Time dependence of the rate and direction of mineral weathering and clay mineral formation with special consideration to kaolinites
This paper summarizes mineralogical results from temperate, subtropical and tropical soil chrono- and climosequences and aims to understand the role of the factor time for the formation of kaolinites. A mid-Pleistocene welded F6-paleosol at Stari Slankamen, Serbia, developed discontinuously in a time span of 140 ka, shows much greater pedochemical weathering and clay mineral formation than Holocene loess soils: more than 40% of the feldspars and almost 80% of the micas are decomposed; mainly smectites dominate the fine clay fraction followed by illites, but no kaolinites have formed. In the Atlantic coastal region of Morocco during the first 100 ka only weathering of calcarenites to Rendzinas (Typic Calcixerolls) has taken place with little formation of 2:1 clay minerals. In a time span of several 100 ka, however, the direction of weathering goes towards strong kaolinite formation. Six “red soils” (Typic Dystrudepts to a Typic Hapludalf) in hyperhermic SW Nepal with 1,500–1,750 mm annual rainfall (five humid months), and a “black soil” (Vertic Haplustoll) near Borada, Gujarat, India with 930 mm annual rainfall were studied. Two of the “Red Soils” have thermoluminescence (TL) ages between 10 and 30 ka, the “Black Soil” has an age of about 10 ka. Surprisingly, little pedogenic clay mineral formation could be identified. The illites and kaolinites are mostly of detrital (inherited) origin. Only in the Vertic Haplustoll, a small increase mainly of smectites but no kaolinites were found, although the content of weatherable minerals is large. In South India, nine soils in a climatic sequence derived from saprolite of weathered granitic gneiss were examined for recent and relict features. Above a threshold of about 2,000 mm in an Udic Rhodustalf deep weathering is a recent process leading to the formation of kaolinites; above 2,500 mm in a Typic Rhodudult it leads also to gibbsite. In Aridic Rhodustalfs (three to one humid months), besides the formation of 2:1 clay minerals, still strong formation of kaolinites has taken place. However, this process has now almost ceased; instead, secondary carbonate is accumulating in the lower part of the profile proving that these Alfisols are relict soils, formed in an earlier period of much moister climate. However, according to our recent results from Morocco and Nepal besides a climatic change, the soil forming factor time is very important: strong pedogenic formation of kaolinites also in the seasonal tropics needs a longer time, probably some 100 ka.