焚烧对植物硅石的溶解行为及硅氧同位素组成的影响

IF 4.5 1区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Geochimica et Cosmochimica Acta Pub Date : 2024-11-07 DOI:10.1016/j.gca.2024.11.007
Andrea J. Prentice, Elizabeth A. Webb
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引用次数: 0

摘要

植物中沉淀的蛋白石-A(硅质植金石)的δ30Si和δ18O值已被证明可用于古环境重建。在此,研究人员考察了燃烧和部分溶解植物碎屑对其同位素组成和溶解行为的影响。先将植生石块加热至 700 °C,然后在不同 pH 值(4-8)和温度(4-19 °C)条件下,在间歇反应器中进行溶解实验。加热导致植金石 δ18O 值发生-2.6 ‰的移动。核磁共振结果表明,加热会减少表面邻接硅烷醇的数量,这很可能会导致形成应变的 SiOSi 键,其中包含了 18O 贫化羟基中的氧。在溶解过程中,灼烧过的植金石的δ18O 最多增加 3.5 ‰(平均 1.8 ‰),直到达到 15-45 % 的饱和度,然后固体表面的二氧化硅吸附开始降低固体二氧化硅的δ18O 值,尽管出现了净溶解。在相同条件下,已烧毁的植金石在溶解过程中δ18O的最大增加值比以前观察到的未烧毁的部分溶解的二氧化硅的δ18O值低1.8‰。加热并没有引起δ30Si值的明显变化,而部分溶解烧毁的植生石块会引起δ30Si值的轻微上升,但上升幅度小于未烧毁的植生石块。在低 pH 值和低温度条件下,烧毁的植生石块的溶解速度比新鲜植生石块的溶解速度慢,但当 pH 值为 6、温度为 19 ℃ 时,烧毁的植生石块的溶解速度比新鲜植生石块的溶解速度快。我们认为,由于烧毁的植金石表面水解位点较少,加热后残留的分离硅烷醇在低 pH 值条件下难以去质子化,导致溶解速度较慢。不过,在较高的 pH 值下,烧毁的植金石中的应变 SiOSi 键断裂可能是其溶解率高于新鲜植金石的原因。我们建议在古气候重建中谨慎使用土壤植生石块的 δ18O 值,因为它们可能在加热和部分溶解过程中发生变化。对于从考古壁炉或易受野火影响的草地采集的植物碎屑,加热导致δ18O值降低,会导致使用古温度计方程高估温度。必须注意识别溶解或焚烧造成的变化,因为这些变化并不总是很明显。
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The effect of burning on the dissolution behaviour and silicon and oxygen isotope composition of phytolith silica
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.
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来源期刊
Geochimica et Cosmochimica Acta
Geochimica et Cosmochimica Acta 地学-地球化学与地球物理
CiteScore
9.60
自引率
14.00%
发文量
437
审稿时长
6 months
期刊介绍: Geochimica et Cosmochimica Acta publishes research papers in a wide range of subjects in terrestrial geochemistry, meteoritics, and planetary geochemistry. The scope of the journal includes: 1). Physical chemistry of gases, aqueous solutions, glasses, and crystalline solids 2). Igneous and metamorphic petrology 3). Chemical processes in the atmosphere, hydrosphere, biosphere, and lithosphere of the Earth 4). Organic geochemistry 5). Isotope geochemistry 6). Meteoritics and meteorite impacts 7). Lunar science; and 8). Planetary geochemistry.
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