Astrochronology and chemostratigraphy for the Frasnian-­‐Famennian boundary section (Late Devonian) at Steinbruch Schmidt

The Late Devonian mass extinction is was the second of five global mass extinctions that shaped life on Earth during the Phanerozoic Eon, and occurred at the Frasnian–Famennian boundary (FFB, 374 372 Ma). Here, we focus on this time interval at the famous Steinbruch Schmidt section in Germany. This section includes the well exposed Kellwasser black shale intervals, the FFB, as well asand a U-Pb dated ash layer. In this study, we sampled an interval of 5.3 m around the FFB, with an average sampling interval of 3 cm, leading to a collection of about 200 samples. On every sample, we measured carbon and oxygen stable isotopes, magnetic susceptibility and XRF elemental geochemistry. Magnetic susceptibility (MS) and detrital-input-related elements such as Ti and Al are higher during the Kellwasser, and there is a good correlation between Al, K, Ti, Fe and MS. We selected the MS and Ti signals for spectral analysis, as we expect these proxies to contain the best-preserved astronomical signal. Both proxies give similar results, with important spectral power at frequencies of 0.5, 2, 5.3 and 5.5 and 9.5 cycles/kyr. The peaks are located at similar frequencies but reach different confidence levels in both signals. Subsequently, we applied the average spectral misfit (ASM) method, which assesses the most probable sedimentary rate and searches for the best fit between the frequencies extracted from the Ti and MS spectral analyses and the expected Milankovitch frequencies in the Devonian. Using the ASM results, we transform the signal from the distance domain into the time domain. Furthermore, there is an ash layer just below the boundary, leading to anchor our floatingdating of the ash layer just below the boundary provides an anchor point for this cyclostratigraphic framework . The well-known biostratigraphy of thise section, as well as the carbon- isotope stratigraphys, will allow to the use of this new time frame at a global scale.