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The anaerobic oxidation of methane (AOM) is a crucial component of the methane cycle, but quantifying its role in situ under dynamic environmental conditions remains challenging. We use sediment samples collected during IODP Expedition 347 to the Baltic Sea to show that relative abundances of 12CH2D2 and 13CH3D in methane remaining after microbial oxidation are in internal, thermodynamic isotopic equilibrium, and we attribute this phenomenon to the reversibility of the initial step of AOM. These data suggest that 12CH2D2 and 13CH3D together can identify the influence of anaerobic methanotrophy in environments where conventional bulk isotope ratios are ambiguous, and these findings may lead to new insights regarding the global significance of enzymatic back reaction in the methane cycle.

Microbial anaerobic oxidation of hydrocarbons is a key process potentially involved in a myriad of geological and biochemical environments yet has remained notoriously difficult to identify and quantify in natural environments. We performed position-specific carbon isotope analysis of propane from cracking and incubation experiments. Anaerobic bacterial oxidation of propane leads to a pronounced and previously unidentified 13C enrichment in the central position of propane, which contrasts with the isotope signature associated with the thermogenic process. This distinctive signature allows the detection and quantification of anaerobic oxidation of hydrocarbons in diverse natural gas reservoirs and suggests that this process may be more widespread than previously thought. Position-specific isotope analysis can elucidate the fate of natural gas hydrocarbons and provide insight into a major but previously cryptic process controlling the biogeochemical cycling of globally significant greenhouse gases.

The concentrations of electron donors and acceptors in the terrestrial subsurface biosphere fluctuate due to migration and mixing of subsurface fluids, but the mechanisms and rates at which microbial communities respond to these changes are largely unknown. Subsurface microbial communities exhibit long cellular turnover times and are often considered relatively static-generating just enough ATP for cellular maintenance. Here, we investigated how subsurface populations of CH4 oxidizers respond to changes in electron acceptor availability by monitoring the biological and geochemical composition in a 1339 m-below-land-surface (mbls) fluid-filled fracture over the course of both longer (2.5 year) and shorter (2-week) time scales. Using a combination of metagenomic, metatranscriptomic, and metaproteomic analyses, we observe that the CH4 oxidizers within the subsurface microbial community change in coordination with electron acceptor availability over time. We then validate these findings through a series of 13C-CH4 laboratory incubation experiments, highlighting a connection between composition of subsurface CH4 oxidizing communities and electron acceptor availability.

Fluids trapped as inclusions within minerals can be billions of years old and preserve a record of the fluid chemistry and environment at the time of mineralization1, 2, 3. Aqueous fluids that have had a similar residence time at mineral interfaces and in fractures (fracture fluids) have not been previously identified. Expulsion of fracture fluids from basement systems with low connectivity occurs through deformation and fracturing of the brittle crust4. The fractal nature of this process must, at some scale, preserve pockets of interconnected fluid from the earliest crustal history. In one such system, 2.8 kilometres below the surface in a South African gold mine, extant chemoautotrophic microbes have been identified in fluids isolated from the photosphere on timescales of tens of millions of years5. Deep fracture fluids with similar chemistry have been found in a mine in the Timmins, Ontario, area of the Canadian Precambrian Shield. Here we show that excesses of 124Xe, 126Xe and 128Xe in the Timmins mine fluids can be linked to xenon isotope changes in the ancient atmosphere2 and used to calculate a minimum mean residence time for this fluid of about 1.5 billion years. Further evidence of an ancient fluid system is found in 129Xe excesses that, owing to the absence of any identifiable mantle input, are probably sourced in sediments and extracted by fluid migration processes operating during or shortly after mineralization at around 2.64 billion years ago. We also provide closed-system radiogenic noble-gas (4He, 21Ne, 40Ar, 136Xe) residence times. Together, the different noble gases show that ancient pockets of water can survive the crustal fracturing process and remain in the crust for billions of years.

