Depth-Dependent Coupling and Decoupling of Paired Carbon Isotopes Across the Ediacaran-Cambrian Transition 18

The BACE (Basal Cambrian carbon isotope Excursion), a pronounced negative shift in carbonate carbon isotopes (δ13Ccarb), occurred globally across the Ediacaran-Cambrian transition. This event could be attributed to widespread oxidation of the marine dissolved organic carbon (DOC) reservoir, which, however, remains debated. Resolving the origin of the BACE is crucial for understanding the coeval rapid decline of the Ediacara Biota and the subsequent emergence and diversification of early animals (e.g., small shelly fossils). Here we present a spatiotemporal comparison of δ13Ccarb and δ13Corg across three sections with varying water depths on two cratons: the Daqiao Mine (DQM; shallow) and Caojiawan (CJW; mid-depth) sections on the Yangtze Block of South China, and the Xiaoerbrak (XBL; mid-depth) section on the Tarim Block. Results show that, during the BACE, δ13Ccarb and δ13Corg exhibited coupled variations in the shallow-water DQM section, but were decoupled in both mid-depth sections (CJW and XBL). Integrating previously reported data from sections across varying water depths on each craton, we identify a consistent basin-scale spatial pattern: “δ13Ccarb-δ13Corg coupling in shallow waters, but decoupling in deep waters”. This pattern supports the DOC oxidation hypothesis for the BACE. Specifically, large-scale oxidation of the marine DOC added abundant 12C-enriched dissolved inorganic carbon to seawater, driving a marked decrease in δ13Ccarb. Meanwhile, residual DOC in deeper waters buffered δ13Corg variation, leading to its decoupling from δ13Ccarb. In shallow, oxygenated waters, DOC was absent or largely exhausted, resulting in the conventional pattern of coupled δ13Ccarb-δ13Corg signals. These processes were likely linked to enhanced continental weathering (supplying oxidants such as sulfate) and active upwelling (transporting deep DOC to shelf regions and promoting its oxidation). Such processes may have profoundly influenced surface environments and biological evolution: the massive release of dissolved inorganic carbon from DOC oxidation raised seawater HCO3- concentration, favoring early biomineralization (e.g., the appearance of small shelly faunas). Conversely, enhanced sulfate reduction could have produced toxic hydrogen sulfide, imposing environmental stress on the Ediacara Biota and contributing to its marked decline.