Anoxic Annulata Events in the Late Famennian of the Holy Cross Mountains (Southern Poland): Geochemical and palaeontological record

The Upper Famennian Annulata Black Shales are exposed in the deep-shelf successions of the famous Kowala Quarry and the Ściegnia outcrop in the Holy Cross Mountains, Southern Poland. At Kowala, the twin Annulata anoxic events are manifest as two organic-rich (TOC up to 23 wt.%), finely laminated, fossiliferous black shales, each up to 0.6 m thick, separated by marl and massive or nodular limestone layers. The bituminous interval is condensed in the Ściegnia outcrop to a single 10 cm thick horizon. In both sections, the ABS levels date to the upper part of the Upper Palmatolepis trachytera conodont Zone, corresponding to the Diducites versabilis–Grandispora famenensis miospore Zone. The Annulata Events are marked by a flourishing pelagic biota (the opportunistic bivalve Guerichia venusta, platyclymenids, the goniatite Erfoudites, entomozoacean ostracods, and prasinophyte algae). The initial phase of anoxia saw a bloom of specialized conodont fauna, mostly deep-water outer-shelf palmatolepids such as Palmatolepis glabra lepta, before the second anoxic pulse caused a significant crisis amongst this community. Isorenieratane and gammacerane biomarkers indicate the development of photic zone anoxia during both phases. Degree of Pyritisation (DOP) values (estimated from the TOC–TS–Fe diagram) and the uranium–thorium proxies, as well as trace metals enrichments suggest that bottom-water conditions became dysoxic during deposition of the Lower ABS, while the more distinctly transgressive Upper ABS records anoxic/euxinic deposition during the later Event. When combined with the framboid data, however, unstable anoxia punctuated by short-term oxygenation events are assumed especially for the initial ABS phase, a crucial factor for effective nutrient recirculation from bottom waters to the photic zone and consequent phytoplankton blooming. This organic-rich level is easily identified amongst the background Upper Famennian rhythmic limestone-shaly succession, which was deposited under dysoxic to oxic conditions, with episodic anoxia developing only in the water column. Other well-known Late Devonian anoxic/high productivity episodes, recorded in the Dasberg, Kowala and Hangenberg black shales, also might partly to follow interglacial deepening pulses. These deepenings episodically reversed the overall regressive trend that resulted from a stepwise long-term climate change towards the end-Devonian Gondwanan glaciation.