Limitations of microbial hydrocarbon degradation at the Amon Mud Volcano (Nile Deep Sea Fan)

The Amon mud volcano (MV), located at 1250 m water depth on the Nile Deep Sea Fan, is known for its active emission of methane and non-methane hydrocarbons into the hydrosphere. Previous investigations showed a low efficiency of hydrocarbon-degrading anaerobic microbial communities inhabiting the Amon MV center in the presence of sulphate and hydrocarbons in the seeping subsurface fluids. By comparing spatial and temporal patterns of in situ biogeochemical fluxes, temperature gradients, pore water composition and microbial activities over three years, we investigated why the activity of anaerobic hydrocarbon degraders can be low despite high energy supplies. We found that the central dome of the Amon MV, as well as a lateral mud flow at its base, showed signs of recent exposure of hot subsurface muds lacking active hydrocarbon degrading communities. In these highly disturbed areas, anaerobic degradation of methane was less than 2% of the methane flux. Rather high oxygen consumption rates compared to low sulphide production suggest a faster development of more rapidly growing aerobic hydrocarbon degraders in highly disturbed areas. In contrast, the more stabilized muds surrounding the central gas and fluid conduits hosted active anaerobic hydrocarbon-degrading microbial communities. Furthermore, within three years, cell numbers and hydrocarbon degrading activity increased at the gas-seeping sites. The low microbial activity in the hydrocarbon-vented areas of Amon mud volcano is thus a consequence of kinetic limitations by heat and mud expulsion, whereas most of the outer mud volcano area is limited by hydrocarbon transport.

(Table 1) AMS radiocarbon ages from ODP Sites 169-1033 and 169-1034

This paper explores the paleoseismic record potentially preserved in the upper 40 m of hydraulic piston cores collected in 1996 at two sites in Saanich Inlet, British Columbia, during ocean drilling program (ODP) Leg 169S. The ODP cores are missing 1-2 m of water-rich sediment directly underlying the seafloor, but this sediment is preserved in shorter piston cores collected in 1989 and 1991. The upper part of the ODP cores consists of rhythmically laminated (varved) marine mud with intercalated massive beds, interpreted to be debris flow deposits. Some of the debris flow deposits are linked to past earthquakes, including the 1946 Vancouver Island earthquake (M7.2), a great (M8-9) plate-boundary earthquake at the Cascadia subduction zone in January 1700, and a large crustal or plate-boundary earthquake about 1000 yr ago. Earthquakes may also be responsible for debris flows in about AD 1600, 1500, 1250, 1150, 850, 450, 350, 180, and BC 200, 220, 500, 900, and 1050. If so, the average recurrence interval for moderate to large earthquakes, which trigger debris flows in Saanich Inlet, is about 150 yr. This recurrence interval is broadly consistent with the frequency of moderate to large earthquakes in the region during the historical period. Debris flows, however, can also be triggered by non-seismic processes, making it difficult to assemble a complete earthquake record from the Saanich Inlet cores. We propose that extensive debris flow deposits, emplaced by single large failures or many smaller coincident failures, probably have a seismic origin.

(Table 2) Physical properties at DSDP Holes 44-391 and 76-534

Laboratory measurements on sediment samples and density well logs run at DSDP Site 534 in the Blake-Bahama Basin were used to establish an in situ velocity and density structure. Synthetic seismograms were generated for comparison to reprocessed seismic reflection data in the vicinity of the Site. Uncertainties in the relative positions of the hole and seismic reflection data, velocity corrections, and the composition of the unrecovered section were evaluated. In light of the errors and compressed section, no unique correlation of the seismic reflection data to the drill hole is completely defensible either in this chapter or elsewhere. The preferred correlation resulting from this exercise is as follows, with the Site 534 report correlation shown in parentheses where different. Horizon beta', 887 m; Horizon beta, 950 m (975 m); Horizon C , 1202 m (1250 m); Horizon C, 1268 m (1340 m); Horizon D', 1342 m (1432 m); Horizon D, 1550 m (1552 m). The major differences in these correlations arise from the use of slightly different velocities and hole location relative to the seismic profiles. The Site 534 report results rely on hole placement on a basement flank, whereas in this chapter we locate it within a basement depression still within the uncertainty of the navigation. The Site 534 report also uses drilling rates, CDP velocity analyses, sonobuoy data, and previous similar drilling correlation methods used at Site 391, along with other geologic considerations in arriving at differing results. Although the correlation method used in this investigation is more objective and the hole location uncertainties better defined, in order to have confidence in any results we will require drilling in areas where reflections are either more widely spaced or where we have better vertical velocity control in the hole.

(Table T1) Gas hydrate fabric and dipping angles of ODP Leg 204 sediments

The sediments of Hydrate Ridge/Cascadia margin contain extensive amounts of gas hydrate. A total of 57 sediment samples including gas hydrate were preserved in liquid nitrogen and have been imaged using computerized tomography to visualize hydrate distribution and shape. The analysis gives evidence that gas hydrate in vein and veinlet structures is the predominant shape in the deeper gas hydrate stability zone with dipping angles from 30° to 90°(vertical).

Iodine ages of pore water at Hydrate Ridge, Cascadian continental margin

Hydrate Ridge off the coast of Oregon, USA, is a prime example for gas hydrate occurrences in active margin settings. It is part of the Cascadia Margin and was the focus of Ocean Drilling Program (ODP) Leg 204, which successfully recovered fluids from nine sites from the southern part of the ridge. Iodide concentrations in pore fluids associated with gas hydrates are strongly enhanced, by factors up to 5000 compared to seawater, which allows the use of this biophilic element as tracer for organic source regions. We applied the cosmogenic isotope 129I (T1/2=15.7 Ma) system to determine the age of the organic source formation responsible for the iodide enrichment. In all sites at ODP Leg 204, 129I/I ratios were found to decrease with depth to values around 250x10**-15, corresponding to minimum ages of 40 Ma, but in several sites, maxima in the 129I/I ratios point to the local addition of young iodide. The results indicate that a large amount of iodide was derived from deep accreted sediments of Eocene age, and that additional source regions provide iodide of Late Miocene age. The presence of old iodide in the pore waters suggests that fluid pathways are open to allow transport over large distances into the gas hydrate fields. The strong correlation between iodide and methane in hydrate fields coupled with the similarity in transport parameters in aqueous solutions suggests that a large fraction of methane in gas hydrates also has old sources and is transported into the present locations from source regions of Eocene age.