Paleomagnetic inclinations and relative paleointensity variations during the Aptian in Poggio le Guaine core, Umbria–Marche Basin, Italy

The onset of the Cretaceous Normal Superchron (CNS) at ~120.96 Ma represents a fundamental transition in geomagnetic field behaviour from a high frequency of polarity reversals to a prolonged interval of stable normal polarity. Although traditionally regarded as a period of exceptional geomagnetic stability, recent studies have identified short-lived polarity reversals, revealing that geomagnetic behaviour during the transition into a superchron was more complex than previously assumed. Here, we present a paleomagnetic study of the Poggio le Guaine (PLG) core, (Umbria–Marche Basin, central Italy), spanning the interval between 116.2 and 120.4 Ma. Samples were collected from an approximately 16-m-long interval of the PLG core, between depths of 79.8 and 95.97 m, and an updated age model was established through astronomical tuning. Paleomagnetic inclinations and relative paleointensity (RPI) variations were analysed using natural remanent magnetization (NRM) and rock magnetic proxies, including magnetic susceptibility (χ) and anhysteretic remanent magnetization (ARM) (NRM_20mT/χ and NRM_20mT/ARM_20mT). Our results identify two distinct groups of reversed polarity samples. The older interval corresponds to Chron M0r at ~120.2 Ma and occurs immediately prior to the deposition of the Selli Level, linking it to the onset of Oceanic Anoxic Event 1a (OAE1a). The younger interval corresponds to the short reversal event ISEA, formally designated as Chron M-1r, at ~117 Ma. A pronounced decrease in geomagnetic field intensity occurs immediately before the M0r reversal (~120.2 Ma), followed by a gradual strengthening and a series of oscillations between ~119.8 and 118.5 Ma. From ~118.5 Ma onward, the record exhibits increased variability and overall lower field intensities, culminating in a subdued geomagnetic field regime around the ISEA reversal (~117 Ma). Following this event, RPI values progressively recover, indicating a gradual strengthening of the geomagnetic field. Altogether, these results emphasize the importance of high-resolution sedimentary archives for resolving short-timescale geomagnetic field variations and provide new constraints on episodic geomagnetic instability during the onset of the CNS, documenting short-lived polarity reversals accompanied by reduced geomagnetic field strength, consistent with reversal processes observed throughout the Phanerozoic.