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Società Geologica Italiana Volume24 31- -Febbraio Luglio 2014 Volume 2013
Climatic and Biotic Events of the Paleogene 2014 CBEP 2014
Selected short notes and abstracts Ferrara, Italy, July 1-6 2014
edited by: G. R. Dickens & V. Luciani A cura di: Domenico Calcaterra & Silvia Fabbrocino
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Increasing atmospheric CO2 prior to the Paleocene-Eocene Thermal Maximum inferred from stomata of Ginkgo adiantoides, Bighorn Basin, Wyoming, USA Richard S. Barclay (a) & Scott L. Wing (a) _____________________________________________________________________________________________________________________________________________________ (a) Paleobiology Department, Smithsonian Institution, National Museum of Natural History, 10th & Constitution, Washington, D.C., 20560-0121, USA. E-mail:
[email protected]
Document type: Short note. Manuscript history: received 15 May 2014; accepted 30 May 2014; editorial responsibility and handling by Gerald R. Dickens & Valeria Luciani. _____________________________________________________________________________________________________________________________________________________
KEY WORDS: Bighorn Basin, carbon dioxide, climate-sensitivity, cuticle, deep-time climate change, paleobotany, stomatal index proxy.
The Paleocene-Eocene Thermal Maximum (PETM) was a geologically brief interval of intense global warming 56 million years ago. Multiple proxies suggest a temperature increase of 5-8 °C within ~20ka (Wing et al., 2005). It is arguably the best geologic analog for anthropogenic carbon emissions. The PETM is marked by a ~4-6‰ negative carbon isotope excursion (CIE) and extensive marine carbonate dissolution (Zachos et al., 2005), which together are powerful evidence for a massive addition of isotopically light carbon to the oceans and atmosphere. In spite of broad agreement that the PETM reflects a large carbon cycle perturbation, atmospheric concentrations of CO2 (pCO2) during the event are not well constrained (Schubert & Jahren, 2013). In this study we reconstructed pCO2 across the Paleocene/Eocene boundary using the stomatal index proxy. Stomatal index is the number of gas-exchange pores (stomata) on a leaf divided by the number of epidermal cells, both counted in a standard area and expressed as a percentage. An inverse relationship between stomatal index and pCO2 is expected because plants lose water as well as gain CO2 through their stomata, a process regulated by a genetic mechanism that balances the cost and the benefit by adjusting stomatal index (Woodward, 1987). Terrestrial sections in the Bighorn Basin, Wyoming, contain macrofossil plants with cuticle immediately bracketing the PETM, as well as dispersed plant cuticle from within the body of the CIE (Wing & Currano, 2013). These fossils allow for the first stomatal-based reconstruction of pCO2 near the Paleocene-Eocene boundary. Cuticle-bearing fossils are used to determine both the relative timing of pCO2 changes and to estimate the magnitude of pCO2 shifts in relation to the CIE that defines the PETM. Several studies have measured the relationship between pCO2 and stomatal index in the living species Ginkgo biloba (Retallack, 2001; Royer et al., 2001; Royer, 2003). The largest and most widely cited calibration of stomatal index and pCO2
in Ginkgo biloba (Royer et al., 2001) shows a strongly linear decrease of stomatal index as pCO2 increases from 290 to 370 ppm, then a near asymptote as pCO2 increases to higher values. There are, however, significant problems with previous calibrations. Either the data are sparse and don’t include plants that grew in pCO2 conditions above 370 ppm (Retallack, 2001), or counts of stomata and epidermal cells were made using multiple techniques for preparing and imaging the leaf epidermis (Royer et al., 2001), which could lead to inconsistency. Therefore we have generated a new dataset examining the relationship of stomatal index vs. pCO2 in Ginkgo biloba (Fig. 1). Leaves were collected from herbarium sheets and living individuals from 1877 to 2013, macerated into lower and upper cuticles using hexavalent chromium. The internal view of the lower epidermis was imaged with an environmental SEM and seven intercostal areas were counted per leaf. This dataset covers a range of pCO2 from 290 to 429 ppm and shows the expected decline in stomatal index with increasing pCO2, but the slope