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/cm 3 /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/cm 3 /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.

Bacterial profiles in Amazon Fan sediments, Sites 934 and 940

“…Additionally, it is difficult to account for such consistent increases in total bacterial and DDC numbers to levels not encountered since 50 mbsf by contamination . Similar increases in bacterial numbers at great sediment depth that are associated with increases in sulfate or methane concentrations have been observed at other ODP sites (Cragg et al, , 1995bParkes et al, 1990).…”

“…2). This relationship has been commonly observed in Pacific Ocean sites Cragg et al, 1992;1995a, 1995bCragg and Parkes, 1994;Cragg and Kemp, 1995). Between 70 mbsf and the last sample at 108 mbsf, DDC were detected only once at 89 mbsf.…”

“…Recent Ocean Drilling Program (ODP) research on marine sediments (Whelan et al, 1986;Tarafa et al, 1987;Parkes et al, 1990Parkes et al, , 1994Cragg and Parkes, 1994;Cragg and Kemp, 1995;Cragg et al, 1990Cragg et al, , 1992Cragg et al, , 1995aCragg et al, , 1995b has confirmed the presence of a deep bacterial biosphere in marine sediments. Previously, this had only been predicted from the extensive amount of indirect geochemical evidence, that is, chemical changes in pore water, gas production, kerogen modification, concretion formation, and isotopic evidence (Krumbein, 1983;Suess and Whiticar, 1989;Kvenvolden and Kastner, 1990;Kastner et al, 1991).…”

“…The thermal gradient of the Earth's crust is ~10°−40°C/km, therefore, in most sediments temperature is unlikely to inhibit bacterial activity until several kilometers below the seafloor as hyperthermophilic bacteria may grow in excess of 110°C (Huber et al, 1989;Pledger and Baross, 1991;Jørgensen et al, 1992). Deep sediment bacterial populations decrease exponentially with depth and have been shown to be responsive to deep geochemical changes in the sediment, such as brine incursions Parkes et al, 1990) and the presence of methane and hydrogen sulfide gas hydrates (Cragg et al, 1995b;1996). Additionally, bacterial sulfate reduction and methanogenic activity has been measured in sediments from depths greater than 500 mbsf Getliff et al, 1992;Parkes et al, 1995).…”

Microbial abundance in lacustrine sediments: a case study from Lake Van, Turkey

“…The reasons can be diverse, e.g. rapid changes in sedimentary organic matter content due to changes in palaeoproductivity and/or preservation like in sapropel layers of the Mediterranean (Coolen et al 2002), due to exceptionally high heat flow in the Nankai Trough (Moore et al 2001), where cell concentration increases again at greater depth due to in situ thermogenic generation of microbial substrates (Horsfield et al 2006) or at the base of continental margins, where mass-wasting events alter sediment accumulation rates (Cragg et al 1995).…”