Carbon and deuterium isotopes and C ratios in gas hydrates of ODP Leg 204 sites

Sediments at the southern summit of Hydrate Ridge display two distinct modes of gas hydrate occurrence. The dominant mode is associated with active venting of gas exsolved from the accretionary prism and leads to high concentrations (15%-40% of pore space) of gas hydrate in seafloor or near-surface sediments at and around the topographic summit of southern Hydrate Ridge. These near-surface gas hydrates are mainly composed of previously buried microbial methane but also contain a significant (10%-15%) component of thermogenic hydrocarbons and are overprinted with microbial methane currently being generated in shallow sediments. Focused migration pathways with high gas saturation (>65%) abutting the base of gas hydrate stability create phase equilibrium conditions that permit the flow of a gas phase through the gas hydrate stability zone. Gas seepage at the summit supports rapid growth of gas hydrates and vigorous anaerobic methane oxidation. The other mode of gas hydrate occurs in slope basins and on the saddle north of the southern summit and consists of lower average concentrations (0.5%-5%) at greater depths (30-200 meters below seafloor [mbsf]) resulting from the buildup of in situ-generated dissolved microbial methane that reaches saturation levels with respect to gas hydrate stability at 30-50 mbsf. Net rates of sulfate reduction in the slope basin and ridge saddle sites estimated from curve fitting of concentration gradients are 2-4 mmol/m**3/yr, and integrated net rates are 20-50 mmol/m**2/yr. Modeled microbial methane production rates are initially 1.5 mmol/m**3/yr in sediments just beneath the sulfate reduction zone but rapidly decrease to rates of <0.1 mmol/m**3/yr at depths >100 mbsf. Integrated net rates of methane production in sediments away from the southern summit of Hydrate Ridge are 25-80 mmol/m**2/yr. Anaerobic methane oxidation is minor or absent in cored sediments away from the summit of southern Hydrate Ridge. Ethane-enriched Structure I gas hydrate solids are buried more rapidly than ethane-depleted dissolved gas in the pore water because of advection from compaction. With subsidence beneath the gas hydrate stability zone, the ethane (mainly of low-temperature thermogenic origin) is released back to the dissolved gas-free gas phases and produces a discontinuous decrease in the C1/C2 vs. depth trend. These ethane fractionation effects may be useful to recognize and estimate levels of gas hydrate occurrence in marine sediments.

Molecular composition in surface and subsurface sediments of the Helgoland mud area, North Sea

The role of microorganisms in the cycling of sedimentary organic carbon is a crucial one. To better understand relationships between molecular composition of a potentially bioavailable fraction of organic matter and microbial populations, bacterial and archaeal communities were characterized using pyrosequencing-based 16S rRNA gene analysis in surface (top 30 cm) and subsurface/deeper sediments (30-530 cm) of the Helgoland mud area, North Sea. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) was used to characterize a potentially bioavailable organic matter fraction (hot-water extractable organic matter, WE-OM). Algal polymer-associated microbial populations such as members of the Gammaproteobacteria, Bacteroidetes, and Verrucomicrobia were dominant in surface sediments while members of the Chloroflexi (Dehalococcoidales and candidate order GIF9) and Miscellaneous Crenarchaeota Groups (MCG), both of which are linked to degradation of more recalcitrant, aromatic compounds and detrital proteins, were dominant in subsurface sediments. Microbial populations dominant in subsurface sediments (Chloroflexi, members of MCG, and Thermoplasmata) showed strong correlations to total organic carbon (TOC) content. Changes of WE-OM with sediment depth reveal molecular transformations from oxygen-rich [high oxygen to carbon (O/C), low hydrogen to carbon (H/C) ratios] aromatic compounds and highly unsaturated compounds toward compounds with lower O/C and higher H/C ratios. The observed molecular changes were most pronounced in organic compounds containing only CHO atoms. Our data thus, highlights classes of sedimentary organic compounds that may serve as microbial energy sources in methanic marine subsurface environments.

