Using stable isotopes and gas concentrations for independent constraints on microbial methane oxidation at Arctic Ocean temperatures

Microbial oxidation of methane in oxic water bodies is an important control on the amount of dissolved methane which is released from the ocean to the atmosphere. We explored the use of stable isotope methane spikes to quantify methane oxidation rates in Arctic seawater samples. A Picarro G2201‐i cavity ring‐down spectrometer was used to determine methane concentration and isotope ratios from headspace samples in foil incubators. The methane mass balance and the change in stable isotope ratios served as independent constraints on methane oxidation. For a fractionation factor of 1.025 oxidation rate constants determined with both methods agreed within 20% for small changes in isotope ratio (e.g., 10‰). For large changes in isotope ratio (e.g., 90‰), which was outside the calibration range, methods diverged. Rate constants down to 0.01 d−1 could be resolved with high statistical support. Stable isotope infrared spectroscopy to determine methane oxidation in foil incubators (ISMOFI) was successfully tested on under ice seawater from Utqiagvik, Alaska, by repeated sampling from each incubation vessel. Depending on the amount of isotope spike added, we determined oxidation rates of 0.15 ± 0.02 nmol L−1 d−1 at in situ methane concentration and a maximal oxidation potential of 271 ± 41 nmol L−1 d−1. The ISMOFI method permits variable incubation durations from days to months in a single incubator. The method is transportable and applicable in a variety of field or seagoing laboratory environments, and it avoids the use of hazardous substances such as radioisotopes and toxic chemicals

Thermoluminescence of shocked granodiorite

The Hardhat event was an underground nuclear detonation in granodiorite at the Nevada test site. Samples of this rock, representing all stages of peak shock pressure from zero to over 100 kilobars, were Investigated by thermoluminescence to determine whether or not the effects of shock on the quartz and feldspar of the granodiorite could be detected by variations In the shape of the glow curves. Natural thermoluminescence glow curves were obtained In order to detect any changes In the number of electrons trapped In the natural environment. Induced thermoluminescence glow curves were obtained for the detection of possible changes In the total number of electron traps In the crystal structure of the major mineral phases. Glow curves recording the natural thermoluminescence of the Hardhat samples generally displayed a single peak at about 225°C, which decreased In height and area Irregularly with decreasing distance from the shot point. Preshot samples showed the largest natural peak, while highly shocked samples showed essentially no natural thermoluminescence. This effect is attributed mainly to thermal draining of electrons from traps by the heat from the blast. Glow curves recording Induced thermoluminescence, l.e, thermoluminescence of samples which were previously Irradiated with gamma rays In the lab, showed variations In shape which are not, In general, correlated with peak shock pressure. The exact cause fdr this lack of correlation Is not known, but three possible explanations are suggested; compensating effects of shock; effect of heat from the blast on Induced thermoluminescence; and traps not of a type that could be affected by shock.Earth and Atmospheric Sciences, Department o