Reading a CO₂ signal from fossil stomata

Area of Science:

• Paleobotany
• Paleoclimatology
• Plant Physiology

Background:

• The stomatal index of plant leaves correlates inversely with atmospheric CO2.
• Fossil plant cuticles are used to reconstruct past atmospheric CO2 levels.
• Contemporary plants show reduced stomatal index sensitivity to CO2 above 340 ppm.

Purpose of the Study:

• Investigate the nonlinear response of stomatal index to atmospheric CO2.
• Assess the reliability of using fossil plant cuticles for high CO2 level estimation.
• Compare genotypic and phenotypic responses in Ginkgo to CO2 enrichment.

Main Methods:

• Review of existing evidence on stomatal index nonlinearity.
• Analysis of new data from fossil Ginkgo cuticles.
• Comparison of reconstructed CO2 values with stomatal ratio and geochemical models.

Main Results:

• The phenotypic response of stomatal index to CO2 is nonlinear, especially above 340 ppm.
• Genotypic response in fossil Ginkgo mirrors phenotypic responses in experiments.
• Reconstructed CO2 from Ginkgo cuticles aligns with other methods.

Conclusions:

• The nonlinear stomatal response limits the accuracy of estimating high past CO2 levels.
• Fossil Ginkgo data show consistency with stomatal ratio and geochemical CO2 estimates.
• While useful, the stomatal index method has limitations for extreme paleo-CO2 reconstructions.