The late Aeronian graptolitesedgwickiiEvent, associated positive carbon isotope excursion and facies changes in the Prague Synform (Barrandian area, Bohemia)

Abstract

AbstractStudy of the lower Silurian black shale succession of the Prague Synform has enabled detailed insight into graptolite faunal dynamics and diversity trends from the mid-Aeronian diversity maximum through to the late Aeronian crisis. Graptolite diversity decreased from 33 taxa in theLituigraptus convolutusBiozone to 17 taxa in the upper part of theStimulograptus sedgwickiiBiozone and newly erectedLituigraptus rastrumBiozone. The graptolite assemblages of the latter biozones exhibit low species richness along with high dominance. Many graptolite species that became extinct in the early part of thesedgwickiiZone were promptly replaced by new forms. In the later part of thesedgwickiiZone, however, replacement of extinct species by new forms considerably decelerated. The increased rate of graptolite extinction recorded in theconvolutus–sedgwickiibiozone boundary beds coincided with subtle changes in black shale lithologies and a positive shift in δ13Corg(of 2.2 ‰) compared to baseline values. Sea-level drawdown can be inferred from siltstones and/or temporary nondeposition in the middlesedgwickiiZone. This level also sees total organic carbon (TOC) fluctuations and a strong positive δ13Corgexcursion with a peak shift of at least 7 ‰. ThesedgwickiiEvent exhibits substantial reorganization of the graptolite fauna, its taxonomic impoverishment and concomitant increase in species dominance rather than a sudden collapse of the pre-extinction assemblage. Associated changes in lithology, TOC and the pronounced δ13Corgexcursion suggest a relatively extended and probably multi-phase period of stressed conditions that affected the pelagic realm inhabited by graptolites in the course of the late Aeronian interval.