Cation exchange retards shell carbonate recrystallization: consequences for dating and paleoenvironmental reconstructions
Cation exchange retards shell carbonate recrystallization: consequences for dating and paleoenvironmental reconstructions
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DOI：
10.1016/j.catena.2016.03.012
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发表时间：
2016-07
期刊：
影响因子：
6.2
通讯作者：
Kazem Zamanian;K. Pustovoytov;Y. Kuzyakov
中科院分区：
文献类型：
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作者：
Kazem Zamanian;K. Pustovoytov;Y. Kuzyakov
放射性碳法是测定软体动物壳体碳酸盐年龄的常用方法。然而，估计年龄的准确性取决于壳碳酸盐重结晶的程度和完整性。尽管环境碳（C）污染贝壳CaCO 3的影响是众所周知的，但Ca 2+在成岩过程中的作用仍不清楚。此外，年轻的C壳成岩过程中发生在土壤溶液中，其中的Ca 2+浓度与交换性Ca 2+和/或风化的含钙矿物平衡。虽然交换过程在几秒钟内发生，但溶解平衡需要更长的时间（大约几个月）。因此，有人推测，溶解和重结晶壳碳酸盐在土壤中具有较高的阳离子交换容量（CEC）应进行缓慢相比，那些与低CEC。目的是确定土壤CEC和交换性阳离子对壳碳酸盐重结晶的影响，使用14 C标记的方法。将双壳类动物的贝壳颗粒与不含碳酸盐的沙子（CEC = 0.37 cmol+kg− 1）（沙子）、壤土（CEC = 16 cmol+kg− 1）（壤土）或KCl饱和的相同壤土混合，其中可交换阳离子被K+取代（交换）。高灵敏度14 C标记/示踪方法被用来确定碳酸盐重结晶速率。壳碳酸盐重结晶120天后，在壤土和交换（0.016和0.024毫克碳酸钙，分别）显示一个数量级低于砂（0.13毫克碳酸钙）重结晶。高水平的土壤交换性Ca 2+降低了壳碳酸盐的溶解度，因此其重结晶，因为交换比溶解快。因此，土壤阳离子交换量和阳离子组成是贝壳碳酸盐重结晶的决定因素。低CEC土壤中的贝壳可能会经历更强烈的重结晶，因此，他们可能需要进一步的预处理前的定年程序。
The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect of contamination of the shell CaCO3with environmental carbon (C) is well known, the role of Ca2+in diagenetic processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca2+concentration is in equilibrium with exchangeable Ca2+and/or weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objective was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using the14C labeling approach. Shell particles of the bivalveProtothaca stamineawere mixed with carbonate-free sand (CEC = 0.37 cmol+kg− 1) (Sand), a loamy soil (CEC = 16 cmol+kg− 1) (Loam) or the same loamy soil saturated with KCl, where exchangeable cations were replaced with K+(Exchanged). The high-sensitivity14C labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystallization after 120 days in Loam and Exchanged (0.016 and 0.024 mg CaCO3, respectively) showed one order of magnitude lower recrystallization than in Sand (0.13 mg CaCO3). A high level of soil exchangeable Ca2+decreased the solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments before the dating procedure.