Genesis of calcic concentrations in Arguidolls of the Argentinian Pampa

摘要

The characteristic morphology of pedogenic calcic accumulations in the Argentinian Arguidolls were studied in the southern east part of the Buenos Aires province; their distribution down the soil profile and mechanism of their formation were examined.In the monolithic samples the carbonate collomorphic (cryptocrystallic) films and carbonate tubes were examined by scanning electronic microscope observations. The origin of these carbonate accumulations is chemical. Spheroid gypsum concentrations (framboids) located on the boundary between the A humus and Btl illuvial horizons are most likely biogenic. Two ways of formation for those spheroid gypsum concentrations are considered: accumulation of crystal "crusts" around less soluble mineral (probably calcite) by spherical cyanobacterial colonies or the spherical form of concentrations has been inherited from framboid pyrite concentrations from sea sediment which decayed by sulfate-reducing bacteria. It is suggested that gypsum cyanobacterial concentrations have seasonal dynamics of formation in the soil profile. They are formed in the soil profile only during dry seasons whereas in humid seasons they are dissolved and disappear from the profile.