Seasonal patterns of carbon chemistry and isotopes in tufa depositing groundwaters of southwestern Japan
Seasonal patterns of carbon chemistry and isotopes in tufa depositing groundwaters of southwestern Japan
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DOI：
10.1016/j.gca.2007.10.025
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发表时间：
2008-01
影响因子：
5
通讯作者：
M. Hori;K. Hoshino;K. Okumura;A. Kano
中科院分区：
文献类型：
--
作者：
M. Hori;K. Hoshino;K. Okumura;A. Kano
每年层状碳酸盐岩，称为凝灰岩，通常在石灰岩地区发展，通常记录氧和碳同位素组成的季节性模式。δ18O值主要受水温季节变化的控制，而δ13C值则是系统中气态、液态和固态碳源之间复杂反应的结果。通过对地球化学数据进行模型计算，我们研究了日本西南部三个凝灰岩沉积点地下水系统中δ 13 C季节性模式的形成过程。地下无机碳物种与气态CO2交换，气态CO2主要通过自然大气通风和土壤空气扩散引入地下水文系统。这些过程控制了δ 13 C的季节变化，即夏季低、冬季高。在我们调查的三个站点中，我们确定了液相和气相之间碳交换程度的两个极端情况。对于高放射性碳组成（Δ 14 C）和低pCO2的情况，由于大气CO2的贡献大，水质量小，因此存在大量的碳交换。在另一个极端情况下，大气CO2的贡献很小，水的质量相对较大，其特征是Δ 14 C低，pCO2高。结果表明，在三个站点中的两个站点，土壤剖面中的水分停留时间超过1a。研究结果还表明，在水量最小的地点，水在土壤剖面中的停留时间较短（小于1年），这与泉水温度的季节变化幅度较大相一致。凝灰岩的Δ14C值与其沉积的水文条件密切相关。如果凝灰岩沉积体系的初始Δ 14 C值稳定，则可利用14 C年代学测定凝灰岩的年代。
Annually laminated carbonates, known as tufas, commonly develop in limestone areas and typically record seasonal patterns of oxygen- and carbon-isotope compositions. δ18O values are principally controlled by seasonal changes of water temperature, whereas δ13C values are the result of complex reactions among the gaseous, liquid, and solid sources of carbon in the system. We examined the processes that cause the seasonal patterns of δ13C in groundwater systems at three tufa-depositing sites in southwestern Japan by applying model calculations to geochemical data. Underground inorganic carbon species are exchanged with gaseous CO2, which is mainly introduced to the underground hydrological system by natural atmospheric ventilation and by diffusion of soil air. These processes control the seasonal pattern of δ13C, which is low in summer and high in winter. Among the three sites we investigated, we identified two extreme cases of the degree of carbon exchange between liquid and gaseous phases. For the case with high radiocarbon composition (Δ14C) and low pCO2, there was substantial carbon exchange because of a large contribution of atmospheric CO2and a small water mass. For the other extreme case, which was characterized by low Δ14C and high pCO2, the contribution of atmospheric CO2was small and the water mass was relatively large. Our results suggest that at two of the three sites water residence time within the soil profile was longer than 1year. Our results also suggested a short residence time (less than 1year) of water in the soil profile at the site with the smallest water mass, which is consistent with large seasonal amplitude of the springwater temperature variations. The Δ14C value of tufas is closely related to the hydrological conditions in which they are deposited. If the initial Δ14C value of a tufa-depositing system is stable,14C-chronology can be used to date paleo-tufas.