A Mid Mesozoic Revolution in the regulation of ocean chemistry

The model results highlight that overall; the time evolution and regulation of Phanerozoic ocean chemistry are dominated by a Mid Mesozoic Revolution in the marine carbonate cycle. Prior to this transition, it was possible for the ocean to attain states of extreme saturation during the Permian and Triassic as well as during the late Precambrian. This is primarily a consequence of low sea-level in restricting the potential area for the deposition of shallow water carbonates, thus requiring a more saturated ocean and higher rate of precipitation per unit area in order to balance weathering input. This is consistent with the occurrence of mineralogically 'anomalous' carbonates during these periods but not commonly at other times. That the modern carbon cycle does not respond to similar tectonic forcings is due to the ecological success of calcifying planktic taxa during the Mesozoic, which in facilitating the creation of a responsive deep-sea carbonate sink enabled a much greater degree of regulation of saturation state than was previously possible. The model results also highlight the primary role of changes in the concentration of CO2 in the atmosphere and of Ca2+ in the ocean in determining surface pH. The uncertainty inherent in paleo CO2 estimates then translates into sufficient uncertainty in reconstructions of Phanerozoic temperature variability that one can only deduce from the carbonate δ18O record that the Cretaceous was generally warmer and the Carboniferous colder than average. The substantially enhanced oceanic carbon inventory predicted for the Paleozoic suggests that previous calculations of methane hydrate release may have substantially underestimated the quantity of clathrate carbon required to explain observed carbon isotopic excursions. In both cases the importance of quantifying Phanerozoic marine chemistry is clear.