The first aspect is the problem of how to gain a better understanding of the distribution and abundance of gas hydrates in continental slope sediments at the present day. Mark Maslin (University College London) and Nick Langhorne (Office of Naval Research, USA) both emphasized that while the potential distribution of hydrates can be defined, based on the pressure and temperature limits of their stability range and the thicknesses of sediments presently located within that stability range, the actual occurrence of hydrates is less well known and so there is a wide range of uncertainty (about an order of magnitude) in estimates of global marine gas hydrate abundance. Furthermore, there is a large uncertainty regarding the amount of gas hydrates in the second major reservoir, the Arctic permafrost, and nothing is known about storage of gas hydrates in Antarctic permafrost. A particular problem for the marine hydrates, noted by Maslin and also Doug Masson (National Oceanographic Center, NOC), is that even where evidence of hydrate occurrence is found, for example in the form of bottom simulating reflectors (BSRs) in seismic profiles, the proportion of sediment pore space actually occupied by hydrate is still undefined. Expanding on this theme, Maslin asked for opinions on whether BSR occurrence was a good proxy for the occurrence of gas hydrates in continental slope sediments. Replying to this, Masson commented that while positive identification of a BSR was a valid indicator of gas hydrates overlying free gas in pore spaces below the BSR, its apparent absence was not a good indicator of the absence of gas hydrates: for example, the hydrate–gas interface might be parallel to bedding and so much less obvious in seismic profiles. Masson also noted that it is not known how much gas hydrate is required to produce a gas-impermeable cap layer within sediments and thus lead to the formation of a BSR as gas accumulates beneath it. Both Maslin and Russell Wynn (NOC) commented that this lack of knowledge reflects a need for in situ investigations and sampling of potentially gas hydrate-bearing sediments to move beyond regions of high gas hydrate abundance (such as the Cascadia--Alaska continental margin) to study other Report of an open discussion session at the 3rd Johnston-Lavis Colloquium, University College London, 15-17 September 2009.
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