The oxygen isotopic composition of phytolith assemblages from tropical rainforest soil tops (Queensland, Australia): validation of a new paleoenvironmental tool

Abstract. Phytoliths are micrometric particles of amorphous silica that form inside or between the cells of higher plant tissues throughout the life of a plant. With plant decay, phytoliths are either incorporated into soils or exported to sediments via regional watersheds. Phytolith morphological assemblages are increasingly used as proxy of grassland diversity and tree cover density in inter-tropical areas. Here, we investigate whether, along altitudinal gradients in northeast Queensland (Australia), changes in the δ¹⁸O signature of soil top phytolith assemblages reflect changes in mean annual temperature (MAT) and in the oxygen isotopic composition of precipitation (δ¹⁸O_{precipitation}), as predicted by equilibrium temperature coefficients previously published for silica. Oxygen isotopic analyses were performed on 16 phytolith samples, after controlled isotopic exchange (CIE), using the IR Laser-Heating Fluorination Technique. Long-term mean annual precipitation (MAP) and MAT values at the sampled sites were calculated by the ANUCLIM software. δ¹⁸O_{precipitation} estimates were calculated using the Bowen and Wilkinson (2002) model, slightly modified. An empirical temperature-dependant relationship was obtained: δ¹⁸O_{wood phytolith-precipitation} (‰ vs. VSMOW) = −0.4 (±0.2) t (°C) + 46 (±3) (R² = 0.4, p < 0.05; n = 12). Despite the various unknowns introduced when estimating δ¹⁸O_{precipitation} values and the large uncertainties on δ¹⁸O_{wood phytolith} values, the temperature coefficient (−0.4 ± 0.2‰ °C⁻¹) is in the range of values previously obtained for natural quartz, fresh and sedimentary diatoms and harvested grass phytoliths (from −0.2 to −0.5‰ °C⁻¹). The consistency supports the reliability of δ¹⁸O_{wood phytolith} signatures for recording relative changes in mean annual δ¹⁸O_{soil water} values (which are assumed to be equivalent to the weighted annual δ¹⁸O _{precipitation} values in rainforests environments) and MAT, provided these changes were several ‰ and/or several °C in magnitude.