Magnesium isotope behaviors in Jinsha River Basin, Southeast Tibetan Plateau
Abstract
Capability of Mg isotopes for constraining dissolution and precipitation reactions during chemical weathering has been well documented. Riverine materials reveal weathering process information on basin and continent scale. However, mechanisms of dissolution and precipitation influence on the riverine Mg isotope composition under different climatic and geology background are not well constrained. δ26Mg values for solute and suspended sediments in Jinsha River Basin were examined here. Dissolved δ26Mg values ranges from -1.67‰ to -0.5‰, and the suspended loads display systematically heavier δ26Mg (-1.15‰ to -0.06‰) relative to dissolved loads. Mass balance and mixing equations indicate that Mg isotope ratios of Jinsha River waters cannot be fully explained with conservative mixing of different rock type end-members only. The strong correlation between Mg concentrations and δ26Mg values in carbonate (calcite and dolomite) oversaturated waters suggests the precipitation of secondary carbonates are the principal mechanism driving the solute Mg isotope composition heavier. During the formation of carbonate minerals, river water pH shows a negatively relationship with δ26Mg values (R2=0.85). Carbonate precipitation affected δ26Mg values of river waters significantly ( 1‰) while its impact on suspended loads was smaller ( 0.5‰). The Mg concentrations and δ26Mg values for the carbonate undersaturated waters are intermediate between those of silicate dominated basin and carbonate oversaturated waters. The results suggest two different mechanisms controlling river solute δ26Mg: fractionation during carbonate precipitation incorporates Mg for carbonate oversaturated waters; and conservative mixing between a new end member created by the former process and rock weathering end members for carbonate undersaturated waters. This study provides new insights in Mg isotope fractionation mechanisms and applications in constraining river water chemistry evolution.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.H11A1147X
- Keywords:
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- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1065 Major and trace element geochemistry;
- GEOCHEMISTRY;
- 1813 Eco-hydrology;
- HYDROLOGY;
- 1832 Groundwater transport;
- HYDROLOGY