Recent interest in methane hydrates has resulted from the recognition that they may hold enormous potential as an energy resource for future exploitation. They may also play important roles in the global carbon cycle and rapid climate change through emissions of methane from marine sediments and permafrost into the atmosphere, and in causing mass failure of sediments and structural changes on the continental slope.
Natural gas hydrates occur widely on continental slope and rise (estimated to contain over 10,000 Gigatons of methane carbon), stabilized in place by high hydrostatic pressure and frigid bottom temperature conditions. Change in these conditions, either through lowering of sea level or increase in bottom-water temperature, may trigger the following sequence of events: dissociation of the hydrate at its base, weakening of the mechanical strength of sediments, major slumping, and release of significant quantities of methane in the atmosphere to affect enhanced greenhouse warming. Thus, gas-hydrate breakdown has been invoked to explain the abrupt nature of glacial terminations, pronounced 12C enrichments of the global carbon reservoir, and the presence of major slides and slumps in the stratigraphic record of the continental margins associated with periods of low seastands.
These ideas, as well as the potential of gas hydrates as a clean-burning fuel with a global pool estimated to be more than twice as large as all known hydrocarbons, can not be assessed accurately without better understanding of the nature of the hydrate reservoir and more meaningful estimates of the total amount of methane it contains. This underscores the need for a concerted research effort by both the industry and the academiaon this issue of significant scientific importance and vital societal relevance.
Bottom simulating reflectors (BSRs) are commonly associated with free gas at the the base of the gas hydrate stability zone (BGHSZ). On a large scale, BSRs offshore central Peru are present on the lower slope, where uplift is occurring, whereas they are mostly absent on the upper slope in Lima Basin, which is rapidly subsiding. Tectonic subsidence leads to an increase of pressure and hence, a downward movement of the BGHSZ relative to the sediment column. Free gas at the BGHSZ is transformed to gas hydrates and thus is no longer available to cause the low-velocity layer that generates the BSR. BSRs have now been detected, however, in a small area in Lima Basin where erosion is taking place. Thermal accommodation to maintain a constant thermal gradient after erosion leads to cooling at a given point in the sediment. Erosion therefore, like subsidence, causes a downward movement of the BGHSZ with respect to the sediment column. The presence of BSRs and hence, most probably free gas, in an area of both subsidence and erosion requires that sufficient amounts of gas have to be constantly supplied to the BGHSZ to sustain BSRs. Supply of free gas must offset the absorption of gas into the hydrate stability zone caused by the downward movement of its base. Ocean Drilling Program Leg 112 showed that methane from gas hydrates off Peru is most probably of biogenic origin. Additional drilling would be required, however, to test whether methane from beneath the BGHSZ in this area in Lima Basin also is of biogenic origin. This would indicate considerable microbial production of methane deep in the sediment column beneath the hydrate stability zone.
A special pattern of two bottom simulating reflections (BSRs) has been observed on seismic profiles from the continental margin offshore Western Norway. One of these reflections extends over large areas, and has the characteristics of the classical BSR that is a phase-reversed reflection from the base of the gas-hydrate stability zone. The second BSR, here called BSR0, occurs at approximately 70 ms two-way travel time beneath the classical BSR. The distribution of BSR0 is more local, and it does not show the phase-reversal relative to the sea floor reflection that is characteristic for a BSR at the gas hydrate - free gas boundary. Results from an industrial borehole penetrating the classical BSR, from full waveform inversion of multichannel seismic data, from high-frequency ocean bottom hydrophones, and interpretation of seismic profiles from the area, clearly indicate that the classical BSR, which occurs at 330 ms beneath the sea floor reflection, is reflected from the base of the methane hydrate equilibrium field. The classical BSR corresponds to an abrupt drop in velocity, from about 1.8 km/s to 1.4 km/s, suggesting the presence of free methane gas in a 8-10 m low-velocity zone. Relatively high velocities in two 8- and 30- m thick zones just above the BSR suggest that the sediment pores of these zones are partially saturated with methane hydrate, a conclusion that is supported by low chloride values. The full waveform inversion results indicate that BSR0 is not a reflection from a single interface, but corresponds to a 16-20 m zone where the velocity drops from about 1.8 km/s to a minimum of 1.4 km/s, suggesting the presence of free gas in the low-velocity zone, and then increases again. The results may support a hypothesis where BSR0 is a reflection from the base of gas hydrates containing hydrocarbons with a heavier molecular weight in addition to methane gas.
ODP Leg 164 has provided an invaluable set of down-hole and core logging data trough a tectonically undisturbed sediment sequence containing gas hydrates. Porosity, density and velocity data have been utilised toe stimate quantitatively gas hydrates concentration by application of two equations following empirical and theoretical approaches independently. The results can be compared to hydrate concentration estimates based on porewater chlorinity.
In the theoretical approach, we used the Gaussman's equations with an explicit dependence on differential pressure and depth. The compressional shear velocities expressed for water filled porous media in which there are two solid components (matrix and gas hydrates), and for fluid filled porousmedia, where the fluid is a mixture of free gas and water, are those given by Tinivella and Lodolo (in press). For computing the theoretical velocities, we used density and porosity trends measured in the Physical Properties laboratory. Concentration of gas hydrates is derived by comparison between theoretical velocity curves in the absence of clathrates, and the actual velocity curves obtained by sonic logging.
