Reconstruction of Late Permian-Mesozoic pCO2 from δC13 Composition of Pedogenic Carbonate, Ordos and Sichuan Basins of Central China

Pedogenic carbonate samples from 28 locations in the Ordos and Sichuan basins were analyzed for C and O isotopic composition in the Stanford University Stable Isotope Biogeochemistry Laboratory using a Finnigan Gasbench II and Delta+XL mass spectrometer. Samples yield d¹⁸O values typical of pedogenic carbonate (20-27‰ V-SMOW), suggesting little diagenetic alteration.  Upper Permian-Lower Triassic samples exhibit d ¹³C values between -3 to -4‰ (V-PDB). Upper Lower Triassic-Middle Triassic samples show an increase to -7 and -10‰ d ¹³C. Middle Jurassic values range from around -5 to -7.5‰ up to -1.5‰ d ¹³C, followed by a decrease to between -4 and -6 ‰ d ¹³C in Upper Jurassic-Lower Cretaceous samples.  Atmospheric pCO₂ (C_a) is modeled following the Cerling method, which assumes isotopic equilibrium of the soil-atmosphere system:

C_a = S_z ((d ¹³C_s-(1.0044*d ¹³C_r)-4.4)/ d ¹³C_atm-d ¹³C_s))

where S_z is soil CO₂ concentration from diffusive mixing of atmospheric and plant-respired CO₂, and d ¹³C_s,d ¹³C_r, d ¹³C_atm are the d¹³C composition of soil CO₂, plant-respired CO₂, and atmospheric CO₂, respectively. Values of d ¹³C_s are determined by adjusting sample d¹³C for temperature-dependent fractionation. An average S_z value of 4500 ppmV is based on diffusion rates of modern soils. Values for d¹³C_s and d¹³C_atm are based on d¹³C composition of coeval terrestrial organic matter from other studies. Modeling results reveal Late Permian-Early Triassic pCO₂ of 2400-6300 ppmV. Subsequently, pCO₂ decreases during late Early Triassic time (~1240 ppmV) and then increases in Early Middle Triassic time (~2040 ppmV). Late Middle Triassic pCO₂ rises from ~990 ppmV to ~3240 ppmV. Jurassic and Cretaceous results show moderate variability, with an average around ~4660 ppmV. The pCO₂ estimates reported here are similar to other estimates from age-equivalent pedogenic carbonates, and support climate models that depict latest Permian warming as a consequence of elevated pCO₂ and not paleogeography.