability, global climate, and carbon storage. However, current understanding of the distribution and amounts of interstitial gases stored in marine sediments is extremely rudimentary. During the more than a quarter century of drilling by the Deep Sea Drilling Project and Ocean Drilling Program, as well as through wire line coring, vigorous gas expansion in cores from continental margins has been commonly observed. Unfortunately, very little has been learned about how much gas was initially in these sediments, because most of the interstitial gas is lost before sediment is available for sampling.

During the last few years, intensive research efforts have been directed toward assessing the microbial gas inventory in the Blake Ridge (a classic gas and gas hydrate-rich area of the continental rise off the southeast United States. In this seminar, the results of these efforts will be summarized. Traditional techniques of gas measurement (headspace gas) will be compared with other sampling techniques (pressure core sampling), geochemical proxies, downhole seismic and logging results. Also, the potential of using sulfate gradients in the upper ~20 meters as an indicator of the size of the underlying gas concentrations will be discussed.

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