New constraints on the absolute age and duration of the Paleocene – Eocene Thermal Maximum (PETM) from ash layer +19 in Denmark

• Ash +19 is a key horizon for dating the Paleocene-Eocene thermal maximum (PETM). • We present a new U-Pb radiometric ash +19 age of 55.331 ± 0.053/(0.060)/[0.080] Ma. • PETM onset predates ash +19 by 528–604 ± 102 kyr, based on PETM duration estimates. • Our revised age model places the Paleocene-Eocene boundary at 55.935 ± 0.102 Ma. • The regionally important ‘Positive ash series’ (Balder Fm.) spans 55.37 to 55.12 Ma. Precise age constraints for the onset and duration of the Paleocene–Eocene Thermal Maximum (PETM) are essential for understanding the mechanisms that triggered and sustained this major climate perturbation. However, establishing a precise PETM geochronology is complicated by uncertainties in orbital tuning and a current lack of precise radiometrically dated marker horizons. An early Eocene rhyolitic ash layer named +19 is a promising marker horizon due to its distinct geochemistry, its occurrence in key offshore archives, and well-preserved outcrops in Denmark such as Stolleklint, where it conformably overlies PETM strata. We present a new high-precision U-Pb zircon age of 55.331 ± 0.053/(0.060)/[0.080] Ma for ash +19, supported by a new Ar-Ar age of 55.424 ± 0.115/(0.116)/[0.320] Ma within uncertainty. The ash +19 U-Pb age enables direct comparison with ash SB01–1 in Svalbard that was erupted during the PETM carbon isotope excursion (CIE), yielding a 454 ± 90 kyr interval between the two layers. This provides a robust geochronological link from the PETM to ash +19 that is independent of stratigraphic interpretations. Using end-member PETM durations (94–170 kyr), we estimate the time between the PETM onset and ash +19 as 528–604 ± 102 kyr, which is significantly shorter than the 862 ± 20 kyr interval derived from astronomical tuning. Adopting the longer helium isotope-based PETM duration yields an onset age of 55.935 ± 0.102 Ma, with a CIE recovery end at 55.700 Ma. A shorter PETM duration requires a younger PETM onset age. The positive ash series in Denmark, correlating with the offshore Balder Formation, is now constrained to a 250 kyr interval between 55.367 and 55.117 ± 0.080 Ma. This refined age model provides a robust framework for testing and improving early Eocene astronomical time scales.