Other
Scientific paper
May 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agusm.v43c..01y&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #V43C-01
Other
0412 Biogeochemical Kinetics And Reaction Modeling (0414, 0793, 1615, 4805, 4912), 0414 Biogeochemical Cycles, Processes, And Modeling (0412, 0793, 1615, 4805, 4912)
Scientific paper
Here we present high-precision measurements of 18O/16O and 17O/16O in samples of tropospheric O2 using a standard calibrated with measurements of terrestrial and extraterrestrial rock samples. These new data provide a measure of Δ17O on an absolute scale that aids in the interpretation of the cause of the disparity in Δ17O between O2 in the troposphere and terrestrial rocks. We measured the isotopic composition of four separate aliquotes of ground-level air O2. Oxygen was isolated from air cryogenically using molecular sieve substrates. Correction was made for the influence of Ar scattered across the Faraday collectors (~0.06 per mil in δ17O) of the gas- source mass spectrometer. The reference gas used as an internal standard was calibrated against terrestrial rock samples and meteorites analyzed using infrared laser heating fluorination. All results are reported as linearized delta values (signified with a prime superscript symbol). With a mean terrestrial rock Δ17O'of 0.00 ‰ ± 0.02 we obtain Δ17O values of -0.25 ‰ ± 0.04 1σ, -0.22 ‰ ± 0.03, and -0.23 ‰ ± 0.05 for 5 mesosiderite meteorites, 7 pallasites, and 12 HED meteorites, respectively. The latter meteorite data are consistent with results from three other laboratories and serve to establish the absolute scale for the air O2 measurements. Our results for the O2 samples give a mean linearized δ18O' of 23.237 ‰ ± 0.008 1 std err (corresponding to a normal, non-linearized δ18O SMOW value of 23.509 ‰), a mean δ17O' of 11.922 ‰ ± 0.018, and a mean linearized Δ17O' of -0.347 ‰ ± 0.018 based on a rock-water terrestrial fractionation reference line with a slope (β) of 0.528. The latter is the exponent in a normal fractionation law described by the relation α17=(α18)β. This result can be reconciled with the suggestion by Young et al (2002) that the whole of the departure in Δ17O' of tropospheric O2 relative to terrestrial rocks can be attributed to respiration (a Δ17O Dole effect). Angert et al. (2003) measured β for respiration processes and obtained a net value that considers both dark respiration and photorespiration of 0.513. A steady state between photosynthesis (source of O2), with little oxygen isotope fractionation relative to waters, and respiration (sink for O2) with a β of 0.513 is -0.349. This value is obtained from the difference between β = 0.528 (rocks) and β=0.513 (respiration) applied over the 23.2 ‰ shift in 18O between ocean water and troposhere O2. The predicted steady-state value is indistinguishible from our measured value of -0.347 ‰. We conclude that there is good reason to believe that the source of the difference in Δ17O' between tropospheric O2 and terrestrial rocks is the Dole effect. It seems that Δ17O of tropospheric O2 as a biosignature in Earth's atmosphere and begs the question of how much of the signal can be the result of photochemistry.
Young Edward D.
Ziegler Karen
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