Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the reliable use of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy have been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with the aerobic oxidation of methane (εH=49.4±5.0 ‰, εC=5.2±0.4 ‰) – the former being the first of its kind ever reported. This εH value is comparable with values reported from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between δ13CCH4 and δDCH4, Λ (9.4 or 8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach to other habitats with methanotrophic activity will help reveal whether the small εH value observed is a ubiquitous feature across all marine systems.

i_NRACM: incorporating ¹⁵N into the Regional Atmospheric Chemistry Mechanism (RACM) for assessing the role photochemistry plays in controlling the isotopic composition of NO_<i>x</i>, NO_<i>y</i>, and atmospheric nitrate

Nitrogen oxides, classified as NOx (nitric oxide (NO) + nitrogen dioxide (NO2)) and NOy (NOx+ NO3, N2O5 HNO3, + HNO4+ HONO + Peroxyacetyl nitrate (PAN) + organic nitrates + any oxidized N compound), are important trace gases in the troposphere, which play an important role in the formation of ozone, particulate matter (PM), and secondary organic aerosols (SOA). There remain many uncertainties in the origin and fate of atmospheric N compounds including the understanding of NOy cycling, NOx emission budgets, unresolved issues within the heterogeneous uptake coefficients of N2O5, and the formation of organic nitrates in urban forests, to name a few. A potential tool to resolve some of these uncertainties are using natural abundance N isotopes in NOy compounds. Here we have developed a photochemical mechanism used to simulate tropospheric photochemistry to include 15N compounds and reactions as a means to simulate δ15N values in NOy compounds. The 16 N compounds and 96 reactions involving N used in the Regional Atmospheric Chemistry Mechanism (RACM) were replicated using 15N in a new mechanism called iNRACM. The 192 N reactions in iNRACM were tested to see if isotope effects were relevant with respect to significantly changing the δ15N values (±1 ‰) of NOx, HONO, and/or HNO3. The isotope fractionation factors (α) for relevant reactions were assigned based on recent experimental or calculated values. Each relevant reaction in the iNRACM mechanism was tested individually and in concert in order to assess the controlling reactions. The controlling reactions and their diurnal importance are discussed. A comparison between iNRACM predictions and observed δ15N NO3- in particulate matter from Tucson, Arizona, suggests the model, and isotope fractionation factors incorporated into it, are accurately capturing the isotope effects occurring during the photochemistry of NOy. The implication is that measurements of δ15N in NOy compounds may be a new way of tracing in situ N chemistry and a means of assessing NOx emission budgets.

Hydrogen and carbon isotope fractionation factors of aerobic methane oxidation in deep-sea water

Isotope fractionation factors associated with various biogeochemical processes are important in ensuring the practicality of isotope tracers in biogeosciences at large. Methane is a key component of the subsurface biosphere and a notable greenhouse gas, making the accurate evaluation of methane cycles, including microbial methanotrophy, imperative. Although the isotope fractionation factors associated with methanotrophy been examined under various conditions, the dual-isotope fractionation factors of aerobic methanotrophy in oxic seawater column remain unclear. Here, we investigated hydrogen and carbon isotope ratios of methane as well as the relevant biogeochemical parameters and microbial community compositions in hydrothermal plumes in the Okinawa Trough. Methanotrophs were found to be abundant in plumes above the Hatoma Knoll vent site, and we succeeded in simultaneously determining hydrogen and carbon isotope fractionation factors associated with aerobic oxidation of methane (εH = 49.4 ± 5.0 ‰, εC = 5.2 ± 0.4 ‰) – the former being the first of its kind ever reported. This εH value is comparable with reported values from terrestrial ecosystems but clearly lower than those from aerobic and anaerobic methanotroph enrichment cultures, as well as incubations of methanotrophic isolates. The covariation factor between δ13CCH4 and δDCH4, Λ (9.4/8.8 determined using two different methods), was consistent with those from methanotrophic isolate incubations. These values determined herein are valuable for understanding dynamics of methane cycling in the marine realm, and future applications of the approach used herein to other habitats with methanotrophic activity will help reveal whether the small εH value observed herein is a ubiquitous feature across all marine systems.

