The δ30Si and δ18O values of opal-A precipitated in plants (silica phytoliths) have been shown to be useful for 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. NMR results suggest that heating reduces the number of surface vicinal silanols, which likely results in the formation of strained SiOSi bonds which incorporate oxygen from 18O-depleted hydroxyl groups. During dissolution, the δ18O of burned phytoliths increased by up to 3.5 ‰ (average 1.8 ‰) until 15–45 % saturation was reached, and then adsorption of silica on the surface of the solid began to reduce the δ18O value of solid silica despite a net dissolution. The maximum increase in δ18O during dissolution of burned phytoliths is 1.8 ‰ smaller than previously observed for unburned silica subjected to partial dissolution under the same conditions. Heating did not cause a significant change in δ30Si values, and partial dissolution of burned phytoliths caused a slight increase in δ30Si values that was smaller in magnitude than for unburned phytoliths. Dissolution of burned phytoliths progressed more slowly than dissolution of fresh phytoliths in low pH and temperature conditions, but was faster than the dissolution of fresh phytoliths when pH > 6 and temperature = 19 °C. We propose that because fewer hydrolysis sites exist on the surface of burned phytoliths that the isolated silanols that remain after heating are difficult to deprotonate at low pH resulting in slower dissolution. However, at higher pH the breakage of strained SiOSi bonds in burned phytoliths may explain their 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. 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 using paleothermometer equations. Care must be taken to identify alteration by dissolution or burning, which may not always be visually evident.

中文翻译：

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燃烧对植硅体二氧化硅溶解行为和硅氧同位素组成的影响


在植物（二氧化硅植硅体）中沉淀的蛋白石 A 的 δ30Si 和 δ18O 值已被证明可用于古环境重建。在这里，研究了植硅体燃烧和部分溶解对其同位素组成和溶解行为的影响。将植硅体加热至 700 °C，然后在 pH 值 （4–8） 和温度 （4–19 °C） 条件下在间歇式反应器中进行溶出实验。加热导致植硅体 δ18O 值发生 -2.6 ‰ 的变化。NMR 结果表明，加热减少了表面附近的硅醇的数量，这可能导致形成应变的 SiOSi 键，这些键包含来自 18O 耗尽的羟基的氧。在溶解过程中，燃烧的植硅体的 δ18O 增加了高达 3.5 ‰（平均 1.8 ‰），直到达到 15-45% 的饱和度，然后二氧化硅在固体表面的吸附开始降低固体二氧化硅的 δ18O 值，尽管有净溶解。在相同条件下，燃烧的植硅体溶解过程中 δ18O 的最大增加比以前观察到的未燃烧二氧化硅部分溶解的二氧化硅小 1.8 ‰。加热不会导致 δ30Si 值的显着变化，燃烧的植硅体的部分溶解导致 δ30Si 值略有增加，其幅度小于未燃烧的植硅体。在低 pH 值和温度条件下，烧焦植硅体的溶解速度比新鲜植硅体的溶解进展慢，但当 pH > 6 且温度 = 19 °C 时，比新鲜植硅体的溶解速度快。 我们提出，由于燃烧的植硅体表面存在的水解位点较少，因此加热后残留的分离硅醇在低 pH 值下难以去质子化，导致溶解速度变慢。然而，在较高的 pH 值下，烧焦植硅体中应变的 SiOSi 键的断裂可能解释了它们相对于新鲜植硅体较高的溶解速率。我们建议在古气候重建中谨慎使用土壤植硅体的 δ18O 值，因为它们在加热和部分溶解过程中都会发生变化。对于从考古炉膛或容易发生野火的草原收集的植硅体组合，由加热引起的 δ18O 值降低将导致使用古温度计方程高估温度。必须注意识别溶出或燃烧引起的改变，这可能并不总是在视觉上很明显。