ALBERT

Description: This study was performed to investigate gas formation and gas saturation conditions related to acoustic turbidity in shallow (∼40 m deep) marine basins. The Arkona Basin, Baltic Sea, with its organic-rich fine-grained surface sediment provides an ideal “Natural Laboratory” to characterise free gas using seismic, geoacoustic, and geochemical methods. The area of acoustic turbidity covers about 1500 km2 of the central Arkona Basin, corresponding to areas where organic-rich post-glacial sediments exceed 4–6 m in thickness. The highest concentration of pore water methane (7660 μmol L−1), found in areas of high acoustic turbidity, was near the calculated lower limit of methane solubility for the measured in situ temperature, salinity, and pressure. Pore water methane concentration decreased to near 4 μmol L−1 in areas outside of the zone of high acoustic turbidity. Stable carbon (−70.7‰ to −92.3‰ PDB) and hydrogen (−124‰ to −185‰ SMOW) isotope values of methane indicate that methane is predominantly formed by microbial CO2 reduction in Arkona Basin surface sediments and rules out significant contributions of other sources.

Description: Hydrous CaMg-carbonate was synthesized at temperatures of 40 degrees, 60 degrees and 80 degrees C in the laboratory. This material has very similar mineralogical characteristics to natural disordered dolomite from the Coorong region in South Australia. Besides the dolomite variable amounts of amorphous carbonate are present in all samples. The oxygen isotope compositions of synthesized bulk carbonate samples (e.g., amorphous carbonate plus dolomite) plot significantly lower than the Northrop and Clayton (1966) dolomite-water equilibrium. Fractionated degassing of the samples, however, revealed relatively low oxygen isotope values for fast-reacting (using 100% H3PO4) amorphous carbonate. In contrast, slow-reacting dolomite has more positive oxygen isotope values, and calculated carbonate-water oxygen isotope fractionation values are close to strongest known dolomite-water oxygen isotope fractionation published earlier on. Variations of reaction/stabilization temperatures during synthesis gave evidence for dolomite formation from hypersaline solutions by a dissolution/reprecipitation process. It is likely that amorphous carbonate has been a problem in defining the dolomite-water fractionation in the past. Moreover, dolomite-associated amorphous carbonate contents probably led to incorrect speculations about lower oxygen isotope fractionation in a so-called protodolomite-water system. Copyright (c) 2005 Elsevier Ltd.

Description: A decrease in temperature (ΔT up to 45.5 °C) and chloride concentration (ΔCl up to 4.65 mol/l) characterises the brine–seawater boundary in the Atlantis-II, Discovery, and Kebrit Deeps of the Red Sea, where redox conditions change from anoxic to oxic over a boundary layer several meters thick. High-resolution (100 cm) profiles of the methane concentration, stable carbon isotope ratio of methane, and redox-sensitive tracers (O2, Mn4+/Mn2+, Fe3+/Fe2+, and SO42−) were measured across the brine–seawater boundary layer to investigate methane fluxes and secondary methane oxidation processes. Substantial amounts of thermogenic hydrocarbons are found in the deep brines (mostly methane, with a maximum concentration up to 4.8×105 nmol/l), and steep methane concentration gradients mainly controlled by diffusive flow characterize the brine–seawater boundary (maximum of 2×105 nmol/l/m in Kebrit Deep). However, locally the actual methane concentration profiles deviate from theoretical diffusion-controlled concentration profiles and extremely positive δ13C–CH4 values can be found (up to +49‰ PDB in the Discovery Deep). Both, the actual CH4 concentration profiles and the carbon-13 enrichment in the residual CH4 of the Atlantis-II and Discovery Deeps indicate consumption (oxidation) of 12C-rich CH4 under suboxic conditions (probably utilizing readily available—up to 2000 μmol/l—Mn(IV)-oxihydroxides as electron acceptor). Thus, a combined diffusion–oxidation model was used to calculate methane fluxes of 0.3–393 kg/year across the brine–seawater boundary layer. Assuming steady-state conditions, this slow loss of methane from the brines into the Red Sea bottom water reflects a low thermogenic hydrocarbon input into the deep brines.

