Paper No. 5
Presentation Time: 9:15 AM
STABLE CARBON ISOTOPIC COMPOSITION OF FOSSIL LAND PLANTS: AN INDICATOR OF DEVONIAN PALEOCLIMATE CHANGE
Research on recent geologic epochs demonstrates a relationship between the stable carbon isotopic composition of plants and atmospheric pCO2. The former is linked to temperature and the stable carbon isotopic composition of marine carbonate, representing a proxy for photosynthetic fractionation of ancient plants that may reveal information about paleoclimate, especially paleotemperature and paleoatmospheric pCO2. This study analyzed δ13C variation in 10 common fossil plant genera through the Devonian Period (417-354 Ma): Psilophyton, Pertica, Pseudosporochnus, Rhacophyton, Archaeopteris, Tetraxylopteris, Sawdonia, Drepanophycus, Leclercqia, and Haskinsia. These fossils exhibit a wide range of δ13C values, from -20‰ to -30‰, although each taxon had its own narrower δ13C range. Significantly, all taxa show similar secular trends for the Devonian as a whole, or for the part of the Devonian represented by a given taxon: δ13C values are higher in the Middle Devonian by 3-4‰ relative to the Early and Late Devonian, implying lower atmospheric pCO2 and cooler climates during the Middle Devonian. The derived proxy Δ13C‰(Atm-Plant) (i.e., photosynthetic fractionation) matches well with secular variation in (1) an independent O-isotope-based paleotemperature record (Joachimski et al., 2009) and (2) δ13C of atmospheric CO2 for the Devonian, according to cross correlation analysis. This analysis also reveals phase shifts of 0±4 Myr between Δ13C‰(Atm-Plant) and paleotemperature, 2.5(-4,+3) Myr between paleotemperature and δ13CAtm, and 4.5±1 Myr between Δ13C‰(Atm-Plant) and δ13CAtm. These relationships suggest that Δ13C‰(Atm-Plant) and paleotemperature are leading indicators of ancient climate change, and that the δ13C of atmospheric CO2 is modified more slowly in response to climate-induced changes in C-cycle fluxes and reservoir masses.