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

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.