BACTERIAL-POPULATIONS AND PROCESSES IN SEDIMENTS CONTAINING GAS HYDRATES (ODP LEG-146 CASCADIA MARGIN)

Bacterial populations and activity were quantified at three sites in t he Cascadia Margin accretionary wedge, off the West Canadian/American coast (ODP Leg 146). At two sites sediments contained gas hydrates, Si te 889/890 had a discrete zone of hydrate approximately 10 m above a b ottom simulating reflector (BSR) at 225 m below sea floor (mbsf) and S ite 892 had disseminated hydrate in the upper 20 mbsf and a BSR at 74 mbsf. Site 888 was a control site without gas hydrates. The control si te (888) and top approximately 90 mbsf of Site 889/890 had bacterial d istributions similar to previous Pacific Ocean sites. In the upper app roximately 30 m of Site 892, however, bacterial populations were much lower, suggesting inhibition by the high concentrations of H2S within the hydrate zone. Below this depth bacterial populations rose to conce ntrations consistent with other sites. The control site was dominated by SO4 reduction and rates of CH4 oxidation in the top 90 m were low ( 0.002-0.033 nmol cm(-3) d(-1)). At Site 889/890 bacterial populations and activity were stimulated in the discrete hydrate zone. CH4 oxidati on rates increased in the middle of this zone to 134.5 nmol cm(-3) d(- 1) (ca. 9 times the average rate at other depths), resulting in a sign ificant (x 10) increase in the total bacterial population. The anaerob ic process(es) responsible for this oxidation remain unclear, despite SO4-reducing bacteria, previously associated with CH4 oxidation, also being stimulated in this zone. Fluid flux into accretionary wedge sedi ments may be an important process in providing electron accepters to m aintain these relatively high rates of CH4 oxidation. This first micro biological study of gas hydrates indicates that bacterial processes ar e influenced by gas and fluid venting, and they play a major role in g eochemical changes within these deep (> 200 mbsf) sediments.