Stomatal density and index data of Platanus neptuni leaf fossils and their evaluation as a CO sub(2) proxy for the Oligocene

Platanus neptuni (Ettingshausen) Buzek, Holy and Kvacek is a deciduous and preferentially azonal taxon of temperate to warm-temperate vegetation in Europe from the Late Eocene to the Late Miocene. The high fossilization potential of its leaves and easily identifiable stomata and epidermal cells make P. neptuni an excellent source of stomatal data that can be utilized as a CO sub(2) proxy. Moreover, it was found in former studies that CO sub(2) data based on stomatal frequency data of P. neptuni overlapped to a high degree with CO sub(2) results which are provided by other, contemporaneous taxa. In this study, the stomatal CO sub(2) signal of P. neptuni is expanded to include the early Oligocene and is analyzed in more detail with three aims: 1) to evaluate the CO sub(2) signal of P. neptuni stomatal data, 2) to check SI and SD data of P. neptuni for consistency, and 3) to contribute additional terrestrial CO sub(2) data to the Oligocene record. During the Oligocene, full scale Antarctic glaciation occurred, punctuated by various distinct glaciation events. There is evidence that Oligocene glaciation was coupled to atmospheric CO sub(2) level. Presently, the main proxy sources for Oligocene CO sub(2) levels are alkenones and boron-isotope data, both obtained from marine sediments. Since P. neptuni is an extinct taxon, CO sub(2) was reconstructed by using an ecophysiological modeling approach to plant gas exchange which utilizes various other data in addition to stomatal density. Material was considered from sites which are dated to the following time intervals: early Oligocene - 33.9 to 32 Ma, late Oligocene - 27 to 26.2 Ma and 25.3 to 23 Ma, and latest Oligocene - around 24 Ma. Comparison of raw SI and SD data of P. neptuni revealed partially conflicting results, with the SD data indicating a decrease in CO sub(2) from the early to the late Oligocene whereas SI data indicate an increase. In contrast, CO sub(2) results calculated with the gas exchange model indicate relatively stable CO sub(2) for the considered time intervals, with levels of about 400 ppm. The reconstructed CO sub(2) data points are similar to other proxy data and are consistent with the general climate development during the Oligocene.