For an empirical approach, we considered the Nobes et al. (1986) equation, which relates porosity with compressional wave velocity in sediments without hydrates. When applied to define velocity vs. depth curves at ODP Leg164 drill sites from porosity data, the sonic log reveals significantlyhigher values than anticipated by the Nobes et al. model below 200 mbsf. Attributing these enhanced velocities completely to hydrate abundance and following a mo. Numerical results are in satisfactory to good agreement with ODP findings as well as with the theoretical approach, except for layers beneath the BSR, where the empirical - simpler - method clearly fails.
Both models show that hydrates are present from about 200 mbsf down to the occurrence of the BSR, and the concentrations increase progressively downwards. Absolute values of percentage of hydrates in the porous space agree within a few percent with the values suggested by pore water chemistry. The theoretical model - in contrast to the empirical approach -allows for further interpretation of the data below the BSR, providing estimates of the free gas trapped below the hydrate cap.del extension to the hydrate case suggested by Lee et al. (1993), a method to calculate the hydrate concentration in pore space was derived, for which densities and compressional velocities of pore fluid, pure hydrate, and the sediment matrix are the only input parameters.
Lee MW, Hutchinson DR, Dillon WP, Miller JJ, Agena WF & Swift BA, Marine and Petroleum Geology, 10, 493, (1993).
Nobes DC, Villinger H, Davis EE & Law LK, JGR, 91, 14033, (1986).
Tinivella U & Lodolo E, Sci. Res. Proc. ODP, 164, in press, (1998).
Gas hydrates were recently retrieved from the sediments of Lake Baikal (Siberia) during deep drilling. This is - to date - the only reported occurrence of hydrates in a confined fresh-water basin. The enclosed gases in the clathrate structure are mainly methane of biogenic origin.
The presence of gas hydrates had first been suggested by the observation of a distinct BSR on Russian-American multi-channel seismic (MCS) data from the Southern and Central Baikal basins. This BSR corresponds with the base of the gas hydrate stability field and marks the interface between higher-velocity, hydrate-cemented sediments above and lower-velocity, hydrate-free but perhaps gas-charged sediments below. The Baikal BSR has a strong amplitude, often cross-cuts the local stratigraphy, has negative polarity and its position is in accordance with theoretical pressure-temperature phase boundary conditions.
Detailed re-interpretation of the MCS lines shows that the gas hydrate layer in Lake Baikal is characterised by strong local variations in thickness that appear to be linked with underlying structural features.
Also, the Baikal BSR does not always appear as a continuous reflector. It is locally displaced in the presence of (recently) active faults. Acoustic anomalies in the water column above these faults suggest gas seeping at the lake floor. No displacement of the BSR is observed along presently passive faults.
On high-resolution single-channel seismic data, the BSR is much less obvious. Where observed, it is made up of different enhanced and closely spaced bits of reflectors of variable polarity.
These observations suggest that the base of the hydrate stability field in natural conditions is a very complicated boundary, determined by local stratigraphy - controlling distribution of porosity and permeability barriers and pathways - and by local tectonic structures - controlling the location of heat and fluid flow sources - and dependent of the different time scales of the various interacting processes.
During R/V A.M. Keldysh cruise in June and July 1998 the Northwestern European continental margin was investigated with respect to activities associated with gas hydrates and methane release. Structures such as the Haakon Mosby Mud Vulcano and pockmark fields at Vestnesa Ridge and the northern rim of Storegga Slide were visited by employing deep sea submersibles. Geophysical, geological, geochemical and biological research was performed at the same sites and the same time by a team of norwegian, russian, american and german scientists. The focus of the german party is to gather information about the stability and climatic relevance of oceanic gas hydrates. For this, newly developed methane sensors were used for the first time under deep sea conditions, measuring methane concentrations directly above the sea floor along the dive tracks. Methane concentrations clearly exceeding background values were obtained. Mooring stations were deployed, measuring current velocity and direction together with CTD data. Water probes were taken and analysed by gas chromatography. On the basis of these data, modelling of a possible methane plume will be carried out in order to gain knowledge of methane release into the oceanosphere.The poster will present first results and data interpretations from the Barent Sea continental slope.
A mud volcano on top of approx. 6 km of sediments is presenting researchers with a very unexpected hydrodynamic and extreme environment on the North Atlantic passive polar margin. The mud volcano was first discovered on side-scan sonar data (Vogt et al., 1997) and was revisited by a team of norwegian, russian, american, and german scientists using the MIR deep-sea submersibles. The focus of the dives was to gather information about (1) white (and so far believed) hydrate mates (2) possible anomalies in the water column (3) possible methane realeases from the volcano, and (4) thermal and hydrate anomalies in the sediment column.The filming of the seafloor environment together with first and preliminary data interpretations of presently and possibly formerly hydrated sediments will be presented and discussed.
