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Abstract The late Palaeocene Thermal Maximum (LPTM) was a brief interval at c . 55 Ma characterized by a −2.5 to −3‰ shift in the δ 13 C of global carbon reservoirs. The geochemical perturbation probably represents a massive input of 12 C-rich carbon to the exogenic carbon cycle. Largely unresolved issues concerning this carbon injection during the LPTM are the rates of carbon input and removal. Simple expressions are developed here to describe a δ 13 C excursion in the exogenic carbon cycle after carbon input. A change in global δ 13 C (dδ Ex /d t ) can be explained to a first approximation by a set of parameters: the initial mass and isotopic composition of the global carbon cycle ( M Ex(o) , δ Ex(o) ), and the fluxes and isotopic compositions of external carbon inputs, outputs and injected carbon ( F In , δ In , F Out , δ Out , F Add , δ Add ). In general, for a given exogenic carbon cycle, a large F Add or low δ Add results in a larger δ 13 C excursion. Likewise, for a given negative δ 13 C excursion, a large M Ex or low δ Ex requires a greater input of 12 C. Differences in F In , δ In , F Out and δ Out cause changes in the response of δ Ex over time. For a negative δ 13 C excursion of given magnitude, a greater F In requires a greater input of 12 C and lessens the time for δ Ex to return to initial conditions. A decrease in δ Out (caused by an increase in the relative output of organic matter and carbonate) has a similar effect. Variable d M Add /d t produces transients in δ Ex that are related to the source function but modified by carbon removal. In theory, a well-dated and representative global δ 13 C excursion could be used to derive the carbon inputs and ouputs. Ocean Drilling Program (ODP) Site 1051 has an expanded early Palaeogene section, and recent work at this location has provided a well-dated δ 13 C record across the LPTM. This δ 13 C record contains transient variations of apparently global nature. These observed transients are best explained by a pulsed injection of CH 4 into an exogenic carbon cycle with a greater carbon throughput or enhanced burial of organic matter after carbon addition.