Sediments in lakes and wetlands preserve important evidence of changes in Earth's climate, vegetation, and ecological dynamics over the past 20,000 years. In this project, researchers will study sedimentary archives to document the ecological and environmental history of the Cuatro CiÃ©negas valley in the Chihuahuan desert of northern Mexico. Cuatro CiÃ©negas is home to a unique desert spring system that supports North America's highest concentration of endemic species, garnering comparisons to the Galapagos Islands. The high endemism of the Cuatro CiÃ©negas biota has been explained as a consequence of long-term isolation and environmental stability in the valley. A pollen record from a sediment core analyzed in the 1970s was originally interpreted as supporting this hypothesis, but recent reevaluation of the original record, along with new pollen data from a nearby packrat midden, challenged the assumption of long-term environmental stability within the valley. A complication with the old record is uncertainty over early radiocarbon dates on bulk sediment. This research project will make use of two new sediment cores that contain macrofossils suitable for AMS dating along with abundant pollen and charcoal that will provide information on both vegetation and fire history. The researchers will test the hypothesis of long-term stability in the Cuatro CiÃ©negas valley by documenting, with secure dating and at higher resolution, past environmental conditions in Cuatro CiÃ©negas. The cores were recovered in 2008 from wetlands in the Reserva Privada Pozas Azules, which contains a large assemblage of sinkhole ponds, or pozas, in various stages of succession. The 12.3 m profile from Poza Cortador spans over 14,000 years and shows evidence of succession from a deep-water poza to a marsh with emergent wetland vegetation. A profile from nearby Poza Tule covers the last c. 5000 years. Continuous depositional environments suitable for pollen preservation are exceptionally rare in the arid regions of North America. The sediment records to be analyzed will provide important evidence of Late Glacial and Holocene conditions in the Chihuahuan Desert and provide insight into the environmental changes that occurred outside of glaciated regions of North America.
The Cuatro CiÃ©negas valley has become a natural laboratory for studying fundamental issues in biogeography, ecology, and evolution, and this research on long-term history will increase the value of this natural laboratory. The results of the research project will inform paleoclimatology and biogeography, and will provide crucial context for the many studies of speciation, coevolution, microbial evolution and astrobiology that focus on the unique species and species assemblages of the Cuatro CiÃ©negas valley. Project results will also inform ongoing efforts to manage, conserve, and restore the natural resources and endemic biodiversity of Cuatro CiÃ©negas. The research project will provide educational opportunities for several graduate and undergraduate students, and for K-12 students and others served by project outreach. This project is jointly supported by the Geography and Spatial Sciences Program and the Population and Community Ecology Program.
Understanding the general formation and development of the American deserts is difficult because of limited locations where environmental histories are recorded. We studied a wetland complex from a valley within the Chihuahuan Desert that has been considered to have been environmentally stable over tens of millennia, while other desert regions have shown significant environmental changes related to wet and dry periods that spanned centuries to millennia. Our analyses of sediment cores from the wetland revealed considerable environmental variability in the Chihuahuan Desert, including the change from diverse forests mixed with present-day desert taxa to the deserts that we have today. We discovered that the desert did not migrate into our study region; rather, forests migrated upslope and north to the western mountains and to the southeastern United States. We also determined that fire may have been more common during wet periods in the past, suggesting that plant density on the landscape is important to widespread fire events. Studies such as ours that focus on past environmental change in desert regions are important because of increased population growth rates in western cities and states. Our study contributes to a greater understanding of natural drought periods that can affect our growing populations and the environments and biota that draw people to these arid western regions.