DEMISE OF THE LATE TRIASSIC MEGAMONSOON IN WESTERN EQUATORIAL PANGEA

Extending from the Late Permian through Jurassic Western Equatorial Pangea (WEP) was under the influence as an arid climate in response to the formation of the super continent in an ice-free world. The Late Triassic was an exception to this climate state because anomalously wet conditions emerged as inferred by sedimentological evidence and confirmed by general circulation and energy balance paleoclimate models. These studies suggest that once Pangea was assembled, cross equatorial air during the summer penetrated WEP bringing moisture laden air, abundant and seasonal rainfall, and uniformly warm temperatures-thus the megamonsoon. Many of these same field-based studies note a gradual shift towards aridity by the latest Triassic to Jurassic, supported by paleoclimate modeling sensitivity tests. Until now the timing and magnitude of this climate shift has been unknown, with two competing theories as to the cause: continental drift northward of the equatorial monsoonal zone or to an emerging convergent magmatic arc complex to the west that blocked the influx of moist tropical air to the region. Here we demonstrate from geochemical and isotopic data of ninety-nine paleosols and a new Pb-U age chronology from a Late Triassic stratigraphic succession in Petrified Forest National Park, Arizona that the demise of the WEP megamonsoon began between 214.7 and 213.1 Myr ago, well before the Triassic-Jurassic mass extinction, as a result of an evolving Cordilleran magmatic arc to the west. It is at this time that MAP and MAT initially declined, and was followed by rising temperatures and pCO₂ concentrations and declining pO₂ levels after 209 Myr. This climate shift is coincident with a major marine C isotopic perturbation and the Manicouagan impact, although the latter does not appear to have affected environmental conditions in the region. Our climate reconstruction also points to: humid, subtropical to warm temperate conditions during the megamonsoon in contrast to modeled biomes of tropical, summer wet; the occurrence of a putative faunal turnover across the climate transition; and a shift of forests to woodland and then desert landscapes. These findings have implications for near- future environmental changes once the Earth has entered its next greenhouse phase.