Modeling the carbon and sulfur isotope compositions of marine sediments: Climate evolution during the Devonian

A model of global biogeochemical cycles coupled to an energy-balance climatic model (modified after the COMBINE model; [Goddéris, Y., Joachimski, M.M., 2004. Global change in the Late Devonian: modeling the Frasnian–Famennian short-term carbon isotope excursions. Palaeogeogr. Palaeoclimatol. Palaeoecol. 202, 309–329]) is used to calculate the short-term evolution of atmospheric pCO2 during the Devonian. The geochemical cycles for carbon, alkalinity, phosphorus, sulfur and oxygen are included in this model, with also 13C and 34S cycles. High-resolution records of δ13C of marine carbonates and δ34S of marine sulfates are used as forcing parameters of the geochemical cycles in an inverse modeling. Atmospheric pCO2 and pO2 at the end of the Silurian are calculated to have been 3000 ppmv and 0.165 bar (0.75 PAL), respectively. A long-term decrease in pCO2 is modeled for almost the entire Devonian. Short-term lowering of pCO2 to concentrations around 2000 ppmv is calculated for the Silurian–Devonian transition and the Pragian. Contents around 900 ppmv are modeled for the Eifelian–Givetian, Givetian–Frasnian and Frasnian–Famennian boundaries as a consequence of enhanced organic carbon burial during deposition of Lochkovian, Eifelian, and Frasnian grey and black shales. Organic carbon burial is enhanced by the increase of phosphorus delivery to the ocean triggered by short-term sea-level falls. The corresponding short-term global climatic cooling at the Silurian–Devonian boundary, at the end of the Pragian, and the Givetian–Frasnian as well as Frasnian–Famennian boundaries reached 2 °C at the equator. The rapid colonization of continental surface by land plants during the Middle and Late Devonian, increasing chemical alteration of the continents and CO2 consumption by silicate weathering, is assumed to have caused cooling of surface seawater, as suggested by the δ18O values of biogenic apatites.