Feedback on Saturation State
The ocean has an inbuilt negative feedback process for stabilising the saturation rate with respect to calcium carbonate (Ω), albeit one that takes many millennia to counteract perturbations. This process is called carbonate compensation (Zeebe and Westbroeck, 2003). It involves automatic adjustment of the CaCO3 lysocline in response to any deviations in the CaCO3 saturation state of seawater. The lysocline is the depth at which significant amounts of calcium carbonate start to dissolve on the seabed. Above the lysocline, nearly all of the CaCO3 that falls to the seafloor accumulates in the sediments. Below the calcite lysocline the waters are corrosive to it, leading to dissolving of some or all of the CaCO3 within the sediments. The CaCO3 saturation state is equal to ([CO32-] [Ca2+] / Ksp).
As illustrated in the diagram to the right, carbonate compensation involves the adjustment of the depth of the CaCO3 lysocline in the ocean so as to regulate Ω. It acts as follows. As the ocean becomes more acid (as is happening at present), carbonate ion concentration ([CO32-]) will drop. Because the vertical profile of the saturating value of [CO32-] for each of aragonite and calcite does not change, the intersection point of the saturating and the actual [CO32-] will move to shallower depths. This will cause a shallowing of the calcite and aragonite saturation horizons (depths at which Ω=1). Jansen et al. (Jansen et al., 2002) give the following equations for calculating the saturating carbonate ion concentration ([CO32-]c(z), μMol kg-1) as a function of depth (z, km):
The depth of the saturation horizon is obtained by finding the value of z (the depth) at which [CO32-]c(z) is the same as the actual deep ocean [CO32-]. Although the lysocline does not lie exactly at the saturation horizon, the two are interlinked. These equations emphasise the sensitivity of saturation horizons to deep water [CO32-]. For instance, even only a 25% drop in average deep water [CO32-], taking it down from about 90 μMol kg-1 today (Broecker, 2003) to 67.5 μMol kg-1 in the future would, according to the equations, cause the global calcite saturation horizon to shallow from an average depth of 4 km to 2.2 km. Much larger decreases in deep [CO32-] are in fact forecast (section 2.1.1). A model of ocean acidification predicts, under almost all emissions scenarios, that the aver-age calcite lysocline depth will be shallower than 0.5 km by the year 2300 (Caldeira and Wickett, 2005).
Shallowing of the lysoclines will increase global dissolution and decrease burial. This will tend to oppose the low [CO32-] values of an acid ocean, by driving an imbalance between the riverine input of dissolved calcium and carbon and their removal in buried calcium carbonate. The chemical equa-tion for calcification and/or dissolution is given by: Ca + 2HCO CaCO3 + CO2(aq) + H2O (1) Dissolution of 1 Mole of CaCO3 (or an equivalent reduction in CaCO3 burial) therefore leads to ad-dition of 1 Mole of dissolved inorganic carbon to seawater, together with a simultaneous addition of 2 Moles (charge equivalents) of alkalinity. When the carbonate chemistry is calculated out, these changes to DIC and alkalinity induce an increase in the carbonate ion concentration, completing the negative feedback. This increase in ocean carbonate ion concentration will continue until the origi-nal, steady-state, carbonate ion concentration is re-attained. The e-folding response time (the time for the deviation of a variable from its equilibrium value to decrease by a factor of e) for carbonate compensation has variously been estimated at between 6 and 14 ky (Archer et al., 1997, 1998; Sundquist, 1986; Zeebe and Westbroek, 2003).
Carbonate compensation as described here is brought about purely by changes in lysocline depths in response to changes in Ω. There are likely to be several other changes to oceanic CaCO3 cycling that will also contribute to restoration of Ω to its equilibrium value: 1. production of CaCO3 may well decline in a more acidic surface ocean, as discussed above,, lead-ing to a lighter rain of particles out of the surface ocean and hence to a lighter rain to the sea floor; 2. CaCO3 that fell to the seafloor in pre-industrial times, before any ocean acidification, may also be affected. As this CaCO3, lying at or near to the surface of the sediments, comes into contact with increasingly acidic deep bottom waters, it is likely that some of it, finding itself now lying beneath a suddenly shallower lysocline, will be chemically eroded via dissolution. 3. the combination of warming, higher CO2 and increased rainfall in a warmer world may also has-ten the rate of weathering of limestone, dolomite and chalk rocks on land, leading to an increased supply rate of calcium and carbon to the oceans.
- R.E. Zeebe & P. Westbroek (2003) A simple model for the CaCO3 saturation state of the ocean: The "Strangelove", the "Neritan", and the "Cretan" Ocean. Geochemistry, Geophysics, Geosystems, 4: article number 1104.