Carbonate saturation state vs carbonate production rates in the geological past: in search for a paradigm

Saturation state with respect to carbonate (Ω) plays a crucial role in influencing carbonate precipitation in seawater, which is one of the fundamental processes of the global inorganic carbon cycle. It has been shown that a broad positive correlation exists between modeled variation of Ω in the past and the abundance of carbonates in the geological record. With the goal of investigating further the influence of Ω on the calcification process in the oceans, we have examined several carbonate platforms, with age ranging from the Devonian to the Neogene, and estimated an average carbonate production rate (G, expressed as a mass/unit time/unit surface) for each case study. This task required estimating the volumes of carbonates deposited in selected time intervals, and hence, platforms were chosen for being well age constrained and for the good outcrop conditions. The investigated platforms cover nearly 400 Myr of the geological record and are characterized by significantly different carbonate factories and modes of carbonate precipitation. We are aware that a number of factors (e.g. the existence of hiatuses) can affect such type of calculations; however, we coped with them by applying some corrections, where possible. For instance, a correction factor was applied to G estimates based on the evidence that the longer the time interval considered, the lower the observed accumulation rates because larger hiatuses are incorporated. When G estimates for the considered carbonate platforms are plotted in a log/log graph as a function of modeled Ω variations in the geological time (Ridgwell, 2005), they appear aligned and identify a logarithmic relationship. Such relationship closely resembles, and displays similar slope to, the empirical kinetic law that links G and Ω. A significant difference observed, however, is that Gs attained by carbonate platforms are higher than those predicted by the empirical kinetic law. Results of our investigation suggest that a power law linking G and Ω may govern precipitation in shallow water carbonate systems regardless of the organisms involved and, therefore, Ω could have been the main driver of global calcification in the geological past. The higher Gs attained by carbonate platforms with respect to those predicted by the empirical G/Ω kinetic law are consistent with laboratory evidence that biologically mediated calcification occurs at higher rates than inorganic precipitation. However, the fact that this is here observed considering ancient carbonate platforms dominated by different organisms, suggests that this biotic effect may be due to mechanisms that could be ecology¬ independent.