Cold seep formation from salt diapir-controlled deep biosphere oases.

Deep sea cold seeps are sites where hydrogen sulfide, methane, and other hydrocarbon-rich fluids vent from the ocean floor. They are an important component of Earth's carbon cycle in which subsurface hydrocarbons form the energy source for highly diverse benthic micro- and macro-fauna in what is otherwise vast and spartan sea scape. Passive continental margin cold seeps are typically attributed to the migration of hydrocarbons generated from deeply buried source rocks. Many of these seeps occur over salt tectonic provinces, where the movement of salt generates complex fault systems that can enable fluid migration or create seals and traps associated with reservoir formation. The elevated advective heat transport of the salt also produces a chimney effect directly over these structures. Here, we provide geophysical and geochemical evidence that the salt chimney effect in conjunction with diapiric faulting drives a subsurface groundwater circulation system that brings dissolved inorganic carbon, nutrient-rich deep basinal fluids, and potentially overlying seawater onto the crests of deeply buried salt diapirs. The mobilized fluids fuel methanogenic archaea locally enhancing the deep biosphere. The resulting elevated biogenic methane production, alongside the upward heat-driven fluid transport, represents a previously unrecognized mechanism of cold seep formation and regulation.

Tracing sources of atmospheric methane using clumped isotopes.

We apply a recently developed measurement technique for methane (CH4) isotopologues* (isotopic variants of CH4-13CH4, 12CH3D, 13CH3D, and 12CH2D2) to identify contributions to the atmospheric burden from fossil fuel and microbial sources. The aim of this study is to constrain factors that ultimately control the concentration of this potent greenhouse gas on global, regional, and local levels. While predictions of atmospheric methane isotopologues have been modeled, we present direct measurements that point to a different atmospheric methane composition and to a microbial flux with less clumping (greater deficits relative to stochastic) in both 13CH3D and 12CH2D2 than had been previously assigned. These differences make atmospheric isotopologue data sufficiently sensitive to variations in microbial to fossil fuel fluxes to distinguish between emissions scenarios such as those generated by different versions of EDGAR (the Emissions Database for Global Atmospheric Research), even when existing constraints on the atmospheric CH4 concentration profile as well as traditional isotopes are kept constant.

Bulk and grain-scale minor sulfur isotope data reveal complexities in the dynamics of Earth's oxygenation.

SignificanceThe permanent disappearance of mass-independent sulfur isotope fractionation (S-MIF) from the sedimentary record has become a widely accepted proxy for atmospheric oxygenation. This framework, however, neglects inheritance from oxidative weathering of pre-existing S-MIF-bearing sedimentary sulfide minerals (i.e., crustal memory), which has recently been invoked to explain apparent discrepancies within the sulfur isotope record. Herein, we demonstrate that such a crustal memory effect does not confound the Carletonville S-isotope record; rather, the pronounced Δ33S values identified within the Rooihoogte Formation represent the youngest known unequivocal oxygen-free photochemical products. Previously observed 33S-enrichments within the succeeding Timeball Hill Formation, however, contrasts with our record, revealing kilometer-scale heterogeneities that highlight significant uncertainties in our understanding of the dynamics of Earth's oxygenation.

Abiotic redox reactions in hydrothermal mixing zones: Decreased energy availability for the subsurface biosphere.

Subseafloor mixing of high-temperature hot-spring fluids with cold seawater creates intermediate-temperature diffuse fluids that are replete with potential chemical energy. This energy can be harnessed by a chemosynthetic biosphere that permeates hydrothermal regions on Earth. Shifts in the abundance of redox-reactive species in diffuse fluids are often interpreted to reflect the direct influence of subseafloor microbial activity on fluid geochemical budgets. Here, we examine hydrothermal fluids venting at 44 to 149 °C at the Piccard hydrothermal field that span the canonical 122 °C limit to life, and thus provide a rare opportunity to study the transition between habitable and uninhabitable environments. In contrast with previous studies, we show that hydrocarbons are contributed by biomass pyrolysis, while abiotic sulfate (SO42-) reduction produces large depletions in H2 The latter process consumes energy that could otherwise support key metabolic strategies employed by the subseafloor biosphere. Available Gibbs free energy is reduced by 71 to 86% across the habitable temperature range for both hydrogenotrophic SO42- reduction to hydrogen sulfide (H2S) and carbon dioxide (CO2) reduction to methane (CH4). The abiotic H2 sink we identify has implications for the productivity of subseafloor microbial ecosystems and is an important process to consider within models of H2 production and consumption in young oceanic crust.

