Carbon has two stable isotopes (12C, 13C) with delta-13C measured relative to VPDB. The carbon cycle creates a fundamental isotopic dichotomy: inorganic carbon (atmosphere, ocean DIC, carbonates) clusters around delta-13C = 0 +/- 5 per mil, while organic carbon (biologically fixed) clusters around -25 +/- 10 per mil due to kinetic fractionation during photosynthesis. This ~25 per mil offset between the organic and inorganic carbon reservoirs is maintained by continuous biological carbon fixation and is one of the most diagnostic isotopic signals on Earth. Perturbations to the global carbon cycle (mass extinctions, volcanic degassing, methane release, organic carbon burial) shift the delta-13C of ocean DIC, recorded in marine carbonates as carbon isotope excursions that are among the most important chemostratigraphic markers in the geological record.
Carbon isotopes trace the most fundamental biogeochemical cycle on Earth -- the cycle that regulates atmospheric CO2, produces atmospheric O2 (through organic carbon burial), and sustains all life. The ~25 per mil offset between organic and inorganic carbon, maintained by photosynthetic fractionation, is the isotopic expression of the biological carbon pump.
The fractionation during photosynthesis is primarily kinetic. The enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), which fixes CO2 in the Calvin cycle, discriminates against 13CO2, producing organic matter depleted in 13C by 20-30 per mil relative to the CO2 substrate. C4 plants partially bypass this by first fixing carbon through PEP carboxylase (which discriminates less), then feeding the concentrated CO2 to RuBisCO -- resulting in less overall fractionation. These plant-specific signatures are preserved in soil organic matter, tooth enamel, and sediments, enabling reconstruction of past vegetation types and dietary sources.
In the marine realm, the delta-13C of dissolved inorganic carbon (DIC) varies with depth due to the biological pump. Surface waters are enriched in 13C (high delta-13C) because phytoplankton preferentially remove 12C during photosynthesis. Deep waters are depleted (low delta-13C) because remineralization of sinking organic matter adds 12C-rich carbon back to the DIC pool. This vertical gradient (~1-2 per mil in the modern ocean) is recorded by foraminifera at different depths, providing a proxy for deep water circulation and biological productivity in the past.
Carbon isotope excursions in the stratigraphic record mark major perturbations to the global carbon cycle. The late Neoproterozoic Shuram excursion (delta-13C to -12 per mil) may record massive oxidation of a dissolved organic carbon pool. The end-Permian negative CIE records volcanic carbon injection. Oceanic Anoxic Events in the Cretaceous show positive CIEs from enhanced organic carbon burial. These events, recognized globally through chemostratigraphy, demonstrate how carbon isotopes integrate biological, tectonic, and climatic processes into a single measurable signal.