Carbon isotope excursions and microfacies changes in marine Permian–Triassic boundary sections in Hungary

Several Permian–Triassic boundary sections occur in various structural units within Hungary. These sections represent different facies zones of the western Palaeotethys margin. The Gárdony core in the NE part of the Transdanubian Range typically represents the inner ramp, while the Bálvány section in the Bükk Mountains of northern Hungary represents an outer ramp setting. The two sections have different patterns for their δ13C values. The Bálvány section shows a continuous change towards more negative δ13C values starting at the first biotic decline, followed by a sharp, quasi-symmetric negative peak at the second decline. The appearance of the δ13C peak has no relationship to the lithology and occurs within a shale with low overall carbonate content, indicating that the peak is not related to diagenesis or other secondary influences. Instead, the shift and the peak reflect primary processes related to changes in environmental conditions. The continuous shift in δ13C values is most probably related to a decrease in bioproductivity, whereas the sharp peak can be attributed to an addition of C strongly depleted in 13C to the ocean–atmosphere system. The most plausible model is a massive release of methane-hydrate. The quasi-symmetric pattern suggests a rapid warming–cooling cycle or physical unroofing of sediments through slope-failure and releasing methane-hydrate. The Gárdony-1 core shows a continuous negative δ13C shift starting below the P–T boundary. However, the detailed analyses revealed a sharp δ13C peak in the boundary interval, just below the major biotic decline, although its magnitude doesnʹt reach that observed in the Bálvány section. Based on careful textural examination and high-resolution stable isotope microanalyses we suggest that the suppression of the δ13C peak that is common in the oolitic boundary sections is due to combined effects of condensed sedimentation, sediment reworking and erosion, as well as perhaps diagenesis.