Validating the deep time carbonate carbon isotope records: effect of benthic flux on seafloor carbonate

It is a consensus that marine carbonate archives the isotopic composition of seawater dissolved inorganic carbon (DIC, δ13Csw), the largest active C reservoir in the hydrosphere. Carbonate carbon isotope (δ13Ccarb) excursions have been used to reflect perturbations of the global carbon cycle and related environmental change. However, the deep time δ13Ccarb records indicate faster and more pronounced perturbations of the carbon cycle compared to the present day. Here, we report δ13Ccarb and elemental compositions of Late Paleozoic carbonate sections from South China, showing negative correlations between δ13Ccarb and Fe2+ content of carbonate (Fecarb). We suggest that, because Late Paleozoic carbonate was mainly produced by benthic carbonate-secreting organisms, δ13Ccarb recorded the isotopic composition near the seafloor, where benthic flux derived from anaerobic organic matter degradation delivers both Fe2+ and 13C-depleted DIC from porewater. The binary mixing between seawater and benthic flux would result in the deviation of δ13Ccarb from δ13Csw. The negative correlation implies that δ13Ccarb is influenced by benthic flux and is affected by the seafloor redox and sedimentation rate. The deep time spatially heterogeneous and temporally oscillatory δ13Ccarb records in the basin-scale could be alternatively attributed to the variations of local environmental factors rather than a δ13Csw depth-gradient. Thus, the seafloor carbonate precipitation is continuously affected by diagenetic reactions in sediments, suggesting that δ13Ccarb recording the seawater DIC composition is conditional. Our study urges that the interpretation of δ13Ccarb should also consider the sedimentary process and depositional environment of marine carbonate.