INTENSIFIED WEATHERING AT THE PERMIAN-TRIASSIC BOUNDARY IN THE BALATON REGION OF HUNGARY

The Permian-Triassic boundary (PTB) mass extinction, the largest biocrisis in Earth history, has been intensively investigated in marine and terrestrial sections in many parts of the world. However, marginal marine deposits, representing the interface between the marine and terrestrial environments, have received relatively little attention. In this study, we analyzed samples from the Balaton region of southwestern Hungary, representing marginal-marine facies (mainly fine-grained limestone and dolostone) of latest Permian-Early Triassic age and open-marine facies (mainly fossiliferous wackestones and packstones) of Middle-Late Triassic age. Redox conditions were uniformly oxic throughout the study interval, as indicated by uniformly low concentrations of pyrite S (<0.05%), trace-metals at or below PAAS levels, and C_org:P molar ratios between 5 and 50. Geochemical proxies such as TOC (avg 0.1±0.1%), P (0.03±0.03%), and excess Ba (~0 ppm) are consistent with generally low productivity in the Balaton region at that time. However, our sampling was not continuous, so short episodes of anoxia or increased productivity would not necessarily have been detected in this study. One proxy that shows significant secular variation in our dataset, however, is the CIA (chemical index of alteration). This is the ratio of Al oxides to Al plus alkali oxides in a sample, with low values (~0.5-0.6) indicative of fresh material and high values (1.0) indicative of strongly weathered and leached material. CIA values are 0.75-0.85 through most of the Late Permian to Late Triassic study interval, but we observed a short-term increase of CIA values to 0.85-0.95 in immediate proximity to the PTB. We interpret this increase to be indicative of a transient episode of strong chemical weathering in conjunction with the end-Permian crisis. A similar pattern has been reported from other PTB sections (e.g., Algeo et al., 2012, GSA Bulletin, 124:1424-1448), although the CIA values documented here are significantly higher, possibly owing to the relative proximity of the study area to strongly weathered continental source rocks. Marginal marine areas such as that of the Balaton region have potential to provide significant new insights into the PTB crisis.