Accumulation of Pedogenic Gypsum in Western Oklahoma Soils

Recognition of pedogenic gypsum is particularly needed in parts of arid and subhumid climates to apply the suffix “y” to master horizons and to understand soil‐forming processes. Soil micromorphology provides correlation between field and laboratory observations, and recognition of pedogenic gypsum for the refinement of the “y” horizon suffix designation. Seven soils were described and sampled to (i) identify pedogenic gypsum, (ii) determine the distribution of pedogenic gypsum across a hillslope, and (iii) explain the occurrence of gypsum in residual‐derived soils. Gypsum content, saturated paste extracts analyzed for Ca2+and SO2‐4, and micromorphology indicated gypsic horizon formation within the summit position. Small amounts of pedogenic gypsum occurring with allogenic and bedrock gypsum were identified in soils using micromorphology. Pedogenic gypsum accumulation was greatest in soils with higher contents of total gypsum and relatively longer periods of soil formation. In situ dissolution of gypsum during wet springs (March–June) followed by reprecipitation during hot dry summers (July and August) is proposed to explain the formation of pedogenic gypsum. An accumulation of pedogenic gypsum in the B horizon may indicate a net upward movement within the solum or a lateral movement of gypsum within the landscape. The compaction of diagenetic gypsum during geologic burial is proposed to explain gypsum form and formation within veins and joints in Cr and R horizons. Rock fragments containing vein and joint fillings of gypsum persisted within the solum and were identified as bedrock gypsum.