Modeling the dynamic behavior of a droplet evaporation device for the delivery of isotopically calibrated low-humidity water vapor

A model is presented that gives a quantitative description of the dynamic behavior of a low-humidity water vapor generator in terms of water vapor concentration (humidity) and isotope ratios. The generator is based on the evaporation of a nanoliter-sized droplet produced at the end of a syringe needle by balancing the inlet water flow and the evaporation of water from the droplet surface into a dry-air stream. The humidity level is adjusted by changing the speed of the high-precision syringe pump and, if needed, the dry-air flow. The generator was developed specifically for use with laser-based water isotope analyzers in Antarctica, and it was recently described in Leroy-Dos Santos et al. (2021). Apart from operating parameters such as temperature, pressure, and water and dry-air flows, the model has as “free” input parameters: water isotope fractionation factors and the evaporation rate. We show that the experimental data constrain these parameters to physically realistic values that are in reasonable to good agreement with available literature values. With the advent of new ultraprecise isotope ratio spectrometers, the approach used here may permit the measurement of not only the evaporation rate but also the effective fractionation factors and isotopologue-dependent diffusivity ratios, in the evaporation of small droplets.

Bromine Isotope Variations in Magmatic and Hydrothermal Sodalite and Tugtupite and the Estimation of Br Isotope Fractionation between Melt and Sodalite

We determined the bromine isotope compositions of magmatic and hydrothermal sodalite (Na8Al6Si6O24Cl2) and tugtupite (Na8Al2Be2Si8O24Cl2) from the Ilímaussaq intrusion in South Greenland, in order to constrain the Br isotope composition of the melt and hydrothermal fluids from which these minerals were formed. Early formed magmatic sodalite has high Br contents (138 ± 10 µg/g, n = 5) and low δ81Br values (+0.23 ± 0.07‰). Late stage hydrothermal sodalite has lower Br contents (53±10 µg/g, n = 5) and higher δ81Br values (+0.36 ± 0.08‰). Tugtupite that forms at even later stages shows the lowest Br contents (26 ± 2 µg/g, n = 2) and the highest δ81Br values (+0.71 ± 0.17‰). One hydrothermal sodalite has a Br concentration of 48 ± 9 µg/g and an exceptionally high δ81Br of 0.82 ± 0.12‰, very similar to the δ81Br of tugtupites. We suggest that this may be a very late stage sodalite that possibly formed under Be deficient conditions. The data set suggests that sodalite crystallises with a negative Br isotope fractionation factor, which means that the sodalite has a more negative δ81Br than the melt, of −0.3 to −0.4‰ from the melt. This leads to a value of +0.5 to +0.6‰ relative to SMOB for the melt from which sodalite crystallises. This value is similar to a recently published δ81Br value of +0.7‰ for very deep geothermal fluids with very high R/Ra He isotope ratios, presumably derived from the mantle. During crystallisation of later stage hydrothermal sodalite and the Be mineral tugtupite, δ81Br of the residual fluids (both melt and hydrothermal fluid) increases as light 79Br crystallises in the sodalite and tugtupite. This results in increasing δ81Br values of later stage minerals that crystallise with comparable fractionation factors from a fluid with increasingly higher δ81Br values.

Measurement on Diffusion Coefficients and Isotope Fractionation Factors by a Through-Diffusion Experiment

For radioactive waste disposal, it is important that local groundwater flow is slow as groundwater flow is the main transport medium for radioactive nuclides in geological formations. When the groundwater flow is very slow, diffusion is the dominant transport mechanism (diffusion-dominant domain). Key pieces of evidence indicating a diffusion-dominant domain are the separation of components and the fractionation of isotopes by diffusion. To prove this, it is necessary to investigate the different diffusion coefficients for each component and the related stable isotope fractionation factors. Thus, in this study, through-diffusion and effective-porosity experiments were conducted on selected artificial materials and natural rocks. We also undertook measurements relating to the isotope fractionation factors of Cl and Br isotopes for natural samples. For natural rock samples, the diffusion coefficients of water isotopes (HDO and H218O) were three to four times higher than those of monovalent anions (Cl−, Br- and NO3−), and the isotope fractionation factor of 37Cl (1.0017–1.0021) was slightly higher than that of free water. It was experimentally confirmed that the isotope fractionation factor of 81Br was approximately 1.0007–1.0010, which is equivalent to that of free water. The enrichment factor of 81Br was almost half that of 37Cl. The effective porosity ratios of HDO and Cl were slightly different, but the difference was not significant compared to the ratio of their diffusion coefficients. As a result, component separation was dominated by diffusion. For artificial samples, the diffusion coefficients and effective porosities of HDO and Cl were almost the same; it was thus difficult to assess the component separation by diffusion. However, isotope fractionation of Cl and Br was confirmed using a through-diffusion experiment. The results show that HDO and Cl separation and isotope fractionation of Cl and Br can be expected in diffusion-dominant domains in geological formations.