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

土壤成因碳酸盐和生物成因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测年和重建古环境的最重要工具。然而，碳酸盐与土壤二氧化碳的重结晶可能会改变其碳和氧同位素特征，从而对重建结果产生偏差。这需要考虑碳酸盐的重结晶进行修正。将根据环境条件，使用自行开发并已得到证实的方法，包括碳酸盐与14CO2的同位素交换，分析最常见的成壤碳酸盐和生物碳酸盐的重结晶率。这将能够评估哪些碳酸盐材料对古环境重建最有用，哪些条件最好地保存了它们的同位素特征。在第一步中，我们将演示非生物因素-土壤二氧化碳浓度、水分、温度、时间和弥漫的地源碳酸盐的存在-对3组成壤碳酸盐(结核、石涂层、层状盖子)和3种生物碳酸盐(贝壳、骨片、钙化种子)重结晶率的影响。此外，我们还将分析生物因素--根的呼吸和分泌物--在4种不同根系特性的植物上的影响。实验获得的重结晶速率将被用于模拟不同实验条件下土壤成因碳酸盐和生物成因碳酸盐的完全重结晶的模型参数。最后，通过将模拟重结晶与考古遗址中土壤和生物成因碳酸盐的放射性碳测年和U/Th测年进行比较，验证了该模型。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土壤成因和生物成因碳酸盐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.