Greenhouse crises of the past 300 million years Gregory J. Retallack† Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA ABSTRACT cate long-term high (>1000 ppmv) CO2 through None of these Quaternary fl uctuations greatly much of the Mesozoic (Fig. 1). High temperature exceeds current levels of warmth and CO2, so Proxies of past CO2 and climate over the and humidity in polar regions are thus viewed as predictions of future warming from those data past 300 m.y. now reveal multiple global cli- long-term global CO2 greenhouse effects (Royer, represent a modest extrapolation. Fortunately, mate change events in unprecedented detail. 2006), and they are attributed to long-term con- the deeper geological record has many exam- Evidence for past CO2 spikes comes from trols on the carbon cycle by tectonic uplift, ples of higher than modern CO2 levels from the expanded and refi ned stomatal index data oceanic gateways (Frakes et al., 1992), or plant- stomatal index of fossil plants (Retallack, 2001) of fossil Ginkgo and related leaves. New animal coevolution (Retallack, 2004). and warmth from degree of weathering of paleo- evidence for synchronous climatic change Exceptions to this long-lived Mesozoic green- sols (Retallack, 2007). comes from paleosols in Montana, Utah, and house, such as tillite in Early Cretaceous marine This paper provides new evidence for the link neighboring states. Each CO2 spike was co- rocks of central Australia (David, 1907), were between atmospheric CO2 and climate in deep eval with unusually clayey, red, and decalci- regarded skeptically as evidence for Mesozoic time in four parts: (1) stomatal index evidence fi ed paleosols that can be traced throughout periglacial paleoclimates until supported by the for atmospheric CO2, and (2) paleosol evidence the Colorado Plateau. Spikes in atmospheric discovery of glendonites, soil involutions, and for paleoclimate, before (3) statistical testing CO2 also were coeval with increases in paleo- ice wedges in Australia (Alley and Frakes, 2003; of their correlation using superposed epoch sol alkali depletion as an indication of high Rich and Vickers-Rich, 2000). Other indications analysis (Prager and Hoenig, 1989), and, fi nally, temperature, and spikes in paleosol base of large spikes (7200 ppmv) in atmospheric CO2 (4) cross-correlation of the two time series to depletion and depth to calcic horizons as in- came from studies of the stomatal index of fossil derive power laws for changes in mean annual dications of high precipitation. In the Colo- plants (McElwain et al., 1999; Retallack, 2001, temperature and precipitation with increased rado Plateau, times of warmer climate were 2002) and paleosol carbon isotope composi- atmo spheric CO2. also more humid, perhaps due to the greater tion (Nordt et al., 2003; Prochnow et al., 2006; moisture potential of warmer air. Seasonal- Montañez et al., 2007). These short-term spikes REFINED GINKGO STOMATAL INDEX ity of climate did not increase during warm- are superposed on a modest (~500 ppmv CO2) PROXY FOR PAST CO2 wet spikes. The Mesozoic greenhouse was not Mesozoic greenhouse between Permian and persistently hot with cool spells, but warm Ceno zoic icehouses (~300 ppmv CO2: Fig. 1). A paleobotanical gauge of atmospheric CO2 with hot fl ashes. These data furnish power The CO2 spikes at the end of the Middle and from stomatal index is based on observations laws predicting the sensitivity and magni- Late Permian correspond with indications from from greenhouse experiments and herbarium tude of change in mean annual tempera- paleo sols of coeval increases in temperature, pre- specimens spanning postindustrial CO2 rise ture (MAT) and mean annual precipitation cipitation, and seasonality (Retallack et al., 2006; (Retallack, 2001). Plant leaves have fewer (MAP) due to rising CO2 in a mid-latitude, Sheldon, 2006a). This paper both expands and stomates when atmospheric CO2 is high than mid-continental region. The magnitude of refi nes the database for stomatal index records of when atmospheric CO2 is low (Wynn, 2003). the coming anthropogenic greenhouse pales atmospheric CO2 and presents new data on paleo- Stomatal index is the number of stomatal open- in comparison with past greenhouse spikes at climatically sensitive features of paleosols in ings as a percent of epidermal cell plus sto- times of global mass extinctions. Utah, which record Mesozoic greenhouse paleo- mate numbers, and it is preferable compared to climate with unprecedented temporal resolution. measures of stomatal density because it is less INTRODUCTION New records presented here demonstrate that affected by competing effects of aridity, salin- global warming due to CO2 rise is not a unique ity, and soil nutrient defi ciency (Beerling and Ever since Heer (1868) and Nathorst (1897) event in Earth history, and they offer the pros- Royer, 2002). Stomatal index may also respond reported fossil ferns, cycads, and dicots of pect of refi ning predictive models of climate to volcanogenic SO2, but an SO2 