A Mid Mesozoic Revolution in the regulation of ocean chemistry

Abstract

The Phanerozoic has seen fundamental changes in the global biogeochemical cycling of calcium carbonate (CaCO 3), particularly the advent of biomineralization during the early Cambrian when the products of weathering could first be removed through metabolic expenditure, and the subsequent ecological success of planktic calcifiers during the Mesozoic which shifted the locus of deposition from the continental shelves to the deep open ocean. These biologically-driven CaCO 3 depositional ‘mode’ changes along with geochemical and tectonic variations in boundary conditions such as sea-level and calcium ion concentrations all affected the carbonate chemistry of the ocean. I employ a model of atmosphere–ocean–sediment carbon cycling to explore the impact of these factors on the saturation state and carbonate chemistry of the global ocean during the Phanerozoic. 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 CO 2 in the atmosphere and of Ca 2+ in the ocean in determining surface pH. The uncertainty inherent in paleo CO 2 estimates then translates into sufficient uncertainty in reconstructions of Phanerozoic temperature variability that one can only deduce from the carbonate δ 18O 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.