Vogt PR, Cherkashev G, Ginsburg G, Ivanov G, Milkov A, Crane A, Sundvor E, Pimenov N, Egorov A, Eos, Transactions, AGU, 78:48, 549,556-557, (1997).
Several geochemical studies have demonstrated the importance of anaerobic oxidation of methane in marine sediments (see review by Hoehler and Alperin, 1996). This process is considered to consume an equivalent of 5 to 20% of the annual flux of methane into the atmosphere (Reeburgh and Alperin, 1988). Based on circumstantial evidence, the most plausible mechanism involves a consortium of methanogenic and sulfate-reducing bacteria, with the methanogens working in a reversed mode (Hoehler et al., 1994). According to this hypothesis, methanogens chemically transform methane using H2O as the formal oxidant, yielding CO2 and H2 as products. By oxidizing H2 and minimizing its concentrations in pore waters, sulfate-reducers maintain thermodynamic control over the process. However, the specific organisms involved have not been identified.
We studied marine sediments taken from cold methane seeps at the Eel River Basin, off northern California, in order to search for molecular signals related to the process of anaerobic methane oxidation. We will present isotopic evidence from methanogen-specific biomarkers, partially supporting the hypothesis of Hoehler et al. (1994). These data indicate that methanogenic bacteria oxidize methane and use it as substrate for growth. Our interpretation is based on the extreme depletion in 13C (13C < -100‰) of specific biomarkers. This signal indicates that Archaea are using isotopically light methane as a carbon source for biosynthesis. Moreover, one of these strongly depleted biomarkers displays highly source-specific structural features that point to the involvement of a particular subgroup of methanogens. More specifically, this biomarker is diagnostic for two closely related genera (out of 17 phylogenetically distinct genera of methanogenic bacteria), a particular subgroup of Methanomicrobiales. This result is confirmed by qualitative and quantitative sequence comparisons using 16S rRNA and showing the predominance of these two genera in the sediment. Working in their normal mode as methanogens, these two genera are distinct in terms of their choice of substrates for growth and energy yield. This feature suggests the possibility of a biochemical pathway for methane oxidation in a consortium of methanogens and sulfate-reducers differing slightly from that proposed by Hoehler et al. (1994).
Hoehler TM & Alperin MJ, Microbial Growth on C1 Compounds, Kluwer Academic Publishers, 326-333, (1996).
Hoehler TM, Alperin MJ, Albert DB & Martens CS, Global Biogeochemical Cycles, 8, 451-463, (1994).
Reeburgh WS & Alperin MJ, SCOPE/UNEP Sonderband, 66, 367-375, (1988).
During ODP Leg 164, authigenic carbonates were recovered from gas hydrate bearing sediments that overlay the Blake Ridge Diapir on the Carolina Rise. The presence of gas hydrate nodules near the sediment surface indicates that, at present, gas hydrates are stable under seafloor p/T conditions in this area. Active chemosynthetic communities on the seafloor are apparently fed by methane-rich fluids originating from beneath a bottom simulating reflector (BSR), which curves upward around the flanks of the diapir. The methane-rich fluids, which migrate upward along a fault extending through the gas hydrate seal, are most likely derived from the decomposition of gas hydrates below the base of the gas hydrate stability zone. One objective of this study was to determine whether the venting of gas hydrate derived fluids at this site causes authigenic mineral precipitation only at the seafloor, or whether the sediments are subject to continuing diagenetic processes with depth.
The authigenic carbonate nodules, recovered from depths between 0 and 52 mbsf (meter below seafloor), are composed of rounded to subangular intraclasts and carbonate cemented mussel shell fragments. Electron microprobe and X-ray diffraction (XRD) investigations show that aragonite is the dominant authigenic carbonate. Aragonite occurs both as microcrystalline, interstitial cement, and as cavity-filling radial fibrous crystals. The 13C values vary between -48.4‰ and -30.5‰ (PDB) indicating that carbon derived from 13C-depleted methane is incorporated into these carbonates. Dissimilarities between the 13C values of carbonate precipitates recovered from below 10 mbsf and 13C values of the associated pore water (sum)CO2 suggest that these carbonates had formed near the seafloor. Measurements of the strontium isotopic composition on 13 carbonate samples show 87Sr/86Sr values between 0.709125 and 0.709206 with a mean of 0.709165, consistent with the approximate age of their host sediment. Furthermore, the 87Sr/86Sr values of six pore water samples from Site 996 vary between 0.709130 and 0.709204. The similarity of these values to seawater (87Sr/86Sr = 0.709175), and to 87Sr/86Sr values of pore water from similar sample depths elsewhere on the Blake Ridge (Sites 994, 995, and 997), indicates a shallow Sr source.
Based on our data, we see no evidence of continuing carbonate formation with depth. Therefore, with the exception of their seafloor expression as carbonate crusts, fossil vent sites will not be preserved. Since these authigenic features apparently form only at the seafloor, their vertical distribution and sediment age imply that seepage has been going on in this area for at least 600,000 years.
