Research

Ground-truthing leaf-wax isotopic proxies in tropical environments

Leaf-wax (long chain n-alkyl lipids) hydrogen and carbon isotope measurements are increasingly used as a proxy for hydrological and ecological change in tropical lake and marine core studies.  However, there have been very few studies of these proxies in modern tropical environments.  I have been analyzing leaf-wax isotope values in lake sediments, soils and plant tissue from a large aridity gradient across southeastern Mexico, Guatemala and Honduras to better constrain leaf-wax isotopic variability in the tropics.  This research highlights the important influence that variability in aridity and vegetation can have on leaf-wax hydrogen isotopes.

In addition, I have been analyzing the compound specific radiocarbon content of leaf waxes from lake sediments from this study area.  These measurements indicate that the mean age of sedimentary leaf wax lipids is significantly older than the age of sediment deposition, suggesting that a large component of leaf waxes are stored for long periods of time in soil organic carbon reservoirs.  I am exploring the environmental factors controlling soil storage of leaf waxes and the implications of this storage for leaf-wax stable isotope paleoclimate studies.

• Douglas, P. M. J., Pagani, M., Brenner, M., Hodell, D. A., and Curtis, J. H., Aridity and vegetation composition are important determinants of leaf-wax dD values in southeastern Mexico and Central America. Geochimica et Cosmochimica Acta, v. 97 p. 24-45 [PDF]

Evaluating ancient human-environment interactions in the Maya Lowlands

There is evidence from lake and cave records in Mexico suggesting that the Terminal Classic decline of the Maya civilization coincided with a series of major multi-decadal droughts.  However there are few paleoclimate records from the Peten region of Guatemala, which was the political and population center of the Classic Maya civilization.  I am applying leaf-wax hydrogen and carbon isotope measurements to lake sediment cores from Mexico and Guatemala to help resolve the impact of drought on the Classic Maya across this region.

I am also exploring the potential for compound specific radiocarbon measurements of leaf waxes in lake sediment cores to constrain long-term changes in soil organic carbon reservoirs in the Maya lowlands.  I am working to created records of the mean soil residence time of leaf waxes for the past 3000 years in lake sediment cores across the Maya lowlands, and to examine how climatic and anthropogenic drivers affected soil storage of leaf waxes through this interval.

 Constraining Eocene ocean temperature gradients with clumped isotope paleothermometry

The Eocene Epoch is a critical period for understanding how the Earth's climate responds to elevated greenhouse gas concentrations.  It is especially important to identify the distribution of temperatures during the Eocene, since this could have implications for polar amplification of modern greenhouse warming.  I have been applying clumped isotope of high and low latitude Eocene bivalve fossils to better constrain meridional temperature gradients during the Eocene.  Clumped isotope paleothermometry is a relatively new technique that does not require any assumptions related to seawater chemistry, and therefore it provides a valuable tool for validating other paleotemperature proxies.

In addition, I am performing clumped isotope measurements in modern coastal bivalves from a wide range of climates in order to better understand clumped isotope paleotemperature estimates.

• Keating-Bitonti, C. R., Ivany, L. C., Affek, H. P., Douglas, P., and Samson, S. D. (2011) Warm, not super-hot, temperatures in the early Eocene subtropics: Geology, v. 39, p.771-774. [PDF]

 Reconstructing early and mid Holocene hydroclimate variability in the West African Sahel

With colleagues from the Yale Anthropology Department and the University of Wisconsin-Madison Gesociences Department I have been working to develop lake sediment cores from Lake Fati in Mali as an archive of Holocene climate variability in the Sahel.  The Sahel is highly susceptible to drought, but there are few records of past climate variability in this region.  We are currently applying elemental and carbonate isotopic proxies to reconstruct climate changes at this site during and at the end of the African Humid Period.