Evaluating online calculations of oxygen clumped isotopes in a global chemistry-transport model for paleoatmospheric applications

The clumped-isotope composition of atmospheric oxygen varies in space and time, reflecting changes to atmospheric ozone concentrations, large-scale overturning rates, and climate. The preservation of a tropospheric clumped-isotope signal in ice cores therefore suggests that these properties are accessible within the atmospheric record. The paleoatmospheric signals, however, appear modest; moreover, the tropospheric clumped-isotope budget was likely different in the past. And while first-order insights on the ancient atmosphere may be gained from geochemical box modeling, online simulations of the clumped-isotope composition of atmospheric oxygen are ultimately required to test the veracity of atmospheric chemistry and climate models.

We report the first online simulations of the clumped isotopes of oxygen using the GEOS-Chem chemical transport model. The results are compared against atmospheric observations of the modern atmosphere from the surface up to 33 km, primarily in the midlatitudes. Measurement-model agreement is excellent in the mid-stratosphere, but disagreements are significant in the lower stratosphere and below. Model sensitivity tests suggest that lower-stratospheric ozone photochemistry and/or stratosphere-troposphere exchange dynamics are the origin of these disagreements. However, the mass-independent fractionation of oxygen isotopes during ozone formation may also play a role. Implications for paleoatmospheric applications will be discussed.