Fluxes of the meteoric cosmogenic radionuclide ¹⁰Be vary with changes in the incoming cosmic rays modulated by geomagnetic field intensity variations. The variability in the ¹⁰Be flux can be used to synchronize ice cores, as well as marine sediments, by comparison with the relative paleointensity variations of the geomagnetic field. However, lock-in of the paleomagnetic signal at some depth below the sediment–water interface in marine sediments through acquisition of a post-depositional remanent magnetization (PDRM) adds uncertainty to synchronization. Despite the long history of such studies, the magnitude of the PDRM lock-in depth remains controversial. In this article, we present clear evidence for a downward offset of the paleointensity minimum relative to the ¹⁰Be flux anomaly at the Matuyama–Brunhes (M–B) geomagnetic polarity boundary, which we interpret to result from a ∼15cm PDRM lock in depth. This lock-in depth indicates that up to several tens of thousands years of age offset probably occurs when a paleomagnetic record is used for dating marine sediments, and the age of the M–B boundary should be revised to ca. 10kyr younger, which is consistent with a younger ice core derived age of 770±6ka (2σ). This cosmogenic age tuning strategy will contribute to refining paleomagnetic-based age models for marine sediments and identifying of lead-lag relationships for global abrupt environmental changes.