QUATERNARY RESEARCH 35, 222-233 (1991) Stable Isotope Ratios of Carbon in Phytoliths as a Quantitative Method of Monitoring Vegetation and Climate Change EUGENE F. KELLY,*,’ RONALD G. AMUNDSON,* BRUNO D. MARINO,?* AND MICHAEL J. DENIRO-P~ *Department of Soil Science, University of California, Berkeley, California 94720; and TDepartment of Earth and Space Sciences, University of California, Los Angeles, California 90024 Received April 10, 1990 The ‘3C/‘2C ratios of occluded carbon within opal phytoliths from the northern Great Plains show potential as a basis for paleoclimatic reconstruction. A significant correlation exists between the carbon isotopic composition of a host plant and that of the organic matter in its phytoliths. The r’C/“C ratios for phytoliths from surface layers of soils along climatic gradients reflect the current proportions of C, and C4 plants. Variations in the 813C values of phytoliths with soil depth are caused by a variety of processes: burial of soil surface by dust, bioturbation, and possible illuvi- ation by percolating water. Also, contributions of phytoliths by dust and roots have unknown isotopic effects. The 613C values of phytoliths from soils increase with decreasing r4C age, sug- gesting that the proportion of C, plants in this region has increased during the Holocene. Phytoliths of apparent mid-Holocene age suggest exclusive dominance by C, plants which agrees with pa- leoclimatic interpretations of an arid middle Holocene climate. 0 1991 University of Washington. INTRODUCTION Opal phytoliths are microscopic opaline particles formed within plant cell walls. Members of the Gramineae produce large amounts of phytoliths (Jones et al., 1963), although phytoliths also occur in other plant families. Following the death and de- composition of a plant, phytoliths disperse and become a ubiquitous component of soils, comprising up to 1% of the total soil mass in grasslands (Jones and Beavers, 1964). Opal phytoliths are resistant to weathering and have been reported in the Pleistocene paleosols of Olduvai Gorge (Hay, 1976), Pliocene sediments in Califor- nia (Nambudiri et al., 1978), Miocene sed- iments in Kansas (Thomasson et al., 1986), and the Oligocene Badlands of South Da- kota (Retallack, 1983). ’ Current address: Department of Agronomy, Colo- rado State University, Ft. Collins, CO 80523. * Current address: Department of Earth and Plane- tary Sciences, Harvard University, Cambridge, MA 02138. 3 Current address: Department of Earth Sciences, University of California, Santa Barbara, CA 93106. Previous studies involving the use of opal phytoliths for paleoenvironmental recon- struction have focused mainly on their mor- phology (Twiss et al., 1969; Twiss, 1987; Piperno, 1988). The relative amounts of opal phytoliths have been used successfully to differentiate grassland soils from those that support forests (Jones and Beavers, 1964) and to separate short grass from tall grass species (Twiss et al., 1969). The use- fulness of morphological studies of opal phytoliths for environmental reconstruc- tion is predicated on the ability to identify phytoliths from ecologically diverse biota. Some complications arise due to the wide variation in phytolith morphology within a given plant, between parts of a given plant, and between members of plant groups, al- though progress is being made in minimiz- ing those difficulties (Rovner, 1986). There is a need for additional quantita- tive tools to reconstruct climate and vege- tation in North American grasslands. In this region palynological information is scarce due to the absence of lakes and bogs (Ritchie, 1976; Webb et al., 1983). In addi- 222 0033-5894191 $3.00 Copyright 8 1991 by the University of Washington. All rights of reproduction in any form reserved.STABLE ISOTOPE RATIOS OF CARBON IN PHYTOLITHS 223 tion, pollen studies cannot distinguish eco- logically diverse groups within the Grumin- eae (Ritchie, 1976), limiting their ability to detect climatically induced vegetation change in grassland environments. Phytoliths have carbon concentrations of 1 to 5 wt% (Jones and Beavers, 1963; Jones and Milne, 1963). A portion of the total or- ganic carbon contained within phytoliths is considered to be “occluded” since it is not susceptible to oxidation and is presumed to be physically protected. Previous work has shown the occluded carbon to be a suitable substrate for 14C dating (Wilding, 1967). The purpose of this work is to explore the possibility of using the stable isotopic com- position of carbon in phytoliths to recon- struct aspects of paleoenvironmental con- ditions. The basis for this approach is related to the influence of climate on the distribution of C3 and C4 grasses. The relative propor- tions of C, and C, grasses growing at a site reflect climatic conditions, with higher min- imum summer temperature and lower soil moisture favoring higher proportions of C, plants (Terri and Stowe, 1976; Ehleringer, 1978; Tieszen et al., 1979). The 613C values (see Methods for definition) of C, and C, plants differ by substantial amounts, with the former being more 13C-depleted than the latter (Bender, 1968; Smith and Ep- stein, 1971). The 613C value of carbon in the soil phytoliths thus should reflect environ- mental conditions if three conditions are met. First, the 6t3C value of the occluded carbon in a phytolith must be related to the 613C value of the host plant. Second, the 613C value of the carbon in soil phytoliths must not be susceptible to diagenetic alter- ation. Third, the ratio of C, to C, phytoliths deposited in a soil must be proportional to the ratio of C, to C4 biomass growing at the site. Laboratory and field studies were con- ducted to establish that the three conditions stated