is shallower than previously reported, with an r-squared value typical of biological datasets. Applying the newly constructed calibration curve from Ginkgo biloba to the fossils of the nearly identical species Ginkgo adiantoides from the late Paleocene and early Eocene suggests at least a doubling of pCO2 prior to the major CIE that defines the PETM. In the late Paleocene, reconstructed pCO2 levels were ~300 ppm, a concentration maintained for at least 200ka, and potentially for 1Ma prior to the PETM (Royer et al., 2001). During the 150ka interval immediately prior to the CIE, inferred pCO2 rose to as much as 700 ppm, and remained at least as high as 600 ppm following the rapid-recovery interval at the end of the CIE. All evidence collected to date suggests a long-term rise in both pCO2 and temperature, accompanied by drying of soils (Kraus et al., 2013) prior to the prominent CIE at the onset of the PETM. The pCO2 doubling coincides in part with a ~5°C temperature increase during the “pre-warming” interval prior to the CIE, documented from δ18O in mammalian tooth enamel (Secord et al., 2010) and from fossil leaves in the Bighorn Basin. The ~5°C temperature increase with an approximate
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CO2 (ppm) Fig.1 – Calibration dataset of extant Ginkgo biloba applied to fossil specimens of the nearly identical species Ginkgo adiantoides to estimate paleo-pCO2. Figure combines historical samples from 1877 to 2013 (290 to 395 ppm) with samples from individual trees living in elevated CO2 conditions of Washington D.C. (429 ppm). Seven stomatal index counts were made per leaf on SEM images, viewed from the interior of the lower epidermis after maceration using an aqueous solution of hexavalent chromium. Samples figured are mean values for each of 38 individual leaves, from 20 different years.
doubling of pCO2, is consistent with previous estimates of full Earth-system temperature sensitivity to pCO2 doublings. Presumably the source for CO2 released prior to the CIE did not significantly alter the isotopic value of atmospheric CO 2, and thus did not produce a significant isotope shift. This is explainable with submarine volcanism because CO2 produced from this reservoir is relatively enriched in 13C compared to other carbon sources. As suggested previously by multiple authors, warming prior to the CIE may have triggered the release of carbon from a source highly depleted in 13C at the onset of the PETM.
REFERENCES Kraus M.J., McInerney F.A., Wing S.L., Secord R., Baczynski A.A. & Bloch J.I. (2013) - Paleohydrologic response to continental warming during the Paleocene–Eocene Thermal Maximum, Bighorn Basin, Wyoming. Palaeogeogr. Palaeoclimatol., Palaeoecol., 370, 196-208. Retallack G.J. (2001) - A 300-million year record of atmospheric carbon dioxide from fossil plant cuticles. Nature, 411, 287-290. Royer D.L., (2003) - Estimating latest Cretaceous and Tertiary atmospheric CO2 from stomatal indices, in Wing, S. L., Gingerich, P. D., Schmitz, B., and Thomas, E., eds., Causes and consequences of globally warm climates in the Early
Paleogene, Geological Society of America Special Paper 369, Boulder, Colorado, 79-93. Royer D.L., Wing S.L., Beerling D.J., Jolley D W., Koch P.L., Hickey L.J. & Berner R.A. (2001) - Paleobotanical evidence for near present-day levels of atmospheric CO2 during part of the Tertiary. Science, 292, 2310-2313. Schubert B.A. & Jahren A.H. (2013) - Reconciliation of marine and terrestrial carbon isotope excursions based on changing atmospheric CO2 levels. Nat. Commun., 4, 1653, 1-6. Secord R., Gingerich P.D., Lohmann K.C. & MacLeod, K.G. (2010) - Continental warming preceding the Palaeocene– Eocene thermal maximum. Nature, 467, 955-958. Wing S.L. & Currano, E.D. (2013) - The response of plants to five degrees of global warming. American Journal of Botany, 100, 1234-1254. Wing S.L., Harrington G.J., Smith F.A., Bloch J.I., Boyer D.M. & Freeman K.H. (2005) - Transient floral change and rapid global warming at the Paleocene-Eocene boundary. Science, 310, 993-996. Woodward F.I. (1987) - Stomatal numbers are sensitive to increases in CO2 from preindustrial levels. Nature, 327, 6123, 617-618. Zachos J.C., Rohl U., Schellenberg S.A., Sluijs A., Hodell D.A., Kelly D.C., Thomas E., Nicolo M., Raffi I., Lourens L.J., McCarren H. & Kroon D. (2005) - Rapid acidification of the ocean during the Paleocene-Eocene thermal maximum. Science, 308, 5728, 1611-1615.
