Research paperZechstein saline brines in Poland, evidence of overturned anoxic ocean during the Late Permian mass extinction event

Bromine concentrations in halite, sulfate isotopes (δ34S and δ18O), and major ion concentrations in primary fluid inclusions from three boreholes in the Late Permian Zechstein evaporites have revealed sharp variations in marine derived brines within the Polish sector of the European Southern Permian Basin. The base of the Older Halite (Na2), during the latest Permian, registers a change from sulfate-rich brines, similar in composition to modern evaporated seawater, to sulfate-depleted brines (calcium-rich). This change coincides with a drop in δ34S to values close to + 9‰, not observed in δ18O counterparts. Opposite isotope (δ34S-δ18O) trends through the Na2 unit cannot be explained by changes in restriction conditions. We propose that the change to sulfate-depleted (calcium-rich) brines during halite deposition of the PZ2 (Stassfurt) cycle is related to the overturn of anoxic sulfidic deep-waters from the Panthalassa stratified superocean coinciding in time with the Permian-Triassic mass extinction event.The reconstruction of chemical changes in brines reveals two major evaporite sequences of increasing concentration that do not match the classic lithostratigraphic cycles. The first evaporite sequence (PZES-1) contain the evaporite units of the PZ1 (Werra) cycle, the PZ2 (Stassfurt) cycle, the Main Anhydrite (A3), and the base of the Younger Halite (Na3) of the PZ3 (Leine) cycle. The second evaporite sequence (PZES-2) is represented by almost the entire Na3 unit and the PZ4 (Aller) cycle.

► Primary brine-inclusions reveal sharp variations in the Late Permian ocean chemistry. ► Marine inflow to Polish Basin fluctuated from SO4-rich to Ca-rich during deposition of the Stassfurt halite. ► Brine-inclusion variations resulted from mixing with overturned anoxic sulfidic deep-waters. ► Late Permian oceanic chemical fluctuations are in turn associated to mass extinction event. ► Two major evaporite sequences can be distinguished from geochemical point of view.