Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Geology

Supervisor

Dr. Elizabeth Webb

Abstract

The δ30Si and δ18O values of silica phytoliths have applications for reconstructing paleoenvironmental conditions. This study examines the effect of partial dissolution and burning of phytoliths on their isotopic compositions, dissolution behaviour, and physical characteristics (specific surface area, mean particle size, and visual appearance) and discusses problems with the use of phytolith δ18O and δ30Si values that have been modified in soils in paleoclimate reconstruction. 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 fresh phytoliths behave similarly as dissolution progresses, with values increasing until the solution is approximately 30-40% saturated with silicic acid. During this phase of the experiment the isotopic composition of the remaining silica is dominated by dissolution preferentially removing the light isotope (16O and 28Si) to the solution. After ~30-40% saturation back reactions begin to affect the isotopic composition of remaining silica, despite net movement in the forward direction (i.e. dissolution). The δ18O values of precipitated silica are determined by the δ18O value of water in the solution and the temperature of the experiment. The δ30Si values of precipitated silica are determined by the δ30Si value of silicic acid. Phytoliths subjected to burning at 700˚C have δ18O values that are 2.6 ‰ lower than unburned phytoliths while their δ30Si values remain unchanged. This suggests that heating results in the incorporation of 18O-depleted hydroxyl groups into the silica structure. Dissolution of burned phytoliths progressed more slowly than dissolution of fresh phytoliths in conditions that are less favourable for dissolution (i.e. low pH and T) and more quickly in conditions that are favourable (i.e. high pH and T). The δ18O values of partially dissolved burned phytoliths follow the same general trend as those of unburned phytoliths but with less overall change in δ18O values. Burning may increase silanol sites that are more susceptible to dissolution. We recommend caution in using the δ18O and δ30Si values of soil phytoliths in paleoclimate reconstructions. Care must be taken to identify alteration by dissolution or burning, which may not always be visually evident.


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