DOI: 10.1594/pangaea.932842
title: Strontium isotopes reveal large connectivity changes during the late Miocene-Pliocene in restricted Dacian Basin (Romania) of the Eastern Paratethys

description: During the late Miocene-Pliocene the Dacian Basin was the western appendix of the Black Sea. The restricted paleogeographical location of the Dacian Basin within the larger Paratethys-Mediterranean system made possible unique recordings of hydrological changes governed by short lived connections to the open marine realm. The data set presented here is based on strontium isotopes (87Sr/86Sr) measured on ostracod shells collected from a well dated Miocene to Pliocene succession exposed along Slanicul de Buzau Valley, located in the Carpathian Foredeep of Romania (part of the Dacian Basin). We measured the 87Sr/86Sr only on ostracods. The choice for ostracods is twofold: 1) they occur abundantly in the fresh to brackish deposits of the Paratethys, while these environments are generally not suitable for the occurrence of foraminifera or other typical marine organisms; 2) ostracods bear in general low ornamentation and they molt their entire shell in one event. We used ostracods from the Slanicul de Buzau section and added 39 new 87Sr/86Sr data points to the earlier acquired results from Rîmnicu Sărat, extending and refining in this way the Dacian Basin 87Sr/86Sr record from the late Miocene (Khersonian) to the Pleistocene (Romanian Stage). Age constraints for Slanicul de Buzau section are provided by biostratigraphy coupled to magnetostratigraphy. Additionally, the section is correlated by magneto- biostratigraphy to the previously investigated Rîmnicu Sărat section, 15 km to the north. The detailed 87Sr/86Sr record for the Dacian Basin provides an exceptional record of basin connectivity from the latest Tortonian (ca. 7.7 Ma) until the early Pleistocene (ca. 1.8 Ma). Relatively high 87Sr/86Sr originated either from marine incursions from water supplied by the Balta delta in the East Carpathian Foreland, or from outflow waters of Lake Pannon and local rivers, while low 87Sr/86Sr likely originated from connectivity with the Black Sea region. Data 87Sr/86Sr record indicates that a late Tortonian transgression (7.6‒7.4 Ma) started with an incursion of Eastern Paratethys waters into the Dacian Basin. Afterwards, local rivers became the dominant source for the mostly freshwater environments of the early Messinian. The regional Maeotian-Pontian transitional interval, between 6.3‒5.9 Ma, is marked by a second incursion of Eastern Paratethys waters, but during this event, an additional marine (Mediterranean) influx coincides with a short-lived salinity incursion. During the Messinian Salinity Crisis of the Mediterranean, the Dacian Basin connected progressively to the Eastern Paratethys (5.9‒5.5 Ma). The Dacian Basin became restricted during the peak Mediterranean lowstand at ~5.5 Ma and filled with Lake Pannon and local river water between 5.5‒5.3 Ma. During the Plio-Pleistocene, the Dacian Basin reconnected with the Black Sea (at that time isolated from the ocean ocean), which shows similar 87Sr/86Sr as in the Last Glacial Maximum. When evaluating the application of 87Sr/86Sr in assessing the connectivity (i.e. to open ocean) we conclude that the Dacian Basin (between 7.6 and 1.7 Ma) is an opposite counterpart to the Mediterranean during the Messinian Salinity Crisis. While the latter was a marine basin affected by relatively short term disconnection form the ocean, the former (the Dacian Basin) was a part of a large brackish-lacustrine isolated domain affected by a very short connection to the marine realm. Samples were collected from Slanic de Buzau (Romania) deposits exposed on the left side of the Slănicul river, along the Mânzăleşti (GPS 45°29'18.74 N; 26°39'10.30E). The dataset include all 87Sr/86Sr measured on ostracods and one foraminifera level.