Chemical variations of loess from the Chinese Loess Plateau and its implications

The Chinese Loess Plateau (CLP) is the largest loess deposit on Earth with expansive surface-exposed source rocks of varying origins, ages, and history. Here, we present abundances of elements on representative loess and palaeosol samples from seven classic sections of the CLP. Most elements, including soluble elements (e.g. Rb and Cs), show significant correlations with La or Al₂O₃. These correlations indicate that these elements are hosted or absorbed in particle minerals during weathering, transport, and deposition (e.g. mica, K-feldspar, and clay minerals). These new observations allow the use of La/X (‘X’ being the element of interest) and the estimated La abundance of 31 ppm in the model upper continental crust (UCC) to estimate the abundances of other elements. The results show higher Cs (Cs = 6.7 ± 1.2 ppm), lower transition metals, Ba, and Ga. Given the high CaO and presence of carbonate in UCC rocks of both vast western China (the primary source for the CLP) and eastern China, we propose that these updates on the element abundances represent a refined model for the carbonate-bearing UCC.

中国黄土高原（Chinese Loess Plateau, CLP）是全球规模最大的黄土沉积区，分布有大量成因、年代与演化历史各异的地表出露源岩。本研究针对中国黄土高原7个经典剖面的代表性黄土与古土壤（palaeosol）样品，测定了其中多种元素的丰度。绝大多数元素（包括铷（Rb）、铯（Cs）等可溶性元素）均与镧（La）或三氧化二铝（Al₂O₃）存在显著相关性。该相关性表明，在风化、搬运与沉积过程中，这些元素赋存或吸附于云母、钾长石（K-feldspar）以及黏土矿物等颗粒矿物之中。基于上述新发现，本研究可通过镧与目标元素的比值（La/X，其中X为待测元素），结合模型上地壳（upper continental crust, UCC）中估算得到的镧含量（31 ppm），推算其余元素的丰度。推算结果显示，铯（Cs）含量偏高（Cs=6.7±1.2 ppm），过渡金属、钡（Ba）以及镓（Ga）的含量则偏低。考虑到作为中国黄土高原主要物源区的中国西部广袤区域，以及中国东部的上地壳岩石均含有高氧化钙（CaO）与碳酸盐矿物，本研究提出的元素丰度修正结果，可作为含碳酸盐上地壳的精细化模型。