2003 Seattle Annual Meeting (November 2–5, 2003)

Paper No. 10
Presentation Time: 10:15 AM

RAINFALL AND THE MIOCENE EXPANSION OF C4 GRASSES: A LEAF WAX HYDROGEN ISOTOPE PALEOHYDROLOGIC PROXY


SMITH, Francesca A., Department of Geosciences, Pennsylvania State Univ, 542 Deike Building, University Park, PA 16802 and FREEMAN, Katherine H., Pennsylvania State Univ - Univ Park, 209 Deike Bldg, University Park, PA 16802-2711, fsmith@geosc.psu.edu

Hydrogen isotope ratios (dD) of individual leaf wax n-alkanes (C27-C33) preserved in sediments have the potential to provide a valuable record of paleohydrologic conditions. The hydrogen in leaf wax n-alkanes is derived from leaf water, which is derived from soil water, which originates as precipitation. The dD of leaf water can be enriched in deuterium by transpiration and indirectly by evaporation of soil waters. The dD of precipitation varies with source of moisture (which can vary seasonally), latitude and degree of continentality. Thus, the dD of n-alkanes should vary with differences in evapotranspiration and precipitation dD values and could provide an integrated signal of changing hydrologic conditions.

To explore the potential for this paleohydrologic proxy, we measured the dD of individual n-alkanes (primarily C29 and C31) from modern grasses from the Great Plains. Samples included 18 C3 grasses (8 species) and 22 C4 grasses (10 species) collected from 8 different sites. Sample localities span approximately an 80‰ gradient in summer precipitation dD values from North to South and a gradient of soil moisture values from East to West. The grass n-alkane dD values vary with precipitation dD values, with results spanning nearly 100‰ from North Dakota to New Mexico. The dD values along the East-West soil moisture gradient show less systematic variations. In general, C4 grasses had more positive dD values than C3 grasses. Although seasonal differences in precipitation dD values may play a role, a more fundamental ecophysiological difference may also contribute to C3-C4 differences. Previous work by Helliker and Ehleringer on the d18O of leaf water and cellulose in C3 and C4 grasses identified a similar trend. They attribute the heavier C4 d18O values to the smaller interveinal distance in C4 vs. C3 grass leaves. Evaporative enrichment through stomata creates an isotopic gradient between stomata and veins. Back diffusion causes mixing of enriched stomatal water with vein water. The degree of mixing is controlled by the back diffusion path length, which is a function of interveinal distance. Characterizing differences in C3 and C4 grass n-alkane dD values and quantifying the relative contribution of C3 and C4 plants using n-alkane d13C are crucial to future applications of n-alkane dD values as a paleohydrologic proxy.