The hydrogen in plant tissues derives ultimately from environmental waters, which have hydrogen isotope ratios (D/H) that are related to temperature, weather patterns, and hydrologic balance. High-molecular weight n-alkanes are constituents of leaf waxes and serve as a biomarker for vascular plants in sedimentary systems. Leaf-wax lipids from modern plants reflect both meteoric water D/H values and relative humidity. Lower relative humidity leads to greater enrichment through transpiration (evaporation from leaves through stomata) and soil evaporation. Craig-Gordon type models for transpiration and soil evaporation quantitatively describe the relationship between relative humidity and leaf water D/H values and can be modified to describe leaf-wax n-alkane D/H values in relation to relative humidity (Smith and Freeman, 2006).

Terrestrial Ecosystem Responses to the Paleocene-Eocene Thermal Maximum

The Paleocene Eocene Thermal Maximum represents a time of extreme warming associated with a massive release of carbon into the ocean-atmosphere system. Ongoing work in the Bighorn Basin, WY, focuses on the carbon and hydrogen isotope ratios of leaf-wax lipids to reconstruct ecologic and climatic changes during this period of global warming. Carbon isotope ratios reveal a substantial increase in carbon isotope discrimination by plants, which is often attributed to increased available moisture. Hydrogen isotope ratios of those same leaf waxes do not indicate increased moisture (Smith, Wing and Freeman, In review). Instead, the increased discrimination is more likely the result of changing plant communities (Wing et al., 2005).

Reconstructing Neogene Grasslands

Carbon isotope signatures of phytoliths (silica bodies produced by grasses) represent a powerful means of reconstructing the photosynthetic pathways of grasses, which is directly linked to climate and atmospheric CO₂ concentrations. C₃ grasses prefer elevated atmospheric CO₂, cooler and moister conditions, whereas the C₄ grasses can tolerate low atmospheric CO₂, warmer and dryer conditions. Although several proxies have been used to reconstruct different aspects of past grasslands (i.e. fossil flora and fauna, paleopedology, carbon isotope rations of tooth enamel and soil carbonate), no other method provides a direct record of C₃ and C₄ grasses distinct from other types of vegetation. Interpreting carbon isotope ratio of fossil phytoliths requires an understanding of how ratios are established in modern systems, both in the plants (setting initial carbon isotope ratios) and in the soils (pedogenic effects). The carbon isotope values of phytoliths from modern grasses and soils provides the basis for isotopic end-members for pure C₃ and C₄ phytolith assemblages, enabling the interpretation fossil phytolith carbon isotope values in terms of relative dominance of C₃ and C₄ grasses (Smith and White, 2004). Carbon isotope signatures of fossil phytoliths indicate that the central Great Plains transformed from C₃ grasslands to C₄ grasslands during the late Neogene. This represents a major transition analogous to the temperate grasslands of Canada becoming like the aridland grasslands of Texas. Because the world’s crops are comprised primarily of domesticated grasses, understanding the effect of climatic and atmospheric changes in past grasslands is fundamental to anticipating the response of future food supplies to global change.