Trace and rare earth elements as the source and transport indicators of different topsoil end-members in the desert peripheral regions of China

Abstract

Solis in grassland regions, located downwind of and adjacent to deserts, display grain-size sorting and weathering features that cause the mineral and chemical elements of their particulate matter to have some characteristics different from those in the source area. This makes it an issue to be explored whether source tracers previously obtained from deserts can be well applied in grassland areas. We collected 40 topsoil samples in the western Inner Mongolia grassland near the Badain Jaran Desert and the Mongolian Gobi and then studied the three grain-size end-members of their <300 μm components. End-member 1 is matched with samples' <30 μm grain-size fractions, end-member 2 and end-member 3 are matched with some samples' 30–300 μm grain-size fractions. By obtaining the trace element and rare earth element characteristics of the two different grain-size fractions, that of the three end- members are obtained. Therefore, the impact of sorting and weathering on different element ratio proxies is discussed from the perspective of end-members. Then element ratio proxies that are more applicable to grasslands on the periphery of deserts are established, where Sm/Gd, Tb/Dy, Nb/Ta, Er/Tm, La/Ce, Sc/V, Yb/Lu, and Ce/Ce* are least affected by sorting and weathering, and they effectively distinguish and identify the sources of each grain-size end-member. Using these proxies, it can be inferred that end-member 1 (mode grain size is 5 μm) in the grassland topsoil comes from a vast area of deserts and Gobi in Mongolian Plateau. End-member 2 (70 μm) and end-member 3 (120 μm) have stronger source affinity with the Badain Jaran Desert and Altay Mountains in the western Mongolian Plateau; and end-member 2 also contains a small amount of material from the local Yinshan Mountains. This result can be further applied to the sediment sequences of the grassland region in the future, which will help to explain the sources of their end-members and the environmental change processes at different time scales implied by end-members.