Zn isotope fractionation during the development of low-humic gleysols from the Mun River Basin, northeast Thailand

Abstract

Zinc (Zn) is an essential micronutrient released into soils during rock weathering, and Zn isotope composition has been emerging as a powerful tracer involved in the Earth’s surface processes. To explore the soil development in dry tropical regions and evaluate the influence on the global Zn cycles, stable Zn isotope data were presented for the two pristine low-humic gleysol profiles from the Mun River Basin (MRB), northeast Thailand. The samples exhibit the δ66Zn (relative to JMC 3-0749L standard) from −0.69‰ to 0.32‰. Therein, the Fe nodule-containing soils exhibit a light shift of δ66Zn relative to parent red siltstones (Δ66Znnodule-containing soil–p of S1 = −0.60‰) and the other soils (Δ66ZnII-other zones = −0.66‰ – −0.50‰), indicating that the preferential retention of light Zn isotopes in residual soils is attributed to the adsorption of heavy Zn isotopes onto the surface of secondary Fe oxides. Due to the water insufficiency in the MRB, the redox conditions to control the oxide precipitation are driven by the seasonal alteration of groundwater levels. With the recession of groundwater, Fe oxides precipitate and adsorb heavy Zn isotopes. Besides, heavy Zn isotopes are also incorporated in clay fraction. On the contrary, the saturated water is likely to disrupt the interactions of Zn with Fe phases and clay minerals. In addition, low content of organic matter and Mn oxides and the absence of sulfides limitedly contribute to the light Zn pool in soils. Conclusively, the isotopic signature of Zn indicates that Fe oxide-rich soils are likely to control the heavier Zn isotope composition of dissolved load in rivers relative to continental rocks in dry tropical regions.