The mid-Tournaisian (Early Carboniferous) anoxic event in the Laurussian shelf basin (Poland): An integrative approach

Abstract

A wide range of proxies (organic and inorganic geochemistry, gamma-ray spectrometry, microfacies, framboidal pyrite analysis) were applied in order to decipher changes in depositional conditions during the mid-Tournaisian anoxic event, referred to as the Lower Alum Shale Event (LASE) in the Polish part of the Laurussia basin. The LASE part of the section, unlike older Tournaisian deposits, is characterised by high concentrations of isorenieratane, palaeorenieratane, and their diagenetic derivatives, the occurrence of 2-methyl-3-iso-butyl-maleimides, and high values of the sterane-to-hopane ratio. The predominance of tiny pyrite framboids, inorganic data such as low values of the Th/U and high values of the Corg/P and V/Cr ratios, and generally higher enrichments of redox-sensitive metals such as U, V, Mo, Pb, and Zn and depletion in Mn, all suggest anoxic/euxinic conditions on the sea-bottom. The results of our investigations, especially Co, Mn, Cd, and Mo distribution patterns, demonstrate that enhanced primary productivity in the water column was responsible for a high degree of organic carbon burial rather than preservation in restricted conditions on the seafloor. This elevated productivity may have been generated by the upwelling of nutrient-rich deep water, an increase in volcanic activity and volcanogenic oceanic fertilisation, or both. Volcanically driven mid-Tournaisian climatic changes (warming), as manifested by high Zr concentrations and Hg anomalies, may have been responsible for the deglaciation of Gondwana resulting in the global crenulata transgression.The global rise in sea level and high level of organic productivity were responsible for the development of anoxic conditions and the halt of carbonate production, as reflected by drastic changes of facies not only in the sections investigated, but in many other parts of the world as well.