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COPYRIGHT: © Author(s) 2012. This work is distributed under the Creative Commons Attribution 3.0 License.
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Copyright Copernicus GmbH 2012
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 δ18 O signature of soil top phytolith assemblages reflect changes in mean annual temperature (MAT) and in the oxygen isotopic composition of precipitation (δ18 Oprecipitation ), 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. δ18 Oprecipitation estimates were calculated using the Bowen and Wilkinson (2002) model, slightly modified. An empirical temperature-dependant relationship was obtained: δ18 Owood phytolith-precipitation ([per thousand] 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 δ18 Oprecipitation values and the large uncertainties on δ18 Owood phytolith values, the temperature coefficient (-0.4 ± 0.2[per thousand] °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[per thousand] °C-1 ). The consistency supports the reliability of δ18 Owood phytolith signatures for recording relative changes in mean annual δ18 Osoil water values (which are assumed to be equivalent to the weighted annual δ18 O precipitation values in rainforests environments) and MAT, provided these changes were several [per thousand] and/or several °C in magnitude.
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