Early Jurassic (late Pliensbachian) CO2 concentrations based on stomatal analysis of fossil conifer leaves from eastern Australia

The stomatal index (a measure of stomatal density) of an extinct Australian Early Jurassic araucariacean conifer species, Allocladus helgei Jansson, is used to reconstruct the atmospheric carbon dioxide concentration (pCO(2)) in the Early Jurassic. The fossil leaves are preserved in a single bed, palynologically dated to late Pliensbachian (similar to 185-183 Mya). Atmospheric pCO(2) is estimated from the ratios between the stomatal index of A. helgei and the stomatal indices of three modern analogs (nearest living equivalent plants). CO2 concentration in the range of similar to 750-975 ppm was calibrated from the fossil material, with a best-estimated mean of similar to 900 ppm. The new average pCO(2) determined for the late Pliensbachian is thus similar to, although similar to 10% lower, than previously inferred minimum concentrations of similar to 1000, based on data from the Northern Hemisphere, but may help constrain pCO(2) during this period. Our results are the first pCO(2) estimates produced using Jurassic leaves from the Southern Hemisphere and show that i) paleo-atmospheric pCO(2) estimates are consistent at a global scale, though more investigations of Southern Hemisphere material are required, and ii) the stomatal proxy method can now be used without the context of relative change in pCO(2) when applying the correct methodology. (C) 2014 The Authors. Published by Elsevier B.V. on behalf of International Association for Gondwana Research.