Geochemical evidence for major environmental change at the Devonian–Carboniferous boundary in the Carnic Alps and the Rhenish Massif

Abstract

A positive carbon isotope excursion is reported for the global Hangenberg Event near the Devonian–Carboniferous boundary, one of the most significant Phanerozoic mass extinction events. The δ 13C excursion occurs both in micritic limestones and in sedimentary organic matter of black shales and limestones from different palaeogeographical regions, which were precisely correlated by conodont biostratigraphy. The excursion indicates global change in the isotopic composition of marine dissolved inorganic carbon and atmospheric CO 2. This resulted from the increased burial of organic matter by globally widespread deposition of black shales. The δ 18O values of conodont apatite indicate that globally widespread black shale deposition was preceded by increasing temperature, probably with a thermal gradient between equatorial and higher latitudes (∼ 20 °S). The main regressive interval of the event, correlated previously with a short-lived glacial episode in Gondwana, yielded no material for geochemical analyses, but low temperatures are recorded in overlying beds and increase again during the terminal Devonian. Early Carboniferous sea-surface temperatures were similar to those of the Late Devonian. This pattern of increased organic carbon burial, and changes in climate and sea level, is similar to that of several other extinction events of the Phanerozoic. It supports the hypothesis that increased organic carbon burial and oceanic anoxia can trigger mass extinctions, glaciations and eustatic sea-level change.