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 plants^1,2,3, 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.