INVERSE COUPLING OF CARBON AND URANIUM ISOTOPE ANOMALIES IN NEOPROTEROZOIC CARBONATES OF SIBERIA

Insofar as animals need O₂ to respire, geobiologists have sought geochemical barometers to measure the oxidation state of seawater during the rise of multicellular organisms. Here we use C and U isotope compositions, as well as rare earth elements (REEs) to evaluate redox conditions associated with both positive and negative extremes in the Neoproterozoic carbon cycle. Our Siberian carbonate successions include the Nikol’skoe and Chencha formations, which are exposed in the Patom/Ura Uplift and preserve a strong negative δ¹³C anomaly attributed to the terminal Ediacaran Shuram Excursion. The other is the Chernaya Rechka Formation, exposed in the Igarka Uplift, which is either terminal Cryogenian or basal Ediacaran and associated with a strong positive δ¹³C excursion. In both examples, the C and U isotope systems show an inverse relationship, with the Nikol’skoe/Chencha preserving negative δ¹³C compositions and high δ²³⁸U values close to or exceeding modern carbonates, while the Chernaya Rechka reveals positive δ¹³C compositions and low δ²³⁸U values, significantly lower than modern carbonates. The latter suggests expansion of global ocean euxinia, with Ce/Ce* consistent with a locally anoxic water column. These observations support the general model of organic carbon burial under anoxic conditions driving high δ¹³C values in the geologic record. In contrast, the former suggests greater degrees of global ocean ventilation, although the preservation of positive Ce/Ce* is consistent with a locally anoxic water column. This is important insofar as biomineralized sponge-grade animal fossils have recently been reported from a ramp-to-rim transition within the Chencha Formation, suggesting that the metabolic activities of sponges could have influenced seawater elemental cycling and oxidation. Biomineralization may have been promoted by high seawater alkalinity during the accumulation of Shuram carbonates worldwide. This is potentially supported by the consistent coupling of δ¹³C and δ¹⁸O in the Nikol’skoe Formation followed by a notable plateau of δ¹⁸O values in the overlying Chencha Formation, suggesting that alkalinity reached an equilibrium saturation state in well-oxygenated global oceans (supported by near-modern δ²³⁸U) immediately before the appearance of the sponge-grade animals.