Processes controlling the Mg isotope behavior during granite weathering

Silicate weathering is a fundamental process for global materials cycling and the climate regulation. Magnesium (Mg) isotope ratios have been widely used as a natural tracer to study weathering processes and biogeochemical cycles in critical zone. The geochemical behavior of Mg isotopes, however, remains unclear during granite weathering. Here, 23 saprolite samples and two bedrocks in one granite profile from Longnan county, Jiangxi province have been analyzed for their Mg isotopic composition, major elemental and REE concentration, to understand Mg isotope behavior and its controlling factors during granite weathering. In general, bulk regolith is enriched in heavier Mg isotope (-0.37%o similar to 0.27%o) compared to the parental granite (-0.42%o) with one exception (-0.53%o). Saprolites have much lower MgO contents ranging from 28% to 49% (normalized to Ti) relative to parental rock. These show that weathering of granite is accompanied by Mg loss and formation of secondary Mgbearing clay minerals, which preferentially adsorb heavy Mg isotope. This is reinforced by the positive relationship between delta 26Mg values and the content of kaolinite and gibbsite during the early weathering stage. Particularly, the soils most enriched in heavy Mg isotope are also characterized by the highest content of gibbsite. While Fe oxides is shown to make a significant influence over the soil delta 26Mg as weathering proceed by adsorbing light Mg isotope. More importantly, our study provides the first evidence that the impact of pedogenic carbonates is significant by shifting delta 26Mg towards lighter values in granite saprolite, based on the covariation of geochemical parameters, such as the CaO content, LOI and REES. This implies that if secondary carbonates are widespread in silicate soils, the contribution of silicate weathering to dissolved Mg fluxes may have been overestimated. And it may be an important C reservoir in terrestrial, which significantly influences the global C cycle.

Automated summary

Silicate weathering is a vital process for global materials cycling and climate regulation. Magnesium (Mg) isotopes are commonly used as a natural tracer to study weathering processes and biogeochemical cycles. However, the behavior of Mg isotopes during granite weathering is still not well understood.