Characteristics of the carbon cycle in late Mesoproterozoic: Evidence from carbon isotope composition of paired carbonate and organic matter of the Shennongjia Group in South China

The late Mesoproterozoic Era (1300–1000 Ma) was a critical period that witnessed the origin and early evolution of multicellular eukaryotes. Multiple studies have reported observations that the carbon isotope composition of carbonate (δ13Ccarb) underwent a substantial increase after ca. 1250 Ma. The high δ13Ccarb values suggest that the fraction of organic carbon burial (forg) had increased, which, consequently, might have promoted the oxidation of surface environments and the diversification of multicellular eukaryotes. However, details of the carbon cycle during this critical time interval remain elusive, which is pivotal to testing this hypothesis. In this study, we analyzed the stable carbon isotope composition of paired organic matter (δ13Corg) and carbonate, as well as the thermal characteristics of the organic matter, from the Shennongjia Group in South China, which has been constrained to an age of ∼ 1340–1150 Ma. A gradual increase in δ13Ccarb, from ∼–2‰ to >+4‰, is observed in the interval from the middle Shennongjia Group. Above which, the δ13Ccarb declines to ∼–2‰ in the uppermost. The paired δ13Corg values show an extensive range of compositions, from ∼ –22 to –32‰. Raman spectral data revealed that the organic matter in the Shennongjia Group experienced a wide range of thermal history, which might be ascribed to the involvement of detrital organic matter. We reconstructed the Δ13Ccarb-org and forg values in two different scenarios: one included all the δ13Corg data, while the other screened the δ13Corg data to minimize the possible effects of detrital organic matter. The results of these two reconstructions show similar temporal trends. Intriguingly, the Δ13Ccarb-org values are generally larger than the modern counterpart, which can be attributed to higher pCO2. The calculated forg of the Shennongjia Group is generally lower than the modern value, 20%. However, the forg experienced a substantial increase between ca. 1250 and 1190 Ma, suggesting enhanced oxidation of surface environments, which might have paved the way for the diversification of multicellular eukaryotes