Bacterial populations and processes in sediments containing gas hydrates (ODP Leg 146: Cascadia Margin)

Abstract

Bacterial populations and activity were quantified at three sites in the Cascadia Margin accretionary wedge, off the West Canadian/American coast (ODP Leg 146). At two sites sediments contained gas hydrates, Site 889/890 had a discrete zone of hydrate approximately 10 m above a bottom simulating reflector (BSR) at 225 m below sea floor (mbsf) and Site 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 site (888) and top approximately 90 mbsf of Site 889/890 had bacterial distributions similar to previous Pacific Ocean sites. In the upper approximately 30 m of Site 892, however, bacterial populations were much lower, suggesting inhibition by the high concentrations of H 2S within the hydrate zone. Below this depth bacterial populations rose to concentrations consistent with other sites. The control site was dominated by SO 4 reduction and rates of CH 4 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. CH 4 oxidation 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 significant (× 10) increase in the total bacterial population. The anaerobic process(es) responsible for this oxidation remain unclear, despite SO 4-reducing bacteria, previously associated with CH 4 oxidation, also being stimulated in this zone. Fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these relatively high rates of CH 4 oxidation. This first microbiological study of gas hydrates indicates that bacterial processes are influenced by gas and fluid venting, and they play a major role in geochemical changes within these deep (> 200 mbsf) sediments.