The Tortonian marls which were deposited in the Lorca basin prior to the Messinian Tripoli Formation, represent a rather homogeneous marly unit of some 1000 m in thickness. They contain discontinuous levels of indurated nodules and massive layers up to a few meters thick of diagenetic carbonates. The diagenetic carbonates are composed of dolomite which is sometimes associated with opale CT. The chemical composition of these dolomites is variable, as indicated by the large range (2.884 to 2.912) of the d(104) values. The departure from stoichiometry of dolomites is due to the presence of excess Ca and/or of Fe in the crystal lattice.The isotopic compositions of the bulk calcite from the marls show a signal typical of normal marine conditions (average 18O = -2; 13C = -1). The isotopic compositions of the diagenetic dolomites (-0.6<18O <3.5 ; 3.4 <13< 8.8) indicate that diagenesis occurred in 13C -rich and 18O to 18O -poor solutions. In organic -rich sediments, methanogenesis is the process mediated by bacteria by which 13C rich CO2 is generated either during fermentation of organic matter or during carbonate reduction (Claypool and Threlkeld, 1983). The methane accumulated in the sediments may form gas hydrates if the pressure-temperature stability conditions are established. During gas hydrate formation, the 18O-rich water enter preferentially in the hydrate and the remaining liquid water become 18O -depleted; conversely, the decomposition of hydrate liberates 18O -rich water in the pore solutions (Ussler and Paull, 1995). The vertical distribution of the isotopic compositions of the dolomites shows a sharp limit between the lower dolomites with high 13C values- high 18O values and the upper dolomites with high 13C values- low 18O values. This boundary may be considered as the lower limit of the gas hydrate stability below which gas hydrates are decomposed due to the increasing temperatures by burial.
Claypool GE & Threslkeld CN, Init. Reports DSDP 76, 391-402, (1983)
Ussler W & Paull CK, Geo-Mar. Let, 15, 37-44, (1995)
Natural gas seepages are widespread on present-day ocean sea-floors and play an important role in the deep sub-seafloor biosphere. Moreover, the contribution of natural gas seeping from sediments to the global methane budget is presently discussed and the relevance to climate evolution of natural methane (an important green-house gas) is largely unknown. The escape of large volumes of natural gas from the sea floor characterizes nowadays strong deformational settings where tectonic "squeezing" of sediments causes upward migration of great amount of gas-rich fluids. Sudden releases of huge volumes of gas can be generated, on the other hand, by the breakdown of gas hydrates within sediments following sea-level changes. This phenomenon may play also a significant role in promoting sediment instability along many continental margins. The expulsion of gas-rich fluids from the sediments has often been registered in the fossil record by peculiar rocks and deposits that were generally overlooked before the recognition of present day analogues. Seep-related rocks recognized in the fossil record are anomalous carbonates often containing remains of chemosymbiotic communities. They are interpreted on the basis of their isotopic composition and through the comparison with present day settings. Monferrato (NW Italy) is one of the first areas in which the fossil record of these phenomena has been recognized and described. Both limestones with fossil remains of chemosymbiotic macro-communities, called Lucina Limestone, and carbonates barren of macrofossils, called Marmorito Limestone, have been interpreted to result from hydrocarbon-rich fluids emissions on Monferrato Miocene sea-floors. In both types of rocks several microstructures point to the activity of rich and diversified microbial communities thriving on the hydrocarbon-rich fluid emissions. Quite recently new outcrops of methane-related carbonates have been reported in Verrua Savoia and the relationships between these rocks and the surrounding sediments suggest a genesis related to a mud volcano activity. In Monferrato, seep related rocks are consistently found along important tectonic structures and could be possibly related to specific stratigraphic levels. From a regional point of vue the recognition of seep-related deposits in particular geological time spans could be helpful to identify periods of intense tectonic activity or sea-level fluctuations. An identification of these rocks on a larger, possibly global scale, could furthermore provide a new approach to paleoclimatic reconstructions and support the traditional paleontological and sedimentological data.
Our research on microbial life in deep granitic rock aquifers (Pedersen 1997) has been performed at several sites at depths down to 3500 m: The Stripa research mine in the middle of Sweden, the Äspö hard rock laboratory (HRL) situated on the south eastern coast of Sweden, on five sites in Finland, the natural reactors in Oklo and Witwatersrand goldmines in South Africa. The ultimate limitation for an active microbial life at depth is suggested to be the availability of hydrogen, and possibly methane, as energy sources over time and these gases have indeed been found in most deep groundwater. The finding of a deep, autotrophic hydrogen-based biosphere (Kotelnikova and Pedersen, 1998) adds a significant but previously overlooked microbial activity to the deep rock aquifers. Sulphate and iron-reducing bacteria use organic material from the autotrophs and expel sulphide and ferrous iron. A good correlation has been shown between viable iron and sulphate reducing bacteria and the amount of ferric iron and sulphide precipitates in groundwater conducting aquifers. Two new species isolated from aquifers in deep fractured granitic rock were recently discovered, Desulfovibrio aespoeensis (Motamedi and Pedersen, 1998) and Methanobacterium subterraneum (Kotelnikova et al. 1998). Both of these species are adapted to life under the prevailing conditions where they were isolated from, i.e. they are most probably intrinsic. If microorganisms have lived deep underground for a long time, it should be possible to detect fossilised microbes in fracture filling material. A successful investigation of drill cores intersecting a granitic aquifer at 207 m depth demonstrated for the first time that this is the case (Pedersen et al. 1997). Fossil microbes were found, and some were organised in biofilms. This observation provides evidence for ancient life in deep granitic rock aquifers and suggests that the modern life found there is indigenous. Recent data from Äspö HRL indicates that in addition to organisms representing the domains Bacteria and Archaea, organisms from the domain Eukarya, yeast and fungi, are also present and probably indigenous as well. Altogether, our results show that there is a very high probability for the existence of an intra-terrestrial biosphere that is driven by hydrogen from the interior of the earth and, therefore, independent of photosynthesis. Our planet apparently has two biospheres, the sun driven biosphere and the recently discovered, unexplored earth driven intra-terrestrial biosphere. Prospective research will aim at exploration of distribution, diversity, in situ activity and biogeochemistry of the intra-terrestrial biosphere and its influence on anthropogenic underground operations.
