THE STABLE ISOTOPIC COMPOSITION OF TAXON-SPECIFIC HIGHER PLANT BIOMARKERS IN ANCIENT PACKRAT MIDDENS: NOVEL PROXIES FOR SEASONAL CLIMATE IN THE SOUTHWEST US

Fossil packrat middens in the southwestern US contribute critical details to our understanding of past climate change and floral migrations across geography and elevation. They provide detailed snapshots of the flora found in a packrat's range and a survey of pollen from a larger region, allowing unambiguous inferences to be made about changes in seasonal climate.

Our work is an intense effort to extend midden analysis to include taxon-specific molecular fossils. Archived midden materials, representing a significant investment of scientific funding and research effort, provide an opportunity to study taxon-specific plant and microbial lipids in well-dated late Pleistocene and Holocene samples with restricted and well-defined biological inputs.

The study is divided into three parts: (1) a chemical survey of common Neotoma food plants, (2) a molecular and isotopic investigation of fecal pellets in modern middens across a range of ecosystems, and (3) a molecular and isotopic study of fossil fecal pellets in ancient midden sequences from southern Arizona.

Preliminary results demonstrate the feasibility of identifying taxon-specific biomarkers for Pinaceae, Cupressaceae, Cactaceae, Zygophyllaceae, and Agavaceae within complex mixtures. Compound specific isotope analysis of these biomarkers reveals the existence of robust environmental signals for conditions during plant growth. Taxon-specific isotope analysis delineates the Bolling Allerød, Younger Dryas, and Holocene Climatic Optimum periods and reflects significant changes in the strength of the North American Monsoon, the southern limit of westerly storm tracks, and seasonal and/or mean temperatures.

The sensitivity of this molecular archive to conditions during plant growth (soil moisture, precipitation, temperature, relative humidity) will make it a useful and non-redundant addition to climate records in the US southwest. Results will compliment paleobotanical studies and constrain interpretations of higher resolution climate archives lacking sensitivity to seasonal climate.