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 Our results indicate that chemical weathering changes were probably superceded by enhanced physical weathering and clay-mineral transport from basin margins at this continental-interior study site.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.

Smectites affecting water and soil exchangeable cations, especially potassium and ammonium related to plants, are common swelling clay minerals in sediments and paleosols that can record paleoenvironmental and paleoclimatic information. Paleoenvironmental and paleoclimatic interpretations are potentially ambiguous, because smectite minerals may have either detrital or pedogenic origins. This study analyzes smectites in Lower Eocene paleosols of the Qaidam Basin, China, with the goal of determining their origin and paleoclimatic significance. Twelve smectite-rich vertisol samples were analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), inductively coupled plasma-mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) in the Dahonggou (DHG) section of the Lulehe Formation. The samples from DHG section contained predominant dioctahedral smectites (60–65%) in the <2 μm clay fractions. Minor dioctahedral illites, dioctahedral interstratified illite-smectite (I-Sm), kaolinite, and traces of dioctahedral chlorites and palygorskites were present. The chemical data and FTIR spectra revealed both tetrahedral substitutions (i.e., Al for Si) and octahedral substitutions (i.e., Mg and minor Fe for Al) in the dioctahedral smectites. Montmorillonite comprised the dominance of the fine-clay fractions (< 0.2 μm) with minor contributions of beidellite, as determined by the Greene-Kelly test (GK-test), geochemical and FTIR analyses. SEM observations revealed two morphological types of dioctahedral smectites, the less abundant one with typical ‘honeycomb’ structure, interpreted as neoformed, and the other occurring as thin, well-defined plates and irregular masses considered to be detrital. DSC revealed two dehydroxylation temperatures (500–550 °C and 600–700 °C), indicating the presence of trans-vacant (tv) and cis-vacant (cv) layers, respectively. The cv layers were attributed to montmorillonite originating as catchment-delivered detritus, as the study Lulehe Formation exhibited no evidence of volcanic ash layers or diagenesis that would have promoted formation of montmorillonite with high dehydroxylation temperatures. The tv layers represented pedogenic beidellite formed via solution crystallization in a floodplain pedogenic environment. The accumulation of abundant inherited montmorillonite and neoformed beidellite in paleosols of the Lulehe Formation was probably promoted by a relatively warm climate with alternating wet-dry seasons during the Early Eocene. Admixture of detrital montmorillonite makes paleoclimatic reconstructions based on bulk-smectites concentrations and isotopic compositions unreliable, showing that integrated clay mineralogical analyses of the type undertaken in this study are necessary to identify the provenance and paleoclimatic significance of smectites in sediments.

Series of transient greenhouse warming intervals in the early Eocene provide an opportunity to study the response of rock weathering and erosion to changes in temperature and precipitation. During greenhouse warming, chemical weathering is thought to increase the uptake of carbon from the atmosphere, while physical weathering and erosion control sediment supply. A large ancient greenhouse warming event is the Paleocene-Eocene Thermal Maximum at 56 Ma. In many coastal sites, an increase in the abundance of kaolinite clay during the Paleocene-Eocene Thermal Maximum is interpreted as the result of reworking from terrestrial strata due to enhanced runoff caused by increased seasonal precipitation and storminess during a time of decreased vegetation cover. In the continental interior of North America, Paleocene-Eocene Thermal Maximum paleosols show more intense pedogenesis and drying, which are indicated by deeply weathered and strongly oxidized soil profiles. The weathering and oxidation could be related to temperature and precipitation changes, but also to increased time available for weathering and increased soil permeability in coarser sediment. Here, we provide evidence for enhanced climate seasonality, increased erosion ofproximal laterites and intrabasinal floodplain soils, and a potential slight increase in chemical weathering during the smaller early Eocene hyperthermals (Eocene Thermal Maximum 2, including H1 and H2) postdating the Paleocene-Eocene Thermal Maximum, for which no previous clay mineral data were available. Hyperthermal soil formation at the site of floodplain deposition causes a similar, insignificant clay mineralogical change as occurred during the background climates of the early Eocene by showing small increases in smectite and decreases in illite-smectite and illite. Remarkably, the detrital sediments during the hyperthermals show a similar pedogenic-like increase of smectite and decreases of mixed-layer illite-smectite and illite, while the kaolinite and chlorite proportions remained low and unchanged. Since sedimentation rates and provenance were similar during the events, enhanced smectite neoformation during soil formation in more proximal settings, and associated reworking, is the likely process causing this clay mineralogical change. The hundreds to thousands of year time scales at which individual paleosols were formed were probably too short for significant alteration of the rocks by in situ chemical weathering despite changing climates during the two post-Paleocene-Eocene Thermal Maximum greenhouse warming episodes. The relatively small signal, however, raises the question of whether increased chemical weathering can indeed be a strong negativefeedback mechanism to enhanced greenhouse gas warming over the time scales at which these processes act.