Weathering Of Clay Minerals From Grain To Watershed In The Critical Zones ——A Case Study Of Chlorite Weathering

Weathering is a process that keeps rocks and minerals in equilibrium with the environment of earth surface,and it shapes the structure of the earth critical zone and influences the material circulation in the geological cycle.The weathering of clay minerals,layered silicates that account for nearly half of the total volume of sedimentary rocks and regolith,strongly affects physicochemical properties of soils in the critical zone,buffers acids in the atmosphere and biosphere,provides nutrients for the plant growth,and influences the global carbon cycle.This paper studies the cross-scale clay weathering through field researches and laboratory experiments,and the focus of this study is on chlorite which is a group of clay mineral easily to be weathered.The field site is located at the Susquehanna Shale Hills Critical Zone Observatory within the Shaver's Creek watershed in central Appalachia of the United States.By studying the geochemical,mineralogical,and microscopic characteristics of samples from seven boreholes at different geomorphic locations at the Shale Hills subcatchment,the transformation pathway and mechanism of chlorite weathering were studied at grain scale,clast scale and watershed scale.The coupling of chlorite and pyrite weathering with hydrogeochemical conditions was explored by monitoring the surface water and groundwater and borehole logging.The study area was then expanded to three subcatchments within the Shaver's Creek watershed(Shale Hills,SH,shales;Cole Farm,CF,calcareous Rock;Garner Run,GR,quartzite)to study clay weathering cross catchments through geochemical,mineralogical and geomorphological methods.Laboratory mixed-flow reaction experiments,reactive transport modeling(RTM)and literature data discussion were carried out to explore the influences of internal factors(type,structure and surface roughness of chlorite)and external factors(temperature and p H)on chlorite dissolution kinetics.The achievements and conclusions are summarized as follows:(1)Chlorite oxidation always occurs earlier than or at the same time with chlorite dissolution at grain,clast and watershed scales.The initial oxidation depths of pyrite and chlorite are similar and close to the regional water table under the ridge.The oxidation depth of pyrite is deeper than that of chlorite under the valley,and the oxidation zone is wider than that under the ridge.At clast scale,chlorite and pyrite are always preferentially oxidized in rock fragments with high fracture densities or high porosities.At grain scale,chlorite from the same depth can be differentially oxidized.(2)Results from field study and crystal diffraction simulation show that chlorite weathering at Shale Hills experienced four sequential transformation reactions:chlorite=>partially oxidized chlorite=>hydroxy interlayered vermiculite=>vermiculite=>goethite+kaolinite+solution.The reaction mechanism of chlorite vermiculitization is the dehydroxylation and dissolution of interlayers to reduce the interlayer filling degree,and finally only exchangeable cations and water molecules remain in interlayers.(3)Differences in shapes and widths of reaction fronts under the ridge(i.e.,recharge area)and valley(i.e.,discharge area)of Shale Hills result from different hydrogeochemical conditions at two sites:under the ridge,flow with high concentrations of dissolved oxygen infiltrates along the vertical direction(1D),generating narrow and sharp chlorite and pyrite oxidation fronts perpendicular to this direction;under the valley,the dissolved oxygen in the flow is mostly consumed and the water flows out in 3D(up,down,and laterally out),resulting in a wide oxidation zone.The rate-limiting step of chlorite oxidation is transport of oxygen and the rate-limiting step of dissolution reaction is kinetics.There is positive feedback between chlorite oxidation and dissolution.The weathering of chlorite,pyrite and carbonate rocks also influences and promotes one other.(4)All the three subcatchments in Shaver's Creek have the same initial clay mineral suite(chlorite and illite),but these clays weather to different stages in the soil.GR has the lowest denudation rate and the longest soil residence time,so the clay mineral weathering degree there is the largest.The soil residence times of CF and SH are similar,but the dissolution rates of chlorite and illite are slowed down by significant carbonate dissolution in CF,thus the weathering degree of clay minerals in CF is less than that of SH.Different critical zone structures lead to different clay distribution patterns in soil and bedrock in three catchments.(5)Results of mixed flow experiments show that the dissolution rate constant of Mg-rich chlorite is 10^-10.51 mol/m~2/s,the reaction order is 0.32 and the activation energy is 42.03 k J/mol at 25?under the acidic condition.Low p H promotes preferentially release of Fe from chlorite interlayers,while high temperature can overcome reactivity difference between TOT and interlayers,thus promoting consistent dissolution.RTM simulation shows that dissolution rate and saturation index of chlorite decrease rapidly and nonlinearly with time at low p H and high specific surface area.Comparative study of literature data shows that,at high temperature,internal and external factors including experimental conditions and chlorite type have little influence on chlorite dissolution rate.However,the dissolution rate of Fe-rich chlorite is faster than that of Mg-rich chlorite at the more common temperature of 25?,and the rate constant of the same type of chlorite correlates positively and linearly with surface roughness.(6)Clay weathering at different scales is intrinsically related with one another.The transformation of mineral dissolution at molecular scale is mainly the coordination and redox changes.Mineral weathering rate at grain scale is a macroscopic representation of dissolution rates of numerous molecules,and is related to the distribution of reactive sites on the particle surface.At clast scale,mineral oxidation is controlled by both oxygen diffusion and fracture development.At borehole scale,factors including flow volume,flow direction and dissolved oxygen concentration all affect the shape and width of the reaction front.At watershed scale,clay weathering can be affected by lithology,geomorphology,denudation,critical zone structure and its evolution.Different clay dissolution rates between laboratory and field are mainly due to the different calculation methods of reactive surface area.The laboratory dissolution rate can be extrapolated to obtain the field weathering rate by fractal method or characteristic factor correction method.Results of this study emphasize that Fe(II)oxidation may be a rate-limiting step of weathering of a large number of Fe(II)rich clay minerals in the earth crust,which is controlled by the concentration of oxygen and the rate it is delivered to the deep by the groundwater under different landscapes.On a larger spatial and time scale,lithology can control both chemical weathering and physical erosion to influence the regional denudation rate,thereby determining soil residence time and controlling the weathering degree of clay minerals.