GLOBAL CHANGE IN THE LATE DEVONIAN: MODELLING THE FRASNIAN-FAMENNIAN SHORT-TERM CARBON ISOTOPE EXCURSIONS

Abstract

A model of the global biogeochemical cycles coupled to a energy-balance climate model (the COMBINE model) is used to identify the causes of two large δ13C value excursions across the Frasnian-Famennian (F-F) boundary. We test a scenario that links the sea-level rise to stratification of the Proto-Tethys ocean through the formation of warm saline deep waters in extended epicontinental seas. Even though this scenario can produce dysoxia below 100 m depth, it fails to increase the global burial flux of organic carbon and thus seawater δ13C values, since stratification of the ocean leads to decreased productivity in surface waters. Several scenarios postulating a continental origin of the perturbations in the Late Devonian biogeochemical cycles are then tested. We found that weathering of platform carbonates exposed during the Early Famennian sea-level fall can account for a maximum positive shift in δ13C value of +0.7%% at the end of the sea-level fall episode. Another +1.0%% increase in δ13C might originate from rapid spreading of vascular land plants near the F-F boundary, postulating that higher plants globally increased the weatherability of continental surface, and that colonized continental area increased by 30% across the F-F boundary. Finally, the δ13C excursion observed at the base of Upper rhenana Zone and the rapid increase of the carbon isotope ratios at the F-F boundary require an increase of phosphorus delivery to the ocean by 40%, coeval with the sea-level rises. Once the calculated δ13C values are in agreement with the measured data, the COMBINE model calculates a decrease in atmospheric pCO2 from pre-perturbation 2925 ppmv in the Lower rhenana conodont Zone to 1560 ppmv in the Upper triangularis Zone. This decrease in pCO2 is due to the increase in burial of organic matter during the Kellwasser events, and increased continental weatherability triggered by the spreading of continental vascular plants. These changes occur within 4 million years. The corresponding global climatic cooling reaches 4.4°C at the pole, and 2.1°C at the equator.