| Summary: | In present-day seas, animals, algae, and protozoa are threatened by ocean acidification, amplified in many regions by seawater warming and hypoxia (Doney et al . 2009 ). Many species may be affected adversely by 21st-century environmental change, but a decade of research suggests that the hypercalcifying animals responsible for reef accretion may be especially vulnerable to an acidity-driven decrease in the saturation state (Ω; see Box 1.1) of surface seawater with respect to calcite and aragonite. The geological record reveals that natural changes in the marine carbonate system have affected the evolution and abundance of calcifying organisms throughout the Phanerozoic Eon (542 million years (Myr) ago to the present). This being the case, we can use our understanding of the dynamic behaviour of the carbon cycle and the stratigraphic comings and goings of reef-building organisms to inform us about what, if any, lessons can be drawn from the long-term past and applied to our nearterm future. If there is one thing that geology makes clear it is that the earth and its biota are in a continual state of change. Because of its relationship to climate, the partial pressure of CO2 (pCO2) in the atmosphere has been of particular interest to geologists and geochemists, but direct measurement of ancient CO2 levels is impossible for intervals older than those recorded in glacial ice preserved today near the poles and at high altitude (Petit et al . 1999). Therefore, deep-time estimates of pCO2 rely on models, broadly constrained by geochemical proxy data. For example, the widely applied models of Berner and colleagues (e.g. GEOCARB III; Berner and Kothavala 2001; Berner 2006; Fig. 4.1C) estimate fluxes of carbon from one reservoir to another, based on geochemical proxies (mainly isotope ratios and abundances of sedimentary carbonate and organic carbon), and then calculate successive steady states of the system through time. Additional parameters are considered, including estimates of carbon fluxes due to erosion, river ...
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