Using Isotope Geochemistry to Examine both Regional Landscape Biomass Composition and Terrestrail Climate Variations from Paleosol Carbonates, Badlands National Park, SD
My summer study, conducted with David L. Fox and Sam D. Matson, used stable carbon (d13C) and stable oxygen (d18O) isotopes from paleosol carbonates sampled in Badlands National Park to examine both regional landscape plant biomass composition and terrestrial climate variation. It is part of a larger project that aims to:
1. Constrain the time of warm-climate grassland (C4) development in
the Great Plains and determine the relative proportions of C3 and C4 plants
in the region over time using paleosol carbonate d13C values.
2. Construct a terrestrial paleoclimate record spanning the Cenozoic
for the Great Plains and intermontane west from d18O data that can be compared
with marine records for the same period.
3. Identify whether an aridity gradient existed from west to east across
the Great Plains during the later Eocene and Oligocene and to what extent
it shifted in extent and magnitude over time based on d13C values along
a longitudinal transect of localities across the region.
4. Determine if carbonate isotope signatures record changes in the
depth of atmospheric CO2 penetration into Great Plains soils and, hence,
changes in atmospheric CO2 concentration and/or soil productivity based
on comparisons of the d13C values of co-occurring paleosol carbonate and
organic matter.
The results demonstrate a clear temporal gradient in the isotopic signatures
of Badlands Oligocene paleosols. Younger strata are more enriched
in both 13C and 18O relative to the older strata, likely indicating a shift
to a warmer and/or more arid climate. When combined with data published
in other studies for the Miocene and Plio-Pleistocene, the Oligocene carbon
isotope values show a temporal gradient across the Great Plains that indicates
a gradual rise in the proportion of C4 plants in the landscape. The
d18O values from the Oligocene are similar to those from Miocene paleosols,
but the Miocene data are more variable. Because the Oligocene data
are all from the same region (all localities are in Badlands National Park),
the variation in d18O values represents a temporal trend. In contrast,
the primary source of variation in the Miocene data is a result of the
large geographic spread of the samples from north Texas to north Nebraska
and indicates a latitudinal gradient.