The effect of progressive dissolution on the oxygen and silicon isotope composition of opal-A phytoliths: Implications for palaeoenvironmental reconstruction

Abstract

The δ30Si and δ18O values of silica phytoliths precipitated in plants can be applied to palaeoenvironmental reconstructions. This study examines the effect of partial dissolution of phytoliths on their isotopic compositions and physical characteristics (specific surface area, mean particle size) and discusses problems with paleoclimate reconstructions using the δ18O and δ30Si values of phytoliths that have been modified in soils. Dissolution experiments were conducted in batch reactors under a range of pH (4–10) and temperature (4–44°C) conditions. The δ18O and δ30Si values of phytoliths behaved similarly as dissolution progressed, with values increasing until the solutions were approximately 30–40% saturated with silicic acid. During this phase the isotopic composition of the remaining silica was primarily affected by dissolution reactions that preferentially removed the light isotopes (16O and 28Si). Partial dissolution resulted in phytolith δ18O values that were up to +3.9‰ higher and δ30Si values that were up to +0.63‰ higher than unaltered material. After ~30–40% saturation is reached, precipitation of new silica begins to reduce the δ18O and δ30Si values of remaining silica, despite a net dissolution. The δ30Si values of precipitated silica were determined by the δ30Si value of silicic acid previously released by dissolution. The δ18O values of precipitated silica were determined by the δ18O value of water and the temperature of the experiment. Silica precipitated in isotopic equilibrium with water acted to reduce phytolith δ18O values as the solution approached saturation. In soil environments, such reactions may result in a silica coating on the phytolith that has a δ18O value in equilibrium with soil water, confounding paleoclimate estimates. This study demonstrates that assessment of the extent of post-depositional alteration of soil phytolith assemblages is essential prior to the use of their isotopic compositions in paleoclimate models.