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Bulk-carbonate carbon isotope ratios are a widely applied proxy for\udinvestigating the ancient biogeochemical carbon cycle. Temporal carbon\udisotope trends serve as a prime stratigraphic tool, with the inherent\udassumption that bulk micritic carbonate rock is a faithful geochemical\udrecorder of the isotopic composition of seawater dissolved inorganic carbon.\udHowever, bulk-carbonate rock is also prone to incorporate diagenetic signals.\udThe aim of the present study is to disentangle primary trends from diagenetic\udsignals in carbon isotope records which traverse the Permian–Triassic\udboundary in the marine carbonate-bearing sequences of Iran and South China. By\udpooling newly produced and published carbon isotope data, we confirm that a\udglobal first-order trend towards depleted values exists. However, a large\udamount of scatter is superimposed on this geochemical record. In addition, we\udobserve a temporal trend in the amplitude of this residual\udδ13C variability, which is reproducible for the two studied\udregions. We suggest that (sub-)sea-floor microbial communities and their\udcontrol on calcite nucleation and ambient porewater dissolved\udinorganic carbon δ13C pose a viable mechanism to induce\udbulk-rock δ13C variability. Numerical model calculations\udhighlight that early diagenetic carbonate rock stabilization and linked\udcarbon isotope alteration can be controlled by organic matter supply and\udsubsequent microbial remineralization. A major biotic decline among Late\udPermian bottom-dwelling organisms facilitated a spatial increase in\udheterogeneous organic carbon accumulation. Combined with low marine sulfate,\udthis resulted in varying degrees of carbon isotope overprinting. A simulated\udtime series suggests that a 50 % increase in the spatial scatter of organic\udcarbon relative to the average, in addition to an imposed increase in the\udlikelihood of sampling cements formed by microbial calcite nucleation to 1\udout of 10 samples, is sufficient to induce the observed signal of carbon\udisotope variability. These findings put constraints on the application of\udPermian–Triassic carbon isotope chemostratigraphy based on whole-rock\udsamples, which appears less refined than classical biozonation dating\udschemes. On the other hand, this signal of increased carbon isotope\udvariability concurrent with the largest mass extinction of the Phanerozoic\udmay provide information about local carbon cycling mediated by spatially heterogeneous\ud(sub-)sea-floor microbial communities under suppressed bioturbation

Latest Permian carbonate carbon isotope variability traces heterogeneous organic carbon accumulation and authigenic carbonate formation