The Effect of Burning on the Dissolution Behaviour and Silicon and Oxygen Isotope Composition of Phytolith Silica

Abstract:

The δ³⁰Si and δ¹⁸O values of silica phytoliths precipitated in plants can be applied to paleoenvironmental reconstructions. Here, the effects of burning and partial dissolution of phytoliths on their isotopic compositions and dissolution behaviour were examined. Phytoliths were heated to 700˚C and then dissolution experiments were conducted in batch reactors under a range of pH (4-8) and temperature (4-19˚C) conditions. Heating caused a -2.6 ‰ shift in phytolith δ¹⁸O values. During dissolution, the δ¹⁸O of heated phytoliths increased by up to 3.6 ‰ until ~30-40% saturation was reached, and then precipitation of new silica reduced δ¹⁸O despite a net dissolution. The peak δ¹⁸O values reached during partial dissolution were 4.3 ‰ lower than observed for unheated phytoliths. Heating did not cause a significant change in δ³⁰Si values, and partial dissolution of heated phytoliths only caused a significant change in δ³⁰Si values in one of the six experiments (pH 8, T 4˚C). Heating reduces the number of surface silanols, resulting in the incorporation of oxygen from ¹⁸O-depleted hydroxyl groups and the formation of strained siloxane bonds. 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). We propose that fewer hydrolysis sites exist on the surface of heated phytoliths because the isolated silanols that remain after heating are difficult to deprotonate at low pH. However, at higher pH the breakage of strained siloxane bonds in burned phytoliths may result a higher dissolution rate relative to fresh phytoliths. We recommend caution in using the δ¹⁸O values of soil phytoliths in paleoclimate reconstructions as they can be altered during both heating and partial dissolution. Additionally, for phytolith assemblages collected from archaeological hearths or grasslands prone to wildfires, the shift towards lower δ¹⁸O values caused by heating would result in overestimations of temperature and would complicate interpretations of relative changes in δ¹⁸O of phytoliths over time. Care must be taken to identify alteration by dissolution or burning, which may not always be visually evident.