Kotelnikova S & Pedersen K, FEMS Microbiology Ecology, 26, 121-134, (1998).
Kotelnikova S, Macario AJL & Pedersen K, International Jourlan of Systematic Bacteriology, 48, 357-367, (1998).
Motamedi M & Pedersen K, International Jourlan of Systematic Bacteriology, 48, 311-315, (1998).
Pedersen K, FEMS Microbiology reviews, 20, 399-414, (1997).
Pedersen K, Ekendahl S, Tullborg E-L, Furnes H, Thorseth I-G & Tumyr O, Geology, 25, 827-830, (1997).
In oceanic lithosphere Fischer-Tropsch Type (FTT) synthesis of organic compounds is probably a significant process. In the commercial Fischer-Tropsch synthesis organic compounds, especially alkanes, alcohols and carboxylic acids, are formed at high temperature from CO and H2 in the presence of a metal or mineral catalyst. On Earth, the mantle is degassed with respect to CO and CO2. Percolation of water leading to serpentinization of Fe(II)-rich minerals, primarily olivine and pyroxenes, in peridotites is efficient. Molecular hydrogen formed from water is an important reaction product when the Fe(II) minerals are oxidized to magnetite during the serpentinization process. Slow-spreading (fractured) ridges like the Mid-Atlantic Ridge (MAR) and the SW Indian Ridge (SWIR) are more likely to reveal abiotic production of organic compounds than fast-spreading ones because of better penetration of water. The classes of organic compounds that are predicted to form in relatively high quantities in these environments are straight-chain hydrocarbons and carboxylic acids. Fluids of two hydrothermal systems of the Rekjanes Peninsula on Iceland have been sampled. Mixed marine-meteoric hydrothermal waters were collected from the wellhead of one drilled well of the Reykjanes system (290°C) at the tip of the peninsula and two wells of the Svartsengi field (240°C) further to the east. Organic compounds potentially present in the fluids were concentrated on different types 'sorbent extraction units'. Analysis results show the presence of C8-C16 monocarboxylic acids at both 240 and 290°C (although the concentrations were much higher at the low temperature). The presence of primarily carboxylic acids is in accordance with thermodynamic calculations, which predict a metastable maximum concentration of carboxylic acids at about 200°C under the redox conditions buffered by the PPM (pyrite-pyrrhotite-magnetite) mineral assemblage. The reason why the samples lack shorter carboxylic acids than octanoic acid is due to the fact that phase separation occurred at the pressure release during sampling and short acids were expelled with the steam phase.
Biological mediation has been suggested as an important control of chemical exchange between the oceanic crust and seawater, but very little is known about its distribution within the oceanic crust and the relative importance of biotic and abiotic processes. Alteration textures in glassy pillow lava margins record the proportions of biotic and abiotic alteration, that may be estimated by petrographic point counting. Application of this method at the deep DSDP/ODP Holes 504B and 896A at the Costa Rica Rift, and Holes 417D and 418A on the southern end of the Bermuda Rise in the western Atlantic Ocean shows that biotic alteration comprises 60-85% of the total glass alteration in the upper 250 m, and decreases to approximately 10% at 500 m depth. Beyond 500 m depth at Site 504B, we have not been able to trace active biotic processes, and at 540 m depth (Site 418A) biotic alteration comprises only 3% of the total alteration of the basaltic glass. The dominance of biotic alteration in the upper 250 m at Sites 504B and 896A correlates with oxidizing alteration conditions, high permeability, porosity and current temperatures of 60-90°C. Our results suggest that basalt-altering microbes thrive at temperatures up to 90°C, but microbial activity is substantially decreased with increasing temperature and possibly reduced availability of oxygen. These results contribute to our understanding of the depths to which the Deep Biosphere extends into the oceanic crust.