In Situ Enhancement and Isotopic Labeling of Biogenic Coalbed Methane.

Subsurface microbial (biogenic) methane production is an important part of the global carbon cycle that has resulted in natural gas accumulations in many coal beds worldwide. Laboratory studies suggest that complex carbon-containing nutrients (e.g., yeast or algae extract) can stimulate methane production, yet the effectiveness of these nutrients within coal beds is unknown. Here, we use downhole monitoring methods in combination with deuterated water (D2O) and a 200-liter injection of 0.1% yeast extract (YE) to stimulate and isotopically label newly generated methane. A total dissolved gas pressure sensor enabled real-time gas measurements (641 days preinjection and for 478 days postinjection). Downhole samples, collected with subsurface environmental samplers, indicate that methane increased 132% above preinjection levels based on isotopic labeling from D2O, 108% based on pressure readings, and 183% based on methane measurements 266 days postinjection. Demonstrating that YE enhances biogenic coalbed methane production in situ using multiple novel measurement methods has immediate implications for other field-scale biogenic methane investigations, including in situ methods to detect and track microbial activities related to the methanogenic turnover of recalcitrant carbon in the subsurface.

Precise Measurements of 12CH2D2 by Tunable Infrared Laser Direct Absorption Spectroscopy.

We present precise measurements of doubly deuterated methane (12CH2D2) in natural methane samples using tunable infrared laser direct absorption spectroscopy (TILDAS). Using a 413 m optical path length astigmatic Herriott cell and two quantum cascade lasers (QCLs) scanning the spectral regions of 1090.46 ± 0.1 and 1200.23 ± 0.1 cm-1, the instrument simultaneously measures the five main isotopologues of methane. The ratios 13CH3D/12CH4 and 12CH2D2/12CH4 are measured at 0.01‰ and 0.5‰ (1σ) instrumental precision, respectively. The instrumental accuracy was assessed by measuring a series of methane gases with a range of δ13C and δD values but with the abundances of all isotopologues driven to thermal equilibrium at 250 °C. The estimated accuracy of Δ12CH2D2 is 1‰ (1σ) on the basis of the results of the heated methane samples. This new TILDAS instrument provides a simple and rapid technique to explore the sources of methane in the environment.

Correct Symmetry Treatment for X + X Reactions Prevents Large Errors in Predicted Isotope Enrichment.

Confusion over how to account for symmetry numbers when reactants are identical can cause significant errors in isotopic studies. An extraneous factor of 2 in the reaction symmetry number, as proposed in the literature, violates reaction equilibrium and causes huge enrichment errors in isotopic analysis. In actuality, no extra symmetry factor is needed with identical reactants.

An oligotrophic deep-subsurface community dependent on syntrophy is dominated by sulfur-driven autotrophic denitrifiers.

