Quantitative reconstruction of early cretaceous paleoclimate using paleosol carbonates in China

Abstract

In this study, an attempt to quantitatively rebuild the Early Cretaceous paleoclimate was executed by employing calcareous paleosols (fossil soils), which have a horizon of pedogenic carbonate accumulation within the profile, in the central Sichuan Basin of southwestern China and Jinlingshi-Yangshan Basin of northeastern China, using the relationships between the climate and geochemical proxies, depth to calcic horizon and oxygen stable composition of pedogenic carbonates in modern soils. During the early Valanginian and early-middle Berriasian ages in the central Sichuan Basin, the climate was dominantly cold and dry, with a mean annual temperature (MAT) within 5 and 9 °C (standard error (S.E.) = ±5.8 °C) and mean annual precipitation (MAP) within ~190 and ~320 mm. During the late Barremian age (~126 Ma), the MAP was within ~200 and ~380 mm, and MAT was between 9 and 13 °C (S.E. = ±5.8 °C) in the Jinlingsi-Yangshan Basin. Noticeably, for the paleosols which formed under the arid climate, alkali and alkaline earth cations were weakly leached from the soil profile, and even accumulated on the soil surface. Therefore, geochemical indices (such as the chemical index of alteration (K2O-free) and alkaline index) to evaluate the chemical weathering, including base cations (Ca2+, Mg2+, Na+ and K+), as the functions of precipitation and air temperature, are unsuitable for paleoclimate reconstruction of paleo-aridisols.