Modern calibration of phytolith carbon isotope signatures for C 3/C 4 paleograssland reconstruction

Abstract

The carbon isotope signature of fossil phytoliths (silica bodies produced in abundance in the tissue of grasses) presents a promising tool for reconstructing the biogeography of C 3 and C 4 grasses during the Neogene. Previously applied isotopic techniques, such as the carbon isotope signatures of soil carbonate, herbivore tooth enamel, and leaf-waxes, are limited in that they cannot distinguish between C 3 grasses and C 3 shrubs/trees. As a result, these records do not provide any information on the relative dominance of C 3 vs. C 4 grasses. Phytolith carbon isotope ratios, on the other hand, have the potential to provide a direct record of the photosynthetic pathway (C 3 or C 4) of grasses specifically. Because this technique is still in its infancy, several modern calibration issues remain and are examined here through the presentation of some new data and the synthesis of existing published data on the δ 13C values of phytoliths from modern grasses and soils. First, fresh phytoliths extracted directly from grasses exhibit depleted isotopic values relative to the source plant and phytoliths from C 4 grasses are more depleted than phytoliths from C 3 grasses. This difference in depletion is most likely the result of lipids present within phytoliths, which are also generally more depleted in C 4 plants than C 3 plants relative to whole plant values. Second, phytolith assemblages from modern soils do not share the same degree of isotopic depletion or compression as fresh phytoliths. This suggests that soil phytolith assemblage δ 13C is affected by pedogenic or taphonomic processes. The weight of current evidence suggests that pedogenic alteration is less of a concern, while selective preservation (taphonomic bias) may play an important role. Even so, comparison of the δ 13C values of phytolith assemblages with δ 13C values of soil organic matter (SOM) from modern soils indicates that phytoliths can record the C 3/C 4 contribution to the soil. It appears that phytoliths retain a longer-term, smoothed record in contrast to the more rapid turnover of SOM pools that can be influenced by even recent land-use changes. Third, the interpretation of fossil phytolith δ 13C in terms of proportion of C 3 and C 4 grasses requires isotopic end-member values. End-members from fresh phytoliths cannot be used because they fail to capture the range of soil phytolith assemblage data. Comparison of soil phytolith δ 13C values with SOM δ 13C yields empirically derived end-member values that take into account any taphonomic effects. These end-members (C 3=−26.8‰, C 4=−15.3‰) capture the range of observed values for soil and paleosol phytolith δ 13C values spanning the last 650,000 years. The modern calibration work presented here provides the necessary groundwork for applying this technique to paleoecological reconstructions of Neogene grasslands.