A global biogeochemical perturbation across the Silurian–Devonian boundary: Ocean–continent–biosphere feedbacks

Abstract

The large-scale global biogeochemical perturbation across the Silurian–Devonian (S–D) boundary, recorded in the major positive excursion of the δ 13C time curve (Klonk Event) of an amplitude of 2.5 to 3.0‰ (max. 4.0‰) in Europe and up to 5.0‰ in North America, reflects a unique combination of palaeogeographic, biogeochemical and evolutionary processes in the late Caledonian geodynamic setting. The steady sulfur isotopic ratios show an overall stability of the S–D oceanic geochemical system as a whole and do not indicate any synchronous changes in anoxic deep oceanic and sediment processes. Therefore this led us to a hypothesis that the crucial changes that contributed to the recorded carbon cycling turnover are related to rapidly evolving ocean–continent–biosphere feedback. Coastal zones of the latest Silurian epicontinental seas accumulated considerable quantities of organic carbon from early vascular vegetation, which explosively expanded to inhabit vast near-coastal shallows and deltas. Large primary production of these early terrestrial plants and rapidly enhanced sedimentary burial of organic carbon were also responsible for CO 2 drawdown, which have resulted in reversed-greenhouse effect and a global climatic cooling tendency. This feedback was blocked when the sea level gradually dropped and led to shrinking of the Silurian epicontinental seas and the growing climatic deterioration during the S–D transition limited primary production. Furthermore, continued processes of regressive abrasion and erosion limited the storage of organic carbon, as well as the efficiency of the carbonate factory. During the following Early Devonian greenhouse interval, marine regression and active latest Caledonian tectonism promoted progressive weathering of the sedimentary organic matter. The oxidation of C-rich deposits caused the subsequent growth of CO 2 levels in the Early Devonian, culminating in the warming of global climate.