Atmospheric CO2 from the late Oligocene to early Miocene based on photosynthesis data and fossil leaf characteristics

Abstract

The late Oligocene represents a comparatively cool phase followed by a warming event, the so-called Late Oligocene Warming that predates the Mi-1 glacial event at the Oligocene–Miocene transition. There is evidence that these climate events were linked to level changes in atmospheric CO2. In this study, atmospheric CO2 from the late Oligocene to the early Miocene is reconstructed by a terrestrial proxy, stomatal parameters of angiosperm leaves. Fossil leaves of six plant species were considered: Laurophyllum pseudoprinceps, L. acutimontanum (both Lauraceae), Platanus neptuni (Platanaceae), Eotrigonobalanus furcinervis, Quercus rhenana and Q. praerhenana (all Fagaceae). The plant material originates from various sites in Germany (late Oligocene to the early Miocene) and Austria (early Miocene). A mechanistic-theoretical approach based on stomatal data, photosynthesis and gas exchange control was used. Since this approach also requires climate data as input parameters for calculating assimilation and transpiration rates, palaeoclimate of these sites were also derived in detail. Considerable differences were found between results obtained by the two Lauraceae taxa and all other taxa, with the Lauraceae indicating a high CO2 level. If Lauraceae are included, a CO2 level of about 700ppm for the late and latest Oligocene is obtained and about 500ppm for the early Miocene. All non-Lauraceae taxa belonging to different families indicate a consistently lower level of CO2 of about 400ppm for all considered sites and time slices. A value of 400ppm is also consistent with other proxy data that are available for that time period. A steady level of 400ppm would indicate a decoupling of CO2 and cooling/warming events from the late Oligocene to the early Miocene. More data provided by different proxies are desirable to unravel the interactions between atmosphere and climate for the late Oligocene and the early Miocene.