Samples of basalts recovered from pillow lava in the Atlantic Ocean, the Leu Basin and the Costa Rica Rift showed evidence for microbial biodegradation of the basaltic glass. The degraded zones contained textures resembling microbes in terms of size and shape. Staining with a DNA specific fluorescent dye (DAPI) showed the presence of DNA in the cells. Hybridization with fluorescent labeled oligonucleotide probes that hybridize specifically to the 16S-rRNA gene of Bacteria and Archaeae showed that both types of microbes were present in the altered glass. Electron microprobe analysis showed that C, N and K accumulated in the altered glass, and the N/C ratios were comparable to those found in nitrogen starved marine bacteria. In samples from the Costa Rica rift microbes were shown to penetrate 290 m into the volcanic basement (Torsvik et al1998)
Carbon isotope analysis showed that disseminated carbonate in the glass contained isotopic signatures that differed significantly from the isotopic signature of vein carbonate. Most glass samples showed low 13C values, indicating that microbial oxidation of organic material to CO2 was a major energy yielding process for microbial activity. Some glass samples from the Atlantic Ocean showed positive 13C values, indicating the presence of Archaeae that produce methane from H2 + CO2
The presence of Bacteria and Archaeae in basalt has been also verified both by culturing and by analysis of DNA extracted directly form rock samples recently retrieved from the Knipovich Ridge. By electron microscope studies microbes were observed both at the outer surface of the altered rims and at the alteration front, associated with the often strongly pitted glass surface. This supports previous observations that microbes may play an important role in the degradation of basaltic glass in the oceanic crust (Thorseth et al 1995).
Torsvik T, Furnes H, Muehlenbachs K, Thorseth IH & Tumyr O, Earth Planet. Sci. Lett, 162, 165-176, (1998).
Thorseth IH, Torsvik T, Furnes H & Muehlenbachs K, Chem. Geol, 126, 137-146, (1995).
A systematic investigation of low-T hydrothermal minerals from subsurface environments ("moss agate", "stalactitic" forms of quartz, vein-type chert, zeolites) and of "stalactitic" oxidation-zone minerals (goethite, duftite, pyromorphite) revealed the very common presence of tubular filaments with core diameters of 1 to 5 mm. Filaments are frequently organized in composite structures with architectures similar to microbial mats. Fabrics of this type are present in samples from >100 localities worldwide. Filamentous subsurface structures commonly formed during aqueous alteration of volcanic rocks (e.g. Cady Mts, California; eastern Iceland) and oxidizable ores (e.g. Tsumeb, Namibia; Bleiberg, Austria) and are similar in size, morphology and construction to fossilized microbes observed in modern thermal springs. Besides straight and branched filaments, twisted stalks similar in size and morphology to the iron oxidizing bacterium Gallionella ferruginea, were identified in some samples. Other occurrences are related to low-T mineralizations in limestones and alteration of ultramafic rocks. The age of the filamentous structures ranges from subrecent to Precambrian. We interpret the filamentous structures as permineralized and encrusted microbial filaments and discount an origin by nonbiologial self-organization based on the following arguments: 1) tubular construction of filaments; 2) constant core diameter of filaments in a range typical of microbes (1-3 micron); 3) the gravity-draping of micron-thin filaments indicates an originally flexible consistency typical of microbial filaments but not of inorganic mineral precipitates; 4) coalescence of filaments to form mat-like structures; 5) occurrence in low-T mineral assemblages. Nonbiogenic self-organization can produce morphologically similar structures, but under geochemical conditions (high silica water) unlikely for some of the environments envisaged (oxidizing ore, low-pH environment). If proven biogenic, this fossil record of subsurface microbial life opens up new perspectives in the study of subsurface paleobiology and in the exploration for fossil life on Mars. If a nonbiogenic explanation is found, careful characterization is needed to allow the recognition of similar features in Martian rocks.
Permafrost regions on Earth are now recognized as the most constant and stable environment for life among deep habitats. The number of viable microorganisms in frozen sedimentary deposits is only one order of magnitude lower than the same parameter in tundra soils.
Analysis of frozen subsoil sediments of different genesis and age of permafrost (from 200 thousand up to 1 million years) revealed high survival of proteolytic, amylolitic and saccharolytic microbial communities. The library of isolated bacterial strains comprises more than 30 genera. IAT Asolated bacteria revealed their activity at a wide range of temperatures (+4°C - +50°C), but the majority of bacterial strains were psychrotrophic and mesophilic. Among those bacteria the non srore-forming strains were predominant.
Microbial communities after their release from frozen state are characterized by high rate of of proliferation in comparison with thawing samples of tundra soil. It is possible that during freezing the vegetative cells pass to a reversible state of low metabolic activity (hypometabolism) under growing influence of different stress factors, including cryobiosis and osmobiosis in permafrost sediments, and develop anti-stress mechanisms ensuring rapid restoration of biological activity after thawing. When the energy resourses are exhausted, the majority of hypometabolic cells can revert to the deep resting state conserving their genome.
