Astrochronology of the Frasnian-Famennian boundary section (Late Devonian) at Steinbruch Schmidt

The Late Devonian mass extinction was the second of five global mass extinctionsthat shaped life on Earth during the Phanerozoic Eon, and occurred at the Frasnian–Famennian boundary (FFB, 372 Ma). Here, we focus on this time interval at thefamous Steinbruch Schmidt section in Germany. This section includes the wellexposed Kellwasser black shale intervals, the FFB, and a U-Pb dated ash layer. In thisstudy, we sampled an interval of 5.3 m around the FFB, with an average samplinginterval of 3 cm, leading to a collection of about 200 samples. On every sample, wemeasured carbon and oxygen stable isotopes, magnetic susceptibility and micro XRFelemental geochemistry.Magnetic susceptibility (MS) and detrital-input-related elements such as Ti and Alare higher during the Kellwasser, and there is a good correlation between Al, K, Ti, Feand MS. 􀀀13C values are high at and around the Kellwasser levels. We selected theMS, Ti/Al and 􀀀13C signals for spectral analysis, as we expect these proxies to containthe best-preserved astronomical signal. We applied the technique developed byMeyers (2015) on the evaluation of eccentricity-related amplitude modulationindependently to the 3 signals. Obtained results for the 3 signals are exactly in thesame range, with a sedimentation rate of 0.47 cm/kyr and it allows thetransformation of the signal from the distance domain (5.3 m) into the time domain(about a 1100 kyr). If we filter precession from signal and look at its envelope, wecan see a clear modulation by 100 kyr cycle, including 5 to 6 precession cycles, asexpected for the Devonian. These results are in agreement with the timing proposedby De Vleeschouwer et al. (2017). Furthermore, as in De Vleeschouwer et al. (2007),we have identified a strong obliquity Power at the FFB, which is associated atSteinbruch Schmidt with a low eccentricity power. This would correspond to a periodwith low seasonality and could have influenced the development of anoxia. This timeframe combined with the dating of the ash layer below the FFB boundary (Percival etal., 2018) provides an anchor point for this cyclostratigraphic framework.De Vleeschouwer, D., Da Silva, A.-C., Sinnesael, M., Chen, D., Day, J.E., Whalen, M.T., Guo, Z., Claeys,P., 2017. Timing and pacing of the Late Devonian mass extinction event regulated by eccentricity andobliquity. Nature Communications 8, 2268.Meyers, S.R., 2015. The evaluation of eccentricity-related amplitude modulation and bundling inpaleoclimate data: An inverse approach for astrochronologic testing and time scale optimization.Paleoceanography 30, 1625–1640.