Recrystallization of shell carbonate in soil: 14C labeling, modeling and relevance for dating and paleo-reconstructions
Recrystallization of shell carbonate in soil: 14C labeling, modeling and relevance for dating and paleo-reconstructions
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DOI：
10.1016/j.geoderma.2016.07.013
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发表时间：
2016-11
期刊：
影响因子：
6.1
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
软体动物贝壳通常存在于广泛的地质和考古背景中。贝壳碳酸盐可用于年龄测定(Δ14C)和古环境指示(δ18O，δ13C)。然而，土壤中的贝壳碳酸盐重结晶可能会使碳(C)同位素特征与土壤二氧化碳重新平衡。由于缺乏合适的实验方法，平衡动力学仍然知之甚少。在这里，我们使用人工~(14)C标记技术研究贝壳碳酸盐重结晶过程随时间的变化。将不含有机和无有机的原生雄蕊壳粒分别与黄土或无碳酸盐壤土混合。混合物被放在密闭的瓶子里，瓶子里的空气含有14CO2(二氧化碳=0.2%)。贝壳的~(14)C活度随时间的变化与贝壳碳酸盐的重结晶有关，贝壳碳酸盐的重结晶在一天后就开始了。重结晶率分别为10−、3%/d−、1.6×10−/d、2%/d、−/d。有机物的去除增加了壳体的孔隙度，从而增加了与土壤溶液交换的接触面。无机壳在黄土中重结晶的速度比其他处理快得多(56d为0.56%)。黄土中的重结晶(有无有机化合物时)是无碳酸盐土壤中重结晶的2到7倍。黄土碳酸盐本身会发生重结晶并在壳层上堆积，导致高估了贝壳碳酸盐的重结晶作用。建立了贝壳碳酸盐重结晶随时间变化的模型。该模型考虑了壳结构中有机化合物的存在或不存在以及埋藏基质中地源碳酸盐的存在或不存在。模型的拟合结果符合R2=100.98。模拟结果表明，黄土中无机壳几乎完全重结晶(95%的壳碳酸盐)所需的模拟时间为88年，无碳酸盐土壤中含有机壳的模型所需时间长达770年。在此之后，原有的同位素签名将完全消失，取而代之的是壳结构中新的δ13C和Δ14C签名。因此，壳层碳酸盐重结晶作用在地质时间上可能进行得相对较快。在解释测年结果和古环境重建时，这一点是必要的。
Mollusk shells are commonly present in a broad array of geological and archaeological contexts. The shell carbonate can serve for numerical age determination (Δ14C) and as a paleoenvironmental indicator (δ18O, δ13C). Shell carbonate recrystallization in soils, however, may re-equilibrate the carbon (C) isotopic signature with soil CO2. The equilibration dynamics remain poorly understood because of the absence of suitable experimental approaches. Here we used the artificial14C-labeling technique to study the process of shell carbonate recrystallization as a function of time.Organic-free and organic-containing shell particles ofProtothaca stamineawere mixed with loess or a carbonate-free loamy soil. The mixtures were placed in air-tight bottles, where the bottle air containing14CO2(pCO2= 2%). The14C activity of shells was measured over time and related to the recrystallization of shell carbonate.Recrystallization of shell carbonate already began after one day. The recrystallization rates were 10− 3% day− 1in organic-containing shell embedded in soil and 1.6 · 10− 2% day− 1in organic-free shells in loess. Removal of organic compounds increased shell porosity, and so, increased the contact surface for exchange with soil solution. Organic-free shells recrystallized much faster in loess (0.56% in 56 days) than in other treatments. Recrystallization was 2 to 7 times higher in loess (in the presence and absence of organic compounds, respectively) than in carbonate-free soil. Loess carbonate itself can recrystallize and accumulate on shells, leading to overestimation of shell carbonate recrystallization. A model for shell carbonate recrystallization as a function of time was developed. The model considers the presence or absence of organic compounds in shell structure and geogenic carbonates in the embedding matrix. The model enabled all results to be fitted withR2= 0.98.The modelled time necessary for nearly full recrystallization (95% of shell carbonate) was 88 years for organic-free shells in loess and up to 770 years for organic-containing shells in carbonate-free soil. After this period, the original isotopic signature will vanish completely and will be replaced by a new δ13C and Δ14C signature in the shell structure. Thus, shell carbonate recrystallization may proceed relatively rapidly in terms of geologic time. This is necessary to consider in the interpretation of dating results and paleoenvironmental reconstructions.