Global 3‐D Simulations of the Triple Oxygen Isotope Signature Δ¹⁷O in Atmospheric CO₂

Abstract

The triple oxygen isotope signature Δ17O in atmospheric CO2, also known as its “17O excess,” has been proposed as a tracer for gross primary production (the gross uptake of CO2 by vegetation through photosynthesis). We present the first global 3‐D model simulations for Δ17O in atmospheric CO2 together with a detailed model description and sensitivity analyses. In our 3‐D model framework we include the stratospheric source of Δ17O in CO2 and the surface sinks from vegetation, soils, ocean, biomass burning, and fossil fuel combustion. The effect of oxidation of atmospheric CO on Δ17O in CO2 is also included in our model. We estimate that the global mean Δ17O (defined as Δ17normalO=lnfalse(δ17normalO+1false)−λRL·lnfalse(δ18normalO+1false) with λRL = 0.5229) of CO2 in the lowest 500 m of the atmosphere is 39.6 per meg, which is ∼20 per meg lower than estimates from existing box models. We compare our model results with a measured stratospheric Δ17O in CO2 profile from Sodankylä (Finland), which shows good agreement. In addition, we compare our model results with tropospheric measurements of Δ17O in CO2 from Göttingen (Germany) and Taipei (Taiwan), which shows some agreement but we also find substantial discrepancies that are subsequently discussed. Finally, we show model results for Zotino (Russia), Mauna Loa (United States), Manaus (Brazil), and South Pole, which we propose as possible locations for future measurements of Δ17O in tropospheric CO2 that can help to further increase our understanding of the global budget of Δ17O in atmospheric CO2.

Triple Oxygen and Clumped Isotope Compositions of CO2 in the Middle Troposphere

“…However, the inter-hemispheric difference of Δ 17 O in meteoric water is not large (Elkins et al, 1996) and CO 2 equilibrates rapidly with the meteoric water. Therefore, we do not expect large difference in the Δ 17 O values of CO 2 from the northern hemisphere and southern hemisphere (also see Koren et al, 2019). Hoag et al (2005) also showed that despite a difference between the two hemispheres in fluxes of ocean sink and source and leaf/soil exchange, the model difference is only about 0.01 ‰.…”

“…This anomalous CO 2 can be transported from the stratosphere to the troposphere by the large scale Brewer-Dobson circulation and synoptic eddy diffusion. The mixing of this downwelling CO 2 with near normal lower level CO 2 may change the isotopic anomaly in the latter, providing an opportunity for determining CO 2 fluxes associated with biogeochemical cycles (Hoag et al, 2005;Koren et al, 2019;Liang, Mahata, Laskar, Thiemens, et al, 2017;Liang, Mahata, Laskar, & Bhattacharya, 2017;Young et al, 2014). A detectable level of anomaly and anomaly gradient would exist in the middle and lower troposphere if the rates of mass dependent surface processes are not fast enough to erase the signature from the stratosphere.…”

Supplementary material to "Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>"

“…Figure S13. A 3D model result for the seasonal cycle of the globalΔ 17 O value of leaf water for the year 2012 (Koren et al, 2019). Note that Δ 17 O value is reported here with l value of 0.5229.…”

“…Figure S14 Annual variability of ci/ca ratio for C4 and C3 plants from SiBCASA model (Schaefer et al, 2008;Koren et al, 2019). Table S1 Reproducibility of the extraction system and the CO2-O2 exchange system.…”

Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ<sup>17</sup>O of atmospheric CO<sub>2</sub>

“…The 17 O-excess of CO2 (Δ 17 O, involving the triple oxygen isotope composition of CO2, see equation 5) has been suggested as additional independent tracer for constraining global GPP (Hoag et al, 2005;Thiemens et al, 2013;Hofmann et al, 2017;Liang et al, 2017b;Koren et al, 2019). Because Δ 17 O is not or only slightly sensitive to mass-dependent fractionation processes acting on CO2 and H2O, its interpretation may be less sensitive to the effects mentioned above.…”