Summary
The decrease of atmospheric CO2 concentration recorded in Antarctic ice around 1600 AD is one of the mostsignificant atmospheric changes to have occurred during the last millennia, before the onset of the industrial period.Together with the temperature decrease, the CO2 drop has been used to derive the sensitivity of carbon storesto climate. However, the cause of it is still under debate because models are not yet able to reproduce either itsmagnitude, or its timing. Here we present new measurements of the CO2 concentration decrease recorded in anice core from a medium accumulation rate site in Antarctica (DML). We show that the new record is compatible(differences <2 ppm) with the CO2 record from the high accumulation rate DSS site on Law Dome (East Antarctica),when the different age distributions are taken into account. We have also measured the d13C-CO2 change inDML ice, filling a gap around 1600 AD in the DSS d13C record. We use a double deconvolution of the CO2 andd13C records together to provide quantitative evidence that the CO2 decrease was caused by a change in the netflux to the terrestrial biosphere. Finally, we provide a new interpretation of a published record showing increasingatmospheric carbonyl sulphide during the CO2 decrease, suggesting that cooler LIA climate affected terrestrial biosphericfluxes. Altogether our findings support the hypothesis that reduced soil heterotrophic respiration is likely tohave given the most significant contribution to the LIA CO2 decrease implying a positive CO2-climate feedback.
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