REDOX INDICATORS IN OCEANIC SEDIMENTS: A CASE STUDY OF THE ANOXIC EVENT 2 IN THE BOHEMIAN CRETACEOUS BASIN, CZECH REPUBLIC

Abstract

One of the most expressive and well documented oceanic anoxic events; the OAE 2, is preserved at the Cenomanian/ Turonian boundary in the Pecínov section of the Bohemian Cretaceous Basin in the Czech Republic. This study aims at recording and discussing the impact of this oceanic anoxia on some potential redox parameters in the sediments of this event. It is focused on the behavior of redox-sensitive chemical elements, stable carbon and sulfur isotopes and type of fossil bacteria in the black marine mudstones and phosphate nodules deposited during the OAE 2. The results show variable concentrations of redox-sensitive elements relative to average shale, negative values of δ¹³C in CO₃-apatite and δ³⁴S in pyrite in addition to dominant sulfur-reducing fossil bacteria. The mudstones are depleted in organic carbon and hence some trace elements (e.g., Cd, Zn, Cu and Co), usually associated with organic matter, are not significant as redox indicators in the case studied. The most prominent geochemical indicators of anoxia are expressed in the elevated Ce-anomaly and Ce/La ratio, and depleted Mn concentrations found in all samples, with the phosphate nodules holding the highest anoxic indicators. The stratigraphic correlation of geochemical redox parameters with the δ¹³C_org curve at the Pecínov section shows general compatibility with some exceptions; two δ¹³C_org excursions have their signature in some geochemical redox indicators. The OAE 2 section at the Pecínov quarry represents an extreme case of reduced concentrations of organic carbon and most redox-sensitive elements relative to other OAE 2 sequences in Europe and the Atlantic Ocean, which can indicate frequent oxygenation events caused by sea-level fluctuation during deposition, marked by bioturbated surfaces.