Early diagenetic greigite as an indicator of paleosalinity changes in the middle Miocene Paratethys Sea of central Europe

Abstract

The Miocene epicontinental Paratethys Sea of central Eurasia has experienced multiple restriction and reconnection events to the open ocean. Magnetostratigraphy is an important dating tool to better understand the temporal and spatial paleoenvironmental variations associated with these changes. Magnetostratigraphy in the Paratethys domain, however, is complicated by the presence of greigite (Fe3S4). Here we report rock magnetic and X-ray fluorescence data of the Tisa section (Romania) which was previously magnetostratigraphically dated at the middle Miocene (base at 12.8 Ma and top at 12.2 Ma). This section comprises the Badenian Sarmatian Extinction Event (BSEE), which is marked by a major salinity change from marine to brackish environments, related to the opening of the connection between the Central and the Eastern Paratethys basins. In the marine Badenian sediments below the BSEE, the pyritization process is shown to be complete because of abundant sulfate supply. In the brackish Sarmatian deposits, four intervals with early diagenetic greigite are observed, and linked to insufficient sulfate in the water column. These four greigite intervals appear to correspond to maxima in the ∼100 kyr eccentricity cycle. We propose that increased fresh water from the Eastern Paratethys basin during eccentricity maxima restricted the sulfate availability in the Tisa area, leading to a reduced HS− production and enhanced greigite preservation. The early diagenetic formation of greigite enables a quasi syn-depositional recording of the paleomagnetic field, which allows reliable paleomagnetic dating in this section. Our results further suggest greigite as a potential indicator for salinity changes during marine/brackish transitions.