Global 3-D Simulations of the Triple Oxygen Isotope Signature Delta O-17 in Atmospheric CO2

The triple oxygen isotope signature Delta O-17 in atmospheric CO2, also known as its "O-17 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 Delta O-17 in atmospheric CO2 together with a detailed model description and sensitivity analyses. In our 3-D model framework we include the stratospheric source of Delta O-17 in CO2 and the surface sinks from vegetation, soils, ocean, biomass burning, and fossil fuel combustion. The effect of oxidation of atmospheric CO on Delta O-17 in CO2 is also included in our model. We estimate that the global mean Delta O-17 (defined as Delta O-17=ln(delta O-17+1)-lambda RL center dot ln(delta O-18+1) with lambda(RL) = 0.5229) of CO2 in the lowest 500m of the atmosphere is 39.6per meg, which is similar to 20per meg lower than estimates from existing box models. We compare our model results with a measured stratospheric Delta O-17 in CO2 profile from Sodankyla (Finland), which shows good agreement. In addition, we compare our model results with tropospheric measurements of Delta O-17 in CO2 from Gottingen (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 Delta O-17 in tropospheric CO2 that can help to further increase our understanding of the global budget of Delta O-17 in atmospheric CO2.