Subsurface lithoautotrophic microbial ecosystems (SLiMEs) under oligotrophic conditions are typically supported by H2 Methanogens and sulfate reducers, and the respective energy processes, are thought to be the dominant players and have been the research foci. Recent investigations showed that, in some deep, fluid-filled fractures in the Witwatersrand Basin, South Africa, methanogens contribute <5% of the total DNA and appear to produce sufficient CH4 to support the rest of the diverse community. This paradoxical situation reflects our lack of knowledge about the in situ metabolic diversity and the overall ecological trophic structure of SLiMEs. Here, we show the active metabolic processes and interactions in one of these communities by combining metatranscriptomic assemblies, metaproteomic and stable isotopic data, and thermodynamic modeling. Dominating the active community are four autotrophic ß-proteobacterial genera that are capable of oxidizing sulfur by denitrification, a process that was previously unnoticed in the deep subsurface. They co-occur with sulfate reducers, anaerobic methane oxidizers, and methanogens, which each comprise <5% of the total community. Syntrophic interactions between these microbial groups remove thermodynamic bottlenecks and enable diverse metabolic reactions to occur under the oligotrophic conditions that dominate in the subsurface. The dominance of sulfur oxidizers is explained by the availability of electron donors and acceptors to these microorganisms and the ability of sulfur-oxidizing denitrifiers to gain energy through concomitant S and H2 oxidation. We demonstrate that SLiMEs support taxonomically and metabolically diverse microorganisms, which, through developing syntrophic partnerships, overcome thermodynamic barriers imposed by the environmental conditions in the deep subsurface.

Vibronic origin of sulfur mass-independent isotope effect in photoexcitation of SO2 and the implications to the early earth's atmosphere.

Signatures of mass-independent isotope fractionation (MIF) are found in the oxygen ((16)O,(17)O,(18)O) and sulfur ((32)S, (33)S, (34)S, (36)S) isotope systems and serve as important tracers of past and present atmospheric processes. These unique isotope signatures signify the breakdown of the traditional theory of isotope fractionation, but the physical chemistry of these isotope effects remains poorly understood. We report the production of large sulfur isotope MIF, with Δ(33)S up to 78‰ and Δ(36)S up to 110‰, from the broadband excitation of SO2 in the 250-350-nm absorption region. Acetylene is used to selectively trap the triplet-state SO2 ( (3)B1), which results from intersystem crossing from the excited singlet ( (1)A2/ (1)B1) states. The observed MIF signature differs considerably from that predicted by isotopologue-specific absorption cross-sections of SO2 and is insensitive to the wavelength region of excitation (above or below 300 nm), suggesting that the MIF originates not from the initial excitation of SO2 to the singlet states but from an isotope selective spin-orbit interaction between the singlet ( (1)A2/ (1)B1) and triplet ( (3)B1) manifolds. Calculations based on high-level potential energy surfaces of the multiple excited states show a considerable lifetime anomaly for (33)SO2 and (36)SO2 for the low vibrational levels of the (1)A2 state. These results demonstrate that the isotope selectivity of accidental near-resonance interactions between states is of critical importance in understanding the origin of MIF in photochemical systems.

Manganese-oxidizing photosynthesis before the rise of cyanobacteria.

The emergence of oxygen-producing (oxygenic) photosynthesis fundamentally transformed our planet; however, the processes that led to the evolution of biological water splitting have remained largely unknown. To illuminate this history, we examined the behavior of the ancient Mn cycle using newly obtained scientific drill cores through an early Paleoproterozoic succession (2.415 Ga) preserved in South Africa. These strata contain substantial Mn enrichments (up to ∼17 wt %) well before those associated with the rise of oxygen such as the ∼2.2 Ga Kalahari Mn deposit. Using microscale X-ray spectroscopic techniques coupled to optical and electron microscopy and carbon isotope ratios, we demonstrate that the Mn is hosted exclusively in carbonate mineral phases derived from reduction of Mn oxides during diagenesis of primary sediments. Additional observations of independent proxies for O2--multiple S isotopes (measured by isotope-ratio mass spectrometry and secondary ion mass spectrometry) and redox-sensitive detrital grains--reveal that the original Mn-oxide phases were not produced by reactions with O2, which points to a different high-potential oxidant. These results show that the oxidative branch of the Mn cycle predates the rise of oxygen, and provide strong support for the hypothesis that the water-oxidizing complex of photosystem II evolved from a former transitional photosystem capable of single-electron oxidation reactions of Mn.

