Permian-Pennsylvanian Climate from Paleosols in North-Central New Mexico, USA

This study presents lithostratigraphic, mineralogic, and stable isotope proxies of equatorial paleoclimate from Upper Pennsylvanian (Missourian-Virgilian) through Lower Permian (Wolfcampian) strata of the Taos Trough, north-central New Mexico, USA.  The Upper Paleozoic sequence includes mixed marine carbonate and terrestrial clastic rocks in Pennsylvanian strata (Madera Fm) which transition to terrestrial-dominated siliciclastic rocks in the Lower Permian (Sangre de Cristo Fm). Paleosol morphologies include eutric Argillisols in Missourian rocks, Calcisols in Virgilian and Wolfcampian rocks, and gypsic subgroups in upper Wolfcampian strata. X-ray diffraction analysis of pedogenic clay minerals shows a shift from kaolinite, smectite, vermiculite, and illite assemblages representative of seasonal, subhumid environments in the lower part of the stratigraphy to poorly-ordered 2:1 phyllosilicate and illite assemblages, representative of increased aridity and decreased seasonality, in the uppermost part of the stratigraphy. The stratigraphic distribution of these morphologies is suggestive of stepped change from a subhumid seasonal climate in Missourian time to a nonseasonal arid climate in end-Wolfcampian time. 

There appears to be no significant stratigraphic trend among organic matter d¹³C values, which range from ~-22.5 to -24‰. However, paleosol calcite d¹³C values are ~-5‰ in Virgilian strata, ~-8‰ in lower Wolfcampian strata, and ~-5‰ in topmost Wolfcampian strata. These data suggest relatively high atmospheric PCO₂ (or low biological productivity) during deposition of Pennsylvanian and upper Wolfcampian strata, and relatively low atmospheric PCO₂ (or high biological productivity) during deposition of lower Wolfcampian strata. The d¹⁸O value of pedogenic calcite ranges from ~-4 to -6.5‰ throughout the Missourian, Virgilian and Wolfcampian strata. However, calcite d¹⁸O values of the top-most Wolfcampian strata range from +1 to +3‰. This stratigraphically abrupt, +5-7‰, shift in calcite d¹⁸O values is consistent with predicted changes in tropical rainfall d¹⁸O values during deglaciation of the ice-sheet in Gondwanaland.