Simultaneous δ2 H and δ18 O analyses of water inclusions in halite with off-axis integrated cavity output spectroscopy.

Stable isotope compositions of ancient halite fluid inclusions have been recognized to be valuable tools for reconstructing past environments. Nevertheless, in order to better understand the genesis of halite deposits, it could be of great interest to combine both δ2 H and δ18 O measurements of the water trapped as inclusions in the defects of the mineral lattice. We developed a method combining off-axis integrated cavity output spectroscopy (OA-ICOS) connected on line with a modified elemental analyzer (EA-OA-ICOS) to perform those measurements. The first step was to test the method with synthetic halite crystals precipitated in the laboratory from isotopically calibrated waters. Water isotopic signatures have been measured with conventional techniques, equilibration for δ18 O and chromium reduction for δ2 H. Then, we modified and optimized a conventional EA to connect it online with an OA-ICOS instrument for H2 O measurements. The technique is first evaluated for calibrated free water samples. The technique is also evaluated for salt matrix effect, accuracy, and linearity for both isotopic signatures. Then, the technique is used to measure simultaneously δ2 H and δ18 O values of halite water inclusions precipitated from the evaporation experiments. Data generated with this new technique appeared to be comparable with those inferred from prior off-line technique studies. The advantages offered by the OA-ICOS technique are the simultaneous acquisition of both isotopic ratios and the substantial reduction of data acquisition time and sample aliquot size. Natural halite samples have been analyzed with this method. Natural halite samples as old as Precambrian have also been analyzed with this method.

Improved online hydrogen isotope analysis of halite aqueous inclusions.

We demonstrate an improved method based on continuous-flow elemental analyser pyrolysis isotopic ratio mass spectrometry (CF-EA-PY-IRMS) to measure the 2 H/1 H ratios of water trapped in halite crystals. Two challenges to overcome are the low hydrogen concentration of samples (10-50 µmol H2 ·g-1 ) and the high chloride concentration released when reacting halite in an elemental analyser. We describe an optimization procedure for determining the 2 H/1 H ratio of this trapped water with an acceptable accuracy. This technique involves the use of a high-temperature Cr reactor to quantitatively convert H2 O into H2 . The initial step was performed on halite crystals precipitated from a water reservoir where 2 H/1 H ratios were monitored from its initial stage until the end of evaporation. The 2 H/1 H isotopic analyses were automated online in continuous-flow mode. Precision of the method was determined for those "synthetic" samples with hydrogen concentrations ranging from 0.2 to 0.5 wt%. 2 H/1 H isotopic ratios of evaporating waters bracket the compositions of water inclusions. The formation of fluid inclusions is not instantaneous and records the isotopic signature of the residual waters across a time range during which the isotopic values of the water still evolve. This property explains why the δ2 HVSMOW standard deviation of ±5‰ (2σ) observed for 10-mg aliquots of halite exceeds the instrumental error (about ±1.5‰ 2σ) determined on the basis of IAEA-CH7, NBS 30, and NBS 22 references along with calibrated waters with and without added halite crystals. We also applied this method to Mesoproterozoic (1.4 Ga) and Neoproterozoic (0.8 Ga) halite samples with relatively low hydrogen concentrations (300-1500 ppm). The measured δ2 HVSMOW values for Precambrian waters range from -89‰ to -54‰. We propose that this technique offers a new perspective and great potential for palaeoenvironmental reconstructions based on the 2 H/1 H analyses of water trapped in halite.

Global hydrogen reservoirs in basement and basins.

BACKGROUND: Hydrogen is known to occur in the groundwaters of some ancient cratons. Where associated gases have been dated, their age extends up to a billion years, and the hydrogen is assumed also to be very old. These observations are interpreted to represent the radiolysis of water and hydration reactions and migration of hydrogen into fracture systems. A hitherto untested implication is that the overwhelming bulk of the ancient low-permeability basement, which is not adjacent to cross-cutting fractures, constitutes a reservoir for hydrogen. RESULTS: New data obtained from cold crushing to liberate volatiles from fluid inclusions confirm that granites and gneiss of Archean and Palaeoproterozoic (>1600 Ma) age typically contain an order of magnitude greater hydrogen in their entrained fluid than very young (<200 Ma) granites. Sedimentary rocks containing clasts of old basement also include a greater proportion of hydrogen than the young granites. CONCLUSIONS: The data support the case for a global reservoir of hydrogen in both the ancient basement and in the extensive derived sediments. These reservoirs are susceptible to the release of hydrogen through a variety of mechanisms, including deformation, attrition to reduce grain size and diagenetic alteration, thereby contributing to the hydrogen required by chemolithoautotrophs in the deep biosphere.

Evidence for Seismogenic Hydrogen Gas, a Potential Microbial Energy Source on Earth and Mars.

