Methane Clumped Isotopologue Variability from Ebullition in a Mid-latitude Lake

Methane is a greenhouse gas and is an important component of carbon cycling in freshwater environments. Isotope ratios of methane (13C/12C and D/H) are used extensively as tracers to identify methane sources. Recent advances in the measurement of clumped methane isotopologues (13CH3D, 12CH2D2) offer new opportunities to constrain sources and sinks of atmospheric methane. Previous measurements of clumped methane isotopologues from freshwater environments have been spatially and temporally limited. The abundance of 13CH3D and methane flux from ebullition in the deep basin of Upper Mystic Lake were measured from May to November 2021 to characterize the source isotopologue signatures and methane fluxes for mid-latitude lakes. The trends in δ13C and δD values support decreased methane oxidation in the early summer compared to fall. The Δ13CH3D values from this study range from 2.0 to 4.2‰, reflecting methane oxidation occurring anaerobically in lake sediments and euxinic bottom waters at sample sites. The relatively large variation in the Δ13CH3D values observed within this lake basin aligns with previous observations of bubbles from arctic lakes. The values of Δ13CH3D do not correlate with methane flux, suggesting that Δ13CH3D measurements from background ebullition are not sensitive as a proxy for ebullition rates. This study presents a uniquely large (n = 40) set of freshwater Δ13CH3D values from a single lake basin, which we use to recommend a sampling strategy of ≥9 samples to constrain the Δ13CH3D source signal within ∼0.5‰ from similar environments. This data demonstrates the utility of clumped methane isotopologues to gain insights into local biogeochemical processes from field studies and points to the challenge of using clumped isotopologue measurements to constrain global freshwater–methane sources to the atmosphere

High-Precision Measurements of ³³S and ³⁴S 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 ³³S and ³⁴S. 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 ³³S/³⁴S 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 Δ³³S values between −0.15 and +0.2‰, seasonal changes in partitioning between oxidation pathways caused average sulfate Δ³³S values of 0‰ throughout the year

Raw data acquired during the growth of <i>E</i>. <i>coli</i>.

The data were not filtered or smoothed. The two insets show the behavior of the sensor at intervals outlined by the two rectangles and share the same units on the x- and y-axes. The zero point was re-calibrated by flushing the chamber with oxygen-free nitrogen after the end of the experiment.</p

Chemical structure of 5,10,15,20-Tetrakis(pentafluorophenyl)-21H,23H-porphyrin palladium(II) [10], the dye used in our sensor patches.

Image generated from PubChem data using OpenBabel [12].</p

A graph illustrating the calibration procedure.

(A) Behavior of luminescence lifetime during calibration (blue). The total volume of liquid injected is shown as red dots, orange crosses indicate injection points on the lifetime graph. After injections, the bottle was flushed with 75 ppm oxygen reference gas and equilibrated with the ambient pressure (green rectangle). (B) The luminescence lifetime of the dye as a function of the effective injected reference liquid volume. Red line is the initial approximation and blue line is the final approximation that compensates for the unintentional inflow of oxygen. (C) Effective amount of oxygen in the gas phase in the units of the injected volume of the reference solution. Red line shows the initial approximation and blue the final one. (D) Calibration curves for the luminescence lifetime as a function of the partial pressure of oxygen with 75 ppm oxygen reference gas in N2 used a calibration point. Red is the initial approximation and blue the final one.</p

General operating principle of luminescence lifetime sensors based on sine wave attenuation.

General operating principle of luminescence lifetime sensors based on sine wave attenuation.</p

Electrical schematics of the main components of the analog board.

A) LED driver circuit. B) Transimpedance amplifier with a programmable gain amplifier. Full schematics are available in the S1 File. Temperature coefficients (in ppm) and dielectric specifications for critical resistors and capacitors are listed in the S1 File as well.</p

Comparison between oxygen sensors talked about in this article.

Comparison between oxygen sensors talked about in this article.</p

Attachment system.

(Left) Close-up rendering of the attachment system for a 250 mL medium bottle. Two vertical rod magnets are used to align the sensor patch. Plastic parts are rendered as partially transparent. (Right) The full set of designed attachments.</p

Finished sensor system design attached to a serum bottle.

Right part shows the screen and highlights the most important information shown. A highly degraded sensor patch was used to make the background clearly visible. The background is negligible compared to the luminescence for a non-degraded sensor patch.</p