Onset of carbon isotope excursion at the Paleocene-Eocene thermal maximum took millennia, not 13 years.

The Paleocene-Eocene thermal maximum (PETM) may represent the best paleo-analog for rapid and massive carbon release to the ocean and atmosphere. Thus, constraining the carbon release rate at its onset is critical. Wright and Schaller (1) use records from apparently rhythmically layered shelf sediments to argue that the layering is annual and that the onset of the carbon isotope excursion (CIE, fingerprint for carbon release) in the surface ocean was complete in 13 y. Using basic carbon cycle and climate considerations , we show this is not feasible. In fact, Wright and Schaller's isotope records indicate that the CIE onset took at least several millennia. This finding rules out a cometary origin of the carbon release. For a near instantaneous release of carbon into the atmosphere, the magnitude of the CIE in the surface ocean directly following the release is