PALEOSOLS AND PEDOGENIC ISOTOPES AS EVIDENCE FOR EXTREME LATE TRIASSIC CLIMATIC EVENTS, CHINLE FORMATION, NORTHERN NEW MEXICO

This study provides a detailed pedogenic evaluation of two age-equivalent Upper Triassic (Late Norian through Rhaetian) stratigraphic intervals in New Mexico in order to assess the climate and ecology preceding the Triassic-Jurassic mass extinction. The two study areas evaluate the Chinle Formation and equivalent strata that are located in north-central and east-central New Mexico. Little pedogenic variation is observed throughout the strata at each location, and a typical paleosol profile is about 1 meter thick and has an AB–Bw–Bk–BC horizon succession. Bkm, Bss, Bssk, or Bssg horizons are present in some paleosols. Micromorphological features suggest dominantly well-drained sola with minor periods of moist or saturated conditions. Depth-to-carbonate functions estimate that mean annual precipitation was between 200 and 450 +/-95 mm. By comparing these paleosols to modern soils, this study demonstrates that the Late Triassic Western Interior during the Late Norian to Rhaetian was arid to semiarid and supported a desert shrub environment that had localized and periodic moist or saturated soil conditions. Carbon and Oxygen isotopes from pedogenic carbonate suggest that there were two extreme Rhaetian climate events characterized by elevated temperatures and pCO2 levels and/or extremely dry intervals. Such climatic events would have caused a severe biotic crisis and may have been a driving mechanism of the Triassic-Jurassic extinction event.