Photo credit: Climate of the Past EGU
Figure 1. Eotrigonobalanus furcinervis (Rossmässler, 1840; Kvacek and Walther, 1989): ˇ (a) mass occurrence of leaves in lignite, Schleenhain opencast mine, Saxony, Germany, site Schleenhain 2, Borna Formation, Bruckdorf Member, late Eocene (Priabonian), SPP zone 18o, MMG PB SchleOE 535; (b) abaxial leaf cuticle with stomata and trichome bases, Schleenhain opencast mine, Saxony, Germany, site Schleenhain 4, Böhlen Formation, Gröbers Member, earliest Oligocene (Rupelian), SPP zone 20A/B, slide MMG PB SchleMO 11/05 from leaf SchleMO 556/2.
Fossil plant stomata indicate decreasing atmospheric CO2 prior to the Eocene–Oligocene boundary
by Steinthorsdottir M., Porter A. S., Holohan A., Kunzmann L., Collinson M., McElwain J. C. (2016)
1Department of Geological Sciences and Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden
2School of Biology and Environmental Science, Earth Institute, University College Dublin, Dublin 4, Ireland
3Museum of Mineralogy and Geology, Senckenberg Natural History Collections Dresden, Dresden, Germany
4Department of Earth Sciences, Royal Holloway University of London, Egham, Surrey, UK
in Clim. Past, 12, 439-454, 2016 – doi:10.5194/cp-12-439-2016 –
A unique stratigraphic sequence of fossil leaves of Eotrigonobalanus furcinervis (extinct trees of the beech family, Fagaceae) from central Germany has been used to derive an atmospheric pCO2 record with multiple data points spanning the late middle to late Eocene, two sampling levels which may be earliest Oligocene, and two samples from later in the Oligocene.
Using the inverse relationship between the density of stomata and pCO2, we show that pCO2 decreased continuously from the late middle to late Eocene, reaching a relatively stable low value before the end of the Eocene.
Based on the subsequent records, pCO2 in parts of the Oligocene was similar to latest Eocene values.
These results suggest that a decrease in pCO2 preceded the large shift in marine oxygen isotope records that characterizes the Eocene–Oligocene transition and that when a certain threshold of pCO2 change was crossed, the cumulative effects of this and other factors resulted in rapid temperature decline, ice build up on Antarctica and hence a change of climate mode.
The new terrestrial stomatal proxy-based pCO2 record presented here, derived from fossil leaves of Eotrigonobalanus furcinervis (extinct Fagaceae, beech tree family) from Saxony, Germany, spans the late middle Eocene to latest Eocene, with two sampling levels which are probably from earliest Oligocene, and two samples from later in the Oligocene. The record indicates that pCO2 decreased continuously and gradually by ca. 40 % during the late Eocene, from ca. 630 ppm in the late middle Eocene to ca. 365 ppm in the late Eocene and ca. 410 ppm near the Eocene–Oligocene boundary. Late and latest Oligocene pCO2 was slightly higher at around 430– 475 ppm. Although the pCO2 values reported here may be artificially low, due to factors inherent to stomatal proxybased calibration, they nonetheless broadly agree with the pCO2 range of previously published Eocene–Miocene stomatal proxy records, indicating that Cenozoic pCO2 may have been considerably lower than previously thought based on marine proxies. The substantial late Eocene decrease in pCO2 reported here is consistent with terrestrial records of vegetation change and reconstructions of coldest month mean temperatures, as well as with marine isotope records of global sea surface temperatures. The substantial drop in temperatures and/or ice sheet growth that defines the Eocene– Oligocene boundary in the marine record is not recorded here. This may be caused by the possibility that the Saxony record does not possess the stratigraphic resolution to record such a change, or indicate that decrease in pCO2 took place before the recorded decrease in global sea surface temperatures. The results reported here lend strong support to the theory that pCO2 drawdown, rather than continental reorganization, was the main forcer of the Eocene–Oligocene climate change, when a “tipping point” was reached in the latest Eocene, triggering the plunge of the Earth system into icehouse conditions.