Qaidam Basin paleosols reflect climate and weathering intensity on the northeastern Tibetan Plateau during the Early Eocene Climatic Optimum

Abstract

As an important geological archive, paleosols have been widely used in reconstruction of paleoenvironmental and paleoclimatic conditions. In this study, we undertook detailed analyses of the clay mineralogy and carbon and oxygen isotopic compositions of pedogenic carbonates in a series of 13 paleosol horizons within a fluvial section of the Eocene (~52–44 Ma) Lulehe Formation in the Qaidam Basin. Pedogenic smectite and illite/smectite (I/S) mixed-layer minerals are the predominant clay minerals in the paleosols, suggesting that the early Eocene paleoclimate of the Qaidam Basin was strongly seasonal with warm-wet summers. The high levels of pedogenic smectite production and high atmospheric pCO2 reconstructed from soil carbonate nodule δ13C (mean −7.1 ± 0.8‰) are consistent with elevated temperatures during the Early Eocene Climatic Optimum (EECO). Coeval paleosol carbonate nodules yield estimated paleo-surface water compositions (δ18Opsw) of −7.9‰ to −6.0‰ (VSMOW), which are significantly higher than the δ18O value of modern summer precipitation in the Qaidam Basin. The estimated temperature difference (~5.0 °C cooling) implies uplift of the study area by nearly 1000 m to its current average elevation of ~3000 m above sea level. This paleoelevation estimate agrees with modern meteoric water isotopes of Global Network of Isotopes in Precipitation (GNIP) sites in Asia in the same elevation range.