Research article
Deep time perspective on rising atmospheric CO2

https://doi.org/10.1016/j.gloplacha.2020.103177Get rights and content

Highlights

  • Ginkgo stomatal index as a guide to atmospheric CO2 is substantially revised.

  • This revision gives comparable results to the paleosol CO2 paleobarometer.

  • Past mass extinctions coincide with CO2 spikes of 1500 ppm or more.

  • Revised CO2 during Late Permian mass extinction was 2109 ± 1267 ppm.

  • Over the past 300 Ma CO2 was never less than 180 ppm.

Abstract

The accuracy of CO2 hindcasting using fossil Ginkgo stomatal index is ripe for revision for three reasons: exponential rise in atmospheric CO2 over the past decade, discovery of a Kew herbarium specimen of Ginkgo picked in 1754, and increased sophistication of a pedogenic CO2 paleobarometer as an independent parallel record. Past mass extinctions coincide with revised CO2 spikes of 1500 ppm or more. Increases such as the middle Miocene level of 640 ± 71 ppm expected before the year 2100 resulted in biome shifts, with expansion of tropical forests northward, and of grasslands into deserts. Deep time records from paleosols and from stomatal index reveal that CO2 levels less than 180 ppm and more than 1500 ppm are toxic to the biosphere.

Introduction

Atmospheric CO2 from fossil fuel burning, forest fires, agricultural ploughing, and oxidation of methane from ruminants and rice paddies has increased dramatically in the past decade (Ciais et al., 2013). Since observations began in 1958 with 316 ppm, atmospheric CO2 levels at Mauna Loa have continually climbed with ±2 ppm seasonal variation to a peak of 415 ppm in May 2019 (NOAA, 2020). By the year 2100, atmospheric CO2 concentrations could reach 936 ± 140 ppm, based on RCP 8.5 emission scenario of a heterogeneous world with continued population growth (Meinshausen et al., 2011). Such changes are unprecedented in Quaternary ice core measurements showing fluctuation between 180 and 280 ppm (Lüthi et al., 2008), and their effects can be assessed by proxies for atmospheric CO2 deep in geological time (Breecker and Retallack, 2014; Franks et al., 2014). Deep time records give a unique perspective on natural variation and limits of atmospheric CO2.

A useful proxy for ancient CO2 is stomatal index, or percent stomata normalized to other cells, of the living fossil Ginkgo and fossils with similar cuticle anatomy (Retallack, 2001). Stomatal index declines with rising CO2 because plants minimize water loss by deploying the fewest stomata needed for CO2 intake to sustain photosynthesis, and is different for different woodland species as discovered by Salisbury (1927). For application to deep time “living fossils” such as Ginkgo have been evaluated for this effect (McElwain and Chaloner, 1996; McElwain, 2018), using live and dated herbarium leaves, as well as leaves from greenhouse experiments under different CO2 levels (Beerling et al., 1998; Retallack, 2001; Royer et al., 2001). Some doubt about greenhouse experiments has arisen because they have been harsh enough to create deformed stomata (Barclay and Wing, 2016). This study is mainly based on herbarium specimens now extending back to 1754 (Fig. 1), but is also well constrained because of non-linear decline in stomatal index of Ginkgo over the past two decades newly presented here (Fig. 2).

Stomatal size also increases with increasing CO2 (Franks and Beerling, 2009a, Franks and Beerling, 2009b), and provides another useful CO2 proxy (Franks et al., 2014), but requires many additional measurements of studied specimens not currently available. Stomatal and cell size also increase with genome size, especially for polyploids (Beaulieu et al., 2008), which can show dramatic stepwise increases within closely related fossil leaves (Roth and Dilcher, 1979). Such dramatic differences in cell size were not seen in our data of Ginkgo and other taxa with similar cuticles, in which cell size differences were only noted at very high levels (>1000 ppm) of CO2.

A significant refinement of the Ginkgo stomatal index proxy is offered here from herbarium specimens of unprecedented temporal range from 1754 to 2015, when observed CO2 had turned the corner to its current dramatic rise (NOAA, 2020). Our new calibration allows a new reconstruction of atmospheric CO2 over the past 300 million years as evidence of past greenhouse crises (Retallack, 2009a), their effects in biodiversity crises (Peters and Foote, 2002), and their role in biogeographic shifts (Retallack et al., 2016).

Section snippets

Material and methods

A recently discovered Kew Herbarium specimen of Ginkgo biloba picked in 1754 is well enough preserved to count stomata, despite fungal damage (Fig. 1). All images counted (Conde, 2016) were scanning electron micrographs using an environmental instrument, which did not require metal sputter coating. Such images show papillae more clearly than standard macerated preparations, which is important because there is not a 1:1 correspondence between papillae and cells (Fig. 1). Our 1754 leaf anchors

Independent validation

CO2 calculated from our new transfer function (Eq. (3) above) can be checked against estimates of CO2 from a revised model of carbon isotopic fractionation in paleosols using new paleosol-specific productivity estimates (Breecker and Retallack, 2014), which considerably improves the correlation of paleosol and stomatal index measures (Beerling and Royer, 2011). Our newly calibrated estimates of atmospheric CO2 from Gingko stomatal index are compared with paleosol estimates for the same

Greenhouse spikes

Numerous transient spikes in CO2 (Fig. 4) are now well controlled stratigraphically not only by improved dating of Ginkgo leaves, but by evidence of warm-wet spikes in sequences of paleosols, and in carbon isotopic time series (Retallack, 2009a). Our result may be contrasted with another pilot study proposing values “below 1000 ppm for most of the Phanerozoic, from the Devonian onwards” (Franks et al., 2014), but that result for the Late Triassic, for example, was obtained by averaging

Conclusions

Coming atmospheric levels of 450–950 ppm CO2 by 2100 due to anthropogenic forcing (Meinshausen et al., 2011) were last seen during the Middle Miocene (Breecker and Retallack, 2014), when transient atmospheric injections of CO2, perhaps from Columbia River Group flood basalt eruptions (Retallack et al., 2016), created modest mammalian and floral overturn (Retallack, 2013). These crises were followed by diversity-enriching polar migrations and expanded carbon sinks from geographical expansion of

Acknowledgements

Greg Bothun, Dan Gavin, and Pat Bartlein offered stimulating discussions. Jonathan Wynn, Joshua Roering, and Edward Davis proffered mathematical advice. Chrissie Prychid and Nicola Kuhn provided SEM images of a Ginkgo leaf of the Kew Herbarium picked in 1754 in Japan. Arne Arneberg provided leaf fragments from the Swedish Natural History Museum picked in 1829 from “Hortus Botanicus Augustinus”, Amsterdam. Sarena Campbell, John Donovan, and Julie Chouinard aided with scanning electron

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