Student: David Valerio
Department: Earth, Environmental and Planetary Sciences
Defense Date: Thursday, August 5th, 2021
Time: 2:00 p.m.
Title: Large contribution of light-dependent oxygen uptake to global O2 cycling
Abstract
The oxygen triple-isotope composition (Δ’ 17 O = δ’ 17 O – θ x δ’ 18 O) of atmospheric O 2 is an
important end-member in isotopic mass-balance proxies of marine gross oxygen productivity
estimated from O 2 dissolved in seawater and of global biospheric productivity quantified using
atmospheric O 2 trapped in glacial ice. We modified an existing chemical reaction network box
model of the Δ’ 17 O budget of atmospheric O 2 to examine the dominant controls on this parameter
and to identify ways to reconcile model predictions of Δ’ 17 O with air observations from
laboratories using different isotopic analysis methods. The model is composed of five boxes: the
stratosphere, the troposphere, the terrestrial biosphere/hydrosphere, and the marine
biosphere/hydrosphere. We identify the isotope effects and global expression of biological O 2
uptake pathways as the dominant control on the Δ’ 17 O of atmospheric O 2 , and find that the
inclusion of Mehler-like reactions in marine cyanobacteria, previously neglected in the global O 2
budget, with an O 2 flux equal to ~40-50% of marine gross oxygen productivity resolves three
problems at once: 1) interlaboratory disagreements about the Δ’ 17 O of atmospheric O 2 , 2)
incompatibility of model predictions of the Δ’ 17 O of atmospheric O 2 with air observations from
two laboratories, and 3) puzzling discrepancies between concurrent measurements of
Δ’1 7 O-based gross oxygen productivity and 1 4 C-based net carbon productivity. The addition of
Mehler-like reactions as a biological O 2 consumption pathway in the global O 2 budget has
important implications for the use of the Δ’ 17 O of O 2 as a proxy of biological productivity in the
modern and on glacial-interglacial cycles. More fundamentally, these findings question what
variations in the Δ’ 17 O of O 2 indicate about global biogeochemical cycling.