Millimeter-wave optical double resonance schemes for rapid assignment of perturbed spectra, with applications to the C̃ (1)B(2) state of SO2.

Millimeter-wave detected, millimeter-wave optical double resonance (mmODR) spectroscopy is a powerful tool for the analysis of dense, complicated regions in the optical spectra of small molecules. The availability of cavity-free microwave and millimeter wave spectrometers with frequency-agile generation and detection of radiation (required for chirped-pulse Fourier-transform spectroscopy) opens up new schemes for double resonance experiments. We demonstrate a multiplexed population labeling scheme for rapid acquisition of double resonance spectra, probing multiple rotational transitions simultaneously. We also demonstrate a millimeter-wave implementation of the coherence-converted population transfer scheme for background-free mmODR, which provides a ∼10-fold sensitivity improvement over the population labeling scheme. We analyze perturbations in the C̃ state of SO2, and we rotationally assign a b2 vibrational level at 45,328 cm(-1) that borrows intensity via a c-axis Coriolis interaction. We also demonstrate the effectiveness of our multiplexed mmODR scheme for rapid acquisition and assignment of three predissociated vibrational levels of the C̃ state of SO2 between 46,800 and 47,650 cm(-1).

Development of a spectroscopic technique for continuous online monitoring of oxygen and site-specific nitrogen isotopic composition of atmospheric nitrous oxide.

Nitrous oxide is an important greenhouse gas and ozone-depleting-substance. Its sources are diffuse and poorly characterized, complicating efforts to understand anthropogenic impacts and develop mitigation policies. Online, spectroscopic analysis of N2O isotopic composition can provide continuous measurements at high time resolution, giving new insight into N2O sources, sinks, and chemistry. We present a new preconcentration unit, "Stheno II", coupled to a tunable infrared laser direct absorption spectroscopy (TILDAS) instrument, to measure ambient-level variations in (18)O and site-specific (15)N N2O isotopic composition at remote sites with a temporal resolution of <1 h. Trapping of N2O is quantitative up to a sample size of ∼4 L, with an optimal sample size of 1200-1800 mL at a sampling frequency of 28 min. Line shape variations with the partial pressure of the major matrix gases N2/O2 and CO2 are measured, and show that characterization of both pressure broadening and Dicke narrowing is necessary for an optimal spectral fit. Partial pressure variations of CO2 and bath gas result in a linear isotopic measurement offset of 2.6-6.0 ‰ mbar(-1). Comparison of IR MS and TILDAS measurements shows that the TILDAS technique is accurate and precise, and less susceptible to interferences than IR MS measurements. Two weeks of measurements of N2O isotopic composition from Cambridge, MA, in May 2013 are presented. The measurements show significant short-term variability in N2O isotopic composition larger than the measurement precision, in response to meteorological parameters such as atmospheric pressure and temperature.

Measurement of a doubly substituted methane isotopologue, ¹³CH3D, by tunable infrared laser direct absorption spectroscopy.

Methane is an important energy resource and significant long-lived greenhouse gas. Carbon and hydrogen isotope ratios have been used to better constrain the sources of methane but interpretations based on these two parameters alone can often be inconclusive. The precise measurement of a doubly substituted methane isotopologue, (13)CH3D, is expected to add a critical new dimension to source signatures by providing the apparent temperature at which methane was formed or thermally equilibrated. We have developed a new method to precisely determine the relative abundance of (13)CH3D by using tunable infrared laser direct absorption spectroscopy (TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one tuned at 8.6 µm to measure (13)CH3D, (12)CH3D, and (12)CH4, and the other at 7.5 µm to measure (13)CH4. With the use of an astigmatic Herriott cell with an effective path length of 76 m, a precision of 0.2‰ (2σ) was achieved for the measurement of (13)CH3D abundance in ca. 10 mL STP (i.e., 0.42 mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at lower precision. The accuracy of the Δ(13)CH3D measurement is 0.7‰ (2σ), evaluated by thermally equilibrating methane with a range of δD values. The precision of ±0.2‰ corresponds to uncertainties of ±7 °C at 25 °C and ±20 °C at 200 °C for estimates of apparent equilibrium temperatures. The TILDAS instrument offers a simple and precise method to determine (13)CH3D in natural methane samples to distinguish geological and biological sources of methane in the atmosphere, hydrosphere, and lithosphere.

