Environmental changes around the Jurassic/Cretaceous transition: New nannofossil, chemostratigraphic and stable isotope data from the Lókút section (Transdanubian Range, Hungary)

Abstract

New biostratigraphical, chemical and stable isotope (C, O) data are presented from the Lókút section (Transdanubian Range, Hungary) representing a ca. 13m thick continuous succession of Lower Tithonian–Lower Berriasian pelagic limestones. The study is conducted to verify timing of nannofossil events and major palaeoenvironmental changes at the Jurassic/Cretaceous transition including lithogenic input, palaeoredox and palaeoproductivity variations. Nannofossil zones from NJT 16b to NKT have been identified in the Lókút section and correlated with magnetostratigraphy, covering an interval from polarity zone M21r to M18r. The nannofossil Zone NJT 16b spans the interval from the upper part of M21r to lowermost part of M19n2n but its lower limit is poorly defined due to large diachronism in first occurrence (FO) of Nannoconus infans in various Tethyan sections. FOs of N. kamptneri minor and N. steinmannii minor are situated in the topmost part of the M19n2n and lowermost part of M19n1r magnetozones, respectively. They are located ca. 2–2.5m above the J/K boundary defined as Intermedia/Alpina subzonal boundary, which falls within the lower half of magnetozone M19n2n. The position of first occurrences of these taxa is similar to that from the Puerto Escaño section (southern Spain) and slightly lower than in Italian sections (Southern Alps). Concentrations of chemical element proxies of terrigenous transport (Al, K, Rb, Th) decrease towards the top of the Lókút section, which suggests a decrease in input of terrigenous material and increasing carbonate productivity during the Early Tithonian and the Berriasian. Slight oxygen depletion at the sea bottom (decrease of Th/U ratio), and large increase in concentrations of productive elements (P, Ba, Ni, Cu) is observed upsection. Nutrients supply via upwelling seems to be the most likely explanation. Increase in phosphorus accumulation rate and a microfacies change from Saccocoma to calpionellid dominated took place in the polarity chron M19r, which apparently coincided with the worldwide Nannofossil Calcification Event, related to a bloom of strongly calcified calcareous nannoplankton taxa. Deposition in the Lókút area was probably affected by long-term climatic trends: aridization and warming. Decreasing δ13C values of bulk carbonates throughout the Tithonian and the Berriasian are interpreted as a result of a global trend of accelerated carbonate productivity supported by local factors such as increased upwelling intensity, and a possible change in the composition of carbonate mud.