Assessment of pretreatment methods on CIA-K and CALMAG indices and the effects on paleoprecipitation estimates
Assessment of pretreatment methods on CIA-K and CALMAG indices and the effects on paleoprecipitation estimates
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DOI：
10.1016/j.palaeo.2022.111102
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发表时间：
2022-06
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影响因子：
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通讯作者：
Lauren A. Michel;N. Sheldon;T. Myers;N. Tabor
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作者：
Lauren A. Michel;N. Sheldon;T. Myers;N. Tabor
古土壤的化学成分经常被用来利用现代土壤的代用关系来重建古气候和古环境条件。然而，随着时间的推移，一些代理关系已被应用于不属于原始训练数据集的古土壤类型，或者在原始研究未采用的预处理步骤之后进行分析，而没有对这些方法变化是否影响了数据的解释进行批判性评估。在这里，我们使用来自加利福尼亚州和德克萨斯州的32种现代土壤的化学成分对预处理方法进行了严格的测试，以评估它们对两个最常用的用于估计平均年降水量的气候函数指数CIA-K和CALMAG的影响。化学数据来自两种不同预处理方式的配对土壤，包括使用弱冰醋酸消化方解石和不使用预处理方法保持土壤中所有钙，包括来自成土碳酸盐。除预处理后氧化钙耗竭外，未经处理的水和经酸处理的水的主要元素氧化物值变化不大。在基质中CaO含量为bb0 ~ 2%的土壤中，酸消化预处理对氧化钙的消耗显著。这种酸消化预处理导致年平均降水量估计值大幅增加，而这与公布的气象值不符。作为CaO损失的函数，降水估计偏向较高值的程度与土壤材料的碳酸钙含量参数相关。这对于适当地应用CIA-K和CALMAG指数作为含有成土和/或成岩碳酸盐的古土壤剖面的古降水估计的可行代理具有重要意义，我们建议在未经处理的样品中使用CaO的定量阈值，以尽量减少潜在的误差来源。
The chemical compositions of paleosols are frequently used to reconstruct paleoclimatic and paleoenvironmental conditions using proxy relationships derived from modern soils. However, through time, some of the proxy relationships have been applied to paleosol types that were not part of the original training data sets, or they have been analyzed following pretreatment steps that were not employed by the original studies without critical evaluation of whether those methodological changes have impacted the interpretation of the data. Here we test pretreatment methods critically using chemical compositions of thirty-two modern soils from California and Texas to evaluate their impact on two of the most commonly employed climofunction indices to estimate mean annual precipitation, CIA-K and CALMAG. Chemical data are generated from paired soils which have underdone two different pretreatment styles, including using weak glacial acetic acid to digest calcite and using no pretreament methods to keep all calcium in the soil, including from pedogenic carbonate. There is little change in major element oxide values from untreated to acid-treated aliquots, except for calcium oxide depletion after pretreatment. In all soils with >2% CaO in the matrix, calcium oxide is significantly depleted by the acid-digestion pretreatment. This acid-digestion pretreatment induces a substantial increase in estimates of mean annual precipitation, which are not in accordance with published meteorological values. The extent to which precipitation estimates are skewed toward higher values as a function of CaO loss is parametrically correlated with calcium carbonate content of the soil material. This has important implications for the appropriate application of CIA-K and CALMAG indices as viable proxies of paleoprecipitation estimates from paleosol profiles that contain pedogenic and/or diagenetic carbonates and we suggest quantitative thresholds for CaO in untreated samples to minimize potential sources of error.