Lower and upper Cretaceous paleosols in the western Sichuan Basin, China: Implications for regional paleoclimate

Abstract

Pedogenic features of paleosols preserved in the lower and upper Cretaceous sedimentary strata of the western Sichuan Basin, China, were described and examined in terms of their macromorphology and micromorphology, mineralogy, and geochemistry. These Cretaceous paleosols (entisols, inceptisols, and aridisols) are analogous to modern soil orders based on soil taxonomy (Soil Survey Staff, 2014). A quantitative reconstruction of paleoclimates inferred from the geochemical composition of paleosol horizons, depth to carbonate nodules, and the stable oxygen isotopic composition of pedogenic carbonates in paleosols indicates that a cool-arid climate associated with short-term subhumid fluctuations prevailed in the early Cretaceous (Hauterivian) and a cool-arid climate regime existed in the late Cretaceous (Maastrichtian) of the Sichuan Basin. Atmospheric CO 2 concentrations estimated from the paleosol barometer indicated values between 156 ± 138 and 753 ± 339 ppmv for the Hauterivian and between 110 ± 71 and 235 ± 77 ppmv for the middle-lateMaastrichtian. An overall coupling between atmospheric pCO 2 and paleotemperature occurred in the Sichuan Basin, while the covariations are likely linked to global cooling events in the Maastrichtian and/or are likely associated with the Faraoni oceanic anoxic event (F-OAE) during the late Hauterivian. Paleoclimate variations in the Sichuan Basin were controlled by regional paleogeography, the prevalence of subtropical high-pressure systems, monsoonal circulation, and the atmospheric CO 2 level in the early and late Cretaceous.

Cretaceous Evolution of the Central Asian Proto‐Paratethys Sea: Tectonic, Eustatic, and Climatic Controls

“…71–70 Ma; Friedrich et al, 2012; Linnert et al, 2014; Miller et al, 1999) (Figure 5). We interpret this as evidence for the Late Cretaceous cooling in Central Asia, which was previously documented in terrestrial settings of eastern China including the Xining‐Minhe, Sichuan, Songliao, and Nanxiaong basins (Gao et al, 2015; Li et al, 2018; Ma et al, 2018; Wang et al, 1990, 2013). The observations align well with Maastrichtian climatic fluctuations observed in intermediate and deep waters of different oceanic basins and indicate parallel climatic evolution between deep ocean waters and the sea surface waters of the epicontinental proto‐Paratethys Sea.…”

Controlling the Iron Migration Mechanism for the Cretaceous Sediment Color Variations in Sichuan Basin, China

“…The weathering leaching index (BA) reflects the relationship between active components and inert components; the smaller the ratio, the higher the degree of leaching of active components and the stronger the chemical weathering. 21 Calcium and magnesium are medium or active elements that are dissolved and transported in semi-arid and semi-humid environments. 50 At the same time, the radius of the calcium ion is greater than that of the magnesium ion.…”

“…It is still controversial that goethite on the surface of clastic minerals or clay minerals will dehydrate and oxidize into hematite with the increase in time and burial temperature because the burial temperature of Paleogene and Neogene red beds has not reached the boundary of transformation. 24 Li reconstructed the mean annual precipitation (MAP) and mean annual temperature (MAT) data of the Early Cretaceous and Late Cretaceous in the Sichuan Basin, 21 indicating that the whole Cretaceous was a long-term semi-arid and arid temperate climate. Previous studies have shown that the climate of the Late Cretaceous was dry and hot through the study of palynology and clay minerals in the Sichuan Basin.…”

“… 18 For a long time, the most mainstream view is that the oxidation environment formed under hot and arid climate conditions leads to the formation of terrestrial red beds, and the formation of its red color is related to the existence of disseminated hematite. 19 − 21 However, researchers have demonstrated that in modern desert environment sediments, the red bed is not typical and, under different climatic conditions such as cold and humid, also can form red deposits. 1 , 22 − 24 It has been confirmed that the formation of red color is related to disseminated hematite; there are still controversies about its origin and coloring principle.…”

“…The red sediments produced in this region are widely used as paleoclimate indicators and for paleomagnetism research studies and geochemistry tests. , Many researchers believe that sediments of different colors represent the paleoclimatic conditions of hot, dry, or cold. , These color changes are related to fluctuations in redox conditions before or after deposition . For a long time, the most mainstream view is that the oxidation environment formed under hot and arid climate conditions leads to the formation of terrestrial red beds, and the formation of its red color is related to the existence of disseminated hematite. − However, researchers have demonstrated that in modern desert environment sediments, the red bed is not typical and, under different climatic conditions such as cold and humid, also can form red deposits. ,− It has been confirmed that the formation of red color is related to disseminated hematite; there are still controversies about its origin and coloring principle. In recent years, some researchers believe that the color of sediments results from the interaction between the regional structure and the global climate cycle , and propose the thermal origin of terrestrial red beds .…”