The devil's in the disequilibrium: sensitivity of ocean carbon storage to climate state and iron fertilization in a general circulation model

Abstract

Ocean dissolved inorganic carbon (DIC) storage can be conceptualized as the sum of four components: saturation (DIC_sat), disequilibrium (DIC_dis), carbonate (DIC_carb) and soft tissue (DIC_soft). Among these, DIC_dis and DIC_soft have the potential for large changes that are relatively difficult to predict. Here we explore changes in DIC_soft and DIC_dis in a large suite of simulations with a complex coupled climate-biogeochemical model, driven by changes in orbital forcing, ice sheets and the radiative effect of CO₂. Both DIC_dis and DIC_soft vary over a range of 40 μmol kg^−1 in response to the climate forcing, equivalent to changes in atmospheric CO₂ on the order of 50 ppm for each. We find that, despite the broad range of climate states represented, changes in global DIC_soft can be well-approximated by the product of deep ocean ideal age and the global export production flux, while global DIC_dis is dominantly controlled by the fraction of the ocean filled by Antarctic Bottom Water (AABW). Because the AABW fraction and ideal age are inversely correlated between the simulations, DIC_dis and DIC_soft are also inversely correlated. This inverse correlation could be decoupled if changes in deep ocean mixing were to alter ideal age independently of AABW fraction, or if independent ecosystem changes were to alter export and remineralization, thereby modifying DIC_soft. As an example of the latter, iron fertilization causes DIC_soft to increase, and causes DIC_dis to also increase by a similar or greater amount, to a degree that depends on climate state. We propose a simple framework to consider the global contribution of DIC_soft + DIC_dis to ocean carbon storage as a function of the surface preformed nitrate and DIC_dis of dense water formation regions, the global volume fractions ventilated by these regions, and the global nitrate inventory. More extensive sea ice increases DIC_dis, and when sea ice becomes very extensive it also causes significant O₂ disequilibrium, which may have contributed to reconstructions of low O₂ in the Southern Ocean during the glacial. Global DIC_dis reaches a minimum near modern CO₂ because the AABW fraction reaches a minimum, which may have contributed to preventing further CO₂ rise during interglacial periods.