Fully automated, high-precision instrumentation for the isotopic analysis of tropospheric N2O using continuous flow isotope ratio mass spectrometry.

RATIONALE: Measurements of the isotopic composition of nitrous oxide in the troposphere have the potential to bring new information about the uncertain N2O budget, which mole fraction data alone have not been able to resolve. Characterizing the expected subtle variations in tropospheric N2O isotopic composition demands high-precision and high-frequency measurements. To enable useful observations of N2O isotopic composition in tropospheric air to reduce N2O source and sink uncertainty, it was necessary to develop a high-precision measurement system with fully automated capabilities for autonomous deployment at remote research stations. METHODS: A fully automated pre-concentration system for high-precision measurements of N2O isotopic composition (δ(15)N(ß) , δ(15)N(α), δ(18)O) in tropospheric air has been developed which combines a custom liquid-cryogen-free cryo-trapping system and gas chromatograph interfaced to a continuous flow isotope ratio mass spectrometry (IRMS) system. A quadrupole mass spectrometer was coupled in parallel to the IRMS system during development to evaluate peak interference. Multi-port inlet and fully-automated capabilities allow streamlined analyses between in situ air inlet, air standards, flask air sample, or other gas source in exactly replicated analysis sequences. RESULTS: The system has the highest precision to date for (15)N site-specific composition results (δ(15) N(α) ±0.11‰, δ(15)N(ß) ±0.14‰ (1σ)), attributed mostly to uniformity of analytical cycles and particular attention to fluorocarbon interference noted for (15)N site-specific measurements by IRMS. Air measurements demonstrated the fully automated capacity and performance. CONCLUSIONS: The system makes substantial headway in measurement precision, possibly defining the limits of IRMS measurement capabilities in low concentration N2O air samples, with fully automated capabilities to enable high-frequency in situ measurements.

High-precision measurements of (33)S and (34)S fractionation during SO2 oxidation reveal causes of seasonality in SO2 and sulfate isotopic composition.

This study presents high-precision isotope ratio-mass spectrometric measurements of isotopic fractionation during oxidation of SO2 by OH radicals in the gas phase and H2O2 and transition metal ion catalysis (TMI-catalysis) in the aqueous phase. Although temperature dependence of fractionation factors was found to be significant for H2O2 and TMI-catalyzed pathways, results from a simple 1D model revealed that changing partitioning between oxidation pathways was the dominant cause of seasonality in the isotopic composition of sulfate relative to SO2. Comparison of modeled seasonality with observations shows the TMI-catalyzed oxidation pathway is underestimated by more than an order of magnitude in all current atmospheric chemistry models. The three reactions showed an approximately mass-dependent relationship between (33)S and (34)S. However, the slope of the mass-dependent line was significantly different to 0.515 for the OH and TMI-catalyzed pathways, reflecting kinetic versus equilibrium control of isotopic fractionation. For the TMI-catalyzed pathway, both temperature dependence and (33)S/(34)S relationship revealed a shift in the rate-limiting reaction step from dissolution at lower temperatures to TMI-sulfite complex formation at higher temperatures. 1D model results showed that although individual reactions could produce Δ(33)S values between -0.15 and +0.2‰, seasonal changes in partitioning between oxidation pathways caused average sulfate Δ(33)S values of 0‰ throughout the year.

Can Isotopologues Be Used as Biosignature Gases in Exoplanet Atmospheres?

