Abstract :
[en] The Late Devonian period was affected by major, irreversible changes including two of the most severe biodiversity crises in Earth’s history, the so-called Kellwasser and Hangenberg Events (respectively at the Frasnian – Famennian and the Famennian – Carboniferous boundaries). Currently, hypotheses for the Late Devonian extinctions include sea-level fluctuations and regression, climate cooling, ocean anoxia, massive volcanism and/or bolide impact. Unfortunately, testing these hypotheses is impaired by a lack of sufficient temporal resolution in paleobiological, tectonic and proxy climate records. Recent advances in astronomical dating have improved the accuracy of the Frasnian time scale and part of the Famennian (De Vleeschouwer et al., 2012, 2013). However, the time duration of the Famennian stage remains poorly constrained even though this is the key to understanding cause-and-effect relationships of the Late Devonian greenhouse–icehouse transition and the onset of the end Famennian glaciation that ultimately led to the Hangenberg Event at the Devonian–Carboniferous boundary.
During the Late Devonian an epieric sea in mid-continent North America occupied the Illinois Basin onto which a Late Frasnian – Early Carboniferous sequence of deep-shelf deposits was archived. A complete record of this sequence, mainly composed of marl, organic-rich shale, silty-shale and carbonate, is captured in three overlapping cores stored in the Iowa Geological Survey (H-30, Sullivan Slough and H-32). H-30 records the Frasnian-Famennian boundary, the Sullivan Slough covers almost all the Famennian (middle triangularis-upper expansa conodont zones) and H-32 spans the junction between the uppermost Famennian and lowermost Carboniferous.
To have the best chance of capturing Milankovitch cycles (long- and short-eccentricity cycles as well as obliquity and precession), 2200 rock samples were collected at 5-cm intervals across the entire sequence. Magnetic susceptibility (MS) was measured on each sample and the preservation of paleoenvironmental/climatic information into the signal was verified through geochemical analyses (Ti, Al, Zr by XRF) and magnetic measurements (low temperature magnetic susceptibility and IRM acquisitions). Correlation and overlap between each core was made using available conodont biostratigraphy and then refined by comparing the trends in the MS and δ13C signals for critical intervals. To estimate the duration of the Famennian stage we applied multiple techniques on the MS signal (multitaper method, evolutive harmonic analysis and adaptive-weighted harmonic F-test). By combining these techniques, we identified highly stable 405-kyr cycles across the Sullivan Slough and the H-32 cores. 405-kyr cycles were not observed in H-30, but 100-kyr cycles were identified. A preliminary estimate of the duration for the Famennian stage using the 405-kyr cycle as chronometer constrains the duration of the Famennian stage to 13 ± 0.4 Myr with an average sedimentation rate of 0.5 cm/kyr across the sequence.
