# Temporal Variations of Sediment Provenance in a Karst Watershed, China
## 1 Introduction  
In Southwest China, large-scale carbonates are distributed over a total area of 550,000 km\({}^{2}\). Because carbonate rock is highly soluble, it commonly leads to the formation of karst areas. In most karst areas of Southwest China, the soil has been severely eroded, exposing the bedrock, resulting in _rocky desertification_. Studies suggest that karst areas suffer from many problems, such as soil erosion, frequent disasters, ecological system degradation, and land productivity decline [1; 2; 3]. During the weathering process, carbonates exposed in a surface karst area are easily broken down, and most toxic metals in the carbonates will be adsorbed by, and concentrated in, secondary clay minerals, resulting in elevated toxic metals in the surface karst area [4; 5; 6]. The low-temperature metallogenic domain of Southwest China, which forms a production base for metal minerals, is mostly located within a karst area. Extensive mining, disordered tailings, sewage, and waste gas emissions may introduce toxic metals into soil, water, and other environmental medium [7]. Therefore, karst areas are at risk of serious toxic metal pollution, due to multiple factors. Toxic metal pollution is not only irreversible, but also does not degrade over time, seriously endangering human health [8; 9]. The unique geological setting of karst areas commonly results in toxic metal pollution being spread over a wide area. As such, it is difficult to manage and can result in great harm to inhabitants of the area [10]. In order to continuously monitor changes in the geochemical environment and elemental spatial distributions in China, three geochemical baseline projects were implemented in 1992-1996, 2008-2012, and 2015-2019, respectively. From 1992 to 1996, the _Environmental Geochemical Monitoring Networks and Dynamic Geochemical Maps (EGMON)_ project was implemented, 845 topsoil samples were collected, and 39 geochemical indexes were analyzed [11; 12]. In 2008-2012, the first round of _China's Geochemical Baselines project (CGB1)_ was conducted, and a nationwide geochemical baseline network was established with a total of 3382 surface samples collected in the catchment area. In 2015-2019, the second round of _China's Geochemical Baselines project (CGB2)_ was completed with 1589 representative surface samples collected. A total of 81 geochemical indexes were analyzed in CGB1 and CGB2 [13]. Accordingly, through the successful implementation of these three projects, China has realized the dynamic monitoring of national scale environmental changes to surface catchment sediments. Catchment sediments may originate from multiple sources, reflecting changes in natural sources and anthropogenic activities. Studies on the provenance of fine-grained sediments began in the 1970s [14]. An unmixing model was introduced to quantitatively calculate the contributions of different sources. Many physical, chemical, and biological properties of sediments have been used as tracers, including color and grain size, clay mineral characteristics, the characteristics of magnetic minerals, geochemical indicators, radioisotopes, cosmogenic radionuclides, and soil enzymes and pollens [15; 16; 17]. Geochemical tracing has been used to determine the source and evolution of elements, minerals, and rocks in geological and geochemical processes, and is an important means of exploring material sources and revealing geochemical processes. Elemental geochemical tracing is based on the varying proportions and properties of elements in different geological bodies [18]. In the process of rock weathering, alkali and alkaline earth metal compounds that are more reactive, such as K\({}_{2}\)O and Na\({}_{2}\)O, are easily released to the dissolved phase, while other elements or oxides that are more stable such as SiO\({}_{2}\), TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\), and Fe\({}_{2}\)O\({}_{3}\), the high field strength elements Nb, Ta, Zr, Hf, and rare earth elements, behave conservatively.