The primary criterion ratified by the International Subcommission on Paleogene Stratigraphy (ISPS) to define the Paleocene-Eocene (P/E) boundary, and the beginning of the Paleocene-Eocene Thermal Maximum (PETM), is the onset of a prominent negative Carbon Isotope Excursion (CIE; Aubry et al., 2007), located in the lower to middle part of Chron C24r, in calcareous nannofossil Zone NP9 and at the base of planktonic foraminiferal Zone E1 of Berggren & Pearson, 2005 (see also Wade et al., 2011), also termed Zone P5 in Aubry et al. (2007). Based on cyclostratigraphy, the CIE is estimated to have spanned 150 ± 20 kyr and would reflect a major perturbation of the global carbon cycle. Organic matter (OM) may be judged as a (very) reliable material for isotopic chemostratigraphy, in both marine and terrestrial settings. Here we show several examples of successions (Belgium, Egypt, France, Spain, Tunisia, USA-Wyoming) where: isotopic analyses on OM are necessary to define the P-E boundary (lack of carbonates and/or diagenetic 1. alteration of the isotopic signal on carbonates, including calcitic shells, bulk rocks and pedogenic nodules), organics are probably not the best material to precise the P-E boundary,2. geological processes, such as hiatuses, and potential reworking of OM in channels and turbidites, may per3. turb the reliability of the carbon isotope results (on both organics and carbonates). Aubry, M.P. et al., 2007. Episodes, 30, 271-286. Berggren, W. A., and Pearson, P. M., 2005, J. of Foraminiferal Research, 35/4, 279–298. Wade, B.S. et al. 2011, Earth Science Reviews 104, 111-142.

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