UNLABELLED: The oxidation of molecular hydrogen (H2) is thought to be a major source of metabolic energy for life in the deep subsurface on Earth, and it could likewise support any extant biosphere on Mars, where stable habitable environments are probably limited to the subsurface. Faulting and fracturing may stimulate the supply of H2 from several sources. We report the H2 content of fluids present in terrestrial rocks formed by brittle fracturing on fault planes (pseudotachylites and cataclasites), along with protolith control samples. The fluids are dominated by water and include H2 at abundances sufficient to support hydrogenotrophic microorganisms, with strong H2 enrichments in the pseudotachylites compared to the controls. Weaker and less consistent H2 enrichments are observed in the cataclasites, which represent less intense seismic friction than the pseudotachylites. The enrichments agree quantitatively with previous experimental measurements of frictionally driven H2 formation during rock fracturing. We find that conservative estimates of current martian global seismicity predict episodic H2 generation by Marsquakes in quantities useful to hydrogenotrophs over a range of scales and recurrence times. On both Earth and Mars, secondary release of H2 may also accompany the breakdown of ancient fault rocks, which are particularly abundant in the pervasively fractured martian crust. This study strengthens the case for the astrobiological investigation of ancient martian fracture systems. KEY WORDS: Deep biosphere-Faults-Fault rocks-Seismic activity-Hydrogen-Mars. Astrobiology 16, 690-702.

Evidence for methane in Martian meteorites.

The putative occurrence of methane in the Martian atmosphere has had a major influence on the exploration of Mars, especially by the implication of active biology. The occurrence has not been borne out by measurements of atmosphere by the MSL rover Curiosity but, as on Earth, methane on Mars is most likely in the subsurface of the crust. Serpentinization of olivine-bearing rocks, to yield hydrogen that may further react with carbon-bearing species, has been widely invoked as a source of methane on Mars, but this possibility has not hitherto been tested. Here we show that some Martian meteorites, representing basic igneous rocks, liberate a methane-rich volatile component on crushing. The occurrence of methane in Martian rock samples adds strong weight to models whereby any life on Mars is/was likely to be resident in a subsurface habitat, where methane could be a source of energy and carbon for microbial activity.

Seaweed attachment to bedrock: biophysical evidence for a new geophycology paradigm.

Seaweeds are amongst the most obvious and ecologically important components of rocky shore communities worldwide but until now little has been known about the processes involved in their attachment. This multidisciplinary study investigated for the first time the interactions between marine macroalgal holdfasts and their underlying substrata, requiring the development of specialized sample preparation techniques to maintain the structural integrity of the holdfast-bedrock interface. Transmitted plane polarized light microscopy, scanning electron microscopy with energy dispersive spectroscopy and structured light illumination microscopy were used in the examination of the interface between Ascophyllum nodosum (Fucales, Heterokontophyta) and crustose red algae Lithothamnion sp. (Corallinales, Rhodophyta) on granite and limestone substrates. The new evidence presented here represents a paradigm shift in the way we view seaweed attachment because results show that the holdfasts exploit the physical characteristics of the rock-forming minerals in order to penetrate the bedrock and thus facilitate the attachment process. Mineral cleavage planes together with intercrystalline and intracrystalline boundaries and fractures provide penetration pathways for the holdfast tissue. This process causes disaggregation of rock-forming minerals to depths <10 mm and therefore assists in the bioerosion of coastal bedrock. It is concluded that seaweeds are able to cause weathering of natural rock and the term 'geophycology' is introduced to describe seaweed-bedrock interactions, including seaweed-induced weathering.

Frequency domain fluorescence lifetime study of crude petroleum oils.

Frequency domain (FD) fluorescence lifetime data was collected for a series of 20 crude petroleum oils using a 405 nm excitation source and over a spectral range of approximately 426 to approximately 650 nm. Average fluorescence lifetimes were calculated using three different models: discrete multi-exponential, Gaussian distribution, and Lorentzian distribution. Fitting the data to extract accurate average lifetimes using the various models proved easier and less time consuming for the FD data than with Time Correlated Single Photon Counting (TCSPC) methods however the analysis of confidence intervals to the computed average lifetimes proved cumbersome for both methods. The uncertainty in the average lifetime was generally larger for the discrete lifetime multi-exponential model when compared to the distribution-based models. For the lifetime distributions, the data from the light crude oils with long lifetimes generally fit to a single decay term. Heavier oils with shorter lifetimes required multiple decay terms. The actual value for the average lifetime is more dependant on the specific fitting model employed than the data acquisition method used. Correlations between average fluorescence lifetimes and physical and chemical parameters of the crude oils were made with a view to developing a quantitative model for predicting the gross chemical composition of crude oils. It was found that there was no significant benefit gained by using FD over TCSPC other than more rapid data analysis in the FD case. For the FD data the Gaussian distribution model for fluorescence lifetime gave the best correlations with chemical composition allowing a qualitative correlation to some bulk oil parameters.