THE APPLICABILITY OF CHEMICAL INDEX OF ALTERATION(CIA)TO REFLECT WEATHERING INTENSITY OF HUMID SUBTROPICAL WEATHERING CRUST SOIL
Weathering crust-soil profile is an important carrier to record the long-term information of the"Earth Critical Zone".However,there are limited studies on the in-situ weathering of a wide variety of weathering crust,and it is debatable whether chemical weathering indexes can accurately reflect weathering intensity.In this paper,six granite weathering crust soil profiles in Guangdong Province's humid subtropical climate area(22.152°~23.579° N,111.787°~115.853° E)were used as the research object,which are Conghua section(CH:23.579°N,113.538°E),Xinxing section(XX:22.647° N,112.206°E),Huidong section(HD:23.020°N,114.679°E),Jiexi section(JX:23.413°N,115.853°E),Boluo section(BL:23.517° N,114.474°E),and Yangchun section(YC:22.152°N,111.787°E).A total of 265 samples were collected with 10cm as the sampling interval,and individual soil layers with larger thickness were separated by 20 cm.The thickness of the collected profiles is between 3.5 m and 5.5 m.Meteorological data for each profile are from the nearest weather station and are averaged over a 30-year period(1981~2010).The major geochemical element and grain size were measured and analyzed to describe the chemical weathering and physical changes of the profiles,and discuss the applicability of CIA values in reflect weathering intensity.The results demonstrated that the contents of Fe,Al,and Ti,which had a negligible effect on migration in the temperate zone,decreased in the upper layer of the subtropical weathering profile.With the increase of regional precipitation,the value of CIA increased from the top to the bottom of the weathering profile,which was contrary to the ideal weathering trend that the top-down weathering intensity of the weathering crust profile decreased.Additionally,there was a good negative relationship between the CIA and climate variables,particularly precipitation(R2=0.77,P<0.01,y=-0.0065x+104.01).Through the discussion of this abnormal phenomenon,the following understandings were obtained:in humid subtropical areas,the surface soil of weathering crust had complex physical and chemical changes,especially the physical erosion caused by high precipitation makes the chemically inactive Fe,Al,Ti oxides and clay particles carried away by surface runoff or infiltrated into deeper soil,resulting in low CIA value of the surface soil,which cannot truly reflect its chemical weathering intensity.Therefore,The CIA index should be utilized carefully to reflect the weathering intensity of humid subtropical region with high precipitation.
major geochemical elementChemical Index of Alterationchemical weathering intensityweathering crust soilhumid subtropical zone