Exploring shallow-water carbonate precipitation rates through geological time

Carbonate precipitation is a key part of the global carbon cycle and the rate at which this process takes place plays a pivotal role at many levels, from the production of shells and mineralized parts in calcifying organisms to determining the formation and evolution of carbonate platforms. To which extent and how biology and seawater chemistry steer carbonate precipitation, however, is still a matter of debate. We tried to explore the link between carbonate production rates on shallow water platforms in the geological past and some of the main parameters of the carbonate system in seawater, focusing in particular on carbonate saturation state (Ω). We examined 12 stratigraphic intervals of different tropical carbonate platforms, ranging from Devonian to Miocene in age, and estimated for each the carbonate production rate (G) as a mass of carbonate precipitated on a unit area in the unit time. As such, our Gs differ from the usually calculated accumulation rates which are instead computed as thicknesses on unit time and that, being influenced by accommodation, can be considered as crude estimates of actual carbonate production rate. Computing G in the fossil record poses challenges that range from the degree of preservation of the examined carbonate platform to the selection of the stratigraphic interval. Having such estimates and referring to modeled Ω in the geological past (Ridgwell, 2005) allows comparison to laboratory (Burton & Walter, 1987; Zhong & Mucci, 1989) and fields studies (e.g. Langdon et al., 2000) that investigate the influence of Ω on carbonate precipitation rates in abiotic and modern natural systems. In such comparison we also considered the influence of temperature – a parameter that strongly influences carbonate precipitation rates – and the issue represented by the fact that the impact of hiatuses in the completeness of the sedimentary record increases with the length of the time interval that is considered. A Generalized Linear Modeling approach (GLM) was applied on a dataset comprising fossil, modern and laboratory data (250 observations) in order to assess the importance and the mutual interaction of the proposed predictors on the carbonate precipitation rates. Results suggest that Ω is the most influential factor in determining the carbonate precipitation rates and a Ω/G relationship of fossil examples – which are characterized by the dominance of different carbonate precipitation modes – can be modeled by a relationship which is close to that that can model Ω/G of shallow water tropical carbonate systems and that both these latter share commonalities and differences with the empirical kinetic law that links carbonate precipitation rate and saturation state in abiotic systems. These results may help shedding light on the role of biotic influence and Ω in driving carbonate precipitation in the oceans through geological time and may offer insights for predicting future responses of marine carbonate ecosystems under changing environmental conditions.