Planetary fertility during the past 400 ka based on the triple isotope composition of O 2 in trapped gases from the Vostok ice core

• Published in: Climate of the Past
• Main Authors: J. C. von Fischer, B. Barnett, T. Blunier, M. L. Bender
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2012
• Subjects: Environmental pollution; TD172-193.5; Environmental protection; TD169-171.8; Environmental sciences; GE1-350; ice core
• Online Access: https://doi.org/10.5194/cp-8-1509-2012; https://doaj.org/article/5f31ab729b9c47b19a01649a5f3d511b

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 O 2 and during respiration affect δ 17 O approximately half as much as δ 18 O. An "anomalous" (also termed mass independent) fractionation process changes δ 17 O about 1.7 times as much as δ 18 O during isotope exchange between O 2 and CO 2 in the stratosphere. The relative rates of biological O 2 production and stratospheric processing determine the relationship between δ 17 O and δ 18 O of O 2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of O 2 production by photosynthesis versus the rate of O 2 –CO 2 isotope exchange in the stratosphere. However, the analysis of the 17 O anomaly is complicated because each hydrological and biological process fractionates δ 17 O and δ 18 O in slightly different proportions. In this study we present O 2 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.