?18O signature of phytoliths from the last interglacial Lynch's Crater sediments (Qld, Australia): insights on changes in precipitation sources

The 65 m continental sedimentary sequence from Lynch's Crater (17°37‧S, 145°70‧E; last 230 ka) has provided one of the main references for late Quaternary environmental changes in northeast Australia and in the Western Pacific area. The site is located at the southern limit of an area that is influenced by the Australian Summer Monsoon (ASM). The climate pattern is controlled by the position of the Intertropical Convergence Zone (ITCZ) and the monsoon circulation. Today, over 80% of mean annual precipitation falls during the November-March interval, supplied by NW monsoonal winds during the earlier phase of the ASM, by S-E trade winds during the later phase of the ASM and by occasional cyclones. Influence of Quaternary Northern Hemisphere glacial/inter-glacial changes on precipitation and rainforest dynamics at Lynch's crater has been previously evidenced from the pollen record. Here, we explore the oxygen isotopic composition of wood phytoliths (δ18Owood phytolith) recovered from 30 samples from the 70-130ka sedimentary section (core LC2-04). Reconstruction of a precise core chronology for the core is in progress. A recent calibration indicated the efficiency of δ18Owood phytolith signatures from Queensland rainforest phytolith assemblages in recording changes in mean annual δ18Osoil water values (assumed to be equivalent to the weighted mean annual δ18Oprecipitation values in rainforest environments) and mean annual temperature, provided these changes were on the order of several ‰ and/or several °C in magnitude. In parallel, through the morphological identification of phytolith types (d/p index), phytolith assemblages recovered from sediments have been shown to be a reliable tool for quantitatively estimating tree cover density at low elevation tropical sites. The Lynch's Crater phytolith record, although limited at this stage by the chronological framework, evidences several key features. 1) Increases in δ18Owood phytolith values are synchronous with an increase in d/p value tracing rainforest expansion, e.g., at the beginning of stage 5e ca. 135ka.. 2) Both records show synchronous ca. 20ka cycles inversely correlated with changes in 17°S summer insolation. Increases in δ18Owood phytolith and indirectly in δ18Oprecipitation cannot be explained by an increase in precipitation amount or in atmospheric temperature. In the light of those results, changes in precipitation sources and continental recycling are discussed.