The Relationship of Climate to the Stable Isotopic Com- position of Hackberry (Celtis) Endocarps: A Potential Means of Paleoenvironmental Reconstruction in the Great Plains

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The long-term goal of the proposed integrated research is to exploit the peculiar composition of hackberry (Celtis) endocarps for an isotopic approach to paleoenvironmental reconstructions. This project will quantify how the stable C and O isotopic composition of organic and inorganic components in the endocarps reflect environmental conditions and how genetic variability affects these relationships. Endocarps of Celtis fruit mainly consist of aragonite, but also contain smaller amounts of organic C. The high mineral content may be one reason why Celtis endocarps are a common, and well-preserved, plant remnant in Cenozoic sediments of the Great Plains. We propose to conduct experiments to determine the effect of individual environmental parameters (light intensity, water potential, temperature, humidity) on the delta C 13 and delta O 18 values of various plant components and on the isotopic composition of organic matter and carbonate in Celtis endocarps. The present relationship of endocarp isotope ratios to regional climate will be examined by collecting Celtis fruit, tissue, and climate data, from many locations in the United States. We will establish predictive relationships between climate and stable isotope values by sampling a subset of Celtis trees from a USDA experiment involving more than 120 sources of germplasm. These experiments at 12 sites in the United States span the entire north-south temperature range as well as the east-west rainfall and atmospheric water deficit ranges. The effects of diagenesis on the isotope ratios, and its implications for the use of Celtis in Cenozoic paleoclimatic studies, will be examined via short-term experiments. Should these studies prove encouraging, we will then submit a proposal to begin a second phase of the project in which we will collect, and isotopically analyze, endocarps from Holocene to Oligocene sediments of the Great Plains.