The geomicrobiology of deep marine sediments from Blake Ridge containing methane hydrate (Sites 994, 995, and 997)

Abstract

Bacterial populations and activity were quantified at three sites on the Blake Ridge, Ocean Drilling Program Leg 164, which formed a transect from a point where no bottom-simulating reflector (BSR) was present to an area where a welldeveloped BSR existed. In near-surface sediments (top ~10 mbsf) at Sites 994 and 995, bacterial profiles were similar to previously studied deep-sea sites, with bacterial populations (total and dividing bacteria, viable bacteria, and growth rates [thymidine incorporation]) highest in surface sediments and decreasing exponentially with depth. The presence of methane hydrate was inferred at depth (~190-450 mbsf) within the sediment at all three sites. Associated with these deposits were high concentrations of free methane beneath the inferred base of the hydrate. Bacteria were present in all samples analyzed, to a maximum of 750 mbsf, extending the previous known limit of the deep biosphere in marine sediments by ~100 m. Even at this depth, the population was substantial, at 1.8 × 10 6 cells mL -1 . Bacterial populations and numbers of dividing and divided cells were stimulated significantly below the base of the inferred hydrate zone, which may reflect high concentrations of free gas. Localized increases in bacterial populations within the hydrate stability zone may also have been associated with free gas. Solid methane hydrate, recovered from 331 mbsf at Site 997, contained only 2% of the predicted bacterial population in a sediment from this depth, suggesting reduced bacterial populations in solid hydrate.Bacterial activity in near-surface sediments was dominated by sulfate reduction. Sulfate reduction rates and pore-water sulfate decreased rapidly with depth, concomitant with an accumulation of solid-phase sulfide in the sediment. Once sulfate was depleted (~20-30 mbsf), methane concentrations, methanogenesis, and methane oxidation all increased. Below 100 mbsf, bacterial processes occurred at very low rates. However, bacterial activity increased sharply around 450 mbsf, associated with the base of the inferred hydrate zone and the free-gas zone beneath; anaerobic methane oxidation, methanogenesis from both acetate and H 2 :CO 2 , acetate oxidation, sulfate reduction, and bacterial productivity were all stimulated (from 1.5 to 15 times), demonstrating that the sediments near and below the BSR form a biogeochemically dynamic zone, with carbon cycling occurring through methane, acetate, and carbon dioxide. At Site 995, pore-water acetate was present in surprisingly high concentrations, reaching ~15 mM at 691 mbsf, ~1000 times higher than "typical" near-surface concentrations (2-20 µM). Potential rates of acetate metabolism were extremely high and could not be sustained without influx of organic carbon into the sediment; hence in situ rates are likely to be lower than these potential rate measurements. However, there is evidence for upward migration of high concentrations of dissolved organic carbon into the sediments at these sites.Rates of acetate methanogenesis below the BSR were 2-...

Microbial methane production rates in gas hydrate-bearing sediments from the eastern Nankai Trough, off central Japan

“…Depth profile of methane production rates at each site did not show any relationship with depth profiles of alkalinity and acetate concentration at each site. The methane production rates were much lower than those previously reported in hydrate-bearing sediments from different sites, such as Cascadia Margin and Blake Ridge Wellsbury et al, 2000), although the data had been obtained by the same method. The results imply that the significant amounts of methane in gas hydrates in the Nankai Trough have not been recently produced by in situ microbial methanogenesis.…”

“…At site A, depth profile of the acetate concentration showed two peaks of 77.3 µM at 47.1 mbsf and 83.4 µM at 334.3 mbsf, just below the BSR. These acetate concentrations are above the 2-20 µM concentrations typical for near-surface sediments (Wellsbury et al, 2000) and above concentrations in other deep subsurface marine sediments (e.g., Wellsbury et al, 2002, <10 µM). Figure 3 shows the depth profiles of methane production rates estimated from tracer experiments using 14 C labeled bicarbonate and acetate.…”

“…Elevated deep acetate concentrations may be a characteristic of deep hydrate formations as they have also been found in Hydrate Ridge sediments (ODP Leg 204, Lorenson et al, 2006). Sediment heating during rapid burial and upward fluid migration have been suggested to be responsible for the increasing acetate concentrations with sediment depth (Wellsbury et al, 1997(Wellsbury et al, , 2000, and these processes may also be occurring at Nankai Trough gas hydrate sites.…”

“…Since the presence of gas hydrates is a major difference between the ODP sites and sites T and A, perhaps this is responsible for the higher rates of acetate methanogenesis at T and A sites. Wellsbury et al (2000) also showed that acetate methanogenesis was elevated in deep, gas hydratebearing sediments of Blake Ridge (ODP Leg 164), and like sites T and A, pore water acetate concentrations were elevated at depth (up to 15 mM and acetate methanogenesis 1.2 nmol/cm 3 /d) above the typical nearsurface concentrations of 2-20 µM. Elevated deep acetate concentrations may be a characteristic of deep hydrate formations as they have also been found in Hydrate Ridge sediments (ODP Leg 204, Lorenson et al, 2006).…”

Methane Hydrate Production From Alaskan Permafrost

“…Culturing and molecular studies have demonstrated the abundance and diversity of bacteria within deep layers of deep-sea sediments (Wellsbury, Goodman et al 2000;Marchesi, Weightman et al 2001;Inagaki, Sakihama et al 2002). Bacterial abundance in marine sediments as determined University of Alaska Anchorage DE-FC26-01NT41331 Appendix D…”