MECHANISMS CONTROLLING OCEAN CARBON UPTAKE AS THE EARTH ENTERED THE LAST ICE AGE

Many mechanisms have been proposed to explain the ∼85–90ppm decrease in atmospheric carbon dioxide (CO₂) during the last glacial cycle, between 127,000 and 18,000yrs ago. When taken together, these mechanisms can, in some models, account for the full glacial–interglacial CO₂ drawdown. Most proxy-based evaluations focus on the peak of the Last Glacial Maximum, 24,000–18,000yrs ago, and little has been done to determine the sequential timing of processes affecting CO₂ during the last glacial cycle. Here we use a new compilation of sea-surface temperature records together with time-sequenced records of carbon and Nd isotopes, and other proxies to determine when the most commonly proposed mechanisms could have been important for CO₂ drawdown. We find that the initial major drawdown of 35 ppm 115,000yrs ago was most likely a result of Antarctic sea ice expansion. Importantly, changes in deep ocean circulation and mixing did not play a major role until at least 30,000yrs after the first CO₂ drawdown. The second phase of CO₂ drawdown occurred ∼70,000 yrs ago and was also coincident with the first significant influences of enhanced ocean productivity due to dust. Finally, minimum concentrations of atmospheric CO₂ during the Last Glacial Maximum resulted from the combination of physical and biological factors, including the barrier effect of expanded Southern Ocean sea ice, slower ventilation of the deep sea, and ocean biological feedbacks.