Carbon and Oxygen Isotope Trends of Upper Toarcian to Lower Aalenian Belemnite Rostra from the Mistelgau Clay Pit, Franconia, Germany

This study investigates the stable isotope geochemistry (δ¹³C and δ¹⁸O) of belemnite rostra collected from the Mistelgau clay pit near Bayreuth, Southern Germany. During the Upper Toarcian, the study area was situated within the subtropical epicontinental Sea of the Western Tethys shelf, which was influenced by sea level changes and complex palaeoceanographic current patterns.

The analyzed material comprises of calcitic rostra collected from the entire Jurensismergel Formation and the lowermost horizons of the Opalinuston Formation. To ensure high temporal resolution, a stratigraphic sampling approach was employed, with specimens collected at 10 cm intervals. This high-resolution sampling strategy allows for the detection of subtle environmental shifts across the Toarcian-Aalenian boundary.

The precise chronostratigraphic framework for this geochemical analysis is based on the foundational research conducted during the first author’s diploma thesis in 1999 (Schulbert 2001) where the stratigraphy was examined using the distribution of ammonite taxa.

The succession at Mistelgau exposes a continuous sequence of argillaceous and marly facies, favouring a good preservation of the belemnite rostra. By utilizing the δ¹⁸O signatures as proxies for palaeotemperatures and δ¹³C for investigating the carbon cycle, this dataset provides insights into the beginning change from the Warm to the Cold Mode of the Late Toarcian into the Early Aalenian.

While still in the phase of screening our material, the δ¹⁸O data displays an approximate one permille negative excursion right at the Toarcian/Aalenian boundary which indicates a temporarily ~ 4°C seawater warming.

The results contribute to a deeper understanding of regional palaeoenvironmental changes within the South German Basin and their correlation with global isotope trends.

Reference

Schulbert, Christian. "Die Ammonitenfauna und Stratigraphie der Tongrube Mistelgau bei Bayreuth (Oberfranken)." Berichte der Naturwissenschaftlichen Gesellschaft Bayreuth 4 (2001): 1-183.