Temporal Variation in the Chemical Index of Alteration in Early Cretaceous Black Shale as a Proxy for Paleoclimate

To better understand Early Cretaceous climate variation in inland East Asia, the temporal variation in the chemical index of alteration (CIA) was reconstructed in black shale from an outcrop of the Fajiaying Formation in the Lower Cretaceous Laiyang Group at Lingshan Island, East China. As the formation height increased, the corresponding CIA increased. The obtained index of compositional variability values and an (Al2O3)–(CaO* + Na2O)–K2O ternary diagram demonstrate that the gathered samples had not evolved into K-metasomatism during diagenesis. Moreover, low Zr concentrations parallel to the magmatic trend in the Th/Sc-versus-Zr/Sc diagram reveal that the sediment sorting and recycling did not affect the paleoweathering proxies. The lack of correlation between the CIA and Al/Si or K/Si further suggests that the paleoweathering proxies were not affected by the sediment sorting. Accordingly, as the CIA and the chemical index of weathering increased, the corresponding chemical weathering increased slightly. This is consistent with the transition from a dry-hot climate to a warm-humid one proposed by paleoclimatic proxies such as Rb/Sr, Sr/Cu, Al2O3/MgO, and clay minerals. In this study, a near-ubiquitous correlation is proposed between the variations of paleoclimate and the CIA of shale, in which a rise in humidity significantly increased the chemical weathering. Meanwhile, it was found that under the dry-hot conditions of the Early Cretaceous, chemical weathering in inland areas far from the ocean was relatively weak and dependent on atmospheric humidity. This analysis validates the CIA, as part of the stratigraphic record, to be a robust proxy for terrestrial paleoclimatic variation during the Early Cretaceous.