Changes in the δ18O and δ30Si values of partially dissolved phytoliths: implications for environmental reconstruction

The oxygen and silicon isotope compositions of phytolith silica preserved in soils can be used to examine past climate and biogeochemical cycles. The δ18O values of phytoliths vary with temperature and the δ18O values of formation water while the δ30Si values of phytoliths depend on silicon availability and weathering rates. However, further investigation into the stability of opal-A phytoliths in soils is necessary, as post-depositional alteration of biogenic silica may alter the isotopic composition and affect paleoenvironmental reconstruction. The effects of partial dissolution on the oxygen- and silicon-isotope composition of phytoliths was assessed through dissolution experiments in silicic acid-free water at a range of temperature (4-44°C) and pH conditions (4-10). After 3-30% dissolution, concurrent oxygen and silicon isotope analyses were obtained for the remaining opal-A. Over the course of the experiments oxygen and silicon isotope values of solid silica changed by up to +4.4 ‰ and +0.55 ‰, respectively. Under most conditions δ30Si values increased during early dissolution, followed by a decrease as dissolution progressed until δ30Si values were similar to, or in extreme cases, lower than the original unaltered phytoliths. This might indicate that both dissolution and re-precipitation contribute to the isotopic composition of the altered phytoliths. Changes in δ18O values of the phytoliths follow a similar pattern. We propose that the outer, reactive surface of a fresh phytolith is depleted of 30Si and 18O relative to the rest of the sample. A decrease in surface area suggests that removal of this layer accounts for the initial increase in δ30Si and δ18O values during early dissolution. Extended reaction times result in lower isotopic values and an unexpected decease in aqueous Si suggesting precipitation or exchange occurs. This study improves our understanding of the role of terrestrial biogenic silica in global Si cycle. Post-depositional change in the isotopic composition of phytoliths complicates paleoclimate models but does not preclude their use if the isotopic alteration of the surface layer can be accounted for, or the altered surface layer can be removed prior to analyses. Like diatoms, phytoliths preserved in sediments likely have already lost their reactive surface layer.