Planetary fertility during the past 400 ka based on the triple isotope composition of O₂ in trapped gases from the Vostok ice core

Abstract. The productivity of the biosphere leaves its imprint on the isotopic composition of atmospheric oxygen. Ultimately, atmospheric oxygen, through photosynthesis, originates from seawater. Fractionations during the passage from seawater to atmospheric O₂ and during respiration affect δ¹⁷O approximately half as much as δ¹⁸O. An "anomalous" (also termed mass independent) fractionation process changes δ¹⁷O about 1.7 times as much as δ¹⁸O during isotope exchange between O₂ and CO₂ in the stratosphere. The relative rates of biological O₂ production and stratospheric processing determine the relationship between δ¹⁷O and δ¹⁸O of O₂ in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of O₂ production by photosynthesis versus the rate of O₂–CO₂ isotope exchange in the stratosphere. However, the analysis of the ¹⁷O anomaly is complicated because each hydrological and biological process fractionates δ¹⁷O and δ¹⁸O in slightly different proportions. In this study we present O₂ isotope data covering the last 400 ka (thousand years) from the Vostok ice core. We reconstruct oxygen productivities from the triple isotope composition of atmospheric oxygen with a box model. Our steady state model for the oxygen cycle takes into account fractionation during photosynthesis and respiration by the land and ocean biosphere, fractionation during the hydrologic cycle, and fractionation when oxygen passes through the stratosphere. We consider changes of fractionation factors linked to climate variations, taking into account the span of estimates of the main factors affecting our calculations. We find that ocean oxygen productivity was within 20% of the modern value throughout the last 400 ka. Given the presumed reduction in terrestrial oxygen productivity, the total oxygen production during glacials was likely reduced.