Ultrastuctural investigations of bacterial strains under the effect of low temperatures both in the samples and pure cultures show the cell differentiation into types of resting forms among non spore- forming bacteria which were characterized by thick cell walls and external capsular layers, formation of cell aggregates in extracellular matrix, decrease in size and coocoid form. Model experiments with autoinducers of anabiosis showed the possibility of the formation of cystlike cells in growing cultures of Arthrobacter spp. - the mostly widespreaded asporogenous bacterium among isolates from permafrost. It is plausible to assume that asporogenous bacteria in permafrost can form hypomethabolic cells with possible following transition to the deep resting state under endogenous regulatory processes during prolonged impact of cold and reveal new forms of anabiotic microbial cells. High biological activity of bacteria from permafrost sediments after thawing testify that low temperatures is a stabilizing factor that sustain life in various resting forms in deep cold biotopes.
The extrusive volcanic rocks that make up the upper half kilometer of oceanic crust have been proposed as an environment that may harbor a substantial microbial biosphere. Although this extrusive basalt layer covers the majority of the surface of the earth, little is known about the potential for this zone to sustain a large population of bacteria. Some of the necessary criteria for a possible sub-surface biosphere include (1) adequate pore space within the crustal rocks, (2) actively circulating hydrothermal fluid (i.e., modified seawater), (3) the presence of chemical gradients, including electron donors and acceptors, (4) the presence of substantial thermal gradients, and (5) initial inoculation of the environment by appropriate biota. These criteria are clearly satisfied within the neo-volcanic zone at mid-ocean ridge spreading centers, where submersible and ROV observations indicate that sub-surface bacterial populations can 'bloom' after recent eruptions or tectonic events. However, the total volume of crust involved in axial hydrothermal vent systems is quite small, and bacterial populations based on transient events are not likely to be significant on a global scale. On the other hand, recent geophysical studies have added new data regarding both axial and off-axis oceanic crust as possible microbial habitats. On-bottom gravity studies have indicated a large-scale (30%) porosity for 1.5 My seafloor that extends at least 500 meters into the crust, and that this porosity is composed in part of large voids (tens of meters in size) that partially collapse with increasing crustal age. Bare rock heat flow data (at Baby Bare Seamount, 3.5 My) and direct fluid flow measurements (at ODP Hole 1026b) indicate a strong tidal modulation to off-axis hydrothermal fluid circulation. This observation requires crustal fluid velocities that are an order of magnitude higher than those associated with thermal convection alone. These new geophysical data, viewed for the first time as constraints on a potential sub-surface biosphere, are providing a new model of entire oceanic crust as a possible habitat for life.
The discovery of in situ dolomite formation in organic carbon-rich deep-sea sediments in Californian margin basins (DSDP Leg 63) and Gulf of California (DSDP Leg 64), and more recently along the Namibian Margin (ODP Leg 175), led to the realization that microbial activity is intimately involved in the dolomite formation. The carbon isotopic signal of such authigenic dolomite, indicating formation under sulphate reducing and methanogenic redox conditions, serves as a microbial tracer. Moreover, in modern hypersaline environments, bacteria have been shown to play an important role in dolomite precipitation. In this study, we propose that intra-crystalline organic matter (OM), trapped in dolomite crystals, may serve as an additional tracer for microbial activity in sedimentary dolomite formation in modern near-shore, shallow-water environments, as well as in deep-sea sediments.
Bacterial body-like structures have been identified entombed in dolomite crystals using high resolution SEM. These observations together with previous organic geochemical investigations indicate that it is possible to find intra-crystalline OM trapped within crystals. In Lagoa Vermelha, a hypersaline coastal lagoon in Brazil, bacterial activity apparently mediates precipitation of primary dolomite. Organic extracts of selected sediment samples from 0-95 cm depth have been analyzed using GC and GC-MS techniques. Intra-crystalline OM, isolated using bleaching, dissolution, and extraction techniques, provides information about the type and the source of organic molecules entombed in the crystals. Although the intra-crystalline OM will be affected only by chemical (i.e. not microbial) alteration, the diagenetic products will remain trapped and preserved in the crystal structure. Thus, being a microbial tracer, the intra-crystalline OM may be a potential source of environmental information. In this study, we compare the lipid signature of intra-crystalline OM with the lipid signature of bacterial cultures from the same sediment. Our long-range goal is to identify tracers for microbial activity which are applicable to authigenic dolomite formation at the Earth's surface, including the deep-sea subsurface biosphere.
Samples of basaltic lava, sediment and water, collected during the summer 1998 from the rift valley of the Knipovich Ridge at 76°.47' N, have been studied by electron microscopy, enrichment cultures and biomolecular methods aimed at describing the microbial population in the basalt and the influence of microbes on the dissolution and alteration of the basaltic glass (Thorseth et al., 1995; Torsvik et al., 1998). The samples were collected at 3500 m below sea level and the ambient sea water temperature was measured to -0.7oC.
On the primary surface and along fractures in the glassy margins of the lava flows, yellow brown to red brown coloured alteration rims, strongly enriched in Fe, P and S, are developed. In samples of recent lava flows the alteration rims are only 1 - 3 µ m thick. In older sediment covered lava flows, the alteration rims are 10 - 40 µ m in thickness and the samples are coated by a 30 µ m thick Fe- Mn- crust. Electron microscopy studies show that a complex microbial society is associated with the alteration of the basaltic glass. The microbes are observed both at the outer surface of the altered rims and at the alteration front, where they are attached to the often strongly pitted glass surface. The cells are usually heavily encrusted with Fe and to some extent Mn.
