Carbon cycle changes during the Triassic-Jurassic transition

The end-Triassic is regarded as one of the five major mass extinction events of the Phanerozoic. This time interval is marked by up to 50% of marine biodiversity loss and major changes in terrestrial ecosystems. Mass extinction events are often marked by changes in the global carbon cycle. The reality and nature of C-cycle changes at the Triassic-Jurassic (T-J) transition are however questioned and its causes are poorly understood. Potential changes in sizes of and fluxes between exchangeable carbon reservoirs, are reflected by changes in the carbon isotopic composition of these reservoirs. Several geological outcrops, representing the T-J transition, have been studied in the Northern Calcareous Alps (Austria). Sediments in these sections were deposited in the intra-platform Eiberg Basin that extended for several hundreds of kilometers on the continental margin of the north-western Tethys Ocean. High resolution C-isotope studies in this basin showed a 5-6‰ end-Triassic carbon isotope excursion (CIE), which is directly preceded by the last occurrence of typically Triassic ammonites and succeeded by the first occurrence of typically Jurassic ammonites and pollen. This strongly suggests a simultaneous occurrence of the end-Triassic mass extinction event and the negative CIE. Compound specific delta 13C measurements on plant-wax derived n-alkanes also show a ~6‰ negative excursion together with the end-Triassic mass extinction event. It suggests global carbon cycle changes caused by 13C depleted carbon release to the atmosphere. These events have been attributed to increased volcanic CO2 emissions related to the break-up of Pangaea, and emplacement of the Central Atlantic Magmatic Province. The duration of increased volcanic activity at the T-J boundary is confined to ~600 kyr. Alternations in relative abundance of limestone versus marl and black shale deposits, are observed in the uppermost Triassic to lower Jurassic shallow marine sedimentary succession of St. Audrie’s Bay and East Quantoxhead (southwest UK). Spectral analyses of high resolution delta 13C (‰), TOC (%) and CaCO3 (weight %) proxy-records from this time interval in these sections, suggest astronomically controlled variations in climate and the palaeo-environment. This allows confining of the duration of the end-Triassic mass extinction and the subsequent recovery interval and the Hettangian stage to ~20-40 kyr, ~140 kyr and ~1.8 Myr, respectively. The short duration of the end-Triassic mass extinction interval and coinciding negative CIE do not match with the astronomically tuned ~600 kyr duration of increased volcanic activity. Simple mass balance calculations show that the end-Triassic global C-cycle perturbation is best explained by rapid and massive release of ~7000 Gt of isotopically depleted carbon from the methane-hydrate reservoir. This research strongly suggests causal relationships between massive end-Triassic carbon release, climate change and highly increased biodiversity-loss. However, extinction levels did not only increase in the uppermost Rhaetian. Also the late Triassic is already marked by enhanced loss of biodiversity in both the marine and terrestrial realm. This study shows that also the late-Rhaetian is already marked by two negative CIEs in marine and continental proxy-records, suggesting 13C depletion of the late Triassic atmosphere. Oxidation of organic carbon by subsurface thermal metamorphism of organic rich strata may be one potential mechanism to transfer isotopically light carbon to the exogenic carbon pool.