High-resolution decording of terrestrial paleo-climate recorder in laminated tufas

凝灰岩是沉积在石灰岩地区的一种淡水碳酸盐，是定量和高分辨率分析陆地古气候的极好工具。研究结果表明：1)凝灰岩的层状结构可视为方解石沉淀速率季节性变化所形成的年度层理；微生物生物量的估算表明，降水不是由生活在凝灰岩表面的蓝藻光合作用引起的。结合化学分析的结果得出结论：3)方解石沉淀中氧稳定同位素和少量金属元素的平衡。这些背景使得利用层状凝灰岩进行古气候分析成为可能，并对从白川县(爱美县)和新密县(冈山县)采集的样品进行了分析。我们发现，氧同位素组成随年分层的变化非常有规律。这种变化反映在水温的有规律的季节性变化上。此外，我们还认识到，降雨事件被记录为富含硅和铝的棕色带。这项研究提供了层状凝灰岩的沉积过程和条件的新方面，并证实了凝灰岩在古气候分析工具中的重要性

A tufa, a freshwater carbonate deposited in a limestone area, is an excellent tool for quantitative and highresolution analysis of terrestrial paleo-climate. Results of our investigation reveal that 1) laminated texture of tufas can be treated as annual layering developed due to seasonal change in precipitation rate of calcite. Estimation of microbial biomass shows that 2) the precipitation is not induced by photosynthesis of cyanobacteria living on the tufa surface. This and results of chemical analyses conduct to the conclusion that 3) equilibrium in terms of oxygen stable isotope and minor metal elements is confirmed in the calcite precipitation.These contexts make paleo-climatic analyses using a laminated tufa possible, and the analyses were performed for the specimens collected from Shirokawa (Ehime Prefecture) and Niimi (Okayama Prefecture). We found that 4) oxygen isotopic composition changes very regularly together with annual layering. The change is reflected from regular seasonal change in water temperature. Furthermore, we realized that 5) rainfall events are recorded as brown-colored bands, which are rich in Si, and Al. Excellent correlation between positions of the bands and periods of the rainfall events was recognized.This study provided new aspects of depositional processes and conditions of laminated tufas, and confirmed importance of the tufas in terms of paleo-climatological analytical tools