Geocheminar winter-2016

Process fingerprinting with triple oxygen and clumped isotopes: new approaches to kinetic isotope effects

Mass-dependent fractionation of stable isotopes can be divided into two categories: equilibrium and kinetic. The studies of novel stable isotopic systems including triple oxygen and clumped isotopes have progressed significantly in the last ten years as new tools to quantity kinetic fractionation in hydrological and biochemical processes.

Recent advances in the ability to measure small differences in ?17O, the deviation from an expected relationship between 18O/16O and 17O/16O ratios, in both waters and low-temperature minerals and rocks (e.g., carbonates, bioapatites, silicates, oxides) present the opportunity to use triple oxygen isotope measurements in hydrological and paleoclimate studies. In particular, the sensitivity of ?17O to kinetic fractionation means that it can be used to constrain the influence of kinetic effects on variations in d18O. I review recently generated datasets on the triple oxygen isotope composition of the hydrosphere and show that there is considerably more variation in ?17O of continental waters than initially proposed, which is due to various processes including evaporation, evapotranspiration and supersaturation during snow formation. I also report the D17O data from an experimental study on synthetic carbonates to evaluate triple oxygen isotope fractionation during the precipitation of carbonate from water, and kinetic effect associated with acid digestion process of the carbonate. This makes it possible to facilitate the use of D17O of carbonate records, in combination with ?18O and ?47 data to permit more accurate reconstruction of d18O in meteoric waters from which the carbonate precipitated.

Finally, I present the first measurements of ?30 (the abundance of 15N15N relative to that predicted by chance alone) using the high-resolution Panorama mass spectrometer, and discuss the potential utility of ?30 as an independent proxy to trace the nitrogen cycle. The parameter ?30 is insensitive to the bulk 15N/14N isotopic ratio of the reservoir; instead, it reflects isotopic ordering in N2, which is altered when N-N bonds are made or broken. Our preliminary measurements of N2 from denitrifying soils and pure cultures of denitrifiers indicate large kinetic isotopic effects during N-N bond formation during denitrification. We also observed a nonstochastic excess in 15N15N was measured in tropospheric N2 [?30 = +19.05 ± 0.12 ‰ (1?)]. This excess likely comes from fixed-nitrogen loss processes in the biosphere. Variations in ?30 of N2 from pure culture experiments (+16.9 to +18.9‰) probably reflect the different isotopic signatures of distinct enzymes that catalyze N-N bond formation. Overall, our results suggest that the degree of isotopic ordering of tropospheric N2 may be applied to reflect the relative contributions of global natural N2 sources.