Aridity Index
The aridity index capitalizes on the different ways that mammals interact with their environment. Oxygen isotopes of mammalian teeth from an environmental gradient in eastern Africa show that mammals can be classified by their isotopic sensitivity to aridity and placed into two groups, evaporation sensitive (ES) and evaporation insensitive (EI).  Tooth enamel δ¹⁸O values of ES animals (e.g., giraffe, gazelle, dikdik, and oryx) increase with aridity, whereas the tooth enamel δ¹⁸O values of EI animals (e.g., hippo, elephant, rhino, and warthog) track local meteoric water δ¹⁸O values. The enrichment between tooth enamel δ¹⁸O values of ES and EI animals can be used as an index of aridity because it records the ¹⁸O enrichment between evaporated water and source water, which is a function of aridity.  By considering the ¹⁸O composition of teeth from different types of mammals as two different geochemical archives, the aridity index isolates two important influences on tooth enamel δ¹⁸O values, the isotopic composition of local waters and environmental aridity.  The development of this new approach makes it possible to use tooth enamel δ¹⁸O as indications of aridity in contexts where the ¹⁸O composition of waters is unknown, like the fossil record.  I'm currently using the aridity index to interpret new data from Plio-Pleistocene fossil fauna from the Turkana Basin in Kenya and to evaluate the role of aridity in the environmental history at Turkana.

Turkana Basin, Kenya

The Turkana Basin, in northern Kenya and southern Ethiopia, contains extensive exposures of Pliocene and Pleistocene sediments (the Omo Group) that fill a depression where the Main Ethiopian and East African rift systems meet. The Turkana Basin is best known for substantial number of hominid fossil remains that have been recovered from the Omo Group deposits, however the ~5,000 km² of sediment exposures also provide an excellent opportunity to examine environmental variability in a large basin through time. My work in the Turkana Basin primarily focuses on using carbon and oxygen isotope ratios of pedogenic carbonates and fossil teeth to study this variability and link it to regional and global climate change during the Pliocene and Pleistocene.  This isotopic work is in collaboration with Frank Brown, Thure Cerling, John Harris, Meave Leakey, Kay Behrensmeyer and David Braun.





Gona, Afar Region, Ethiopia
Gona is best known for archaeological sites that contain the oldest evidence for stone tools in the world, however there is also a rich hominid fossil record that spans the late Miocene through the Pleistocene.  In addition to paleoanthropological, archaeological, and paleontological resources, Gona contains an excellent sedimentary archive of >6 million years of environmental change in an active rift basin.

I have worked as geologist on the Gona Paleoanthropological Research Project (GPRP) since 2001. My work at Gona includes stratigraphy, mapping, and isotopic studies of soil carbonates, fossil teeth and mollusks.  These studies are part of an initiative to build a complete paleoenvironmental story at Gona, that provides an environmental context for human evolution and an anchor point for characterizing local terrestrial responses to regional climate change in eastern Africa since the late Miocene.  My work at Gona is primarily in collaboration with Sileshi Semaw (project director), Scott Simpson, Jay Quade and Stephen Frost, although there are many other scientists who contribute to the research at Gona.  All of our work is done with the help of the Afar people.



Isotopic Composition of Ethiopian and Kenyan Waters

The stable isotopic composition of waters offers an opportunity to use isotopic distributions today as frameworks for interpreting oxygen isotopes archived in the geologic record.  With Thure Cerling, I am building an isotopic database of surface and shallow ground waters from Ethiopia and Kenya.  From the >350 waters that we've collected thus far, we see significant isotopic differences between waters from Kenya and Ethiopia today.  In collaboration with Ed Zipser, we have used TRMM (Tropical Rainfall Measuring Mission) and NCEP (National Centers for Environmental Prediction) data products (wind vectors and equivalent potential temperature) to show that isotopic differences between waters in Ethiopia and Kenya can be explained by different moisture sources.  This meteorological framework is becoming an integral part of how we view the oxygen isotopic reocrd from fossil teeth and pedogenic carbonates in Kenya and Ethiopia.


Isotopic Survey of Micromammals from Ea

stern Africa
Despite the widespread distribution of micromammals in modern and fossil terrestrial ecosystems, micromammal teeth are rarely used in isotope (paleo)ecology because they are too small to be studied using conventional methods (e.g., H₃PO₄ acid digestion, Ag₃PO₄ precipitation).  Recent advances in laser ablation GC-IRMS by Passey & Cerling (2006) make it possible to determine δ¹³C and δ¹⁸O values of micromammals teeth that are otherwise too small to measure conventionally.  With colleagues, Fredrick Manthi, Prince Kaleme and Thure Cerling, I've been using these new methods to study variation in δ¹³C and δ¹⁸O values among micromammals (mostly rodents) from a range of environments in eastern Africa, including primary tropical rainforest in Kahuzi-Biega National Park in eastern Congo (ca. 2100 m), moorlands in the Bale Mountains in Ethiopia (ca. 3800 m), and in Kenya, tropical rainforest in the Kakamega Forest (ca. 1550 m), mesic bushlands and thickets near Nairobi (ca. 1650 m), xeric bushlands and thickets in Olorgesailie (ca. 600 m), and xeric grasslands and shrublands at Turkana (ca. 400 m).  From the 118 individuals analyzed thus far we find a 20‰ range in δ¹³C values, which matches the magnitude of the range in plant δ¹³C values in the sampled environments.  A similarly large gradient in δ¹⁸O values (16‰) of the sampled teeth likely reflects both differences in aridity between sampling locations and variation in water-use strategies among taxa.  This project has just begun.  Fredrick Manthi and Prince Kaleme are continuing to trap animals, and from these collections we'll be better able to characterize the geographic, seasonal and taxonomic variation recorded in δ¹³C and δ¹⁸O values of micromammal teeth.  When we have a strong enough control on the modern systems, we plan to use the same methods to study the paleoecology of fossil rodents from eastern Africa.


Holocene Environmental Change in Oaxaca, Mexico



I recently started a project with Art Joyce and Ray Mueller in Oaxaca, Mexico, in which we're working towards building a paleoenvironmental record from the stable isotopic composition of soil carbonates and soil organic matter.  This project is part of a larger endeavor to build the environmental context for the intensification of agriculture in the Oaxaca region. The multidisciplinary effort also includes Bill Middleton and Michelle Goman, whose teams are investigating the phytolith and pollen records, respectively.