Refined chronologies of magnetochron M0r reveal asynchronous terrestrial and marine carbon isotope responses to Oceanic Anoxic Event 1a

The base of magnetochron M0r has been informally used to calibrate the onset of Oceanic Anoxic Event 1a (OAE1a) and place the Barremian-Aptian boundary; however, its precise age remains a subject of debate. Here, we present a 1497.5-m continuous drill core from the Lower Cretaceous lacustrine Jiufotang Formation in North China. High-resolution cycle-calibrated magnetostratigraphic analyses precisely constrain the termination of Chron M0r to 121.26 ± 0.38 Ma. The carbon isotope excursion associated with OAE1a in the same core postdates the termination of Chron M0r by 1.24 ± 0.4 Myr, substantially longer than ~0.3 to 0.66 Myr offset observed in marine records. This refined chronology provides direct evidence for asynchronous carbon cycles between terrestrial and marine systems during OAE1a, thereby challenging the prevailing hypothesis of rapid atmospheric CO₂ emissions at the onset of OAE1a.