Ventilation of the deep ocean constrained with tracer observations and implications for radiocarbon estimates of ideal mean age

Abstract

Ocean ventilation is the process that transports water and climatically important trace gases such as carbon dioxide from the surface mixed layer into the ocean interior. Quantifying the dominant source regions and time scales remains a major challenge in oceanography. A mathematically rigorous approach, that accounts for the multiplicity of transport pathways and transit times characteristic of an eddy-diffusive flow such as the ocean, is to quantify ventilation in terms of a probability distribution that partitions fluid parcels according to the time and location of their last surface contact. Here, we use globally gridded radiocarbon data in combination with other transient (CFCs) and hydrographic (temperature, salinity, phosphate, and oxygen) tracer data to estimate the joint distribution of age and surface origin of deep ocean waters. Our results show that ~40% and 26% of the global ocean was last in contact with the Southern Ocean and North Atlantic, respectively. Some 80% of the global deep ocean below 1500m is ventilated from these high latitude regions. However, contrary to the classical description of the deep ocean as a roughly equal mixture of “northern” and “southern” source waters, we find a significantly higher contribution from the Southern Ocean relative to the North Atlantic. We estimate the mean transit time from the surface to the deep North Pacific at 1360±350y, intermediate between two widely used radiocarbon-based estimates. To reconcile our estimate of the ideal mean age with ventilation age estimates based on radiocarbon, we apply the estimated distribution function to construct a 3-dimensional distribution of the water mass fraction-weighted surface “initial” radiocarbon concentration that can serve as an accurate reservoir age. Radiocarbon ages corrected for this initial reservoir age are found to be in good agreement (within 5%) with our ideal age estimate, demonstrating that it is essential to take into account the spatially variable surface radiocarbon field when computing ventilation ages using radiocarbon. A wide spectrum of ages contributes to the mean age, providing evidence for the fundamentally eddy-diffusive nature of the large-scale general circulation of the ocean.