HR: 10:35h
AN: PP42A-02    [Abstracts]
TI: Hydrogen Isotope Ratios of Leaf Waxes in C3 and C4 Grasses Record Meteoric Water and Aridity Signatures
AU: * Smith, F A
EM: fsmith@geosc.psu.edu
AF: Smithsonian Institution and Penn State University, Dept. of Geosciences, University Park, PA 16802 United States
AU: Freeman, K H
EM: kate@essc.psu.edu
AF: Penn State University, Dept. of Geosciences, University Park, PA 16802 United States
AU: Ehleringer, J R
EM: ehleringer@biology.utah.edu
AF: University of Utah, Department of Biology 257 South 1400 East, Salt Lake City, UT 84112 United States
AU: Helliker, B
EM: helliker@stanford.edu
AF: International Atomic Energy Agency, Isotope Hydrology Section Wagramer Strasse 5, P.O. Box 100, Vienna, A-1400 Austria
AB: Hydrogen isotope ratios of sedimentary n-alkanes (C27-C33) from vascular plants potentially provide a valuable record of past hydrologic conditions. To explore this, we analyzed grasses grown in a greenhouse and calculated fractionation factors (epsilon) between source water and n-alkane for each sample. An average difference of 21 permil is observed between C3 and C4 grasses, which is comparable to that determined for grasses collected from the Great Plains. The more positive isotope values in C4 grasses likely reflects smaller interveinal distance compared to C3 grass leaves, allowing greater back-diffusion of transpirationally enriched water from stomata, as documented with the oxygen isotope ratios of grass leaf water and cellulose by Helliker and Ehleringer (2000). The oxygen isotope difference is magnified at low relative humidity, when transpiration rates are higher. A similar effect is expected in hydrogen isotope ratios of leaf water and plant compounds. However, preliminary results from grasses grown hydroponically at different relative humidities suggest that there may be a decoupling of the hydrogen isotope ratio of leaf-wax n-alkanes and the oxygen isotope ratio of leaf water and cellulose. To examine the effects of source water delta D and climate on n-alkane delta D values, we analyzed grasses collected from the Great Plains. We use river water delta D values as a proxy for source water and the epsilon values determined in the greenhouse experiments, to predict expected values for C3 and C4 grass lipids. Measured values compare well to predicted values, with the exception of two semi-arid sites where evapotranspiration may have led to leaf-waters that are enriched in deuterium. Residual delta D values (measured-expected) correspond strongly with measures of aridity, such as annual precipitation and recipitation/evaporation ratios.
DE: 4853 Photosynthesis
DE: 4870 Stable isotopes
DE: 3344 Paleoclimatology
DE: 1818 Evapotranspiration
DE: 1851 Plant ecology
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting