Recrystallization of pedogenic and biogenic carbonates in soil: environmental controls, modelling and relevance for paleoenvironmental reconstructions

成土碳酸盐和生物碳酸盐的同位素特征是土壤放射性碳定年和重建半干旱和干旱气候下古环境的最重要工具。碳酸盐与土壤CO2的再结晶，但是，可能会改变他们的C和O同位素的签名，因此偏见的重建结果。这需要考虑碳酸盐的重结晶进行校正。自主开发并已被证明的方法，涉及同位素交换的碳酸盐与14 CO2将被用来分析重结晶速率的最常见形式的土壤和生物成因的碳酸盐取决于环境条件。这将有助于评估哪些碳酸盐物质对古环境重建最有用，以及哪些条件最能保留其同位素特征。在第一步中，我们将展示非生物因素的影响-土壤CO2浓度，水分，温度，时间和存在的扩散地质碳酸盐-对重结晶速率的3组成壤（结核，石涂层，层状帽）和3生物碳酸盐（壳，骨片，钙化种子）。此外，我们将分析生物因素的影响-根呼吸和分泌物-在实验中与4种植物具有不同的根系特性。实验获得的重结晶速率将被用来参数化的模型模拟在各种实验条件下的土壤和生物成因的碳酸盐岩的总重结晶。最后，该模型将被证明通过比较模拟重结晶与放射性碳和U/Th测年的土壤和生物成因的碳酸盐岩样品在考古遗址。该项目的结果将使研究人员能够根据环境和生物因素选择最合适的成土和生物碳酸盐岩进行测年和古环境重建，并将为可能的重结晶提供可靠的校正。

Isotopic signatures of pedogenic and biogenic carbonates are the most important tools for radiocarbon dating of soils and for reconstructing paleoenvironments under semiarid and arid climates. Recrystallization of carbonates with soil CO2, however, may shift their C and O isotopic signatures and therefore bias the reconstruction results. This calls for corrections considering the recrystallization of carbonates. The self-developed and already proven approach involving isotopic exchange of carbonates with 14CO2 will be used to analyze recrystallization rates of the most common forms of pedogenic and biogenic carbonates depending on environmental conditions. This will enable to evaluate which carbonate materials are the most useful for paleoenvironmental reconstructions and which conditions best preserve their isotopic signature. In a first step we will demonstrate the effects of abiotic factors - soil CO2 concentration, moisture, temperature, time and the presence of diffuse geogenic carbonates - on the recrystallization rates of 3 groups of pedogenic (nodules, stone coatings, laminar caps) and 3 biogenic carbonates (shells, bone pieces, calcified seeds). Furthermore, we will analyze the effects of biotic factors - root respiration and exudation - in experiments with 4 plants with different root properties. The experimentally obtained recrystallization rates will be used to parameterize the model simulating total recrystallization of pedogenic and biogenic carbonates under various experimental conditions. Finally, the model will be proven by comparing simulated recrystallization with radiocarbon and U/Th dating of pedogenic and biogenic carbonates sampled in archaeological sites. The results of this project will allow researchers to select the most suitable pedogenic and biogenic carbonates for dating and paleoenvironmental reconstructions depending on the environmental and biotic factors, and will provide reliable corrections for possible recrystallization.