Strontium isotope compositions of Late Permian evaporites from the northernmost Thuringian Basin (Germany) indicate continental influence on the marine Zechstein Sea

Abstract

In the Late Permian Zechstein Sea of Central Europe, up to 2000 m of evaporitic rocks were deposited in at least four consecutive cycles. The age of these evaporitic rocks could not yet be precisely determined, because they are virtually fossil-free and do not contain radiometrically datable volcanic layers. A chemostratigraphic age of the succession can be determined by comparing ⁸⁷Sr/⁸⁶Sr ratios of marine gypsum and anhydrite to the worldwide marine strontium evolution curve. Unfortunately, published ⁸⁷Sr/⁸⁶Sr data of the Zechstein succession are characterized by frequent outliers towards higher ratios, making an age assignment challenging. The scatter in ⁸⁷Sr/⁸⁶Sr ratios might be induced by different processes like the contribution of meteoric water to the brine, in-situ Rb decay, or post-depositional hydrothermal or diagenetic overprint. Here, we present a dataset of 26 new gypsum and anhydrite ⁸⁷Sr/⁸⁶Sr ratios from drill cores situated at “Alter Stolberg” in the northernmost Thuringian Basin. Evaporites of the Werra-, Staßfurt-, and Leine cycles were sampled. The close proximity of the drillings allows a very accurate assignment of the stratigraphic position of each sample, so that trends and outliers in ⁸⁷Sr/⁸⁶Sr ratios can easily be recognized. While the entire Werra Formation obviously revealed non-marine ⁸⁷Sr/⁸⁶Sr ratios, the lowermost ⁸⁷Sr/⁸⁶Sr ratios in the Staßfurt and Leine Formations can be assumed to represent marine ratios and allow estimating a chemostratigraphic age of 257‒254 Ma. The combination of the ⁸⁷Sr/⁸⁶Sr data with the mineral composition of the samples suggests a contribution of meteoric water, probably river water, to the Zechstein Sea as the main reason for the observed increase in ⁸⁷Sr/⁸⁶Sr ratios. Additional in-situ Rb decay, related to the riverine input of clay minerals, cannot be excluded. Modelling the amounts of sea water and meteoric water in the brine indicates that 83‒99% of meteoric water would be necessary to explain the highest ⁸⁷Sr/⁸⁶Sr ratios observed in the Werra Formation.

Graphical abstract