Fluctuations of Carbon Dioxide Concentrations in the Middle Cretaceous and its Paleoclimate Response—— Evidence from Paleosol Carbonates in Qamdo Basin

Key words: 
• pedogenic carbonate /
• Mid-Cretaceous /
• pCO2 /
• carbon cycle perturbations /
• climate sensitivity

Abstract: [Objective]The reconstruction of atmospheric carbon dioxide concentrations (pCO2) during the Cenomanian-Turonian (C-T) has been previously limited to stage-scale temporal resolutions, which has greatly constrained its effectiveness in unveiling the extreme greenhouse climate of the C-T period and perturbations in the global carbon cycle. [Methods]To enhance our understanding of pCO2 fluctuations and the paleoclimatic response during the C-T "greenhouse climate," this study reconstructed high-resolution atmospheric pCO2 and mean annual precipitation (MAP) during the C-T period by analyzing the stable carbon and oxygen isotopes of calcareous nodules from the Upper Cretaceous Nanxin Formation in the Qamdo Basin, southeastern Tibet. [Results and Discussions]Our results reveal a gradual decline in pCO2 during the early Cenomanian period, followed by an increase in the middle and late Cenomanian stages. Moreover, we observed significant fluctuations in pCO2 during the early and middle Turonian stage. These findings align with the variations in pCO2 throughout the C-T period that have been estimated using the stomata ratio method of plant fossils and the geochemical models. In addition to the long-term pCO2 fluctuations, the paleosoils of the Nanxin Formation also documented two crucial carbon cycle perturbations during the Cenomanian period: Oceanic Anoxia Event 2 (OAE2) and Mid-Cenomanian Event (MCE). The atmospheric pCO2 rose by 309 ppmV and followed by a rapid decline of 520 ppmV across the initiation of MCE, accompanied by a positive shift of 1.25‰ in organic carbon isotopes (13Corg). During OAE2, there were two distinct CO2 pulses, with the maximum pCO2 concentration coeval with the lowest 13Corg. The pCO2 increased up to approximately 1300 ppmV at the OAE2 boundary, followed by a remarkably reduction of around 17% due to substantial organic carbon burial during OAE2. The increased concentration of CO2 throughout the mid-late Cenomanian intensified the greenhouse effect and elevated atmospheric humidity, resulting in significant negative shifts in 13Corg that were decoupled with inorganic carbon isotopes. Our conclusion further suggests that there would be an increase of 511 mm/yr in MAP as the atmospheric pCO2 elevated from 500 ppmV to 1000 ppmV during the C-T period. [Conclusions]We assumed that the elevated atmospheric pCO2 and climatic sensitivity during the mid-Cretaceous considerably intensified the hydrological cycle, contributing to an extreme greenhouse climate period.