Telling Bromacker time: Time-series analysis of redbed cores from the Early Permian of the Tambach Basin, Thuringian Forest, Germany

Research boreholes Fb Altenbergen 01/2022 and Fb Tambach-Dietharz 01/2023 from the perimontane Tambach Basin (Artinskian; 290.1–279.3 Ma) flank the Bromacker excavation site with 2.6 and 1.5 km distance and recovered 250 and 200 m, respectively, of excellent core in fluvial (channel, overbank, floodplain) and conglomeratic alluvial facies. High-resolution analysis of climate-relevant proxies was based on color (CIEL*a*b color space), volume-specific magnetic susceptibility, and carbonate content (of Fb Altenbergen, 92.33 – 198.59 m; 33 cm spacing), supported by gamma-ray logs. Lithologies and measurements readily correlate with each other over the 4 km distance, with the Bromacker site approximately midway. A time-series analysis (spectral analysis, Fourier analysis; multi-taper method) based on the proxy measurement series (with and without conglomerate) shows numerous cycles. The dynamic-chaotic fluvial environment and the climate-independent weathering of the lithic, Fe-rich minerals of proximal volcanic provenance primarily affect high-frequency "short" cycles. Low-frequency “long” cycles, in contrast, consistently show a high statistical significance. We interpret them to represent periodic climatic changes with stronger bioturbation and soil formation, correlated to warmer periods of higher precipitation. Using the frequency-ratio method, cycles compare well to Early Permian orbital (Milankovic) cycles of known astrochronological periods, best to the long eccentricity cycle of 405 ka in the Permian. Fb Altenbergen 01/2022 thus appears to represent a stratigraphic record of ca. 2.13 Ma, Fb Tambach-Dietharz of ca. 2.16 Ma, and a mean sedimentation rate of ca. 12 and 10 cm / ka, respectively, for both compacted and decompacted profiles.