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

Abstract The δ 30 Si and δ 18 O 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 δ 18 O and δ 30 Si 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 δ 18 O and δ 30 Si 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 ( 16 O and 28 Si). Partial dissolution resulted in phytolith δ 18 O values that were up to + 3.9‰ higher and δ 30 Si 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 δ 18 O and δ 30 Si values of remaining silica, despite a net dissolution. The δ 30 Si values of precipitated silica were determined by the δ 30 Si value of silicic acid previously released by dissolution. The δ 18 O values of precipitated silica were determined by the δ 18 O value of water and the temperature of the experiment. Silica precipitated in isotopic equilibrium with water acted to reduce phytolith δ 18 O values as the solution approached saturation. In soil environments, such reactions may result in a silica coating on the phytolith that has a δ 18 O 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.