The Impact of Fluid and Gas Venting on Bacterial Populations and Processes in Sediments from the Cascadia Margin Accretionary System (Sites 888–892) and the Geochemical Consequences

Abstract

Bacterial populations and activity were quantified at four Ocean Drilling Program Leg 146 sites in the Cascadia Margin accretionary wedge, off the west Canadian/American coast. At two sites the sediments contained gas hydrates: Site 889/890 had a discrete hydrate zone for -10 m above the bottom-simulating reflector (BSR) at 225 m below the sea floor (mbsf), and Site 892 had disseminated hydrate in the top -20 mbsf and a BSR at 73 mbsf. Site 891 had fault zones and discrete zones of active fluid venting. The other site, 888, was a control site without gas hydrates or substantial fluid venting. Bacterial populations were present at all sites with deepest samples from Site 888 at 564 mbsf. The control Site 888 and the top -90 mbsf of Site 889/890 had bacterial distributions similar to previous sites studied in the Pacific Ocean. In the top -20 m of Site 892, however, bacterial populations were much lower than in the other two sites; they may have been inhibited by the high concentrations of hydrogen sulfide within the hydrate zone. Below this depth, bacterial populations increased to concentrations consistent with other sites, apart from local decreases in Hole 892A associated with thermally generated aromatic hydrocarbons, which might be toxic to a portion of the bacterial community. Near-surface bacterial populations at 891A were -4.5 times higher than expected, probably reflecting fluid venting near this site. Below about 50 mbsf at this site (Hole 891B), bacterial populations were also elevated and peaks corresponded to geochemical anomalies resulting from fluid venting. Bacterial processes generally decreased with increasing depth at the control site, which was dominated by sulfate reduction, and rates of methane oxidation in the top 90 m were low, ranging from 0.002 to 0.033 nmol/cm3/d. Fluid and gas venting have a marked effect on deep bacterial processes at Sites 891 and 889/890: in contrast to the control site, maximum bacterial activities occur and bacterial populations are elevated at depth (sub-200 mbsf), reflecting marked increases in methane oxidation. At Site 889/890, bacterial populations and activity were stimulated in the discrete hydrate zone. Methane oxidation rates increased in the middle of this zone to 134.5 nmol/cm3/d, and this resulted in a significant increase in the total bacterial population. The anaerobic process(es) responsible for methane oxidation remain unclear, but fluid flux into accretionary wedge sediments may be an important process in providing electron acceptors to maintain these high rates of methane oxidation. Variation in bacterial profiles between cores at the same site (891 and 892), only a few meters apart, closely reflect spatial heterogeneity in geochemistry and fluid expulsion in these accretionary sediments, demonstrating that bacterial processes are a sensitive index for the fluid and gas venting that may drive many of the geochemical changes.