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

Automated summary

The δC-13(carb) records of late Paleozoic carbonate sections from South China show a negative correlation with Fe2+ content in the carbonate, suggesting influences from benthic flux and seafloor conditions on carbon isotope composition. This indicates that the interpretation of δC-13(carb) should consider sedimentary processes and depositional environments in marine carbonate.