Triple-isotope composition of atmospheric oxygen as a tracer of biosphere productivity

Abstract

Oxygen has three naturally occurring isotopes, of mass numbers 16, 17 and 18. Their ratio in atmospheric O2 depends primarily on the isotopic composition of photosynthetically produced O2 from terrestrial and aquatic plants1,2,3, and on isotopic fractionation due to respiration4. These processes fractionate isotopes in a mass-dependent way, such that 17O enrichment would be approximately half of the 18O enrichment relative to 16O. But some photochemical reactions in the stratosphere give rise to a mass-independent isotope fractionation, producing approximately equal 17O and 18O enrichments in stratospheric ozone5 and carbon dioxide6,7, and consequently driving an atmospheric O2 isotope anomaly. Here we present an experimentally based estimate of the size of the 17O/16O anomaly in tropospheric O2, and argue that it largely reflects the influences of biospheric cycling and stratospheric photochemical processes. We propose that because the biosphere removes the isotopically anomalous stratosphere-derived O2 by respiration, and replaces it with isotopically ‘normal’ oxygen by photosynthesis, the magnitude of the tropospheric 17O anomaly can be used as a tracer of global biosphere production. We use measurements of the triple-isotope composition of O2 trapped in bubbles in polar ice to estimate global biosphere productivity at various times over the past 82,000 years. In a second application, we use the isotopic signature of oxygen dissolved in aquatic systems to estimate gross primary production on broad time and space scales.