New Geomagnetic Polarity Scale for the Late Cretaceous to Eocene

The Geomagnetic Polarity Time Scale (GPTS) is fundamental for dating geological events and reconstructing tectonic plate movements. Traditionally, the GPTS is based on records of magnetic anomalies created by the spreading of the ocean floor, calibrated by a few radioisotopic dates. Recently, the construction of the GPTS has focused on astrochronology, where magnetostratigraphy is dated from astronomical cycles recorded in selected sedimentary sequences. A new study proposes a GPTS for the Late Cretaceous to Eocene period, utilizing three data sources: 20 radioisotopic dates, 97 magnetochron durations from astrochronology, and 154 magnetic anomaly tracks from thirteen oceanic ridge flank regions ¹.

Construction Method

The research team argues that the combination of different types of data, each with independent errors, provides a better-defined GPTS. They used a Bayesian formulation and a Markov Chain Monte Carlo (MCMC) algorithm to generate a large sample of GPTSs that fit the radioisotopic and astrochronological data, minimizing fluctuations in global ocean floor spreading rates. The best estimates of the ages and durations of the GPTS are the averages of the values sampled by the MCMC, and the uncertainties are quantified by the standard deviations of the samples ¹.

The new geomagnetic polarity scale confirms a significant change in tectonic plate movements between 50 and 45 million years ago. During this period, the collision between India and Eurasia reduced the spreading rate in the Indian Ocean, while spreading rates increased in the North Pacific and South Atlantic ¹.

The approach used in the study can serve as a model for the construction of future geological time scales. Furthermore, it demonstrates that high-resolution time scales with durations derived from astrochronology can be reliable for periods where there are no marine magnetic anomalies, such as most of the geological record ².