A comparative study of total organic carbon-δ13C signatures in the Triassic–Jurassic transitional beds of the Central European Basin and western Tethys shelf seas

Abstract

Stratigraphic studies are an integral component in understanding the chronology of events that led to the end-Triassic mass extinction, by resolving causal relationships between environmental upheavals and biotic response. Successful correlation of Triassic–Jurassic (Tr–J) successions is complicated by the disappearance of macro-fossils that are otherwise central components in stratigraphic studies. This problem is exacerbated in multiple Tr–J sections situated in Europe, where the so-called “Event Beds” – assumed to demarcate the extinction interval – are virtually devoid of fossils. An alternative stratigraphic approach entails the reconstruction of carbon isotope records, where stratigraphic fluctuations in carbon isotope composition are considered to track changes in the global biogeochemical carbon cycle. The predominance of carbonate-lean sections has prompted the reconstruction of total organic carbon (TOC)-based carbon isotope records. However, bulk rock derived TOC is the diagenetically stabilized remnant of organic components that accumulated on the sea floor, and which can originate from multiple sources. In this study, we assess long-term TOC-based carbon isotope trends at two sites: Bonenburg (Central European Basin) and Kuhjoch (the Tr–J Global Stratotype Section and Point; western Tethys shelf seas). We focus on the TOC 13C-enrichment of the Event Beds with the aim of deciphering stratigraphic fluctuations in relation to their main driver (the exogenic carbon pool versus organic matter source changes). By studying the systematic co-variance of several sedimentary parameters (TOC, total nitrogen [TN], and the palynomorph composition), we infer that the TOC composition is possibly characterized by insignificant organic matter source changes in terms of the marine and terrestrial organic carbon contributions. By contrast, a clay mineralogical shift to more K-depleted minerals as well as the elevated occurrence of wood fragments in the Event Beds suggest a terrestrial organic matter source shift from immature substrates to substrates predominated by “pre-aged” or “fossil organic matter” under a changing continental weathering regime. This outcome urges for reservations when interpreting TOC-based carbon isotope records in terms of global C-cycle perturbations, especially when coinciding with lithological and mineralogical changes. On a more positive note, the shift towards positive carbon isotope values appears to be a recurring feature, possibly testifying to a globally significant climate-controlled weathering regime shift.