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Soils on the Kane alluvial fans, in the northeastern Bighorn Basin near Lovell, Wyo., accumulate gypsum (CaSO^'ZH^O) over time. This gypsum is added chiefly as eolian dust derived from both local and long-range sources. The arid but cool climate has permitted gypsum to accumulate continuously for the past 600 ka (thousands of years). The linear increase in pedogenic gypsum with time suggests that effective moisture in the Kane area has never increased substantially in the past 600 ka. The dates when the fan surfaces stabilized and, thus, the times when the soils began to form are estimated by various dating methods, including tephrochronology, paleontology, and stream-incision rates. The gypsum content influences the development of other soil properties. At the Kane fans, pedogenic or secondary gypsum has accumulated at annual rates of 1.1 X 10~6 to 120 X 10~6 g/cm2 ; modern aerosolic gypsum measured m dust traps is being added at annual rates of 26 X 10~6 to 60 X 10~6 g/cm2 . Secondary calcium carbonate (CaCO-j) does not increase in soils older than 100 ka, despite the availability of CaCO-j in eolian dust and in the parent material, probably because gypsum suppresses the solubility of CaCO-,. Total texture (a measure of pedogenic silt and clay) and pH increase over time in soils younger than 300 ka, but decrease or show irregular trends in older soils because of large accumulations of pedogenic gypsum. In contrast to soils in most environments, rubification (reddening and brightening of soil colors) in the study area increases with depth. Gypsum also causes physical disintegration of gravel clasts in the soils by crystallizing in cracks and along grain boundaries. Depth to gypsum in soils on the Kane fans is controlled by climate, soil age, and the hygroscopic properties of gypsum. The youngest soil containing visible gypsum and those soils older than 100 ka have their maximum gypsum accumulations at the average depth (2.6 cm) to which the modern precipitation moistens the soil. The 100-ka-old soils have their maximum gypsum accumulations at a greater depth (more than 5.5 cm). This disparity between gypsum depths probably reflects both differences in climate and the near-surface accumulation of gypsum during interglacial periods. Buildup of pedogenic gypsum at shallower depth during interglacials may have hindered subsequent leaching and reprecipitation of gypsum at greater depth during glacial periods of increased soil moisture, because the gypsum can hygroscopically hold the increased moisture. Thus, the near-surface accumulations of gypsum in soils older than 100 ka mainly reflect interglacial wetting depths. INTRODUCTION Studies of soils formed on the Kane alluvial fans1 , on the northeastern margin of the Bighorn Basin, yield valuable information on the processes and rates of soil development in a cool, arid climate. This study assesses the effects of time on soil development in a chronologic sequence of soils. To ascribe the characteristics of soils to their ages, it is necessary to demonstrate conclusively that the other four important soil-forming factors parent material, topography, vegetation, and modern climate, as defined by Jenny (1941) are constant or have little effect on these characteristics. Many studies of aridic soils in the southwestern United States (for example, Gile and others, 1966; Gardner, 1972; Bachman and Machette, 1977; Shlemon, 1978; Machette, 1985) have demonstrated that the dominant pedogenic process in that region is the accumulation of calcium carbonate (CaCOo), most of which is derived from dust and rainfall, m contrast, gypsum (CaSO,*2H2 O) accumulates in soils in the arid environment of the Kane fans, and secondary CaCO3 is negligible. Accumulation of soluble salts characterizes hot desert soils (Watson, 1979) and polar desert soils (Bockheim, 1980a), but gypsic soils have been little studied in the United States (Nettleton and others, 1982). There are few studies of soils in arid parts of the intermontane basins of the northern Rocky Mountains (for example, Reider and others, 1974, in the Laramie Basin, Wyo.). Information on the soils formed on the Kane fan may help to link soil-forming processes in warm deserts and polar deserts. Interpretations in this paper rely chiefly on field data and gypsum analyses. Some particle-size and chemical data are used to discuss the parent material. Reheis (1984) discussed other laboratory data, including particle size and chemical and mineralogic analyses. Acknowledgments. My greatest appreciation goes to P. W. Birkeland, who critically read most of this manuscript, for discussion and thoughtful evaluation of many ideas. K. L. Pierce, D. W. Moore, and L. D. McFadden also contributed valuable perspectives. The late D. E. Marchand coerced me into studying gypsic soils. Able field assistance was provided by Kim Jones. *The Kane alluvial fans are informally named for an abandoned railway station across the Bighorn River, which also gives its name to the Kane, Wyo., 7 1/2minute quadrangle within which the fans lie. Gypsic Soils on the Kane Alluvial Fans, Big Horn County, Wyoming Cl GEOMORPHIC AND CLIMATIC SETTING

Gypsic soils on the Kane alluvial fans, Big Horn County, Wyoming