Clumped isotopes of atmospheric trace gases

Transcription

1 Clumped isotopes of atmospheric trace gases Thomas Röckmann Institute for Marine and Atmospheric research Utrecht (IMAU) Utrecht University, The Netherlands Thanks to: Magdalena Hofmann, Dipayan Paul, Elena Popa Overview Traditional vs. clumped isotope signatures Processes affecting clumped isotope anomalies Applications to atmospheric research The MAT 253 Ultra instrument at Utrecht University Outlook 1

2 Use of isotope measurements Additional (not always) independent observables But also additional unknowns (e.g. 18 O exchange with leaf water) Second order isotope effects sometimes less affected e.g. clumped isotopes Gillon and Yakir, Plant Phys, 2000 Clumped isotopes Molecules with multiple heavy isotopes CO 2 : C 18 O 16 O 12 C 18 O 18 O CH 4 : CH 3 D 12 CHD 2 N 2 O: N 15 N 18 O 15 N 15 N 16 O O 2 : 17 O 18 O 18 O 18 O N 2 : 15 N 15 N 2

3 Abundance of clumped isotopes Abundance CO 2 with C: 1.1% CO 2 with 18 O: 0.2 % CO 2 with C & 18 O:?? 1.1 % 0.2 % ( 2) stochastic (random) distribution almost, but not precisely Notation C= Traditional δ values for CO 2 C 12 C sample C 12 C VPDB O= 18 O 16 O sample 18 O 16 O VSMOW -1 Analogous definition C 18 O 16 O 12 C 16 O 16 O 47 = C 18 O 16 O 12 C 16 O 16 O sample reference Clumped isotope signature C 18 O 16 O 12 C 16 O 16 O C 18 O 16 O 12 C 16 O 16 O sample randomized sample -1 Deviation from random isotope distribution independent of bulk isotopic composition 3

4 What affects a clumped isotope signature? Thermodynamic equilibrium T information Non-equilibrium processes, e.g. Diffusion Chemical reactions Substrates (acetate-methyl vs. CO 2 /H 2 for CH 4 ) Biology/reversible reactions Statistics Temperature dependency of CDH 3 abundance Δ 18 ( ) equil. Homogenous isotope exchange reactions CDH CH 4 CH CDH 3 12 CD 2 H CH 4 12 CDH CDH 3 Low T: Higher abundance of CDH 3 Due to higher bond strength of doubly substituted isotopologue. High T: Stochastic composition C and D are randomly distributed within all CH 4 isotopocules. Clumped isotope thermometer Young et al.,

5 Clumped isotope thermometer and deviations Formation temperature of CH 4 Biological (kinetic) processes Δ 18 : Primarily C-D clumping in CH 4 Stolper et al., Science, 2014 Yeung et al., Science, 2015 Kinetics Whenever clumped isotope effect is NOT the sum of the individual isotope effects ε 1,2 ε 1 + ε 2 change in clumped isotope signal k heavy k light 1 Example: Isotope fractionation in diffusion of CO 2 in air: Fractionation C 16 O 16 O C 18 O 16 O -8.7 sum -.1 C 18 O 16 O difference

6 Thermodynamics and diffusion: Photosynthetic CO 2 47 out 47 in sunflower H. annuus H. hibernica ivy Z. mays maize signal independent of 18 O of H 2 O C m C a Overall effect: Photosynthesis lowers 47 of atmospheric CO 2 Hofmann et al., in prep, 2016 Statistics Two indistinguishable atoms (e.g. 18 O 18 O, 12 CH 2 D 2 ) fundamental issue with definition of Δ O 18 O 16 O 16 O 18 O 18 O 16 O 16 O sample randomized sample -1 Product of individual abundances not known for indistinguishable atoms Assign average value Error!! Negative clumping signal 6

7 O 2 from photosynthesis Starting atmospheric O 2 > 1.5 S h 4 S 3 Thermodynamic equilibrium 1.5 h O OA OB B H 2 O S 2 O 2 h S 1 h S 0 A H 2 O light dark O 2 formation at the oxygen evolving complex (5-step Kok cycle for the H 2 O splitting reaction 2H 2 O + 4 hν O2 + 4 H e - ) Photosynthetic O 2 << 1.5 Yeung et al., Science, 2015 Statistical negative clumped isotope anomalies Applicable to all systems with indistinguishable atoms! Information on heterogeneity of isotopic pools δ 1 vs. 1 vs. δ average Yeung et al results: Δ obs δ heter 50 heterogeneity Röckmann et al

8 Measurement difficulties abundance / precision Isotopocule Abundance C 18 O 16 O 4.6 * C 18 O 18 O 4.4 * 10 6 CH 3 D 6.6 * CHD * N 15 N 16 O 1.4 * N 15 N 18 O 1.6 * O 18 O 4.4 * 10 6 Measure these low abundances to better than precision 17 O 17 O 1.4 * 10 7 Measurement difficulties mass resolution Isotopocule Mass Isotopocule Mass ΔM CH 3 D CHD

9 The MAT Ultra IRMS at Utrecht University MAT 253 ULTRA Double focusing machine (electrostatic + magnetic) Mass resolution up to ! Resolves e.g. 14 N 2, 12 C 16 O, 12 CH 4 - High sensitivity with electron multipliers - Many double and potentially multiple substituted molecules Achieved in first weeks CO 2 : C 18 O clumping Δ 47 = O 18 O clumping Δ 48 = 0.03 O 2 : 17 O 18 O clumping Δ 35 = O 18 O clumping Δ 36 = O 17 O clumping Δ 34 = 0.1 CH 4 : C D clumping coming next Outlook Applications to atmospheric sciences CH 4 : Formation temperatures of methane thermogenic versus biogenic sources fundamental origin of Arctic methane CO 2 : constraints on photosynthesis? O 2 : tracer for oxidative processes? N 2 O: production pathways: nitrification denitrification Challenge for atmospheric measurements: large samples needed! 9

10 Explanation: different O sources in photosystem II S h 4 S 3 h O OA OB B H 2 O S 2 O 2 h S 1 h S 0 A H 2 O O 2 formation at the oxygen evolving complex (5-step Kok cycle for the H 2 O splitting reaction 2H 2 O + 4 hν O2 + 4 H e - ) Δ obs δ heter 50 Yeung et al., Science,

11 Temperature dependency of 18 O C 16 O abundance Enriched abundance of 18 O C 16 O Due to higher bond strength of doubly substituted isotopologue. Homogenous isotope exchange reaction C 16 O 16 O + 12 C 18 O 16 O 12 C 16 O 16 O + C 18 O 16 O Stochastic composition 18 O and C are randomly distributed within all CO 2 isotopologues. Clumped isotope thermometer Dennis et al.,