Stable hydrogen and oxygen isotopes in mineral-bound water and the indication for chemical weathering intensity

• Different types of mineral-bound water from a weathering profile are discriminated.
• Different mineral-bound waters have variable H-O isotopic compositions.
• Hydrogen and oxygen isotopes of constitution water indicate weathering intensity.
• The isotopic tracing implication of different mineral-bound waters is discussed.

Chemical weathering plays a key role in the long-term climate change and global biogeochemical cycle. In this study, a 5.1 m-long basalt-developed weathering profile from Hunan Province, China, was selected for the analyses of major elements, grain size, pH, total organic carbon, thermogravimetric (TG) and differential thermogravimetric properties (DTG), as well as stable isotopes (δD and δ18O) in mineral-bound waters. The major purpose is to distinguish different types of mineral-bound water and to verify whether hydrogen and oxygen isotopes of mineral-bound water can indicate chemical weathering intensity. Based on the TG and DTG results, different types of mineral-bound water were extracted by a sequential heating extraction technique (40–120 °C, 120–300 °C, 300–600 °C and 600–900 °C). The mineral-bound water lost below 120 °C is absorbed water, a mixture of interlayer and crystal waters occurs at 120–300 °C, and above 300 °C is mostly constitution water. Hydrogen and oxygen isotopic ratios at top ~ 2 m are subject to the influences of eolian and pedogenic processes. The absorbed water has been exchanged and reached isotopic equilibrium with ambient soil water, while the crystal water and interlayer water cannot be separated by heating method in this study. Hydrogen and oxygen isotopes of constitution water can indicate chemical weathering, showing positive relationships with weathering intensity. Although the mechanism for their indication to chemical weathering remains further investigation, we suggest that the isotopic fractionation between the formation of clay minerals and surface water should be taken into account. This research sheds new light on mineral-bound water phase and a potential proxy for tracing the process of chemical weathering.