Clay Minerals In Loess On The Loess Plateau And Its Environmental Significance

The complete set of "loess-paleosol" sequences on the Loess Plateau, China, as one ofthe three records for paleoenvirenment during the Quaternary period, record the biologic,climatic and environmental changes of the Loess Plateau, even of the Eurasian since2.5Ma B.P.. Soils derived from the Loess Plateau of China are regionally important andexpression of the soil properties along the soil profile may be directly related to climatechanges. Clay minerals are derived from weathering of parent material and/or precipitatedfrom the soil solution. They reflect successive stages of mineralogical evolution dependingon the various environmental conditions that have prevailed during soil formation.Therefore, this study focused on the mineralogy in the Holocene loess-paleosol in theWugong, Luochuan and Ansai sections and the fifth paleosol (S₅) in the Wugong section,together with the specific properties (e.g. grain size, carbonate content and magneticsusceptibility),¹⁴C dating by the accelerator mass spectrometry method (AMS) and fieldobservations to access the clay mineral transformation, in relation to the pedogenesis andthe regional climate changes. The main results are as follows:1) The main stratigraphic subdivisions of the Holocene loess-paleosol in the Wugongsection clearly identified in the field were confirmed by analytical data with single-weldedpaleosol S0(8,500-3,100year B.P.) and a cover loess L0(3,100–0year B.P.) and atransitional loess (L_T)(11,500–8,500year B.P.). They presented a pretty classic picture asa complete episode of the Holocene climate change, corresponding to a glacial-interglacialclimatic fluctuation on the southernmost Loess Plateau. The intensive decalcification,enrichment of clay content and increasing of magnetic susceptibility in the paleosol (S₀,50-150cm) signified the strong pedogenesis with respect to the Malan loess. Thecalculated from the magnetic susceptibility and the depth and the total amount of carbonateleaching suggested that the average annual precipitation during the mid-Holocene was up to800mm. And the climate type in this period was analogous with the modern climateconditions in the northern subtropical zone (the southern slope of Qingling Mountains) forthe formation of Udic Luvisols (Brown Soil).2) Mineralogy of the Holocene loess-paleosol in the Wugong section on thesouthernmost Loess Plateau suggested that clay mineralogical compositions were similar inthe stratums with the illite in the majority throughout the profile. The changes of thephyllosilicate minerals were consistent with the soil genetic horizons and with thevariations in CaCO₃content, particle size distribution, and magnetic susceptibility. Arelatively high vermiculite content and the presence of hydroxyl-interlayered mineral (HIM)occurred in the most weathered palaeosol (S₀). According to the XRD analysis before andafter sodium citrate extraction, the HIM in the paleosol was mainly composed of thehydroxy-interlayered smectite (HIS) and the hydroxy-interlayered vermiculite (HIV). Thehydroxyl-interlayers common present in Alfisol of southern China, occurred in the paleosolsignified the dominant forest landscape palaeoecology and the acidic paleosol environmentduring the mid-Holocene. The contradiction between the type clay minerals present and themeasured alkaline soil pH values in the palaeosol could be understood by therecalcification caused by the post-pedogenic leaching from the overlying loess. A mostlikely explanation for the occurrence of HIM and transformation of phyllosilicate mineralsin the paleosol stratum derived from loess was suggested.3) The relatively high values of half height width (HHW) and integral breath (IB) andthe relatively low values of chemical index of illite (ICI) represent relatively low illitecrystallinity and intensive weathering and pedogenesis in the Holocene paleosol (S0) withrespect to the overlying and underlying loess. However, the values of HHW, IB and ICIshowed no significant variations throughout the profile ranging from0.25to0.30(o2θ),0.4to0.6(o2θ) and0.25to0.40(o2θ) throughout the profile, respectively, whichimplied a weak chemical weathering. The successive adding of eolian sediments withabundant CaCO₃restrained the intensity of mineral weathering and pedogennesis.Generally, the depotassication