How is the Ocean Carbon Reservoir Filled?

Since the beginning of the industrial era humankind has been changing atmospheric composition by emitting large amounts of carbon dioxide. About a quarter of these emissions are taken up by the ocean. The rate limiting step of this uptake is the transport of the anthropogenic carbon (C_ant) from the surface where it is absorbed, to the deep where it is stored. The transport and distribution of C_ant remains uncertain, since C_ant cannot be measured directly and the calculations require complex methods. Here we use a transport matrix from an inverse mixing model to reconstruct the penetration of C_ant into the ocean. The time-evolving surface boundary condition for the C_ant was determined by using surface pCO₂ growth rates from the Norwegian Earth System model and climatological distributions. The transport matrix connects 11113 possible surface sources to the ocean interior assuming steady state circulation. With this approach, we are able to calculate the total amount of C_ant that has penetrated the ocean from each surface source. Preliminary results show that the Antarctic Subtropical Front, together with the Ross sea are the main conduits for the transport of C_ant into the ocean interior, and to a lesser extent, the Nordic and Labrador seas. Since the approach does not rely on chlorofluorocarbons (CFCs) for estimating the surface-to-interior transit times—these are constrained by radiocarbon—we avoid possible biases from the different behavior of CFCs and C_ant regarding solubility and rate of gas exchange, as well as the different atmospheric time-scales. We aim to improve the current estimates of C_ant transport and provide high resolution view of its entry pathways in the ocean.