Ice core record of CO variations during the last two millennia: atmospheric implications and chemical interactions within the Greenland ice

In order to study in detail the pre-industrial CO level during the last two millennia and its temporal variations, several ice cores from Greenland and Antarctica were analysed. Our Antarctic CO results remain very close to those observed previously for the last 150 years and suggest that carbon monoxide concentration did not change greatly over Antarctica during the last two millennia. Between 1600 and 1800 AD, CO concentrations obtained in the Greenland ice are also very close to those already reported for the 1800–1850 AD period. In contrast, the oldest part of the Greenland CO profile exhibits high CO levels (100–180 ppbv) characterised by a strong variability. This part of the Greenland record likely does not reflect the true atmospheric CO concentrations. We discuss the possible processes which could have altered the atmospheric CO signal either before or after its trapping in the ice. The oxidation of organic material in the oldest part of the investigated Greenland ice appears as the most likely explanation. Because there are strong similarities between the Greenland CO and CO₂ concentration profiles for the 1000–1600 AD period, mechanisms involved in both cases could be at least partly the same. Therefore, oxidation of organic materials is a serious candidate for in-situ CO₂ production in the Greenland ice. Due to the fact that the Antarctic ice contains much less impurities and show no peculiar variability in CO concentrations, we are more confident about the atmospheric significance of our Antarctic CO concentration profile.