Description: Hydrothermal gases from shallow seafloor vents in the Bay of Plenty, New Zealand contain CO2, CH4, and the higher gaseous hydrocarbons up to i-, n-C4H10. The gases are similar to those discharged at fumaroles on the nearby White Island. Carbon isotope compositions for CO2 fall between −3.4‰ and −5.5‰ PDB and reflect a shallow magmatic carbon source. The δ13C values of CH4 range from −24.6‰ to −28.9‰ PDB and the δD values vary between −122‰ and −135‰ SMOW. The CH4 isotope values and the presence of the higher hydrocarbon compounds such as C2H6 and C3H8 with δ13C values near −20‰ PDB suggest hydrocarbon production by high-temperature maturation of sedimentary organic matter and mixing (∼1:1) of the thermogenic CH4 with abiogenic CH4. Long-chained hydrocarbons occur in dredged samples close to the active vents. Their n-alkane distribution has a high to moderate odd–even predominance and an extensive hopane series, indicative of higher land-plant waxes and prokaryotic membranes in the source. Substantial amounts of unsubstituted polynuclear aromatic hydrocarbons (PAH) mark the transition from aliphatic- to aromatic-dominated bitumens, consistent with extensive source maturation resulting from thermal stress. The bitumens are interpreted as pyrolysates derived from buried near-coastal vegetation and terrestrial detritus under various thermal regimes, mixed with immature seafloor organic matter.

Description: As a result of extensive hydrocarbon exploration, the North Sea hosts several thousand abandoned wells; many believed to be leaking methane. However, how much of this greenhouse gas is emitted into the water column and ultimately reaches the atmosphere is not known. Here, we investigate three abandoned wells at 81-93m water depth in the Norwegian sector of the North Sea, all of which show gas seepage into the bottom water. The isotopic signature of the emanating gas points towards a biogenic origin and hence to gas pockets in the sedimentary overburden above the gas reservoirs that the wells were drilled into. Video-analysis of the seeping gas bubbles and direct gas flow measurements resolved initial bubble sizes ranging between 3.2 and 7.4mm in diameter with a total seabed gas flow between 1 and 19 tons of CH4 per year per well. Estimated total annual seabed emissions from all three wells of ~24 tons are similar to the natural seepage rates at Tommeliten, suggesting that leaky abandoned wells represent a significant source of methane into North Sea bottom waters. However, the bubble-driven direct methane transport into the atmosphere was found to be negligible (〈2%) due to the small bubble sizes and the water depth at which they are released.

Description: Hydrocarbon gases have been sampled from both cold-seeping and heat-venting areas in the New Ireland fore arc basin in the vicinity of Lihir Island. Highest concentrations of up to 10 μl/l CH4 with a δ13CCH4 value of −54.9‰ PDB have been measured in the deep ocean water within a long and narrow deep sea basin located between Edison Seamount and an uplifted structure named “Mussel Cliff.” Surface sediments of the seep area were covered with chemoautotrophic deep sea fauna such as Calyptogena species and tube worms. Large authigenic calcite concretions occur in the sediments between 50- and 200-cm sediment depth. The carbon isotopes of the carbonates in the concretions range from −15‰ to −40‰ PDB indicating a mixture of two CO2 sources: normal marine-inorganic carbon fixed in biogenic shells and CO2 from anaerobe bacterial oxidation processes of methane. Accordingly, 14C-AMS dating suggests that authigenic calcite mineralisation incorporated relatively “young” carbon from methane oxidation. In contrast, the C1/C2 ratio of 234 and the δ13CCH4 value of −24.1‰ PDB in the hot hydrothermal vent of Lihir Harbour indicates a mixture of a major abiogenic carbon source for methane formation related to magmatism associated with Lihir Volcano. The observed variable fluid characteristics within only 20-km distance between hot hydrothermal-venting and the methane-seeping deep sea area indicates highly variable heat flow situations and/or sediment distributions which control the gas geochemical characteristics in the New Ireland fore arc basin.

