CARBON ISOTOPE COMPOSITION OF ATMOSPHERIC CO2 DURING THE LAST ICE-AGE FROM AN ANTARCTIC ICE CORE

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 C-13 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 delta-C-13 was more negative than pre-industrial values by 0.3+/-0.2 parts per thousand. 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.