Anaerobic enrichment cultures show the presence of methanogenic Archaeae in rock, sediment and water samples. Methane is produced from H2 + CO2, methanol and three methylamine. DNA has been extracted from the samples and and from primary enrichment cultures. 16S-rRNA genes were amplified by the polymerase chain reaction (PCR) and separated according to their base composition by denaturing gradient gel electrophoresis (DGGE). DNA sequencing of the dominant DGGE bands indicate the presence of psycrophilic marine Bacteria and confirm the presence of Archaeae in the samples.
Thorseth IH, Torsvik T, Furnes H & Muehlenbachs K, Chem. Geol, 126, 137-146, (1995).
Torsvik T, Furnes H, Muehlenbachs K, Thorseth IH & Tumyr O, Earth Planet. Sci. Lett, 162, 165-176, (1998).
The Dutch-French expedition MEDINAUT (November-December 1998) conducted Nautile dives on seven mud volcanoes in the Eastern Mediterranean, namely Napoli, Milano, Maidstone and Moscow in the Olimpi mud field on the crest of the Mediterranean Ridge south of Crete, and Amsterdam, Kazan and Kula in the area of the Anaximander mountains south of Turkey. All visited mud volcanoes show evidence of active fluid and gas seepage. As a result of hypersaline fluid seepage, spectacular brine pools have accumulated over large areas of the summit of Napoli mud volcano. The pools, less than one meter deep, are located in shallow depressions of the seafloor on the summit of the volcano. They appear to have collected brines that were expulsed through the seafloor at a number of vents, observed as darkish spots, 10 to 30 cm across, with white precipitates (salt? sulfide? or bacterial mats?) deposited at their rims. Temperatures of the brines are close to that of the bottom seawater. Temperature gradients in sediments at the bottoms of pools reach locally 1°C/m. Other seep features identified on several mud volcanoes include areas of reduced dark sediment, in cases with lateral extents as wide as several metres, sometimes with a white nebulous concentration of bacterial filaments at the water-sediment interface. Carbonate crusts of variable thicknesses are widespread at the surfaces of most mud volcanoes, with particularly thick crusts observed on parts of both Napoli and Amsterdam. Carbonate crusts form mounds inhabited by tube worms, small bivalves (Lucinidae, Vesicomyidae, Mytilidae), sea-urchins and crabs (a probably chemosynthetic-based community) at several sites of active seepage. There are millions of shells of dead bivalves on the seafloor. All mud volcanoes expulse large quantities of methane with methane concentrations in the water column up to several hundreds of microlitres per litre, i.e. over 10 000 times the normal seawater concentration. Methane expulsion could be partly associated with the presence of methane gas hydrate, which would account for the abnormally low salinities (down to 9‰) of pore waters sampled on Milano, Amsterdam, Moscou, and Kazan.
Direct observations were made from the submersible Nautile of eastern Mediterranean fluid seeps and mud volcanoes in the Olimpi and Anaximander areas (south of Crete and southwestern Turkey, respectively) during the late 1998 French-Dutch MEDINAUT expedition. Most of the mud volcanoes studied are associated with linear structural features and fault zones. Fluid seeps were found not only on the mud volcanoes but also elsewhere in both areas along faults observed on the seafloor. An important result is that mud volcanoes are not the only source of methane from the Mediterranean Ridge. The Anaximander mud volcanoes tend to lie along zones of structural offset or discontinuity. One such zone produces a scarp with vertically outcropping strata where fault geometry could be studied. Slickensides were seen on some faults. Seep activity was clear from large communities of live vestimentiferan worms along the cliff face, and evidence of seep-related bivalve communities on sedimented surfaces, especially near the top of the section. In this area there is an apparent intersection of NW-SE faults continuous with the southern boundary of the Florence Rise to the southeast, E-W faults, and NE-SE faults (direction of relative plate movement). On the west margin of Amsterdam mud volcano is an active seep site on top of a parasitic mud cone lying along a NE-SW fault inferred from deep tow sidescan data, and lying along strike from the Faulted Ridge site. In the Olimpi area, the mud volcanoes Maidstone and Moscow lie along a common NW-SE structural feature inferred to be a fault. The Maidstone and Napoli mud volcanoes have similar NW-SE axes of symmetry suggesting parallel development. Between Moscow and Maidstone lies a linear N-S valley with brine lakes. Fault scarps observed on the seafloor in the valley had predominantly N000 to N020 strikes, small offsets, and anastomosing appearance; but there were also NW-SE trends (and minor NE-SW trends) seen nearer the south end where the valley intersects the Maidstone-Moscow lineament and where the main brine lakes are found. High levels of methane were measured in the water from the brine lake. An empty brine lake nearby was examined and found still to have tube worms living in a build-up of carbonate crust typical of all seeps found during the expedition. Scarps in relatively fresh mud flows on Maidstone and Moscow, and older looking ones on Napoli, show trends both consistent with the assumed tectonic control and expected from multibeam and deep tow data here.
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