Silica phytoliths, which are deposits of opal-A that precipitate in the intra- and intercellular spaces of plant tissues during transpiration, commonly contain small amounts of occluded organic matter. In this paper, we investigate whether the δ
13C values of phytoliths from a C
4 grass,
Calamovilfa longifolia, vary in response to climatic variables that can affect the carbon-isotope composition of plant tissues. There is no significant correlation (
r2 < 0.3) between climate variables and the δ
13C values of
C. longifolia tissues (average δ
13C
tissue = −13.1 ± 0.6 ‰;
n = 70) across the North American prairies. However, plant tissue δ
13C values are lower for grasses collected in populated areas where the δ
13C value of atmospheric CO
2 is expected to be lower because of fossil fuel burning. Phytolith δ
13C values are more variable (δ
13C = −27.3 to −23.0‰; average = −25.1 ± 1.3‰;
n = 34) and more sensitive to changes in aridity than whole tissue δ
13C values. The strongest correlations are obtained between the δ
13C values of stem or sheath phytoliths and humidity (
r2 = 0.3), latitude (
r2 = 0.4) and amount of precipitation (
r2 = 0.5). However, use of these relationships is limited by the wide spread in δ
13C values of phytoliths from different plant tissues at the same location. We have been unable to infer any relationship between δ
13C values of phytoliths and expected variations in the δ
13C values of atmospheric CO
2. The
C. longifolia phytoliths are depleted of
13C relative to tissue carbon by 10-14‰. This means that the phytoliths examined in this study have carbon isotopic compositions within the range reported previously for phytoliths from C
3 plants. This observation may further limit the usefulness of soil-phytolith assemblage δ
13C values for identifying shifts in grassland C
3:C
4 ratios.
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