金沙江巧家段河流阶地记录的干热河谷形成时代研究

The middle and lower reaches of Jinsha River have the largest area of Dry-hot Valley landscape in Southwest China. The age of this special natural landscape has not been made clear at present. In the studies of previous natural science foundation, we found that the history of the Dry-hot valley evolution can be partly recorded by strata, but not enough ideal stratigraphical sections meet the research needs. Searching for some records except strata to construct a complete evidence chain would help to comprehensively understand the evolution history of the Dry-hot valley with multi angle view. The river terraces preserved on the both sides of Qiaojia valley of Jinsha River provides an important opportunity for these studies. These terraces distribute from the bottom of Dry-hot Valley to the area above its top, where subtropical forest is the dominating landscape, so the whole evolution history of the Dry-hot Valley landscape could been recorded by these terraces. Our project will dating these terraces with the C-14, optically stimulated luminescence (OSL), Al-26/Be-10, and electron spin resonance(ESR) dating techniques, extract environmental proxies, such as sporo-pollen, clay mineral and elemental geochemistry characteristic from them and then calculate the age of the Jinsha River Dry-hot Valley landscape. The implementation of our project will provide scientific materials to further interpret the evolution history of the Quaternary landscape pattern in Southwest China.

金沙江中下游分布着我国西南地区面积最大的干热河谷。关于这一特殊自然地理景观出现于何时，目前缺少定论。申请者在前一个项目研究中发现，地层记录虽然能够部分解决这一问题，但理想剖面不多。寻找地层以外的其它记录，形成完整的证据链是全面系统认识干热河谷形成历史的基础。金沙江巧家段河谷两侧广泛发育的多级阶地为这方面研究提供了良好契机。这些阶地从干热河谷底部到之上的亚热带森林都有大量分布，完全可以记录干热河谷景观从诞生到发展的整个历史过程。项目拟通过对这些阶地开展系统的年代学（包括：C-14、OSL、Al-26/Be-10和ESR）和环境代用指标（包括：孢粉、黏土矿物、元素地球化学等）研究工作，综合分析确定金沙江干热河谷景观的形成时代，为深入研究中国西南地区第四纪以来景观格局的演变提供重要线索。

金沙江干热河谷景观出现于何时，目前尚无定论。缺少长序列连续的地层记录促使这一问题的解决需要寻找地层以外的线索。项目利用发育在金沙江巧家段的多级阶地从地貌记录的角度开展研究，获得如下重要结果；（1）金沙江巧家段发育至少9级阶地，由老到新分别为1.59Ma、1.29Ma、1.20Ma、0.44Ma、0.29Ma、0.22Ma、0.17Ma、0.08Ma和0.03Ma。（2）利用河谷下切速率外推确定金沙江下切至海拔1300m深处形成孕育干热河谷景观的焚风效应的时代为3.74～2.77Ma。（3）粒度和微形态特征揭示堆积在各级阶地上的黄土状物质为风成成因，属干热河谷景观的相关沉积。（4）最高阶地年龄约束的最老黄土状物质指示巧家段因持续干热化堆积风成沉积物的历史不晚于1.6Ma。（5）综合阶地记录结果，项目认为孕育巧家段干热河谷景观的地形和局地环流格局形成于上新世晚期，而河谷持续干热化形成黄土状物质堆积的历史不晚于早更新世中后期。项目的研究结果首次将金沙江干热河谷景观的形成历史追溯到了上新世，也为西南地区现代地貌景观的形成研究提供了重要资料。