Amenability to radiometric dating. Unknown, but absolute cyclostratigraphic dating is possible. Amenability to magnetostratigraphy. Yes, chrons C22n, C21r, C21n and C20r have been identified. Amenability to chemostratigraphy. Yes, but work is still in progress. Accessibility. Excellent. Free access. Yes, the section is located in a public beach. Permanent protection of the site. Yes. The whole coast is protected by the Spanish Littoral Law (22/1988, July 28); the Gorrondatxe beach is specially protected in order to preserve the endangered Natterjack toad (Epidalea calamita) from regional extinction. Local institutions (town and regional councils, University of the Basque Country) are willing to cooperate. Possibility to fix a permanent marker. Yes. Once the GSSP is approved, a permanent marker (a metal plate) will be fixed. Local institutions (town and regional councils, University of the Basque Country) are willing to cooperate.

Thirteen Lower–Middle Eocene (Ypresian–Lutetian) successions, including the Gorrondatxe section in the western Pyrenees, show biomagnetostratigraphic correlation schemes that do not agree with the current standard framework. The main discrepancy concerns the position of the boundary between planktonic foraminiferal Zones P9 (=E7, approximately) and P10 (=E8, approximately), which was thought to occur within calcareous nannofossil Subzone CP12a and at the boundary between magnetic polarity Chrons C22n and C21r. However, in the differing correlation scheme the boundary between Zones P9 (=E7) and P10 (=E8) occurs close to the base of Subzone CP13a and to the boundary between Chrons C21n and C20r. An attempt at a new Ypresian–Lutetian boundary biomagnetochronology is made based on data from the Gorrondatxe section, which shows that the boundary between Zones P9 (=E7) and P10 (=E8) is 3.1 Myr younger than hitherto considered. Therefore, the duration of the Early Eocene, most commonly defined according to this planktonic foraminiferal zonal boundary, has generally been underestimated over the last four decades.

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A distinctive low‐carbonate interval interrupts the continuous limestone‐marl alternation of the deep‐marine Gorrondatxe section at the early Lutetian (middle Eocene) C21r/C21n Chron transition. The interval is characterized by increased abundance of turbidites and kaolinite, a 3‰ decline in the bulk δ13C record, a >1‰ decline in benthic foraminiferal δ13C followed by a gradual recovery, a distinct deterioration in foraminiferal preservation, high proportions of warm‐water planktic foraminifera and opportunistic benthic foraminifera, and reduced trace fossil and benthic foraminiferal diversity, thus recording a significant environmental perturbation. The onset of the perturbation correlates with the C21r‐H6 event recently defined in the Atlantic and Pacific oceans, which caused a 2°C warming of the seafloor and increased carbonate dissolution. The perturbation was likely caused by the input of 13C‐depleted carbon into the ocean‐atmosphere system, thus presenting many of the hallmarks of Paleogene hyperthermal deposits. However, from the available data it is not possible to conclusively state that the event was associated with extreme global warming. Based on our analysis, the perturbation lasted 226 kyr, from 47.44 to 47.214 Ma, and although this duration suggests that the triggering mechanism may have been similar to that of the Paleocene‐Eocene Thermal Maximum (PETM), the magnitude of the carbon input and the subsequent environmental perturbation during the early Lutetian event were not as severe as in the PETM.