FOSSIL LEAVES AS BIOSENSORS OF EOCENE PALEOATMOSPHERIC CO2

Abstract

During the Cretaceous, as broad-leaved flowering plants evolved leaf forms similar to those seen in many flowering plants today, they had to develop physiological responses to changes in the supply of basic resources such as [CO2]. As the amount of [CO2] varied, the anatomy of the leaves accommodated these fluctuations, displaying physiologically determined signals that can be used to discriminate atmospheric levels of [CO2]. A decreasing number of stomata (air exchange holes in a leaf) on leaves of broad-leaved trees, as determined by stomatal index, indicates an increase in [CO2]. The stomatal index represents the ratio of the number of stomata per unit leaf area to the number of total epidermal cells per unit leaf area, thus expressing frequency independently of variation in epidermal cell size, serving as a sensitive parameter for detecting stomatal frequency changes. In comparing the stomatal indexes of leaves of the loblolly bay (Gordonia lasianthus) with those of 100-year-old leaves and of related Eocene leaves, significant differences were observed between these groups which appear to be directly related to the atmospheric [CO2] present when the leaves grew. Our preliminary results indicate that the Eocene p[CO2] was the order of 450–500 ppmv at a time when the earth was significantly warmer than today. Analysis of stomatal indexes over time provides a useful method for quantifying past environmental changes of [CO2].