『Abstract
Soils derived from the Loess Plateau of China are regionally
important and expression of the soil properties along the soil
profile may be directly related to climate changes. The objective
of this research was to analyze the clay mineral transformation
of loess from the southernmost Loess Plateau, in relation to the
pedogenesis and the regional climate changes. The mineralogy of
the soil profile at the site was studied by using X-ray diffraction.
The results showed that 2:1 type minerals dominated with the illite
the majority throughout the profile. The changes of the soil minerals
were consistent with the soil genetic horizons and with the variable
CaCO3 content, particle size distribution,
and variations of magnetic susceptibility. A relatively high vermiculite
content and the presence of hydroxyl-interlayered mineral (HIM)
occurred in the most weathered horizon (50-150 cm) corresponding
to the buried palaeosol (So) formed during
the ‘optimum’ Holocene (8400-3100 B.P.yr) which indicated the
acidic conditions with low organic matter in this period. During
the formation of palaeosol, the expected transformation of illite
and chlorite into vermiculite through the formation of mixed layers
has occurred. The distribution of kaolinite was uniform with depth
in the palaeosol suggesting inheritance from the original eolian
deposition materials. Generally, the depotassication of illite
and the degradation of chlorite were the major mineral transformation
processes that occurred with soil-formation. Calculation of the
total amount of carbonate leaching of the palaeosol suggested
that the annual precipitation during the ‘optimum’ Holocene was
probably approximately to 880 mm. The regional climate conditions
in the northern subtropical zone (the southern slope of Qingling
Mountains) for the formation of Udic Luvisols (Brown Soil). However,
as a result of its specific pedogenic pattern, the pedogenic strength
of the palaeosol did not reached the level of Udic Luvisols (Brown
Soil). The hydroxyl-interlayered mineral presence in the 50-150
cm horizon only, also illuminated that the palaeosol was buried
rapidly by post depositional loess during the abrupt cold-dry
climate in the late Holocene. The contradiction between the type
clay minerals present and the measured alkaline soil pH values
in the palaeosol could be understood by the recalcification caused
by the post-pedogenic leaching from the overlying loess.
Keywords: Clay mineral; X-ray diffraction; Mineral evolution;
Pedogenesis; Regional climate』
1. Introduction
2. Materials and methods
2.1. Site descriptions and soil sampling
2.2. Analytical methods
2.2.1. Soil physicochemical properties}
2.2.2. Soil mineralogy
3. Results
3.1. Soil chemical and physical characteristics
3.2. Composition of phyllosilicate mineral
3.3. Illite weathering
4. Discussion
5. Conclusions
Acknowledgements
References