Valanginian isotopic and palaeoecological signals from the Bucegi Mountains, Southern Carpathians, Romania

Abstract

AbstractThe first carbon- and oxygen-isotope records from the Valanginian to earliest Hauterivian (Early Cretaceous) in the Southern Carpathians, Bucegi Mountains (Romania), are discussed. Stable-isotope (δ13Ccarb, δ18Ocarb) curves from bulk sedimentary carbonate have been generated from three sections. δ13Ccarb values fluctuate between 2.1 and 2.7‰ within the late Early Valanginian (middle NK3A–base NK3B Calcareous Nannofossil Subzone, that corresponds to late B. campylotoxus–early S. Verrucosum Ammonite Zone), and between 1.1 and 1.7‰ within the Late Valanginian–earliest Hauterivian (NK3B–base NC4A Calcareous Nannofossil Subzone, that corresponds to ? uppermost S. verrucosum–N. peregrinus–C. furcillata–earliest Acanthodiscus radius Ammonite Zone). The total organic carbon (loss on ignition method (LOI)) content fluctuates between 1.6 and 2.8% within the late Early Valanginian, and from 0.6 to 3.3% within the Late Valanginian–earliest Hauterivian. These results suggest the presence of Valanginian organic-rich deposition and a concurrent positive δ13Ccarb excursion, which has been interpreted to represent an increase in surface water productivity. The abundance of the nannofossils, Biscutum constans, Discorhabdus rotatorius, Zeugrhabdotus erectus and Diazomatolithus lehmanii suggest higher fertility and eutrophic conditions. Low oxygenation conditions leading to organic matter preservation are suggested by the presence of conical and trochospiral inflated benthic foraminiferal specimens. Furthermore, bottom-water dysaerobic conditions are suggested by the dominance of small Spirillina.High fertility in the ocean could be related to a rise in atmospheric pCO2 levels, globally linked to the formation of the Paranà–Etendeka continental flood basalts and increased rates of oceanic crust production. Moreover, the palaeoenvironmental changes represented in the Southern Carpathians coincides with the initiation of carbonate platform drowning, as recorded globally in many sections.