Atmospheric CO2 history of the late Permian and Early Triassic

The end-Permian mass extinction, the largest biological crisis in Earth history, is currently understood in the context of Siberian Traps volcanism introducing large quantities of greenhouse gases to the atmosphere. We reconstructed the late Permian to Middle Triassic atmospheric CO2 record by applying the carbonate paleosol pCO2 barometer to soil carbonates from sections in northwest China (Xinjiang Province), north China (Henan and Shanxi Provinces), Russia (South Ural foreland basin), South Africa (Karoo Basin), and the United Kingdom (Dorset). Atmospheric pCO2 shows an approximate 4-fold increase from mean concentrations of 412–919 ppmv in the late Permian (Changhsingian) to maximum levels between 2181 and 2610 ppmv in the Early Triassic (late Griesbachian). Mean CO2 estimates for the later Early Triassic are between 1261–1936 ppmv (Dienerian) and 1063–1757 ppmv (Spathian). Significantly lower concentrations ranging from 343 to 634 ppmv are reconstructed for the latest Early to Middle Triassic (Anisian). In parallel to the reconstructed rise in greenhouse gas levels, low-latitude sea surface temperatures (SST) increased by 7–10 °C, from 25–28 °C to >35 °C (Joachimski et al., 2020). With the decrease in pCO2 in the late Spathian to Anisian, SSTs decreased as well (Sun et al., 2012). Thus, pCO2 as well as SSTs persisted at high levels for almost 5 m.y. In contrast, pCO2 reconstructed using the photosynthetic carbon isotope fractionation suggest much lower atmospheric pCO2 (e.g. Shen et a. 2022), inconsistent with significant warming, while modeling studies suggest up to a 13-fold increase in pCO2 (e.g. Cui et al. 2021). Most important, the 5 m.y. long episode of elevated pCO2 suggests that negative feedback mechanisms such as silicate weathering, the most effective mechanism by which to extract CO2 from the atmosphere and to buffer Earth’s climate, were not effective enough to reduce atmospheric pCO2 to pre-crisis levels. Instead, marine authigenic clay formation (i.e., reverse weathering) may have been an important component of the global carbon cycle keeping atmospheric pCO2 at elevated levels during this critical time interval. References: Cui et al. 2021, PNAS, V. 118, No.37, e201470118; Joachimski et al. 2020, GSA Bull., 132, 427-443; Shen et al. 2022, Nat. Geosc., 15, 839-844; Sun et al. 2012, Science, 338, 366-370.