Organic geochemistry of Late Cretaceous to Paleogene sediments from MITI Sanriku-oki borehole

Perylene is present in high concentration in Paleogene sediments from the Sanriku-oki borehole of the Ministry of International Trade and Industry (MITI), northeastern Japan. The borehole penetrates a thick sequence of Late Cretaceous to Neogene sediments deposited under a range of conditions, including fluvial-deltaic and shallow marine. Organic petrological and geochemical data show the sediments to be rich in organic matter (OM) derived from higher plants. Biomarker analysis of aliphatic and aromatic hydrocarbons confirms a significant input from higher plants, with extracts dominated by numerous gymnosperm- and angiosperm-derived biomarkers such as diterpanes, oleanenes, des-A-triterpanes and their aromatized counterparts. The highest concentration of perylene occurs in Middle Eocene sediments deposited in a relatively reducing environment. Stable carbon isotope compositions show 13C enrichment in perylene compared to gymnosperm and angiosperm biomarkers, consistent with a fungal origin. This elevated abundance of sedimentary perylene could relate to a Paleogene continental climate where fungi probably flourished.

Composition of particulate lipids in the intermediate zone between the Kara and Laptev Seas

Chemical group composition of particulate lipids from the intermediate zone between the Kara and Laptev Seas is studied by thin-layer chromatography with flame ionization detection (IATROSCAN TH-10). Hydrocarbons and complex polar lipids similar to those found in the previously studied southeastern area of the Kara Sea are basic components of particulate lipids. High content of triglycerides in the upper layers of the water mass north of the Severnaya Zemlya Islands is a characteristic feature of group composition of particulate lipids. Distribution of triglycerides correlates with localization of the ice cover boundary and complies with process of phytoplankton blooming in the ice edge zone. Distribution of lipid concentration depends on water stratification in the intermediate zone between the Kara and Laptev Seas.

Molecular and isotopic properties of gases from ODP Holes of ODP Leg 204

We report and discuss molecular and isotopic properties of hydrate-bound gases from 55 samples and void gases from 494 samples collected during Ocean Drilling Program (ODP) Leg 204 at Hydrate Ridge offshore Oregon. Gas hydrates appear to crystallize in sediments from two end-member gas sources (deep allochthonous and in situ) as mixtures of different proportions. In an area of high gas flux at the Southern Summit of the ridge (Sites 1248-1250), shallow (0-40 m below the seafloor [mbsf]) gas hydrates are composed of mainly allochthonous mixed microbial and thermogenic methane and a small portion of thermogenic C2+ gases, which migrated vertically and laterally from as deep as 2- to 2.5-km depths. In contrast, deep (50-105 mbsf) gas hydrates at the Southern Summit (Sites 1248 and 1250) and on the flanks of the ridge (Sites 1244-1247) crystallize mainly from microbial methane and ethane generated dominantly in situ. A small contribution of allochthonous gas may also be present at sites where geologic and tectonic settings favor focused vertical gas migration from greater depth (e.g., Sites 1244 and 1245). Non-hydrocarbon gases such as CO2 and H2S are not abundant in sampled hydrates. The new gas geochemical data are inconsistent with earlier models suggesting that seafloor gas hydrates at Hydrate Ridge formed from gas derived from decomposition of deeper and older gas hydrates. Gas hydrate formation at the Southern Summit is explained by a model in which gas migrated from deep sediments, and perhaps was trapped by a gas hydrate seal at the base of the gas hydrate stability zone (GHSZ). Free gas migrated into the GHSZ when the overpressure in gas column exceeded sealing capacity of overlaying sediments, and precipitated as gas hydrate mainly within shallow sediments. The mushroom-like 3D shape of gas hydrate accumulation at the summit is possibly defined by the gas diffusion aureole surrounding the main migration conduit, the decrease of gas solubility in shallow sediment, and refocusing of gas by carbonate and gas hydrate seals near the seafloor to the crest of the local anticline structure.

General descriptions, organic contents and ratios of geolipid components at DSDP Leg 74 Holes

Two samples of Miocene sediments from Site 525 and four samples of sediments ranging in age from Pleistocene to Miocene from Site 528 have been analyzed for concentrations of organic and carbonate carbon, carbon/nitrogen ratios of organic matter, and extractable hydrocarbons and fatty acids. Organic carbon concentrations average 0.32% and show a diagenetic decrease with greater sediment age. Distributions of n-alkanes and n-alkanoic acids give evidence of considerable microbial reworking and of eolian contributions of terrigenous components. Organic contents of these sediments reflect a history of low marine productivity and poor preservation of organic matter in the eastern South Atlantic since middle Miocene times.