Silicate Weathering Rate And Its Controlling Factors:The Study From The Different "Small Watershed Systerms"

Studying the controlling factors in the chemical weathering of silicate minerals is crucial to understanding the regulation of silicate weathering for the global climate,and a small 'mono-lithology' watershed was used as the main medium for this purpose.River water samples were collected 1-2 times/month from small granitic watersheds in the Jiuhua Mountains during one hydrological year to examine the silicate weathering rate and its controlling factors.By using the mass balance method,the calculated contribution of cations from agricultural activities,atmospheric input,silicate weathering and carbonate weathering were found to be 2.3%,18.9%,51.3%and 28.3%,respectively.A disproportionate contribution from rock outcrops containing less carbonate(<5%)does show the importance of trace carbonate weathering in these small granitic watersheds.The average silicate weathering and CO2 consumption rates of ten small granitic watersheds in the Jiuhua Mountains are 9.2 t km-2 y-1 and 3.12×105 mol km-2 y-1,respectively.The apparent activation energy(Ea)value for global granitic watersheds is 46.6 kJ mol-1.The positive effects of temperature and runoff on silicate weathering rate(SWR)is visible from a global perspective through compiling data from granitoid watersheds in global different climatic zones.Binary linear regression analysis for SWR and temperature/runoff has been performed,and the adjusted R-Square is 0.61,showing that climatic factors(temperature and runoff)can explain 61%of the SWR change.Meanwhile,the R-Square value increases significantly when the elevation difference(a relief parameter)is higher than 400 m,indicating that temperature is the first-order controlling factor in alpine areas with a weathering-limited regime.Therefore,it is necessary to distinguish the weathering regime when discussing the climatic factors of silicate weathering rate.With the purpose of studying the effect of lithology on chemical weathering rate,we selected an area near Beihai,Tengchong,Yunnan province as our study site.In this site that covers dozens of km2,there are only two kinds of silicates,granite and andesite and the granite watersheds are entirely consistent with the andesitic watersheds in tectonic events,climatic conditions include temperature,precipitation and runoff,and the development of vegetation.Under this condition,the difference of weathering rate between these two watersheds mainly due to their lithological differences.We collected samples of river water and rain water in July 2017 and calculated out that atmosphere input,silicate weathering and carbonate weathering contributes 10.9%,73.8%,16.4%and 5.4%,79.2%,15.5%of the cation in granite and andesite watersheds respectively,by using the mass balance method..The weathering rate of silicate and carbonate and the rate of CO2 consumption is 5.68 t km-2 y-1,2.68×105 mol km-2 y-1 and 9.96 t km-2 y-1,0.29 ×105 mol km-2 y-1,respectively.Among this,the weathering rate of granite and andesite is 3.26 t km-2 y-1 and 8.14 t km-2 y-1.The weathering rate of andesite is 2.5 times as much as that of the granite.This result reveals the lithology plays the main role when other weathering-influencing factors are consistent.The characteristics of major element of granite regolith of Jiuhua Mountains indicate that Na2O,CaO,and K2O are evidently depleted relative to the bedrock,and the depleted degree is the highest in the middle of the profile.Fe2O3 is remarkably enriched in the profile.This trend is similar with those of chemical index of alteration(CIA)and loss on ignition(LOI),which shows that the Jiuhua Mountains area in the subtropical zone experiences strong chemical weathering and show the soil-forming process of desilicating and ferrallitic weathering.The ratio of 87Sr/86Sr of weathering regolith,in generally,has an increasing tendency with the weathering age.From the top to bottom of the weathering regolith,the weathering of K-feldspar and biotite,plagioclase and trace carbonate,the leaching materials from the upper strata dominate the 87Sr/86Sr ratio of the granite regolith,respectively.