The Jurassic extinction events and its relation to CO2 levels in the atmosphere : a case study on Early Jurassic fossil leaves

Abstract: The Mesozoic was a so-called “hothouse” time period in Earth’s history, with high concentrations of atmospheric CO2 and elevated temperatures, but the exact climate evolution is not fully known for the entire inter-val. The Early Jurassic for instance is believed to have been a warm period, flanked by two episodes of extremely high CO2, coinciding with environmental degradation and mass extinctions: the Triassic-Jurassic (200 Ma) and the early Toarcian (183 Ma) mass extinction events. Mass extinctions are often related to variations in climate condi-tions, which in turn are linked to the global CO2 concentration, and it is therefore important to understand the levels of CO2 before, during and after such events. Here, atmospheric CO2 levels were reconstructed for the late Pliesbachian (ca.185 Ma), an Early Jurassic time period that is not well understood in terms of climate. The sto-matal proxy method of palaeo-CO2 reconstructions was applied, using fossil leaves derived from the Clarence- Moreton Basin in eastern Australia. The stomatal proxy relies on the inverse relationship between the density of plant leaf stomata and atmospheric CO2 concentrations. The ten exceptionally well-preserved fossil leaves derived from the araucarian conifer species Allocladus helgei. Two different methods of calibrations, using the nearest liv-ing equivalent species Athrotaxis cupressoides, were applied to the stomatal data and CO2 concentrations in the range 600 – 750 ppm were found. These values are high compared to present day CO2 concentration (396 ppm in May 2012), but are lower than values usually cited for this period, as interpolated between CO2 concentrations known from the flanking mass extinction events of 1000 – 2000 ppm. The results found here therefore indicate that the Pliensbachian was a cooler period than previously assumed, but that it was still a “hothouse” warm period with higher global temperatures than today.