Nd isotope constraints on the Middle Miocene oceanography of the northern Paratethys

Neodymium (Nd) isotopes, tracers of seawater composition and mixing, measured in calcitic bivalve shells and calcitic micrite from the Middle Miocene deposits in Poland and Ukraine, were used to reconstruct the seawater circulation and the evolution of seawater geochemistry in the Fore-Carpathian Basin, the northern peripheral part of the Paratethys. The data revealed a wide variation in ε Nd values for the Badenian seawater, ranging from −12.1 to −6.6. This is in contrast to the more radiogenic composition of the Sarmatian seawater, with Nd isotope signatures ranging from −8.7 to −6.4. The shift towards more radiogenic ε Nd values during the Sarmatian aligns well with the opening of a wider gateway connecting the Central and Eastern Paratethys, which facilitated an inflow of waters from the Eastern Paratethys. The significant variation in the Nd isotope composition of the Badenian and Sarmatian seawater provides evidence for the mixing of relatively radiogenic open sea water with unradiogenic riverine waters. Results from this study indicate that the largest influx of unradiogenic Nd by riverine waters came from the crystalline basement of the Ukrainian Shield and the Paleozoic substrate of the Holy Cross Mountains in Poland. There is no evidence that riverine waters draining the Carpathian orogen constituted an essential source of radiogenic neodymium. The inflow of radiogenic open sea water from another domain of the Central Paratethys seems more likely. The lateral distribution of Nd isotope signatures of seawater in the basin points to a circulation pattern consisting of a northwest-oriented current associated with mesoscale anticyclonic offshoot eddies. • Nd isotope record of the Middle Miocene seawater in the Fore-Carpathian Basin is presented. • Variation in Nd isotope signatures documents the mixing of radiogenic open seawater with unradiogenic riverine waters. • Largest influx of unradiogenic Nd from the basement of the Ukrainian Shield and the Paleozoic of the Holy Cross Mountains. • A seawater circulation pattern with a northwest-oriented current is postulated.