Description: Oxygen isotope ratios were obtained from authigenic clinoptilolites from Barbados Accretionary Complex, Yamato Basin, and Exmouth Plateau sediments (ODP Sites 672, 797, and 762) in order to investigate the isotopic fractionation between clinoptilolite and pore water at early diagenetic stages and low temperatures. Dehydrated clinoptilolites display isotopic ratios for the zeolite framework (δ18Of) that extend from +18.7‰ to +32.8‰ (vs. SMOW). In combination with associated pore water isotope data, the oxygen isotopic fractionation between clinoptilolite and pore fluids could be assessed in the temperature range from 25°C to 40°C. The resulting fractionation factors of 1.032 at 25°C and 1.027 at 40°C are in good agreement with the theoretically determined oxygen isotope fractionation between clinoptilolite and water. Calculations of isotopic temperatures illustrate that clinoptilolite formation occurred at relatively low temperatures of 17°C to 29°C in Barbados Ridge sediments and at 33°C to 62°C in the Yamato Basin. These data support a low-temperature origin of clinoptilolite and contradict the assumption that elevated temperatures are the main controlling factor for authigenic clinoptilolite formation. Increasing clinoptilolite δ18Of values with depth indicate that clinoptilolites which are now in the deeper parts of the zeolite-bearing intervals had either formed at lower temperatures (17–20°C) or under closed system conditions.

Description: In order to develop the potential tool of diatom oxygen isotopes for paleoenvironmental studies we compared oxygen isotopes of natural marine diatoms sampled from ocean surface water, sediment traps and surface sediments with oxygen isotopic fractionations determined for laboratory diatom cultures. Freshly grown natural diatoms (phytoplankton samples and sediment trap material) and cultured diatoms reveal similar oxygen isotope fractionation factors. The fresh diatoms have 3 to 10 parts per thousand lower isotope fractionation factors than fossil (sedimentary) diatoms. A temperature-related oxygen isotope fractionation could not be established for the laboratory cultures (and the natural phytoplankton samples), and there is evidence that diatom growth rate until reaching the stationary growth state also controls the measured silica-water oxygen isotope fractionation factor. It is possible, however, that slow diatom growth in sea surface water may well lead to a temperature-dependent silica-water oxygen isotope fractionation which is the prerequisite for a use of diatom oxygen isotopes in palco-surface water studies. FTIR-spectroscopic analyses of various diatomaceous materials revealed that the ratio of integrated peak intensities for Si-O-Si/Si-OH correlates with the 3 to 10 parts per thousand delta O-18(silica) increase from fresh to fossil diatoms. Open-system (flow-through) silica dissolution experiments suggest that the diatom frustules are isotopically homogenous and that the increase in O-18 is therefore not due to dissolution of isotopically light surficial Si-OH groups. It is concluded that slow internal condensation reactions during silica maturation in surface sediments cause both an increase in the intensity ratio of Si-O-Si/Si-OH and the O-18 content of framework oxygen. These findings also indicate that the oxygen isotope compositions of marine sediment diatoms do not indicate sea surface water temperature but rather reflect variable O-18 contents of surface sediments. Copyright (C) 2001 Elsevier Science Ltd.

Description: Hydrothermal brines from the Atlantis II Deep, Red Sea, have been sampled in situ and analyzed for noble gases. The atmospheric noble gas concentrations (Ne, Aratm, Kr, Xe) in the deepest layer (LCL) are depleted by 20 to 30% relative to the initial concentrations in ambient Red Sea Deep Water without a systematic mass fractionation between the different noble gases. Sub surface boiling during the hydrothermal circulation and subsequent phase separation is shown to be a consistent explanation for the observed depletion pattern. Using a conceptual model of phase separation under sub-critical conditions, in which gases are partitioned according to Henry's Law, we reconstruct the fluid history before injection into the Atlantis II Deep: after having circulated through evaporites and young oceanic crust, where it becomes enriched in HeMORB and ArMORB, the ascending fluid boils, and the residual liquid becomes depleted in noble gas concentrations. The depleted fluid rises to the sediment surface and feeds the Atlantis II basin. The relatively low boiling degree of about 3% (i.e., the percentage of fluid removed as vapor) derived from the model indicates that the Atlantis II system represents an early stage of boiling with relatively small gas loss, in contrast to hydrothermal systems at sediment-free mid-ocean ridges.