First record of Oceanic Anoxic Event 2 in the Eastern Carpathians: implications for chemostratigraphic and biostratigraphic correlations

This study aims to investigate a marlstone and claystone succession located at the southern end of the Eastern Carpathians, a region where oceanic anoxic event 2 (OAE2) has not been pointed out so far. Toward the upper half of this succession, a 17-cm-thick black shale was identified. The investigated depositional interval lies within the late Cenomanian-early Turonian, encompassing the UC3d up to UC7 nannofossil zones. The delta C-13 values fluctuated between 2.06 parts per thousand and 3.89 parts per thousand, showing a positive isotope excursion that was assigned to OAE2. The delta C-13 isotope curve displays the following intervals: pre-excursion, first build-up, trough, second build-up, plateau, and post-excursion. Within the second build-up interval of OAE2, a substantial shift in CaCO3 values, accompanied by high concentrations of total organic carbon and a significant decline in the abundance and diversity of calcareous nannofossil assemblages, was observed. The nannofossil turnover related to OAE2 climax revealed predominance of Watznaueria barnesiae and temporary disappearance from the record of surface-water higher fertility taxa, such as Biscutum constans, Zeugrhabdotus erectus, and Discorhabdus ignotus. Above OAE2, peaks of Eprolithus floralis, followed by increased abundance of Eiffellithus turriseiffelii and Nannoconus spp., were identified. In the lower part of the studied succession (i.e., the upper Cenomanian UC3d nannofossil subzone), during the pre-excursion characterized by low delta C-13 values and less negative delta O-18 values, a small group of nannofossils more related to mid-and highpaleolatitudes, such as Crucibiscutum salebrosum, Repagulum parvidentatum, and Seribiscutum primitivum, is present, always showing a low abundance.

Automated summary

This study investigates a marlstone and claystone succession in the Eastern Carpathians, finding evidence of the second oceanic anoxic event (OAE2). The study shows a significant change in the calcareous nannofossil assemblages and a decrease in their abundance and diversity during OAE2. Additionally, the study identifies some nannofossil species related to mid- and high-latitudes.