Online-Geokolloquium: Montag 13.12.2021

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Reading geologic time in the Devonian sedimentary record from the Rhenish Massif (incl. the Ardennes)

Dr. Anne-Christine Da Silva

University of Liège

Dr. David De Vleeschouwer

MARUM Bremen

Several major ocean anoxic events punctuated the Devonian Period, alongside with ecological crises, shifts in ocean chemistry and climatic changes. One of the key-obstacles in understanding these events lays in the difficulty of providing precise estimates of the duration represented by a sequence of Devonian sedimentary rocks. This lack of temporal precision severely hampers the evaluation of forcing mechanisms and rates of climatic, ecological or biogeochemical change. To improve the precision and accuracy of Devonian chronologies, and thus to unravel the dynamics of Late Devonian environmental change, we apply cyclostratigraphy as a tool to read time in geologic sequences.

The cyclostratigraphic technique is based on the detection of Milankovitch cycles in the sedimentary record. Those cycles result from periodic variations in the Earth-Sun system, affecting the distribution of solar energy over the planet, influencing Earth’s climate on time-scales between 104 and 106 years. Through the integration of astronomical timescales with biostratigraphy and radio-isotopic dating of different section in the Rhenish Massif (including the Belgian Ardennes), we document astronomical forcing regimes for the Frasnian – Famennian Kellwasser Crisis, as well as for the Famennian – Tournaisian Hangenberg Crisis. We conclude that astronomical insolation forcing is unlikely to be the main cause of widespread ocean anoxia, however it might have played an important role in determining the timing and pacing of Late Devonian environmental crises.