effect cannot Malay sian affi nities in Jurassic and Cretaceous response to CO2 forcing. Climate effects of yet be disentangled from that of more abun- rocks of Spitsbergen and Greenland, cosmo- anthropogenic CO2 production have been pre- dant volcanogenic CO2 (Tanner et al., 2007). politan Mesozoic fossil fl oras have been taken dicted from the single, incomplete natural ex- Stomatal response to atmospheric CO2 is taxon as evidence that Mesozoic paleoclimates were periment represented by “hockey-stick” graphs specifi c, so attention has focused on Ginkgo, warm and humid (Vakhrameev, 1991) and of recent temperature and CO2 rise above natural which has an exceptionally long fossil record. lacking in large glaciers even at high latitudes variation of past centuries (Alley et al., 2007). A Ginkgo biloba is very similar morphologically (Frakes et al., 1992). This view is refl ected also more representative sample of eight completed and paleoecologically to Ginkgo adiantoides in computer models (Berner, 2006), which indi- global experiments of the past comes from back to the Late Cretaceous (Royer et al., 2003). glacial-interglacial variation in ice-core proxies The genus Ginkgo, defi ned on the basis of both † E-mail: [email protected]. for CO2 and temperature (Augustin et al., 2004). morphology and cuticular structure, goes back GSA Bulletin; September/October 2009; v. 121; no. 9/10; p. 1441–1455; doi: 10.1130/B26341.1; 10 fi gures; 2 tables; Data Repository item 2009025. For permission to copy, contact [email protected] 1441 © 2009 Geological Society of America Retallack PALEO. MESOZOIC CENOZOIC Permian Triassic Jurassic Cretaceous Tertiary Q. 8000 Ginkgo stomatal index 6000 (ppmv) 2 GEOCARBSULF 4000 2000 Atmospheric CO Atmospheric 0 300 250 200 150 100 50 0 Age (Ma) Figure 1. Alternative atmospheric CO2 time series from mass balance model of carbon burial (gray—GEOCARBSULF of Berner, m 2006) and stomatal index of Ginkgo and μ Lepidopteris (black—Retallack, 2001, 2002; A 1 cm 100 μm see also GSA Data Repository [see text foot- BC 100 note 1]). to the Triassic (Anderson and Anderson, 1989), where the oldest Ginkgo leaves in this compila- tion have the same cuticular structure and sto- matal index as Lepidopteris leaves in the same deposits. Lepidopteris stomatal indices in this D 1 cm compilation are no younger than latest Jurassic, when the genus became extinct. Lepidopteris and allied plants have been used to extend the paleobotanical CO2 paleobarometer back into the Permian (Retallack, 2001, 2002). New Data EF1 cm 100 μm G μ Fossil cuticles are widely used in taxonomic 100 m studies of fossil plants, and additional relevant accounts of fossil plants have been published Figure 2. Jurassic and Cretaceous leaves from Montana, and their cuticles: (A–C) “Ginkgoites ” since recent compilations of stomatal index cascadensis from the Late Jurassic (Tithonian, 150 Ma) upper Morrison Formation near data (Retallack, 2001, 2002). Also, new data Belt, Montana. (D–G) Ginkgo pluripartita from the Early Cretaceous (Aptian, 125 Ma) from museum specimens and preparations have Kootenai Formation near Great Falls. Photomicrographs B and F are upper cuticles been added, especially Jurassic and Cre ta ceous lacking stomates, and C and G are lower cuticle with stomatal apertures overhung by leaves from Montana, USA (Fig. 2), Cre ta- dark papillae. ceous leaves from Victoria (Australia), and Permian leaves from Russia (Retallack et al., 2006). The number of stomatal index records herbarium specimens spanning the postindus- The standard error of this relationship (±37 ppmv) meeting leaf number (5) and cell number (500) trial rise of CO2, with addition of published is trivial compared with standard deviation of rarefaction criteria for quality now stands at 146, greenhouse experimental results (Beerling individual stomatal index measurements (aver- 20 of which are new, and 18 of which have been et al., 1998). That original curve-fi t, and others, aging ±0.9% and resulting in an average error adjusted for refi nements in geological age dat- were reassessed by Wynn (2003), who proposed +2947/–473 ppmv for all 146 determinations). ing (see GSA Data Repository1). a new transfer function, which was modeled on fundamental equations of Fickian diffusion but Limitations and Comparisons of the New Transfer Function which still was based on herbarium and green- Ginkgo CO2 Proxy house data used by Retallack (2001). Additional The transfer function used by Retallack herbarium and greenhouse data published by The highest calculated levels of CO2, a stag- (2001) to infer atmospheric CO2 from Ginkgo Beerling and Royer (2002) now call for minor gering 7200 ± 3000 ppmv at the end-Permian stomatal index was based on measurements of revision of the transfer function (Fig. 3). Esti- mass extinction, are now confi rmed by replicate mation of atmospheric CO2 (O in ppmv) from data from fossil leaves of Russia, India, and stomatal index (I in %) is now based on the fol- Australia (Retallack et al., 2006).