Abstract

Elemental analyses of paleosol B horizons in the Upper Jurassic Morrison Formation of the western United States provide estimates of mean annual precipitation (MAP) and allow determination of humidity regimes. Samples were collected from the lower Morrison Formation in New Mexico and the upper Morrison Formation in Wyoming and Montana. The chemical index of alteration minus potassium (CIA – K) and the calcium and magnesium weathering index (CALMAG) were used to estimate paleorainfall. CIA – K values calculated for paleosols without shrink-swell (vertic) features correspond to MAP estimates between 800 and 1100 mm yr–1, with an average of 1000 mm yr–1. CALMAG values, calculated for vertic paleosols, correspond to MAP estimates between 50 and 1200 mm yr–1, with an average of 700 mm yr–1. MAP estimates from the older New Mexico strata indicate that early Morrison environments were relatively arid. MAP estimates from the younger Wyoming and Montana deposits reflect wetter conditions in the northernmost part of the Morrison Formation, but the transition from arid interior environments was abrupt. Humidity provinces inferred from geochemical proxy-based estimates of evapotranspiration and energy influx from precipitation range from semiarid to superhumid, suggesting wetter conditions than the MAP estimates, but supporting the relative differences in moisture among the three study areas. Paleoprecipitation patterns within the Morrison depositional basin do not match the modern latitudinal distribution of rainfall that arises from zonal atmospheric circulation. Comparison with the Upper Jurassic Lourinhã Formation in Portugal and the Vega Formation in Spain reveals that MAP in Late Jurassic Portuguese environments was similar to that in the wet northern part of the Morrison Formation, although more arid conditions prevailed in some areas of Portugal. Inferred humidity regimes for the Lourinhã Formation, which range from semiarid to superhumid, also indicate small-scale geographic variability in climate, although less pronounced than that observed in the Morrison Formation. Paleoenvironments in northern Spain were similar to the drier Morrison environments in the continental interior. Given the abrupt climatic transitions inferred here for the Morrison Formation, paleoprecipitation estimates derived from a geographically restricted sample may reflect only local conditions and should not necessarily be extrapolated to larger areas.

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