The exchange of CO₂ between the ocean and atmosphere is important for regulation of the Earth’s climate on centennial and millennial timescales. Ice core records reveal that over the past million years the Earth’s climate has been characterized by oscillations between cold, glacial climates with low atmospheric CO₂, and warm, interglacial climates with high atmospheric CO₂. Recent modeling studies and paleoceanographic data suggest that these variations in atmospheric CO₂ might be due to variations in the ventilation of deep ocean waters, with cold glacial states characterized by sluggish ventilation, causing buildup of respired carbon in the deep ocean.

This dissertation presents three related studies that reveal how the ventilation state of the ocean may be inferred from both modern and paleoceanographic data, and how the ventilation state of the ocean may affect the ability of the ocean to sequester CO₂. The first study shows how changes in low-latitude sea-surface temperature may have contributed to the observed glacial-interglacial CO₂ variations. A novel diagnostic formula is used to show that the sensitivity of atmospheric CO₂ to changes in low-latitude temperature depends on how the interior ocean is ventilated. In particular, low-latitude sensitivity is enhanced when large portions of the ocean are ventilated from the sub-tropical and North Atlantic regions. In the second study, a new method is presented for inferring paleo-ventilation ages from sediment-core radiocarbon data. The new method is validated against results from an ocean general circulation model, and shown to be much more accurate than existing methods. Application of this method to sediment-core radiocarbon data from the deep northeast Pacific ocean reveals that ventilation ages were most likely older than present during the last glacial maximum, but that significant uncertainties still remain. In the third study, a global inverse ocean circulation model is developed that constrains ventilation rates and pathways for the modern ocean. Unlike previous inverse models, the model estimates are constrained not only by observed tracer distributions, but also by dynamical principles. The model reveals that most of the interior ocean is ventilated through the Southern Ocean, and that the oldest ocean waters are found in the mid-depth North Pacific where ventilation ages are around 1300-1400 years.