We analysed shallow (to ~1 km) and deep fracture waters (to > 3 km) from the Witwatersrand Basin, South Africa for their noble gas isotopic composition. Their neon signature clearly differentiates a group of typical crustal fluids from another one with a significantly enriched nucleogenic neon signal with the highest 21Ne/22Ne ratios (0.160 +/- 0.003) ever reported in groundwater [1]. Fluid inclusions in adjacent rocks yield even higher 21Ne/22Ne ratios between 0.219 and 0.515, consistent with an extrapolated 21Ne/22Ne value of 3.3 +/- 0.2 at 20Ne/22Ne = 0. We show that this enriched nucleogenic neon end-member represents a fluid component that was produced in the fluorine-depleted Archaean formations and trapped in fluid inclusions > 2 Ga ago [1]. The observation of enriched nucleogenic neon signatures in deep fracture water implies the release of this billion year old neon component from the fluid inclusions and its accumulation in exceptionally isolated fracture water systems. The observed association of this Archaean neon signature with H2-hydrocarbon-rich geogases of proposed abiogenic origin [2] dissolved in the same deep groundwater suggests that the fracture systems have also allowed for the accumulation of various products of water-rock reactions throughout geologic times. One of these fracture systems contained a chemolithotrophic, single species ecosystem surviving on radiolytically produced H2 and sulfate completely independent of the surface photosphere [3,4]. [1] Lippmann-Pipke et al. 2011 Chem. Geol. 283, 287-296 [2] Sherwood Lollar et al. 2002 Nature 416(6880) 522-524 [3] Lin L.-H. et al. 2006 Science 314(5798), 479-482 [4] Chivian, D. et al. 2008 Science 322(5899), 275-278

Introduction The deep gold mines of the Witwatersrand Basin, South Africa, have gained recent attention because of reports of deep microbial communities persisting to depths of >3 kilometers - an exotic outpost of the Earth's deep biosphere [1,2]. Prerequisites of such investigations are studies on the age of the deep fracture water [3] and the origin of associated abiogenic CH4- [4] and H2-rich gases [5]. Here we report about insights obtained from two new gas data sets (I, II) with respect to the dynamic of the ultra-deep fluid system [6,7]. Methods (I) The isotopic composition of neon dissolved in deep fracture water and in fluid inclusions of quartz host rock was analyzed, and the results correlated with the isotopic signatures of the biogenic and abiogenic H2- and CH4-rich sample aliquots. (II) In the other project the geogas composition of the free gas was monitored by means of a mass spectrometer and gas sensitive sensors installed just inside the Pretorius Fault Zone at 3.45 km depth in the TauTona Mine. Data from a 4-year observation period are cross-correlated on the minute-by-minute basis with seismic data obtained from a 3-dimensional array of highly sensitive seismometers. Results (I) Highest ever observed 21Ne/22Ne ratios combined with an anomalous 21Ne/22Ne production ratio imply that the dissolved neon is a 2 Ga metamorphic signal stored in fluid inclusions and accumulating in fracture water since geological times as a result of water rock reaction processes. (II) Geogas concentration variations (4He, H2, CH4, CO2) strongly correlate with seismic signals resulting from mining related blasts and induced earthquakes in the TauTona Mine. The fracture gas permeability of ~ 5*10e-10 m² is estimated on the basis of observed gas flux rates. Conclusion (I) 21Ne/22Ne ratios may serve as search proxy for regions of the Archean Earth's crust where investigations of the deep biosphere might be focused. (II) Methodological improvements enabled the simultaneous ga

A summary is provided of the first of a series of proposed Integrated Science Initiative workshops supported by the UNESCO International Hydrological Programme. The workshop brought together hydrologists, environmental chemists, microbiologists, stable isotope specialists and natural resource managers with the purpose of communicating new ideas on ways to assess microbial degradation processes and reactive transport at catchment scales. The focus was on diffuse contamination at catchment scales and the application of compound-specific isotope analysis (CSIA) in the assessment of biological degradation processes of agrochemicals. Major outcomes were identifying the linkage between water residence time distribution and rates of contaminant degradation, identifying the need for better information on compound specific microbial degradation isotope fractionation factors and the potential of CSIA in identifying key degradative processes. In the natural resource management context, a framework was developed where CSIA techniques were identified as practically unique in their capacity to serve as distributed integrating indicators of process across a range of scales (micro to diffuse) of relevance to the problem of diffuse pollution assessment