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Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere

Yeung, L. Y., L. T. Murray, J. L. Ash, E. D. Young, K. A. Boering, E. L. Atlas, S. M. Schauffler, R. A. Lueb, R. L. Langenfelds, P. B. Krummel, L. P. Steele, and S. D. Eastham, “Isotopic ordering in atmospheric O2 as a tracer of ozone photochemistry and the tropical atmosphere,” J. Geophys. Res. Atmos. 121 (2016) doi: 10.1002/2016JD025455.

JGR Editor’s highlight:

Yeung et al report novel observations of the oxygen isotopes of O2 that provide information about the evolution of ozone in Earth’s atmosphere. The measurements span from near the surface to 33 km, and both a box model and 3D chemical transport model help indicate where and how isotopic signatures are reset. Such a proxy will be helpful to chemistry-climate models investigating the evolution of ozone over time, and as they have shown it can be interpreted using models without needing to incorporate the isotope tracers into the models themselves.

Abstract
The distribution of isotopes within O2 molecules can be rapidly altered when they react with atomic oxygen. This mechanism is globally important: while other contributions to the global budget of O2 impart isotopic signatures, the O(3P) + O2 reaction resets all such signatures in the atmosphere on subdecadal timescales. Consequently, the isotopic distribution within O2 is determined by O3 photochemistry and the circulation patterns that control where that photochemistry occurs. The variability of isotopic ordering in O2 has not been established, however. We present new measurements of 18O18O in air (reported as Δ36 values) from the surface to 33 km altitude. They confirm the basic features of the clumped-isotope budget of O2: Stratospheric air has higher Δ36 values than tropospheric air (i.e., more 18O18O), reflecting colder temperatures and fast photochemical cycling of O3. Lower Δ36 values in the troposphere arise from photochemistry at warmer temperatures balanced by the influx of high-Δ36 air from the stratosphere. These observations agree with predictions derived from the GEOS-Chem chemical transport model, which provides additional insight. We find a link between tropical circulation patterns and regions where Δ36 values are reset in the troposphere. The dynamics of these regions influences lapse rates, vertical and horizontal patterns of O2 reordering, and thus the isotopic distribution toward which O2 is driven in the troposphere. Temporal variations in Δ36 values at the surface should therefore reflect changes in tropospheric temperatures, photochemistry, and circulation. Our results suggest that the tropospheric O3 burden has remained within a ±10% range since 1978.

Laurence Yeung wins 2016 F. W. Clarke award from the Geochemical Society

Yeung headshotClarke medal

Laurence Yeung, assistant professor of Earth Science, will be awarded the F. W. Clarke medal from the Geochemical Society at this year’s V. M. Goldschmidt meeting in Yokohama, Japan. The award is named after Frank Wigglesworth Clarke, who determined the composition of the Earth’s crust and is considered by many to be the father of Geochemistry. From the Geochemical society’s announcement:

The Clarke Award recognizes an early-career scientist for a single outstanding contribution to geochemistry or cosmochemistry published either as a single paper or a series of papers on a single topic. Prof. Yeung is recognized for developing, both experimentally and theoretically, a new clumped isotopologue system with applications to natural systems.

With Dr. Yeung’s award, the Department of Earth Science now has three F. W. Clarke medalists: Profs. Cin-Ty Lee (2009), Rajdeep Dasgupta (2011), and Laurence Yeung (2016). We are tied (with Caltech) for the most Clarke medalists in any department in the world. Here’s to many more!

Link to story on Rice News

Combinatorial effects on clumped isotopes and their significance in biogeochemistry

A new paper from Laurence Yeung on the fundamentals of “clumped-isotope” fractionation, was recently accepted in Geochimica et Cosmochimica Acta. It shows, through simple theoretical arguments, the factors influencing the occurrence of rare-isotope pairs in molecules when they are made. One might be able to base future tracers of biogeochemistry on these principles.

One of the findings is also a convenient practical summary: When playing Craps, never bet on snake eyes if you suspect the dice are loaded―it, along with the other hard rolls (double threes, double fours, etc.) are less likely to come up when the dice are not evenly weighted.

doi: 10.1016/j.gca.2015.09.020

Abstract

The arrangement of isotopes within a collection of molecules records their physical and chemical histories. Clumped-isotope analysis interrogates these arrangements, i.e., how often rare isotopes are bound together, which in many cases can be explained by equilibrium and/or kinetic isotope fractionation. However, purely combinatorial effects, rooted in the statistics of pairing atoms in a closed system, are also relevant, and not well understood. Here, I show that combinatorial isotope effects are most important when two identical atoms are neighbors on the same molecule (e.g., O2, N2, and D-D clumping in CH4). When the two halves of an atom pair are either assembled with different isotopic preferences or drawn from different reservoirs, combinatorial effects cause depletions in clumped-isotope abundance that are most likely between zero and –1‰, although they could potentially be –10‰ or larger for D-D pairs. These depletions are of similar magnitude, but of opposite sign, to low-temperature equilibrium clumped-isotope effects for many small molecules. Enzymatic isotope-pairing reactions, which can have site-specific isotopic fractionation factors and atom reservoirs, should express this class of combinatorial isotope effect, although it is not limited to biological reactions. Chemical-kinetic isotope effects, which are related to a bond-forming transition state, arise independently and express second-order combinatorial effects related to the abundance of the rare isotope. Heteronuclear moeties (e.g., C–O and C–H), are insensitive to direct combinatorial influences, but secondary combinatorial influences are evident.

In general, both combinatorial and chemical-kinetic factors are important for calculating and interpreting clumped-isotope signatures of kinetically controlled reactions. I apply this analytical framework to isotope-pairing reactions relevant to geochemical oxygen, carbon, and nitrogen cycling that may be influenced by combinatorial clumped-isotope effects. These isotopic signatures, manifest as either directly bound isotope “clumps” or as features of a molecule’s isotopic anatomy, are linked to molecular mechanisms and may eventually provide additional information about biogeochemical cycling on environmentally relevant spatial scales.

Read more about the research in the Yeung Lab at yeunglab.org.

New stable isotope ratio mass spectrometer from Nu instruments installed in the Yeung Lab

New stable isotope ratio mass spectrometer from Nu instruments installed in the Yeung Lab. Head over to their blog for updates!

SCIENCE: Biological signatures in clumped isotopes of O2

New faculty paper out! See Laurence Yeung’s paper on a new type of biological signature from the “clumping” of rare isotopes in O2, published in Science. [article]

Abstract

The abundances of molecules containing more than one rare isotope have been applied broadly to determine formation temperatures of natural materials. These applications of “clumped” isotopes rely on the assumption that isotope-exchange equilibrium is reached, or at least approached, during the formation of those materials. In a closed-system terrarium experiment, we demonstrate that biological oxygen (O2) cycling drives the clumped-isotope composition of O2 away from isotopic equilibrium. Our model of the system suggests that unique biological signatures are present in clumped isotopes of O2—and not formation temperatures. Photosynthetic O2 is depleted in 18O18O and 17O18O relative to a stochastic distribution of isotopes, unlike at equilibrium, where heavy-isotope pairs are enriched. Similar signatures may be widespread in nature, offering new tracers of biological and geochemical cycling.