Neodymium and strontium isotopes track the origin of parent brines of primary gypsum deposits (Miocene, Fore-Carpathian Basin)

Neodymium (Nd) isotopes constitute a versatile tracer frequently used in oceanography to identify water masses and track their circulation, both in the modern and ancient oceans and seas. In this research, for the first time, primary marine gypsum is being used as a new archive preserving Nd isotope signatures of past seawater and brines in very shallow-water settings. Nd isotope signatures (epsilon Nd values) coupled with Sr isotope ratios (87Sr/86Sr) allowed to constrain mixing processes and composition of brines in a marine evaporite basin that developed in the Fore-Carpathian Basin during Badenian (Middle Miocene) time. We measured the Nd and Sr isotope composition of selenite crystals collected from gypsum successions, well-exposed in southern Poland and western Ukraine. The crystals reveal wide temporal variations in epsilon Nd(t) values, from -14.4 to -6.1, contrasted by relatively uniform 87Sr/86Sr ratios, from 0.7089 to 0.7093, and provide evidence for the mixing of sea and riverine waters during the brine formation. Gypsum precipitation taking place in very small sub-basins (evaporite pans) was controlled by episodic, significant inflows of riverine waters that triggered the formation of dissolution surfaces and cyclicity within the sabre gypsum successions. Riverine-water inflows into the Fore-Carpathian Basin are postulated from two main directions, from the Ukrainian Shield (western Ukraine) and the Holy Cross Mountains (southern Poland). In conclusion, we show that Nd isotopes are much more sensitive than Sr isotopes in detecting marine vs non -marine contributions in the composition of brines and gypsum deposits.