The response of Sr isotopic and elemental compositions of Zipingpu Reservoir sediments to the 2008 Wenchuan earthquake

Abstract

2008年5月12日里氏7.9级汶川大地震引发了超过56000多个山体滑坡，在龙门山地区产生巨量的碎屑物质，极大地影响了流域的地表过程.然而，地震对河流碎屑物质组成及沉积过程的改变却知之甚少.紫坪铺水库超高沉积速率（平均74 cm/a）的沉积物完整记录了汶川地震前后的沉积过程，为研究2008年汶川地震对流域地表物质搬运及沉积物组成的影响提供了宝贵的素材.本研究测试了汶川地震前后紫坪铺水库沉积物的Sr同位素以及元素组成，结果表明其在地震之后均发生了显著变化.震后沉积物⁸⁷Sr/⁸⁶Sr与Rb/Sr比值同步降低，并且和沉积物化学蚀变指数（CIA）呈正相关性，反映了地震滑坡导致的新鲜物质对沉积物的贡献，特别是地震后强径流的2010—2011年.地震前后沉积物组成的差异性，为利用沉积物反演水文气候和构造事件及其环境效应提供了基础和潜在指标.;The 2008 M_w 7.9 Wenchuan earthquake triggered more than 56000 landslides and induced huge clastic sediment along the Longmen Shan orogen, which had impacted earth surface processes within the catchment. However, little is known about the changes in riverine sedimentary compositions and depositional processes associated with the earthquake. Characterized by high sedimentation rate (averaging 74 cm/a), the continuous sediment in the Zipingpu Reservoir, the lower reaches of the Minjiang River provides valuable records of sediment transfer and compositions before and after the earthquake. In this paper, the strontium isotopic ratio (⁸⁷Sr/⁸⁶Sr) and elemental concentrations of reservoir sediments were measured. The results showed that there are significant differences in both ⁸⁷Sr/⁸⁶Sr ratios and elemental compositions before and after the earthquake. Both ⁸⁷Sr/⁸⁶Sr and Rb/Sr ratios decreased after the earthquake, with a positive correlation with CIA, implying increased contribution of fresh debris to the river sediments by the earthquake-triggered landslides, in particular during 2010 and 2011 when there had intense monsoonal rainfall. The difference of sedimentary compositions before and after the earthquake provides a basis of potential tracers for reconstructing hydroclimatic and tectonic events and their environmental impacts using sedimentary records.