Elemental and strontium isotopic geochemistry of the soil profiles developed on limestone and sandstone in karstic terrain on Yunnan-Guizhou Plateau, China: Implications for chemical weathering and parent materials

The limestone and yellow sandstone soil profiles from SW China were measured for chemical and Sr isotope compositions of the bulk soils and their sequential leachates (labile, carbonate, and residue or silicate fraction), aiming to characterize the parent materials of the soils, to understand the soil weathering and formation processes, and to discuss the origin of the red residua (terra rossa). The studied yellow sandstone soil, yellow limestone soil, and black limestone soil show different pH values, SiO2 contents, Rb/Sr abundance ratios, and 87Sr/86Sr ratios. The sequential leachates of different soil types also have different 87Sr/86Sr and Ca/Sr ratios. The major chemical compositions of the studied soil profiles suggest that all the sandstone and limestone soils are developing at a stage that feldspar is exhausting and the clay minerals are changing from smectite to kaolinite and gibbsite. As compared with the red residua distributed in the karst region, the soils studied here show lower CIA values (58–84), but both higher Na2O/K2O (0.9–2.7) and Na2O/Al2O3 concentration ratios (0.07–0.26) on average, suggesting a lower weathering intensity than that of the red residua. The depth profiles of soil CIA values, Na2O/K2O and Rb/Sr ratios, and 87Sr/86Sr ratios indicate that the weathering intensity is slightly lower for the upper and higher for the deeper soils, which suggest that the sandstone and limestone soil profiles were formed through both accumulation and weathering of in situ weathering residue and input of external detritus or soil from upper land. During weathering of the soils, preferential release of Ca and retention of Sr in soil result in higher Ca/Sr ratios in both labile and carbonate fractions than those in the residue fractions of all soil profiles. The co-variations of Hf/Nb and Zr/Nb ratios, together with those Rb/Sr and 87Sr/86Sr ratios of limestone soils, sandstone soils, and the red residua, demonstrate that their parent materials are distinct, and support the point that the widely distributed red residua is originated from the weathering residua of both carbonate and silicate clastic rocks, and further weathering of the weathering residua resulted in intensive release of Si, Na, Ca and relative enrichment of Al, K and other immobile elements in the red residua.

► A geochemical study has been conducted on the sandstone and limestone soil profiles in SW China.
► The depth profiles of Rb/Sr and 87Sr/86Sr ratios of the two types of soils are different.
► The sequential leachates vary differently in Ca/Sr and 87Sr/86Sr ratio along the soil profiles.
► The soil profiles were not formed from in situ weathering of bed rocks.
► The red residua was the weathering residua of both carbonate and silicate clastic rocks.