of illite and the degradation of chlorite were the majormineral transformation processes that occurred with soil-formation of the Holocenepalaeosol on the southernmost Loess Plateau and the pedogenic strength did not reachedthe level of Udic Luvisols (Brown Soil). Due to the special pedogenesis pattern, the palaeoclimate would be warmer and wetter than the climate that was derived from themineral weathering and soil evolution characteristics in paleosol.4) Analysis of the Holocene loess-paleosol at Ansai, Luochuan and Wugong in anorth-south transect and in geological age from the late Epipleistocene (0-23860±90yearB.P.) suggested that the mineralogy were similar over the Loess Plateau, China. However,in detail, there were significant temporal and spatial variations in analytical results ofpedogenesis and mineralogy of the profiles in the three sections along the climate gradient.Mineral weathering and pedogenesis resulted in a vital increase of vermiculite in paleosolof Wugong section with more clayey, intensive decalcification and increasing of magneticsusceptibility. The subtle decrease of kaolinite and smectite in paleosol southward resultedfrom the resilication by strong plant/clay interaction and leaching of soil base ions withlush vegetation and increasing precipitation during the mid-Holocene. The pedogeneticcyclicity and mineral weathering were progressively weakened with the trendingsouth-north across the Loess Plateau signified the weaker climate fluctuation since11,500years and the decreased influence of summer monsoon on pedogenesis and mineralweathering intensity northward. In general Holocene loess in Ansai section preserved inthick eolian deposits was found to be relatively homogeneous as far as grain size and claymineralogy. There was no significantly variation in phyllosilicate composition with depthwhich coincided with more coarse grains and constant CaCO₃content signified the leastweathering and pedogenesis under stable and dry climate and pedogenic environment overthe last11,500years. The mineralogical composition in loess-paleosol across the CLPrecords precise temporal and spatial variability of the palaeoenvironment. The traditionalscheme of the Holocene climatic change is not a unique phenomenon across the LoessPlateau.5) The fifth paleosol (S₅) is a complex paleosol essentially composed of threewell-developed reddish soil pedons (S_5-1, S_5-2, S_5-3). The carbonate of each paleosol pedonswas absolutely leached from the paleosol during the early stage of soil formation underintensive eluviation condition, and deposited beneath the pedons of30-50cm in thicknesswhich can easily be recognized as stratigraphic markers between each pedons. The claycontent and magnetic susceptibility in the S5-1pedon were absolutely higher than those inthe S_5-2and S_5-3pedons which signified more intensive physical and biochemical weathering and pedogenesis in the S_5-1with respect to the S_5-2and S_5-3pedons. The clay/siltratio was higher in the S5than those in the overlying and underlying loess (L5and L6) andclay content in the S5was60-100%higher than those in the L5and L6. Particle sizedistribution illustrated that the intensified clayification in situ after deposition in the S5than that in the Holocene paleosol (S₀). It was possible that the S5with clay content higherthan that in yellow-brown earth is belong to the soil type formed in the subtropical zone.Magnetic susceptibility in the S5varied from120to310SI(10^-8m³kg^-1)was well abovethe turnout of the S0, signified intensive pedogenesis under much warmer and wetterclimatic condition than that during the mid-Holocene.Nevertheless, the results of mineralogy analysis showed the similar composition ofphyllosilicate minerals throughout the complex paleosol (S_5-1, S_5-2, S_5-3). Mineralweathering processes in the S5were depotassication and hydration of primary minerals anddegradation of chlorite. The S5did not shown more intensive mineral weathering than theL5and L6which was incompatible with the results of CaCO₃content, particle sizedistribution and magnetic susceptibility. One of the possible reasons was that the trappingaffection of harder and denser of CaCO₃illuvial horizon and argillic horizon in eachpaleosol pedons on the water percolation which listened for the loss of alkali and alkalineearth elements and restrained the transformation of soil mineral.