Plot showing methane isotopologue compositions and apparent 13CH3D equilibrium temperatures for various sources of methane in the environment (modified from Wang et al., 2015).
Methane clumped isotopologue
Currently the hottest topic in our laboratory is the measurement and interpretation of the doubly-isotope substituted “clumped” methane isotopologue, 13CH3D. Measurement of this rare isotopologue (a molecule with different isotope configurations) is of great interest since its abundance is expected to reflect the formation temperature of methane. We have been applying tunable laser spectroscopy, an emerging new technology in stable isotope research (Ono et al., 2014). We used this new tool to test the origin of methane in serpentinization sites (Wang et al., 2015), and apply research around deep biosphere (Inagaki et al., 2015), and microbial and atmospheric methane cycling (Wang et al., 2016, Whitehill et al., 2017).
Photochemistry to geology Archean sulfur mass-independent isotope fractionation
The sulfur isotope system provides a unique window to study the early evolution of microbial life and its impacts on atmospheric and ocean chemistry. Sulfur isotope evidence suggests that the sulfate-reducing metabolism dates back as old as 3.47 Ga, and that the Earth’s atmosphere was devoid of oxygen for about 2 billion years. We’re studying detailed sulfur isotope systematics of Archean rocks in South Africa and Western Australia. We’re also carrying out some photochemical experiments to identify the source reaction(s) of Archean sulfur mass-independent fractionation.
Physiology and sulfur isotope effects by sulfate reducing bacteria
Development of quantum cascade laser isotopomer instrument for tropospheric nitrous oxide monitoring
Nitrous oxide (N2O) is both a significant greenhouse gas (radiative forcing in 2009 ~ 0.17 W/m2) and a large contributor to the catalytic destruction of the stratospheric ozone layer. Its mole fractions in the atmosphere continue to rise from a natural preindustrial value of ~270 ppb to ~322 ppb today. We have funding from NSF (PI-Ron Prinn, Co-PI Ono, MIT, and David Nelson at Aerodyne Research) to develop and deploy an automated laser absorption spectroscopy to monitor isotopomer ratios of toropospheric nitrous oxide. The new instruments will monitor four isotopologues/isotopomers of nitrous oxide (15N14N16O, 14N15N16O, 14N14N18O and 14N14N16O) for every 30 min.
Image: Tropospheric N2O concentrations measured at AGAGE sites
Sulfide in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and recycled seawater-sulfate that is reduced during high temperature water rock interactions. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance between the two sources because δ34S values of vent fluid H2S and chimney sulfide minerals may reflect not only the mixing ratio but also the isotope exchange between sulfate and sulfide. We show that Δ33S (≡ δ33S – 0.515 δ34S) values can decouple isotope mixing and exchange because they result in different Δ33S values of up to 0.04 ‰ even if δ34S values are identical.
Multiple-S isotope probing for the deep biosphere
Recent studies suggest the presence of an extensive subsurface deep biosphere. The metabolic rates there are very slow such that it requires a very sensitive and robust technique to detect how active they are. The main objective of this project is to assess if basalt + seawater can sustain lithoautotrophic sulfate reduction during low-T ocean crust weathering. In my NSF funded research, I use high-precision S-33 analysis to tackle this problem. The basis for this work is our new S-33 data that shows that biogenic sulfide tends to be enriched in S-33 by ~0.1 permil. Our technique is precise enough to tell this small difference.