The oxygen isotopic composition of phytoliths from tropical rainforest soils (Queensland, Australia): application of a new paleoenvironmental tool
Authors: Alexandre A. , Crespin J. , Sylvestre F. , Sonzogni C. , Hilbert D. W. 
Published: May 2011

Abstract. Variations in the oxygen isotopic composition of precipitation (δ18Oprecipitation) in inter-tropical areas mainly record variations in water sources, amounts of precipitation, and atmospheric temperature and provide information regarding local climate and regional atmospheric circulation changes. On continents, fossil biogenic minerals and speleothems formed in isotopic equilibrium with water can produce continuous δ18O records and are becoming increasingly valuable for reconstructing past climate changes. Here, we explore the efficiency and limitations of using the oxygen isotopic composition of wood phytoliths (δ18Owood phytolith) from tropical rainforest soils as a suitable proxy for atmospheric temperature and δ18Oprecipitation values, under conditions that are assumed to be non-evaporative. Soil phytolith assemblages, that should contain 100s of years of phytolith production, were collected along four altitude, temperature, and precipitation gradients in the Queensland rainforests (Australia). Oxygen isotopic analyses were performed on 1.6 mg phytolith samples, after controlled isotopic exchange (CIE), using the IR Laser-Heating Fluorination Technique. Long-term mean annual precipitation (MAP) and mean annual temperature (MAT) values at the sampled sites were obtained using a regional GIS database. The δ18Oprecipitation values were estimated. The δ18Owood phytolith values from the leeward slopes were scattered but recorded the modern combination of weighted mean annual δ18Oprecipitation values and MAT. The empirical relationship was &amp;Delta18Owood phytolith-precipitation (‰ vs. VSMOW) = −0.4 (±0.2) t (°C) + 46 (±3) (R2 = 0.4, p<0.05; n=12). δ18Oprecipitation estimates were close to estimates for δ18Oforming water when using the temperature-dependant relationships previously described for sedimentary diatoms and natural quartz. However, they were 3 ‰ higher than estimates for δ18Oforming water when using the fractionation relationship obtained for harvested grass phytoliths. The shift may result from estimates uncertainties and/or partial dissolution of soil phytolith assemblages. At this stage, the absence of a uniform temperature-dependant relationship for the different silica-water couples prevents the reconstruction of precise values for δ18Oforming water and temperatures from fossil samples. At the same time, the obtained temperature coefficient of −0.4 (±0.2) ‰/°C is in agreement with the fractionation coefficients previously obtained for harvested grass phytoliths, sedimentary and fresh diatoms, and natural quartz. The consistency supports the efficiency of δ18Owood phytolith signatures in recording relative changes in mean annual δ18Osoil water values (assumed to be equivalent to the weighted mean annual δ18Oprecipitation values in rainforest environments) and MAT, provided these changes were on the order of several ‰ and/or several °C in magnitude. While rainforest dynamics can be revealed by morphological phytolith indices, the δ18O analysis of rainforest phytolith assemblages from continuous sedimentary sequences should simultaneously provide insights into terrestrial climate changes.