Isotopologue ratios are anticipated to be one of the most promising signs of life that can be observed remotely. On Earth, carbon isotopes have been used for decades as evidence of modern and early metabolic processes. In fact, carbon isotopes may be the oldest evidence for life on Earth, though there are alternative geological processes that can lead to the same magnitude of fractionation. However, using isotopologues as biosignature gases in exoplanet atmospheres presents several challenges. Most significantly, we will only have limited knowledge of the underlying abiotic carbon reservoir of an exoplanet. Atmospheric carbon isotope ratios will thus have to be compared against the local interstellar medium or, better yet, their host star. A further substantial complication is the limited precision of remote atmospheric measurements using spectroscopy. The various metabolic processes that cause isotope fractionation cause less fractionation than anticipated measurement precision (biological fractionation is typically 2 to 7%). While this level of precision is easily reachable in the laboratory or with special in situ instruments, it is out of reach of current telescope technology to measure isotope ratios for terrestrial exoplanet atmospheres. Thus, gas isotopologues are poor biosignatures for exoplanets given our current and foreseeable technological limitations.

Reconciling discrepant minor sulfur isotope records of the Great Oxidation Event.

Understanding the timing and trajectory of atmospheric oxygenation remains fundamental to deciphering its causes and consequences. Given its origin in oxygen-free photochemistry, mass-independent sulfur isotope fractionation (S-MIF) is widely accepted as a geochemical fingerprint of an anoxic atmosphere. Nevertheless, S-MIF recycling through oxidative sulfide weathering-commonly termed the crustal memory effect (CME)-potentially decouples the multiple sulfur isotope (MSI) record from coeval atmospheric chemistry. Herein, however, after accounting for unrecognised temporal and spatial biases within the Archaean-early-Palaeoproterozoic MSI record, we demonstrate that the global expression of the CME is barely resolvable; thereby validating S-MIF as a tracer of contemporaneous atmospheric chemistry during Earth's incipient oxygenation. Next, utilising statistical approaches, supported by new MSI data, we show that the reconciliation of adjacent, yet seemingly discrepant, South African MSI records requires that the rare instances of post-2.3-billion-year-old S-MIF are stratigraphically restricted. Accepting others' primary photochemical interpretation, our approach demands that these implied atmospheric dynamics were ephemeral, operating on sub-hundred-thousand-year timescales. Importantly, these apparent atmospheric relapses were fundamentally different from older putative oxygenation episodes, implicating an intermediate, and potentially uniquely feedback-sensitive, Earth system state in the wake of the Great Oxidation Event.

Effects of iron and nitrogen limitation on sulfur isotope fractionation during microbial sulfate reduction.

Sulfate-reducing microbes utilize sulfate as an electron acceptor and produce sulfide that is depleted in heavy isotopes of sulfur relative to sulfate. Thus, the distribution of sulfur isotopes in sediments can trace microbial sulfate reduction (MSR), and it also has the potential to reflect the physiology of sulfate-reducing microbes. This study investigates the relationship between the availability of iron and reduced nitrogen and the magnitude of S-isotope fractionation during MSR by a marine sulfate-reducing bacterium, DMSS-1, a Desulfovibrio species, isolated from salt marsh in Cape Cod, MA. Submicromolar levels of iron increase sulfur isotope fractionation by about 50% relative to iron-replete cultures of DMSS-1. Iron-limited cultures also exhibit decreased cytochrome c-to-total protein ratios and cell-specific sulfate reduction rates (csSRR), implying changes in the electron transport chain that couples carbon and sulfur metabolisms. When DMSS-1 fixes nitrogen in ammonium-deficient medium, it also produces larger fractionation, but it occurs at faster csSRRs than in the ammonium-replete control cultures. The energy and reducing power required for nitrogen fixation may be responsible for the reverse trend between S-isotope fractionation and csSRR in this case. Iron deficiency and nitrogen fixation by sulfate-reducing microbes may lead to the large observed S-isotope effects in some euxinic basins and various anoxic sediments.