Planetary fertility during the past 400 ka based on the triple isotope composition of O2 in trapped gases from the Vostok ice core 1Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
10 Oct 2012
2Department of Geosciences, Princeton University, Guyot Hall, Princeton, NJ 08544, USA
3Department of Biology, Colorado State University, Ft. Collins, CO 80523, USA
*now at: Nicholas School of the Environment, Duke University, Durham, NC 27708, USA
Received: 02 Jan 2012 – Published in Clim. Past Discuss.: 30 Jan 2012 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
the total oxygen production during glacials was likely reduced.
Revised: 15 Aug 2012 – Accepted: 25 Aug 2012 – Published: 10 Oct 2012
Citation: Blunier, T., Bender, M. L., Barnett, B., and von Fischer, J. C.: Planetary fertility during the past 400 ka based on the triple isotope composition of O2 in trapped gases from the Vostok ice core, Clim. Past, 8, 1509-1526, doi:10.5194/cp-8-1509-2012, 2012.