Grassland-shrubland transitions in Quaternary, New Mexico: is aridity a root cause?

Indiana University, Bloomington IN

Investigators

Abstract

Shrublands have encroached on grasslands in the present-day Southwest US, threatening livelihoods and altering natural water and carbon cycles. Though shrubland expansion has been a focus of many present-day ecologic studies, the relative importance of rainfall, temperature, fire, and other environmental factors remains debated. This project will provide a long-term perspective on the causes and reversibility of natural variations in shrub-grass dominance by studying repeated fluctuations that occurred throughout the past 30,000 years. The project will use geochemical data recorded in ancient soils to investigate the relationship between heat (air temperature), dryness (evaporation), and shrubland expansion. The project will provide professional development to educators and students in the geosciences. The results will be disseminated to high school classrooms through a new workshop for K-12 teachers developed in collaboration with Educating for Environmental Change, an Indiana University School of Education program. This project will provide hands-on training in traditional and novel techniques to undergraduate and graduate students, contributing to training the next generation of leaders in laboratory-based geosciences. This project will investigate Quaternary grassland-shrubland transitions using pedogenic carbonate preserved in paleosols within the Jornada Long Term Ecological Research (LTER), New Mexico. The hydrologic factors that cause shifts between the dominance of shrubs vs grasses (detected via δ13C discrimination in the C3 vs. C4 photosynthetic pathways) has not been definitively addressed in the geologic record due to ambiguities inherent in δ18O of carbonate. The researchers will study two types of vegetation transitions that have been observed in the stable carbon isotopes of paleo-pedogenic carbonate: an abrupt shift from grass to shrubland in the mid-Holocene, and subsequent, episodic fluctuations in dominant vegetation in a mixed ecosystem in the late Holocene. Two emerging stable isotope techniques will be used to assess water stress in the paleo-rhizosphere: clumped isotope geochemistry (∆47) will be used to estimate temperature, and triple oxygen isotope geochemistry (∆'17O) will be used to assess evaporation. The geochemical techniques and interpretive framework developed in this work will have broad implications for understanding C3-C4 transitions at grass-shrub and grass-forest ecotones, and at timescales from the present to the Cenozoic. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.