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

Abstract

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 CO 2 . Both DIC dis and DIC soft vary over a range of 40 µmol/kg in response to the climate forcing, 5 equivalent to changes in atmospheric CO 2 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 10 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 15 extensive sea ice increases DIC dis , and when sea ice becomes very extensive it also causes significant O 2 disequilibrium, which may have contributed to reconstructions of low O 2 in the Southern Ocean during the glacial. Global DIC dis reaches a minimum near modern CO 2 because the AABW fraction reaches a minimum, which may have contributed to preventing further CO 2 rise during interglacial periods.

The influence of the ocean circulation state on ocean carbon storage and CO<sub>2</sub> drawdown potential in an Earth system model

“…DeVries and Primeau (2009). The effect of the air-sea disequilibrium of CO 2 and its connection with biological carbon sequestration was studied by Ito and Follows (2013) and in recent work by Eggleston and Galbraith (2017). Our theoretical approach is similar to those taken by Marinov et al (2008a), Marinov et al (2008b) and Kwon et al (2011).…”

“…Marinov et al, 2008b;Duteil et al, 2012;Bernardello et al, 2014;Eggleston and Galbraith, 2017), but in this case we expand this by using as many as four preformed tracers simultaneously. This eliminates errors that would result from using a linear regression for A pre (Lauderdale et al, 2013;Eggleston and Galbraith, 2017) or from the presence of oxygen disequilibrium in the calculation of AOU (Eq. B3), as discussed by Ito et al (2004).…”

“…OVT gr = ψ max gr − ψ min gr (5) The OVT measures the amount of flushing of the deep water, which is important for the carbon storage (Eggleston and Galbraith, 2017). Another important factor influencing the carbon pools is which overturning cell is dominating in terms of volume (Eggleston and Galbraith, 2017): the northern cell (producing North Atlantic Deep Water, NADW) or the southern cell (producing Antarctic Bottom Water, AABW, and Circumpolar Deep Water, CPDW).…”

“…Another important factor influencing the carbon pools is which overturning cell is dominating in terms of volume (Eggleston and Galbraith, 2017): the northern cell (producing North Atlantic Deep Water, NADW) or the southern cell (producing Antarctic Bottom Water, AABW, and Circumpolar Deep Water, CPDW). Since these water masses are of different origin, they will differ in properties, such as water temperature and nutrients, and will therefore have different capacities for holding C sat , C soft and C dis .…”

“…In a set of idealised GCM simulations, Eggleston and Galbraith (2017) show that CO 2 disequilibrium can be as important as C soft . In our ensemble, the effects of C dis appear to be particularly important in simulations with a lot of sea ice (e.g.…”