The role of calcification in carbonate compensation

The long-term recovery of the oceans from present and past acidification is possible due to neutralization by the dissolution of biogenic CaCO 3 in bottom sediments, that is, carbonate compensation. However, such chemical compensation is unable to account for all features of past acidification events, such as the enhanced accumulation of CaCO 3 at deeper depths after acidification. This overdeepening of CaCO 3 accumulation led to the idea that an increased supply of alkalinity to the oceans, via amplified weathering of continental rocks, must accompany chemical compensation. Here we discuss an alternative: that changes to calcification, a biological process dependent on environmental conditions, can enhance and modify chemical compensation and account for overdeepening. Using a simplified ocean box model with both constant and variable calcification, we show that even modest drops in calcification can lead to appreciable long-term alkalinity build-up in the oceans and, thus, create overdeepening; we term this latter effect biological compensation. The chemical and biological manifestations of compensation differ in terms of controls, timing and effects, which we illustrate with model results. To better predict oceanic evolution during the Anthropocene and improve the interpretation of the palaeoceanographic record, it is necessary to better understand biological compensation. Changes in calcification of marine organisms must be considered to explain the deepening of carbonate accumulation during ocean recovery from acidification events. According to a literature synthesis and modelling, dissolution of sedimentary carbonate is not sufficient to explain observations.