Abstract

During the Late Triassic, western equatorial Pangea, in the present-day American Southwest, was unusually humid, as indicated by sedimentological evidence and inferred from general circulation models. These studies show that once equatorial Pangea was assembled, cross-equatorial summer air flow penetrated into western equatorial Pangea, bringing abundant megamonsoon rainfall and warm temperatures. However, many of these investigations indicate that gradual aridification began sometime during the Late Triassic to Jurassic. We show from field properties, geochemical transfer functions, and isotopic analysis of paleosols in the Chinle Formation of Petrified Forest National Park, Arizona, that the western equatorial Pangea megamonsoon collapsed by 214.7 Ma during the “middle Norian climate shift.” Paleosols include Entisols, Inceptisols, Vertisols, Aridisols, and Alfisols, with transfer functions for mean annual precipitation (chemical index of alteration minus potassium [CIA–K]) and mean annual temperature (NAK) (salinization) developed from analogous modern data sets. The most notable shift is the appearance of carbonate-enriched paleo-Inceptisols and paleo-Vertisols after 214.7 Ma and paleo-Aridisols after 210 Ma.

Prior to the middle Norian climate shift, the region is classified as humid (humidity province) based on rainfall estimates. Initiation of the middle Norian climate shift was characterized by an increase in paleosol carbonate content and rapidly declining rainfall as the region shifted to subhumid, and eventually to semiarid and arid after 210 Ma. Paleosol-derived temperatures are indicative of warm temperate to subtropical ecozones, whereas general circulation models show higher temperatures and tropical conditions, perhaps because boundary conditions were set to atmospheric pCO2 4–5× present. Elevations of >1.5–2 km adjacent to the Cordilleran magmatic arc complex located ∼400 km west of the study area may account for nontropical temperatures. After 210 Ma, however, temperatures began to increase, possibly as a result of escalating atmospheric pCO2 from shale oxidation during marine regression and from coal and paleosol organic matter oxidation during prolonged aridification.

Paleomagnetic studies have suggested northward continental drift of western equatorial Pangea to outside of the Intertropical Convergence Zone as the cause of monsoonal collapse during the Late Triassic to Early Jurassic. The formation of a rain shadow as a result of the evolving Cordilleran magmatic arc as the cause of aridification is supported by recent magnetostratigraphic work substantiating that the region remained in the tropics through the Triassic. According to our age model, the middle Norian climate shift is dated to near the same time as the Manicouagan impact crater, but there is no evidence of such an event in the study area, either geochemically or sedimentologically at our scale of observation. However, a regionally defined faunal turnover may have been a response to rapidly changing climates in the region.

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