Molecular and petrographic indicators of redox conditions and bacterial communities after the F/F mass extinction (Kowala, Holy Cross Mountains, Poland)

Pyrite framboid diameter analysis and organic geochemistry of the triangularis/crepida boundary section at Kowala (Holy Cross Mountains, Poland) imply suboxic to oxic and sporadically euxinic bottom waters during the Lower Famennian. In addition, morphological web-like structures typical for microbial mats, as well as the recognition of 2α-methylhopanes and monomethyl-alkane cyanobacteria biomarkers is evidenced of microbial activity after the global Frasnian/Famennian (F/F) extinction event. The presence of cyanobacterial mats also suggests suboxic to oxic environments and at the same time photic bottom water conditions. However, isorenieratane and its derivatives were detected in almost all samples. The presence of this well-known biomarker of green sulfur bacteria implies that euxinic conditions were present in the upper part of the water column at least intermittently or that temporal euxinia occurred in the water column. Presence of euxinic conditions is confirmed by the occurrence of small-sized pyrite framboids which were particularly dominant in the lower part of the section. The shift towards low δ13C values in both micritic limestones and in sedimentary organic matter seen at the beginning of the period of diminished photic zone, might reflect an influx of newly respired CO2 to surface waters, caused by enhanced respiration at depth after the F/F transition in the Chęciny–Zbrza basin.