FOSSIL PLANT CUTICULAR EVIDENCE FOR HIGH ATMOSPHERIC CARBON DIOXIDE DURING THE MIDDLE MIOCENE THERMAL OPTIMUM

A middle Miocene (16 Ma) paleoclimatic thermal optimum is indicated by a variety of lines of evidence: particularly the unusually high paleolatitudes of lateritic paleosols, Liquidambar, and large foraminifera. It is also compatible with evidence for high atmospheric CO₂ from the stomatal index of fossil Ginkgo leaves. Ginkgo occidentalis leaves from Zakarnatskie, Russia, have a low stomatal index indicating high CO₂, but these leaves are not dated any more precisely than Miocene. Fortunately, there are other middle Miocene Ginkgo cuticles from 49 Camp, Nevada, precisely dated between tuffs with radiometric ages of 16.28 ± 0.02 and 15.84 ± 0.05 Ma. I recently counted the stomatal index of these cuticles at 7.6 ± 1.3 (average of 34 visual fields on 4 cuticle fragments, totalling 11623 cells and 945 stomates). Following the transfer function of Royer et al.(Science 292:2310) this indicates 1.6 +2.7/-0.3 PAL (441 +758/-86 ppmV) CO₂, and following my transfer function (Nature 411:287), indicates 2.7 +3.3/-1.3 PAL (747 +951/-425 ppmV) CO₂. The 49 camp Ginkgo adiantoides cuticles are unusual in showing numerous large (1 x 0.3 mm) elliptical hydathodes, presumably reflecting unusually high transpiration. The associated 49 Camp fossil flora, like other middle Miocene floras such as those of Fingerrock, Pyramid, Mascall and Latah, indicates an unusually warm and wet paleoclimate for western North America for a brief time at around 16 Ma, followed by marked climatic drying and cooling by 15 Ma. The narrow temporal window of the middle Miocene climatic optimum is easy to miss in short and sparse Miocene time series. The idea that high atmospheric CO₂ is decoupled from warm paleoclimate thus derives no support from stomatal index studies, and is based instead on carbon-isotopic reconstructions of former atmospheric CO₂, which may be compromised by methane-hydrate dissociation events. The greenhouse gases CO₂, H₂O and CH₄ appear to have played an important role in climatic warmth and humidity for at least the past 300 million years.