The impact of Miocene atmospheric carbon dioxide fluctuations on climate and the evolution of terrestrial ecosystems

Abstract

The Miocene is characterized by a series of key climatic events that led to the founding of the late Cenozoic icehouse mode and the dawn of modern biota. The processes that caused these developments, and particularly the role of atmospheric CO2 as a forcing factor, are poorly understood. Here we present a CO2 record based on stomatal frequency data from multiple tree species. Our data show striking CO2 fluctuations of approximately 600-300 parts per million by volume (ppmv). Periods of low CO2 are contemporaneous with major glaciations, whereas elevated CO2 of 500 ppmv coincides with the climatic optimum in the Miocene. Our data point to a long-term coupling between atmospheric CO2 and climate. Major changes in Miocene terrestrial ecosystems, such as the expansion of grasslands and radiations among terrestrial herbivores such as horses, can be linked to these marked fluctuations in CO2.

Fig. 1.

Training dataset of mean SI values of L. nobilis (filled circles) and O. foetens (open circles) vs. atmospheric CO₂ levels with 95% prediction interval. Error bars show standard deviation.

Fig. 2.

CO₂ inference model based on historical series of herbarium sheets for L. nobilis (filled circles) and O. foetens (open circles).

Fig. 3.

Late Oligocene–Miocene stomatal index records, inferred atmospheric CO₂ fluctuations, and effects on global temperature compared with major events in terrestrial ecosystems. (A) SI of fossil leaf remains between 25 and 12 Ma (late Oligocene until late middle Miocene. For a list of locations and their age assessment, see SI Table 3). The lines represent trends in SI: blue, L. pseudoprinceps; black, L. nobilis; red, O. foetens; green, G. biloba. The values represent means per stratigraphic unit, with error bars indicating the standard deviation of the SI. The age error bars indicate the minimum and maximum ages of the sample. The stratigraphic framework is established by vertebrate biostratigraphy and magnetostratigraphy (see also SI Table 3). (B) Reconstructed late Oligocene–middle Miocene CO₂ levels based on individual independently calibrated tree species. The error bars of the species-specific CO₂ estimates are based on the standard deviation of the SI measurements on individual fossil leaf samples. The gray band indicates the envelope as determined by the minimum and maximum CO₂ levels inferred from all individual samples per stratigraphic unit. (C) Modeled temperature departure of global mean surface temperature from present day, calculated from mean CO₂ estimates by using a CO₂–temperature sensitivity study (46). Also indicated are the major Miocene climate key events and the position of the Miocene cooling events Mi1/1a, Mi1b, Mi2, and Mi3/4 known from the marine oxygen isotope record (1). The effects of pCO₂ level changes on the global mean land surface temperature were estimated assuming a radiative relationship between CO₂ mixing and global air temperature inferred from climate–CO₂ sensitivity models expressed as ΔT = 4ln (C/CO), where C is the mixing ratio and CO is the preindustrial CO₂ mixing ratio of 278 ppmv (46). (D) Major events in the terrestrial ecosystems in response to the Miocene CO₂ trends, such as changes in terrestrial herbivore communities (–5), the expansion of Miocene grasslands (2, 37, 38), and evidence for C₄ biomass from paleosols (43).

Fig. 4.

Predicted CO₂ concentrations calculated from Eqs. 3 and 4 for L. nobilis (filled circles) and O. foetens (open circles), plotted vs. the actual atmospheric CO₂ levels with a linear extrapolation to elevated CO₂. The arrows indicate the value of the corrected CO₂ level for the climatic optima in the middle Miocene (1) and latest Oligocene (2).