| Abstract: | Terrestrial plants play a unique role in the Earth
System, in that they are in constant and direct contact
with the atmosphere, as they metabolize carbon dioxide
in order to synthesize their own tissues. Because the
carbon of the paleoatmosphere is preserved as plant
fossil tissue through most of the Phanerozoic, I have
worked to quantitatively assess the relative contribution
of factors internal to plants, as well as changing
environmental conditions, to plant tissue d13C value. I
use these relationships to reconstruct environmental
information for the Cretaceous from plant fossil isotopic
analyses. Land plants sample the isotopic composition
of atmospheric CO2 directly during
photosynthesis. For C3 plants, which dominated
Cretaceous ecosystems, isotopic fractionation during
carbon assimilation is influenced by the discrimination
due to differential diffusion of
12CO2 versus
13CO2 in air,
discrimination imparted by the primary carbon fixation
enzyme, and ecophysiological factors that balance
carbon gain with water loss in the leaf . In an analysis
of a large data set (519 published d13Cplant measurements on 176 C3
species) Arens, et al. (2000) showed that the
relationship between the d13C value of
plant tissue and atmospheric CO2 is
linear and significant (r2 = 0.91 for the
full data set). Using a subset of these data, Arens, et al.
(2000) proposed an empirically-derived relationship
[d13Catmosphere = (d13Cplant + 18.67)/ 1.10] that could be
used to estimate d13C of
atmospheric CO2 from preserved
terrestrial plant material. In actualistic tests to verify the
predictive ability of this method, Arens, et al. (2000)
showed that for most mesic environments, the
relationship predicted both ancient and modern
atmospheric d13C values
within the defined confidence interval. In addition, this
finding has been confirmed through the analysis of
modern accumulating sediments from several sites, all
of which yielded an atmospheric prediction within
1.0 of the true modern value. I have
reconstructed the d13C of
Cretaceous atmosphere using d13C values of isolated cuticle (a
component unique to land plants) from two regions:
three localities in the Cordillera Oriental of the
Colombian Andes, South America and at the United
Clay Mine locality of the Arundel Formation of Western
Maryland, North America. Both terrestrial records show
a ~5 negative excursion during the Aptian of the
Early Cretaceous that correlates with excursion of
lesser magnitude observed in marine records. In
presentation, I will evaluate potential causes of this
excursion, and discuss its possible relationship to
widespread oceanic anoxia recognized during the
Aptian.
Arens, N.C., Jahren, A.H., and Amundson, R., 2000,
Can C3 plants faithfully record the carbon isotopic
composition of atmospheric carbon dioxide?:
Paleobiology, v. 26, p. 137-164. |