DO FOSSIL PLANTS SIGNAL PALAEOATMOSPHERIC CO2 CONCENTRATION IN THE GEOLOGICAL PAST

Fossil, subfossil, and herbarium leaves have been shown to provide a m orphological signal of the atmospheric carbon dioxide (CO2) environmen t in which they developed by means of their stomatal density and index . An inverse relationship between stomatal density/index and atmospher ic CO2 concentration has been documented for all the studies to date c oncerning fossil and subfossil material. Furthermore, this relationshi p has been demonstrated experimentally by growing plants under elevate d and reduced CO2 concentrations. To date, the mechanism that controls the stomatal density response to atmospheric CO2 concentration remain s unknown. However, stomatal parameters of fossil plants have been suc cessfully used as a proxy indicator of palaeo-CO2 levels. This paper p resents new estimates of palaeoatmospheric CO2 concentrations for the Middle Eocene (Lutetian), based on the stomatal ratios of fossil Laura ceae species from Bournemouth in England. Estimates of atmospheric CO2 concentrations derived from stomatal data from plants of the Early De vonian, Late Carboniferous, Early Permian and Middle Jurassic ages are reviewed in the light of new data. Semi-quantitative palaeo-CO2 estim ates based on the stomatal ratio (a ratio of the stomatal index of a f ossil plant to that of a selected nearest living equivalent) have in t he past relied on the use of a Carboniferous standard. The application of a new standard based on the present-day CO2 level is reported here for comparison. The resultant ranges of palaeo-CO2 estimates made fro m standardized fossil stomatal ratio data are in good agreement with b oth carbon isotopic data from terrestrial and marine sources and long- term carbon cycle modelling estimates for all the time periods studied . These data indicate elevated atmospheric CO2 concentrations during t he Early Devonian, Middle Jurassic and Middle Eocene, and reduced conc entrations during the Late Carboniferous and Early Permian. Such data are important in demonstrating the long-term responses of plants to ch anging CO2 concentrations and in contributing to the database needed f or general circulation model climatic analogues.