Origin of annual laminations in tufa deposits, southwest Japan
Origin of annual laminations in tufa deposits, southwest Japan
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DOI：
10.1016/0031-0182(02)00717-4
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发表时间：
2003-02-15
影响因子：
3
通讯作者：
Fujii, H
中科院分区：
文献类型：
--
作者：
Kano, A;Matsuoka, J;Fujii, H
对日本西南部(爱美县白川县)的层状凝灰岩和一条凝灰岩沉积流进行了为期3年的月度研究。来自凝灰岩的一系列样品清楚地揭示了年度纹层的模式。根据水化学计算，年度分层模式主要受方解石沉淀速率的变化控制。由于地下空气中CO2分压的变化，与降水速率相关的溶解CaCO3浓度在夏秋季较高，冬春季较低。地下PCO_2的季节性变化可能是土壤CO_2扩散和地下空气通风两个温度相关过程的结果。这些变化，除了水温的变化外，还改变了降雨率，降雨率具有明显的季节性模式，特别是在下游。夏秋季节降水速率高，冬春季节降水速率低，与凝灰岩的季节分层规律一致。采集的样品结构表明，纹层由密集钙化的夏秋(6-10月)纹层和轻度钙化的冬春(11-5月)纹层组成。我们推测，沉淀率的增加刺激了蓝藻细丝上厚厚的方解石结壳，从而产生了致密的质地。这一解释得到了夏秋季较低的有机/无机碳产生率的支持。蓝藻组合的季节变化是存在的，但不能反映季节分层模式。由于相关的过程与温度有关，白川的季节性层理模式被认为普遍适用于其他在温带气候下沉积的层状凝灰岩。然而，当地和气候条件可能会导致相反的模式。由于冬季地下二氧化碳含量低，方解石降雨量大，白川泉源附近冬季沉积了致密的纹层。欧洲西北部报告的反转模式可能受到季节性降雨的影响，这反映在水文条件上。(C)2002 Elsevier Science B.V.保留所有权利。
Laminated tufas and a tufa-depositing stream in SW Japan (Shirokawa, Ehime Prefecture) were studied monthly over a 3-yr period. A series of samples from the tufa clearly reveals the pattern of annual laminations. The annual layering pattern was primarily controlled by changes in the rate of calcite precipitation, as calculated from water chemistry. The concentration of dissolved CaCO3, which correlates with the precipitation rate, was high in summer-autumn and low in winter-spring, owing to changes in the partial pressure of CO2 in underground air. Regular seasonal changes in underground PCO2 probably resulted from two temperature-dependent processes, the diffusion of soil CO2 and the ventilation of underground air. These changes, in addition to water temperature changes, altered the precipitation rate, which has a clear seasonal pattern, especially in the lower stream. The seasonal precipitation rate was high in summer autumn and low in winter-spring, which is consistent with the seasonal lamination pattern seen in the tufas. The textures of collected samples show that the laminations consist of densely calcified summer-autumn (June-October) laminae and lightly calcified winter-spring (November-May) laminae. We infer that the increased precipitation rate stimulated thick calcite encrustation on cyanobacterial filaments to produce the dense textures. This interpretation is supported by the lowered organic/inorganic carbon-production ratio in summer autumn. Seasonal variations in cyanobacterial assemblages are present, but do not reflect the seasonal lamination pattern. Because the relevant processes are temperature dependent, the seasonal lamination pattern at Shirokawa is thought to generally apply to other laminated tufas deposited in temperate climates. However, a reversed pattern can result from local and climatic circumstances. Dense laminae were deposited in winter near the source of the spring at Shirokawa, because calcite precipitation was high owing to low underground PCO2 in winter. Reversed patterns reported from northwestern Europe were probably influenced by seasonal rainfall, which is reflected in hydrological conditions. (C) 2002 Elsevier Science B.V. All rights reserved.