| 文石石笋发生矿物重结晶的影响因素及其古气候意义 |
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| 引用本文： | 李嘉燕,田怡苹,光凯悦,朱珊莹,李云霞,高永利,饶志国.文石石笋发生矿物重结晶的影响因素及其古气候意义[J].中国岩溶,2022,41(4):648-659. |
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| 作者姓名： | 李嘉燕 田怡苹 光凯悦 朱珊莹 李云霞 高永利 饶志国 |
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| 作者单位： | 1.湖南师范大学地理科学学院, 湖南 长沙 410081 |
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| 基金项目： | 国家自然科学基金（4200180）；湖南省自然科学基金（2021JJ40349）；湖南省教育厅项目（20B351） |
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| 摘 要： | 洞穴沉积物—石笋已成为研究岩溶区环境气候变化历史的重要载体。在我国湘西地区，某些洞穴石笋原始沉积多为不稳定的文石矿物，极易发生重结晶，可能使石笋中相关化学元素含量最终偏离原生矿物的特征，限制了文石石笋某些代用指标在古气候研究中的应用。文章以前人研究成果为基础，总结梳理了文石石笋发生重结晶的影响因素及其对石笋记录古气候的影响:（1）石笋剖面特征、XRD结果、显微镜观察和地球化学元素特征等可作为石笋发生重结晶的判别依据；（2）洞穴滴水和石笋孔隙水饱和度、文石晶体缺陷和晶体之间的方解石胶结物以及岩溶水体中Mg2+浓度等均会影响文石石笋的矿物转变；（3）在文石向方解石转变过程中，石笋铀含量会有一定程度的流失，可导致放射性铀系定年的异常或年代倒序；（4）矿物重结晶可导致δ18O、δ13C及石笋微量元素浓度（或比值）等指标发生改变，其变化特征因洞穴而异，从而影响其作为环境指示器的可靠性；（5）湖南龙山惹迷洞石笋（RM2）发生了不均一的矿物重结晶，自顶部至20.3 cm以放射状为主，20.3 cm至底部主要为糖粒状，并结合年代结果发现文石重结晶对石笋铀系定年产生了影响，而重结晶作用对该石笋其他指标的影响还有待进一步研究。
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| 关 键 词： | 洞穴石笋 重结晶 古气候 铀系定年 代用指标 |
| 收稿时间： | 2022-02-18 |
Factors influencing the recrystallization of aragonite stalagmites and their implications for paleoclimate |
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| Institution: | 1.College of Geographic Science, Hunan Normal University, Changsha,Hunan 410081, China2.Department of Earth and Planetary Sciences, University of Texas at San Antonio, San Antonio, TX 78249, USA |
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| Abstract: | The karst geomorphology is widely distributed in central south and southwestern China, especially in western Hunan Province, where speleothems (especially stalagmites) have become one of the most important archives for high-resolution paleoclimatic studies. Stalagmites are mainly composed of calcite and aragonite, and aragonite stalagmites can provide precise chronology with high uranium content. However, aragonite is easily transformed to calcite if continuously infiltrating and leaching by dripping water in wet cave environment. Some stalagmites in western Hunan of China initially deposited in aragonite minerals, which are prone to recrystallization (especially transforming into calcite), and could change the relevant chemical element signals, limiting the application of some aragonite stalagmite proxy indicators in paleoclimate research. Here, we summarize and sort out the influencing factors of the recrystallization of aragonite stalagmites and their influence on the stalagmites paleoclimate from published literature to define the applications of aragonite stalagmites in paleoclimate research.The determination of mineral phases and recrystallization of aragonite stalagmites is the first prerequisite for stalagmites paleoclimatic research. The stalagmite profile characteristics, XRD results, microscope observations and geochemical element characteristics could be used as the basis for discriminating the recrystallization of stalagmites.The influencing factors of the recrystallization of aragonite stalagmites include, (1) The saturation of cave dripping water and pore water in stalagmite. When the water is in the state of unsaturation for aragonite and infiltrates into the porous aragonite stalagmites, it will dissolve aragonite and reprecipitate to calcite. (2) Aragonite crystal defects and the existence of calcite cement between crystals can facilitate aragonite transform to calcite. (3) The recrystallization of aragonite stalagmite normally occurs in lower concentration of Mg2+ in karst water. (4) Other factors, such as organic matter and α-recoil, will also affect the mineral transformation of aragonite stalagmites.Besides, the recrystallization of aragonite stalagmites can modify some geochemical signals initially preserved in aragonite. (1) Due to the difference of crystal fabrics of calcite and aragonite, the uranium element will be lost when aragonite is transformed to calcite, and the losing will cause abnormal or reverse chronology. (2) The recrystallization of aragonite stalagmites can result in depleted or abnormal δ18O signals, and δ13C values of recrystallized calcite present more complex characteristics, which will be depleted or similar to primary aragonite. (3) Compared with recrystallized calcite, the primary aragonite is enriched in Sr but depleted in Mg in some caves. However, trace element concentration of recrystallized calcite in other caves does not differ greatly with that of primary aragonite. In brief, the changes of these proxies before and after recrystallization may vary between caves. Consequently, the process of aragonite to calcite transformation will weaken the accuracy of dating and the reliability of these proxies as environmental indicators. (4) Due to the different precipitation conditions of aragonite and calcite, the variation of stalagmite mineral phase may indicate climate and environmental evolution, but more studies are needed to confirm for recrystallization stalagmites.Finally, we present results of mineral analysis and 230Th dating of RM2 stalagmite with 200 cm length from Remi cave, Longshan, Hunan Province. RM2 stalagmite has undergone inhomogeneity recrystallization process. Meanwhile, we find that recrystallization does have a certain influence on the 230Th dating. However, the mechanism of recrystallization and the effect of stalagmite recrystallization on other proxies needs further studies. |
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