Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core

Abstract

Reconstructions of atmospheric CO2 concentrations based on Antarctic ice cores(1,2) reveal significant changes during the Holocene epoch, but the processes responsible for these changes in CO2 concentrations have not been unambiguously identified. Distinct characteristics in the carbon isotope signatures of the major carbon reservoirs (ocean, biosphere, sediments and atmosphere) constrain variations in the CO2 fluxes between those reservoirs. Here we present a highly resolved atmospheric delta C-13 record for the past 11,000 years from measurements on atmospheric CO2 trapped in an Antarctic ice core. From mass-balance inverse model calculations(3,4) performed with a simplified carbon cycle model, we show that the decrease in atmospheric CO2 of about 5 parts per million by volume (p.p.m.v.). The increase in delta C-13 of about 0.25 parts per thousand during the early Holocene is most probably the result of a combination of carbon uptake of about 290 gigatonnes of carbon by the land biosphere and carbon release from the ocean in response to carbonate compensation of the terrestrial uptake during the termination of the last ice age. The 20 p.p.m.v. increase of atmospheric CO2 and the small decrease in delta C-13 of about 0.05 parts per thousand during the later Holocene can mostly be explained by contributions from carbonate compensation of earlier land-biosphere uptake and coral reef formation, with only a minor contribution from a small decrease of the land-biosphere carbon inventory.