The Paleocene-Eocene Thermal Maximum (PETM) global warming event at ∼56 million years before present changed catchment weathering and erosion. Increased chemical weathering of silicate minerals is thought to be an important process removing CO₂ from the atmosphere. However, changes in clay mineralogy can often be explained by enhanced erosion of catchment laterites during the event. Here, we investigate chemical and physical weathering and erosive flux changes through the PETM interval in the Bighorn Basin, Wyoming, a Laramide foreland basin, in a proximal continental-interior alluvial setting. These show an increase of detrital smectite with a lag time of 20-kyr after the main onset the PETM. The smectite increase continued for at least 50-kyr after the event. In-situ, post-depositional pedogenic clay mineral formation is similar between pre-PETM and PETM soil profiles, despite large macroscopic differences between soils that formed before and during the event. Drier, hotter summers during the PETM probably caused decreased vegetation cover that, in concert with more frequent and heavier rainstorms, intensified the erosion of smectite-rich Cretaceous bentonites on the margins of the catchment, which exceeded changes in chemical weathering within the catchment. The lagged response in reaching full PETM clay mineral values can be explained by the time required for upstream sediment to reach the catchment basin floodplain. The prolonged nature of smectite enhancement after the PETM event may again relate to signal propagation times that are now even longer due to lower fluvial recycling rates. Our results indicate that chemical weathering changes were probably superceded by enhanced physical weathering and clay-mineral transport from basin margins at this continental-interior study site.

In recent years, some sapphires were found to fade in sunlight and to increase their color after UV irradiation. This unstable color phenomenon is attributed to the photochromism of corundum. The photochromic effect seriously affects the grading and evaluation of sapphires, although its mechanism is still uncertain. Here, we performed a set of photochromic experiments on sapphire specimens using a 254 nm shortwave UV light source and a D65 light source (which simulates sunlight) to generate different color states exhibiting characteristic absorption, emission, and excitation spectra. We observed that, for different color states, variation in the intensity of the absorption band at ~460 nm was consistent with that of orange fluorescence at 500–800 nm. This observation indicates a relationship between color instability and orange fluorescence. Peaks in excitation spectra at 320, 420, 490, 560, and 637 nm provide insight into the source(s) of excited orange fluorescence, which are related to different types of F-centers and Mg-trapped holes. We propose an explanation for the photochromic phenomenon: the color of photochromic yellow sapphire is the result of a variety of defects that release orange fluorescence simultaneously. Further, we hypothesize that the mechanism of photochromism in yellow sapphires is linked to electron transfer between F-centers and Mg-trapped holes.