Student: David Valerio
Qualifying Exam Date: Thursday, July 1st, 2021
Time: 4:00 p.m.
The oxygen triple-isotope signature (Δ’17O = δ’17O – θ x δ’18O) of atmospheric O2 is an important end-member in isotopic mass-balance proxies of marine gross oxygen productivity estimated from O2 dissolved in seawater, global biospheric productivity measured from atmospheric O2 trapped in ice-cores, and planetary fertility determined from O2 evolved from ancient sulfates. We modified an existing chemical reaction network box model of the Δ’17O budget of atmospheric O2 to examine the dominant controls on this parameter and to identify ways to reconcile model predictions of Δ’17O with laboratory observations from two laboratories using distinct methodologies. The model is composed of five boxes: the stratosphere, the troposphere, the geosphere, the terrestrial biosphere/hydrosphere, and the marine biosphere/hydrosphere. We identify the isotope effect and global expression of biological O2 uptake pathways to be the dominant control on the Δ’17O of atmospheric O2, and find that the inclusion of Mehler-like reactions in marine cyanobacteria, previously neglected in the global O2 budget, with an O2 flux equal to ~40% of marine gross oxygen productivity resolves three separate problems at once: 1) interlaboratory disagreements about the Δ’17O of atmospheric O2, 2) incompatibility of model predictions of Δ’17O of atmospheric O2 with observations from two laboratories, and 3) significant discrepancies between concurrent measurements of Δ’17O-based gross oxygen productivity and 14C-based net carbon productivity. The addition of Mehler-like reactions as a biological O2 pathway relevant to the global O2 budget has important implications for the use of the Δ’17O of O2 as a proxy of biological productivity in the modern, on glacial-interglacial, and on geological timescales, and, more fundamentally, questions what variations in the Δ’17O of O2 indicate about global biogeochemical cycling.