抚仙湖全新世自生碳酸盐及其区域气候和湖泊水位指示意义

抚仙湖是我国面积、蓄水量最大的高原湖泊之一。通过对抚仙湖FXH-B2钻孔岩芯296 cm沉积物的矿物组成和碳酸盐含量分析,结合岩芯XRF扫描数据和AMS^14C年代的测定,探讨了抚仙湖湖泊沉积物中碳酸盐含量与方解石矿物及沉积物Ca元素相对含量之间的关系及其环境指示意义,在对比和参考水下地形测量、地球物理勘探、水质和地球化学监测和不同部位钻孔岩芯的分析结果的基础上,重建了抚仙湖全新世以来气候和湖泊水位的变化。结果指示,抚仙湖碳酸盐含量与方解石的XRD信号强度、沉积物Ca元素的XRF扫描数据之间存在很高的相关性,沉积物中碳酸盐类物质主要为方解石(其他碳酸盐类矿物相对含量极微甚至可以忽略不计),沉积物中Ca元素主要来自碳酸盐,因此其含量由碳酸盐、即方解石矿物含量决定。沉积物中碳酸盐含量的变化可以用沉积物中Ca元素XRF扫描结果进行表示。由于抚仙湖处于亚热带季风气候区,以10月至次年4月降水量极少而蒸发强烈为特点的干季和以5月至9月降水为主的湿季所形成的干湿变化控制了湖泊演化的主要过程,区域气候变化是湖泊演化和水位变化的主要动力。结合湖泊沉积碳酸盐稳定同位素δ^18O和δ^13C、有机质含量及其同位素δ^13C、抚仙湖北部边缘水下侵蚀地形测量、地球物理勘探和沉积地层年代的确定和讨论,明确了抚仙湖沉积碳酸盐含量指示湖泊水位的变化,并重建了抚仙湖过去约12 ka以来水位变化的历史。结果显示,在约12 cal.ka B.P.至2.2 cal.ka B.P.期间的湖泊水位变化主要经历了波动式降低的过程,其中4.37~2.2 cal.ka B.P.期间高CaCO3含量、偏正的碳酸盐δ^18O、δ^13C值指示抚仙湖一度出现低于现代湖面约30 m左右的低水位,可能记录了抚仙湖流域极端的干旱时期,在2.2~2.0 cal.ka B.P.期间抚仙湖水位经历了快速升高的变化事件,期间湖水位快速上升达到现代湖水水位,揭示了印度季风控制区区域降水的特殊性和气候变化的突发性。

The composition of carbonate matters in the sediments from Lake Fuxian and significance of paleoclimate and water level changes

Lake Fuxian(24°21'13"~24°37'57"N, 102°49'14"~102°57'33"E) is one of the deepest inland large lakes in China. In this paper, the carbonate contents, calcite mineral compositions by XRD and element Ca by XRF on the core FXH-B2 drilling from southern center of the lake was analyzed,. The studied core FXH-B2, located at 24°24'57.48"N, 102°52'06.24"E is 296 cm long. Combing with the chronology established by 9^14C dates using AMS^14C dating method, stable isotope analyses on carbonate and organic matters and geophysical data, the contents of carbonate and calcite minerals and element Ca, and their relationships, the significances to reveal water level fluctuations were discussed. Based on these discussions, the water level change history of Lake Fuxian since the last ca. 12 ka was reconstructed. It was calculated that the studied core had a sedimentation rate of 0.24 mm/a. The analysis results show that there is a high correlation between carbonate content and the XRD signal strength of calcite(R^2=0.87) and XRF scanning data of sediment Ca elements(R^2=0.85). The carbonate in the sediments of Lake Fuxian are composed of mainly calcite(the relative content of other carbonate minerals is low or even negligible). The high correlations between carbonate contents, the calcite contents and the relative content of Ca element indicate that in Fuxian Lake sedimentation the Ca element mainly come from calcite. Therefore, their contents were determined by the content of carbonate. Changes in carbonate content in sediments can be expressed by the results of XRF results of Ca elements in sediments. Because Lake Fuxian is located in the subtropical monsoon climate zone, dry and wet oscillations of the climate control the main process of lake evolution process, the lake sedimentary proxies, such as the Ca element, and or carbonate and calcite contents mainly reveal the water level change history in the past. Based on this assumption, the reconstructed lake water level changes during last 12 ka demonstrates a gradual decrease 12.04 cal.ka B. P. to 2.20 cal.ka B. P. During this processes, the high CaCO3 content, more positive δ^18O and δ^13C values reveal a lowest lake water level period between 4.37~2.20 cal.ka B. P., during this period the water level decreased by ca. 30 m, indicating a driest event since 12 ka B. P. This water level decrease is further conformed by the geophysical measurement results in the northeastern of the lake where the underwater human relics was found and the subwater landscape measured erosion features. An abrupt water level rising occurred between 2.2~2.0 cal.ka B. P. and reached the level almost continuous to the present. This abrupt raise of the water level might influence the human activities in the area that need further study.