Characterization of organic compounds in phytoliths:: Improving the resolving power of phytolith δ13C as a tool for paleoecological reconstruction of C3 and C4 grasses

Carbon isotope ratios of phytolith assemblages reflect the C3/C4 ratio of past grassland ecosystems (Kelly et al., 1991; Fredlund, 1993), but the resolution of this method is Limited by the compression of the phytolith delta(13)C scale relative to the whole plant scale. The first step toward decompressing the phytolith delta(13)C scale is to characterize the organic compounds in phytoliths. This study characterizes and compares the organic compounds in C3 and C4 grass phytoliths, and C3 grass tissue, using Tetramethylammonium (TMAH) thermochemolysis and gas chromatography-mass spectrometry (GC-MS). This method is particularly well suited for detecting lignin and lipid, but cannot detect carbohydrates (i.e. cellulose, hemicellulose). Contrary to previous work suggesting that phytoliths might contain lignin (Kelly et al., 1991), the phytoliths analyzed contained only lipid and no lignin. The C3 and C4 phytoliths analyzed did not differ in the compounds that they contained. The compounds in the C3 phytoliths did differ, however, from the compounds in the C3 grass tissue. The presence of Lipid in phytoliths explains the depletion of delta(13)C in phytoliths relative to the whole plant tissue values, but cannot explain the compression of the isotopic scale. The explanation may lie in the presence of carbohydrates in phytoliths that would not be detectable by this derivatization technique. The phytolith isotopic scale compression could be the result of differential incorporation of lipids and carbohydrates (i.e. cellulose or hemicellulose) in C3 and C4 grass phytoliths. Based on these results, future work will investigate the potential for compound-specific isotope ratio measurements in an effort to decompress the phytolith delta(13)C scale.