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半干旱和干旱气候下土壤放射性碳定年和重建古环境的最重要工具。然而，碳酸盐与土壤CO2的再结晶可能会改变它们的C和O同位素特征，从而使重建结果产生偏差。考虑到碳酸盐的再结晶，这需要修正。该方法涉及碳酸盐与14CO2的同位素交换，将用于分析最常见的成土碳酸盐和生物碳酸盐的再结晶速率，这取决于环境条件。这将有助于评估哪些碳酸盐材料对古环境重建最有用，以及哪些条件最能保存它们的同位素特征。在第一步中，我们将展示非生物因素——土壤CO2浓度、湿度、温度、时间和弥漫的地球成因碳酸盐的存在——对3组成土碳酸盐（结核、石膜、层流帽）和3组生物成因碳酸盐（贝壳、骨块、钙化种子）的再结晶率的影响。此外，我们还将以4种不同根系特性的植物为试验材料，分析根系呼吸和根系渗出对植物生长的影响。实验得到的再结晶速率将用于参数化模型，模拟不同实验条件下的成土碳酸盐岩和生物碳酸盐岩的总再结晶。最后，通过将模拟重结晶与在考古遗址取样的成土碳酸盐和生物碳酸盐的放射性碳和U/Th定年进行比较，对该模型进行验证。该项目的研究结果将使研究人员能够根据环境和生物因素选择最适合的成土和生物成因碳酸盐进行定年和古环境重建，并将为可能的再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.