| Calcium and magnesium are critical elements in the global carbon cycle,due to their roles in carbon sequestration via silicate weathering and carbonate formation.Here we apply calcium(δ44/40Ca),magnesium(δ26Mg)and strontium(87Sr/86Sr)isotopes to explore such chemical weathering processes in river systems draining a diverse range of geologic and climatic environments,specifically the Jinsha and the Yalong River.The two rivers originate on the Tibetan Plateau and represents the upper reaches of the Changjiang River,China.The results show that the water chemistry of the Jinsha mainstream is dominated by evaporite weathering,which has a low 87Sr/86Sr（0.7098 to 0.7108）and a wide range of Sr contents（2.70 to 9.35μmol/L）.In contrast,tributaries of the Jinsha River have higher 87Sr/86Sr（0.7090 to 0.7157）and lower Sr contents（～1μmol/L）.Moreover,the Ca isotopic compositions in the Jinsha mainstream（0.87-1.11‰）（relative to the NIST standard SRM 915a）are higher than the tributaries（0.68-0.88‰）and could not be attributed to the conventional mixing of different sources.We suggest that secondary carbonate precipitation fractionates Ca isotopes in the Jinsha River,and the best fit fractionation factor value is 0.9998 for Rayleigh fractionation model and 0.9996 for equilibrium fractionation model.At least 71%of dissolved Ca is removed from the mainstream of the Jinsha River through secondary mineral precipitation,which is higher than the Ca removal in the tributaries.The results highlight that evaporite weathering results in more carbonate precipitation influencing Ca transportation and cycling in the riverine system.The Ca isotopic composition of the dissolved load of the Yalong River ranges from0.60‰to 1.02‰.Higherδ44/40Ca values were found in the plateau and lowland rivers,with lower values in the mountainous rivers.Correlations between riverine dissolvedδ44/40Ca values,Sr/Ca ratios,and carbonate saturation index indicate that the precipitation of secondary carbonates governs the Ca isotopic composition and carbon transformation in most of this river system.However,such correlations are not seen in the lowland tributaries,where the relationship betweenδ44/40Ca and lithium（Li）isotopes instead suggests a control by topography and climate,via secondary clay mineral formation.Specifically,heavy rainfall in the lowland regions lowers the p H of the soil solution,which inhibits the precipitation of secondary carbonate.In addition,the flat terrain and thick soils increase the time for water-rock interaction,which favours the formation of secondary clay minerals that preferentially incorporate the lighter Ca isotopes.Overall,this study highlights the potential of stable Ca isotopes,when used in combination with Sr and other isotope systems（e.g.Li isotopes）,to quantify secondary mineral formation processes and to trace the rates and intensity of silicate weathering in large river basins.To access how chemical weathering processes respond to climatic parameters,we studied temporal variation in stable calcium(δ44/40Ca),magnesium(δ26Mg)and radiogenic strontium(87Sr/86Sr)isotopes in the Jinsha and Yalong river waters for one hydrological year.Based on variations in elemental concentration,chemical weathering rates,and isotope values along with the river discharge,three periods can be marked out.The low runoff period（Q<2000m3/s）,the medium runoff period（2000 m3/s<Q<4000 m3/s）,and the high runoff period（Q>4000 m3/s）.The results show that during both low and high runoff periods,the evolution in riverine chemical compositions is dominated by secondary processes rather than initial rock dissolution.During the low runoff period,meltwater flushes out soil solution,which preserve the isotopic（Sr,Ca,and Mg）signatures of recharge meltwaters and fractionation signal of Ca and Mg from precipitation of secondary minerals,regulating the chemical composition of the rivers.When storms arrive（Q>4000 m3/s）,rapid overland flow transfers large amounts of soil into the river,causing an increase in the suspended content of the river water.The interaction between particles（including suspended matter,riverbed sediments）and river water is enhanced.During this time,the Ca,Sr isotopic evolutions in river waters are attributed to secondary minerals dissolution and particle-water cation exchange,which also lead to the overestimation of silicate weathering rates at a catchment scale.Only during the medium runoff period did primary rock dissolution governs the chemical composition in the river waters.At this stage,the 87Sr/86Sr,δ44/40Ca andδ26Mg values in the Jinsha river waters are similar to that of headwater,which was dominated by evaporite dissolution,whereas the Yalong shows greater carbonate contribution than silicate. |
