02154nas a2200349 4500000000100000000000100001008004100002260000800043653004600051653001400097653002300111653000900134653000900143653000900152653001000161653008500171653006400256100001500320700002300335700002100358700001400379700001500393700001800408700002400426700001800450700001800468245005700486856004300543490000700586520119700593020001401790 2006 d bAGU 10 a1615 Global Change: Biogeochemical cycles 10 aprocesses 10 aand modeling (0412 10 a0414 10 a0793 10 a4805 10 a4912) 10 a3002 Marine Geology and Geophysics: Continental shelf and slope processes (4219) 10 a3004 Marine Geology and Geophysics: Gas and hydrate systems 1 aAngus Best 1 aMichael Richardson 1 aBernard Boudreau 1 aAlan Judd 1 aIra Leifer 1 aAnthony Lyons 1 aChristopher Martens 1 aDanial Orange 1 aSimon Wheeler 00 aShallow seabed methane gas could pose coastal hazard uhttp://dx.doi.org/10.1029/2006EO220001 0 v87 3 aAbnormally high levels of methane gas in seafloor sediments could pose a major hazard to coastal populations within the next 100 years through their impact on climate change and sea level rise. Marine scientists have known for many years that biogenic methane (CH4) is generated in shallow seabed sediments on continental margins, especially in rapidly deposited muddy sediments with high organic matter content (see Methane Flux Control in Ocean Margin Sediments (METROL) project in Mienert et al., [2004]).Gassy sediments are found in river deltas, estuaries, and harbors, but also in deeper waters on continental shelves and slopes. Human activities can accelerate natural seafloor gas generation by increasing the supply of sediments and organic matter from rivers through deforestation and intensive farming, and also by the disposal of human waste at sea. When this extra organic matter becomes buried to about one meter beneath the seabed, biogeochemical processes start to convert it to CH4 [Floodgate and Judd, 1992]. The impact of this extra CH4 could be felt within the next 100 years, assuming a one-centimeter-per-year sediment accumulation rate. a0096-3941