Middle Triassic carbon isotope fluctuations in the Mecsek Mountains, Hungary

摘要

The end-Permian mass extinction and the subsequent Early Triassic is associated with major upheavals of the carbon cycle, an interval of approximately 5 million years, which have facilitated stratigraphic correlations in the absence of stratigraphically important index fossils. The Middle Triassic, an interval of approximately 10 million years, records relatively steady carbon isotope profiles, which, however, limits the utility of carbon isotopes for stratigraphical correlations. Here we investigated the stratigraphic utility of carbon isotopes for the Middle Triassic succession of the Mecsek Mountains (Hungary), and if there are perturbations that can be correlated either regionally, or globally. Our results show that the Mecsek carbon isotope record shows similarities with the Germanic Basin and Dinaridic-Carpathian-Alpine region that can be correlated regionally. Two correlative positive excursions are recognized within the Lapis Limestone Formation, first peaking at 3.8% in the middle part of the formation around the Bithynian/Pelsonian substage boundary and a subsequent peak of 4.2% just below the Lapis Limestone/Zuhanya Limestone formation boundary at the base of the Pelsonian Binodosus ammonoid zone. We infer a decoupling of isotopic composition of the northwestern Tethys and the open ocean during the Middle Triassic, which, owing to the high-continentality of the Germanic Basin and Dinaridic-Carpathian-Alpine region, led to higher variability in isotope signatures. This means that positive isotope perturbations recognized across central Europe can be correlated regionally, but not globally. The Middle Triassic succession from the Mecsek Mountains, therefore, shows a high degree of environmental variability, which in turns coincides with carbon isotope perturbations rather than a stable record, together this suggests that a causal link between a balanced carbon cycle and accelerated biotic recovery does not exist in Hungary.