Late Oligocene atmospheric carbon dioxide concentrations reconstructed from fossil leaves using stomatal index

Ancient atmospheric CO2 can be reconstructed using various climate proxies; stomata from fossil leaves are one of the climate proxies that provide critical information about past climatic conditions of the Earth. Exceptionally well–preserved fossil leaves found in overbank deposits in Chilga of Northwest Ethiopia were used to estimate late Oligocene atmospheric CO2 values using stomatal index. The age of the fossils, 206Pb/238U: 27.23 ± 0.03 Ma, was determined from zircons in an ash deposit comprising the matrix deposited contemporaneously with the fossil leaves. Stomatal indices were calculated from both the fossil leaves and nearest living relatives of the fossils. Corresponding atmospheric CO2 values for the nearest living relatives of the fossils were assigned from historical records from the Mauna Loa Observatory. This produces a calibrating curve that shows variation of atmospheric CO2 over time, and late Oligocene atmospheric CO2 values were quantified from the calibrating curve. The quantified late Oligocene atmospheric CO2 values are about 343 ± 11 ppm which show a 12 % decrease when they are quantified using a leaf gas exchange method. This is consistent with the idea that stomatal–index method underestimates CO2 values compared to the leaf gas exchange method. The late Oligocene was colder than both its preceding Eocene and its following Miocene epochs, and the results are incongruent with the cold Oligocene period. These results for this particular geologic time provide opportunity to examine how plants responded to climate changes in the past and have important implications for the study of current and future climate changes.