六盘山朝那湫湖泊沉积物Rb/Sr比值及其古气候意义

湖泊沉积物Rb/Sr比值通常被视为流域化学风化强度的代用指标, 然而不同湖泊沉积物Rb/Sr比值的古气候意义有时也存在一定差异。本研究以六盘山朝那湫湖泊73cm长度(1743~2012A.D.的沉积序列)岩芯沉积物Rb/Sr比值为分析对象, 结合同一岩芯其他主要元素以及古气候代用指标(TOC、C/N比值、δ¹³C_org等)指示的环境变化信息, 对朝那湫湖泊沉积物Rb/Sr比值在十年/数十年尺度上的古气候意义进行综合判别。研究结果显示: 朝那湫湖泊沉积物中的Rb、Sr元素主要以残渣态赋存, Rb/Sr比值与其他陆源组分元素有较好的相关性, Rb/Sr比值变化主要受控于陆源碎屑物质输入。Rb/Sr比值与同一岩芯TOC、C/N比值、δ¹³C_org等降水代用指标以及邻近区域树轮等古气候记录的对比结果表明: 在十年/数十年尺度上, 气候偏干期, 陆源碎屑物质输入较多, 沉积物Rb/Sr比值减小; 气候偏湿期, 陆源碎屑物质输入较少, 沉积物Rb/Sr比值增大。这主要是由朝那湫流域降水特点和植被覆盖共同决定的, 同时也暗示朝那湫流域物理风化和化学风化强度可能呈反向变化。因此, 在将Rb/Sr比值作为流域化学风化强度的代用指标之前, 有必要对湖泊沉积物的Rb、Sr元素的赋存形态进行辨别, 并结合湖泊自身沉积环境以及其他古气候指标进行综合分析, 方可获得更加全面准确的流域物理侵蚀搬运或化学风化信息。

The Rb/Sr ratios of the bulk sediments in Lake Chaonaqiu of Liupan Mountains,the western Chinese Loess Plateau and its paleoclimate implications

The Rb/Sr ratios in lacustrine sediments have been used to indicate the intensity of chemical weathering in many works, namely, lower Rb/Sr ratios reflect stronger chemical weathering, while higher Rb/Sr ratios indicate weaker chemical weathering. However, the geochemistry of Rb/Sr ratios in lacustrine sediment reveal a complex spatial pattern in different lakes due to site-specific conditions, such as the local climates, the chemical/physical property of the bedrock, vegetation cover, and human activities, and other factors. Here, the paleoclimate implications of Rb/Sr ratios on decadal timescales of bulk sediments in Lake Chaonaqiu were discussed, based on the characteristics, the geographical location and the significances of multiple proxy indices from lake sediments of Lake Chaonaqiu. Lake Chaonaqiu, a small open alpine lake in the Liupan Mountains(lake area is ca. 0.02km2 and the catchment area is ca. 0.2km2, altitude in 2430m a. s. l.), is located on the western Loess Plateau(LP), China. A 73-cm long sediment core(CNQ12-1), the sedimentary sequence during the period 1743~2012A.D., was obtained in the centre of the lake(35°15'53.08'N, 106°18'35.99'E)using a 60-mm UWTTEC gravity corer, in September 2012. A total of 37 samples were collected at 2cm intervals for traditional XRF analysis. The Rb/Sr ratios of the bulk sediments were negatively correlated with Sr(Pearson correlation coefficients r=-0.703)and positively correlated with Rb(Pearson correlation coefficients r=0.623). The Rb/Sr ratios are therefore predominantly controlled by Sr activity within the Lake Chaonaqiu catchment, and Sr likely enriched in Albite. The first principal component(PC1)accounted for 44.372 % of the total variance and therefore predominantly contributed toward the variability in the XRF elemental dataset. PC1 had a strong positive loading from terrigenous fractions elements(Si, Ti, K, Al, Rb, Sr, and Fe). We find that the Rb/Sr ratios of the bulk sediments correlates well with the terrigenous fractions, these suggest that Rb/Sr ratios of the bulk sediments are closely related to terrigenous input from the catchment. Based on the comparison of Rb/Sr ratios with paleoclimate proxies(TOC, C/N ratios and δ13Corg)of core CNQ12-1 and the records of tree rings and historical documents in neighboring regions, we considered that the densely vegetation cover in the catchment during wetter period(when precipitation increased)not only stabilizes the surface soil, but also weakens the load capacity of surface runoff, less terrigenous detritus would come into the lake, resulting in higher Rb/Sr ratios of the bulk sediments. On the contrary, the sparsely vegetation cover during drier periods(when precipitation decreased)exposes more soil surface area, and produces a less cohesive soil, even relatively low-intensity rainfall could create a stronger surface runoff load capacity, more terrigenous detritus would enter into the lake, resulting in lower Rb/Sr ratios of the bulk sediments. The result of this study suggests that physical weathering and chemical weathering in Lake Chaonaqiu catchment have opposite influence on Rb/Sr ratios of the bulk sediments, and they compete in dominating the Rb/Sr ratios of lake sediments on decadal scales. Therefore, it is necessary to study the geochemistry of Rb/Sr ratio of lake sediments(especially that on short term timescales)particularly before it is used as an indicator of weathering intensity of the catchment.