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

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 delta O-18 signature of soil top phytolith assemblages reflect changes in mean annual temperature (MAT) and in the oxygen isotopic composition of precipitation (delta O-18(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. delta O-18(precipitation) estimates were calculated using the Bowen and Wilkinson (2002) model, slightly modified. An empirical temperature-dependant relationship was obtained: Delta O-18(wood phytolith-precipitation) (% vs. VSMOW) = -0.4 (+/- 0.2) t (degrees C) + 46 (+/- 3) (R-2 = 0.4, p < 0.05; n = 12). Despite the various unknowns introduced when estimating delta O-18(precipitation) values and the large uncertainties on delta O-18(wood phytolith) values, the temperature coefficient (-0.4 +/- 0.2% parts per thousand f 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% ffi C-1). The consistency supports the reliability of ffi delta O-18(wood phytolith) signatures for recording relative changes in mean annual delta O-18(soil water) values (which are assumed to be equivalent to the weighted annual delta O-18(precipitation) values in rainforests environments) and MAT, provided these changes were several % and/or several ffi C in magnitude.