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)excur-sions 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 δ13-Ccarb and Fe2+content of carbonate(Fecarb).We suggest that,because Late Paleozoic carbonate was mainly pro-duced by benthic carbonate-secreting organisms,δ13Ccarb recorded the isotopic composition near the seafloor,where benthic flux derived from anaerobic organic matter degra-dation delivers both Fe2+and 13C-depleted DIC from porewater.The binary mixing between seawater and ben-thic flux would result in the deviation of δ13Ccarb fromδ13Csw.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 aδ13Csw depth-gradient.Thus,the seafloor carbonate pre-cipitation is continuously affected by diagenetic reactions in sediments,suggesting that 813Ccarb recording the sea-water DIC composition is conditional.Our study urges that the interpretation of δ13Ccarb should also consider the sedimentary process and depositional environment of marine carbonate.