Hematite reconstruction of Late Triassic hydroclimate over the Colorado Plateau.

Hematite is the most abundant surficial iron oxide on Earth resulting from near-surface processes that make it important for addressing numerous geologic problems. While red beds have proved to be excellent paleomagnetic recorders, the early diagenetic origin of hematite in these units is often questioned. Here, we validate pigmentary hematite ("pigmentite") as a proxy indicator for the Late Triassic environment and its penecontemporaneous origin by analyzing spectrophotometric measurements of a 14.5-My-long red bed sequence in scientific drill core CPCP-PFNP13-1A of the Chinle Formation, Arizona. Pigmentite concentrations in the red beds track the evolving pattern of the Late Triassic monsoon and indicate a long-term rise in aridity beginning at ∼215 Ma followed by increased oscillatory climate change at ∼213 Ma. These monsoonal changes are attributed to the northward drift of the Colorado Plateau as part of Laurentia into the arid subtropics during a time of fluctuating CO2 Our results refine the record of the Late Triassic monsoon and indicate significant changes in rainfall proximal to the Adamanian-Revueltian biotic transition that thus may have contributed to apparent faunal and floral events at 216 to 213 Ma.