Carbon isotope composition of atmospheric CO₂ during the last ice age from an Antarctic ice core

Abstract

BUBBLES of ancient air in polar ice cores have revealed that the atmospheric concentration of CO₂ during the Last Glacial Maximum was 180–200 p.p.m.v., substantially lower than the pre-industrial value of about 280 p.p.m.v. (refs 1, 2). It is generally thought that this reduction in atmospheric CO₂ during glacial time was driven by oceanic processes. The most likely explanations invoke either a decrease in dissolved CO₂ in surface waters because of a more efficient 'biological pump' transporting carbon to deep waters, or a higher alkalinity in the glacial ocean as a consequence of changes in carbonate dissolution or sedimentation^3. Because isotope fractionation during photosynthesis depletes ¹³C in the organic matter produced, changes in the biological pump would alter the carbon isotope composition of atmospheric CO₂, whereas changes in alkalinity would in themselves have no such effect. Here we report measurements of the carbon isotope content of CO₂ in ice cores from Byrd Station, Antarctica, in an attempt to distinguish between these mechanisms. We find that during the ice age the reduced isotope ratio δ¹³C was more negative than pre-industrial values by 0.3 ±0.2%,. Although this result does not allow us to discriminate definitely between the two possible causes of lower glacial atmospheric CO₂. it does indicate that changes in the strength of the biological pump cannot alone have been responsible.