Wildfires impact a large and increasing proportion of the Earth’s surface. With documented soil surface temperatures of up to ∼850 °C, wildfires may fundamentally alter the mineralogy and geochemistry of soils and regolith, more conventionally thought to be dominated by low temperature weathering processes. Here we use an experimental approach to test the effect of temperature on the formation of pyrogenic minerals, and on the distribution and mobility of major, trace and rare earth elements following post-fire chemical weathering. We focus on ferruginous nodules, common Fe-oxide cemented components of soils, which transform from non-magnetic to maghemite-bearing, magnetic nodules under wildfire conditions. These transformations provide a valuable record of fire impacts and facilitate the study of thermal processes and element mobility. Our results show heating produces a typical pyrogenic mineral assemblage of hematite, maghemite, metakaolin and transition alumina. At 900 °C the high temperature Fe2O3 polymorph luogufengite forms, which has never been reported in natural fire-affected substrates and therefore places an upper boundary on palaeowildfire temperatures at the soil-fire interface. Chemical leaching, employed to simulate the impacts of post-fire weathering, demonstrates that formation and subsequent breakdown of these pyrogenic minerals results in increased mobility of several elements including Li, Si, Sc, Cr, Co, Cu, Zn, Rb, Cs, La, Pb and U. Further, we propose that incongruent dissolution of pyrogenic metakaolin may be responsible for the formation of fusic material, an aluminous cement commonly found in soils. We conclude by discussing the significance of these results for the release of potentially toxic metals following a fire, identify trace elements that have the greatest potential to be used as palaeowildfire geochemical proxies (decreased alkali metal concentrations, decreased U/Th ratios, and decreased La compared to other rare earth elements), and the potential impact of wildfire on global geochemical cycles.

中文翻译：

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温度和热原矿物组合在野火诱发的主量和微量元素动员中的作用的实验限制


野火影响的地球表面很大一部分，而且比例越来越大。由于记录在案的土壤表面温度高达 ∼850 °C，野火可能会从根本上改变土壤和风化层的矿物学和地球化学，通常认为土壤和风化层主要由低温风化过程主导。在这里，我们使用一种实验方法来测试温度对热原矿物形成的影响，以及火灾后化学风化后主要、痕量和稀土元素的分布和迁移率。我们专注于铁质结核，这是土壤中常见的氧化铁胶结成分，在野火条件下，这些结核从非磁性转变为含磁赤铁矿的磁性结核。这些转换提供了火灾影响的宝贵记录，并有助于研究热过程和元件迁移率。我们的结果表明，加热会产生赤铁矿、磁赤铁矿、偏高岭土和过渡氧化铝的典型热相矿物组合。在 900 °C 时，高温 Fe2O3 多晶型罗古峰石形成，这在自然受火影响的基质中从未有过报道，因此在土火界面处为古野火温度设定了上限。用于模拟火灾后风化影响的化学浸出表明，这些热解矿物的形成和随后的分解导致多种元素的流动性增加，包括 Li、Si、Sc、Cr、Co、Cu、Zn、Rb、Cs、La、Pb 和 U。此外，我们提出，热原偏高岭土的不一致溶解可能是形成熔融材料的原因，熔融材料是一种常见于土壤中的铝水泥。 最后，我们讨论了这些结果对火灾后释放潜在有毒金属的重要性，确定了最有可能用作古野火地球化学替代物的微量元素（与其他稀土元素相比，碱金属浓度降低、U/Th 比值降低和 La 降低），以及野火对全球地球化学循环的潜在影响。