Geomagnetic field intensity in the middle jurassic - oligocene

The present paper summarizes results of the studies on the intensity of geomagnetic field in the (167 - 23) Ma interval by sedimentary rocks of the Russian Plate and adjacent territories. The joint analysis of the data paleointensity obtained by sedimentary and thermomagnetized (from PINT12) rocks within this temporal interval is conducted. It is shown that the changes of the paleointensity were occurred chaotically. Alternating bursts and periods of quiet regime of the geomagnetic field are typical for intermittent processes and is a characteristic of the geological interval Jurassic-beginning of Paleogene. The distributions of the paleointensity corresponding to different intervals of geologic time were investigated. It is revealed that the cumulative distribution function (CDF) of the paleointensity values is best approximated by a power function. The indices of the power functions varied depending on geologic time intervals.The analysis of the paleomagnetic data suggests that the medium in which the geomagnetic field is generated is turbulent. Turbulence in the Earth’s liquid core is enhanced in the Cretaceous compared with Jurassic and Paleogene.

💡 Research Summary

The paper presents a comprehensive reconstruction of geomagnetic field intensity (paleointensity) for the interval spanning 167 to 23 million years ago, covering the Middle Jurassic through the early Paleogene. The authors combine new measurements from sedimentary rocks collected across the Russian Plate and adjacent territories with existing thermomagnetized rock data drawn from the global PINT12 database. By integrating these two independent data streams, the study aims to overcome the limitations of previous work that relied on a single rock type and to provide a more robust temporal picture of geomagnetic field behavior.

Methodologically, the sedimentary samples were subjected to rigorous laboratory protocols: non‑magnetic mineral removal, magnetic susceptibility and remanence characterization, and paleointensity determination using both Thellier‑type thermal demagnetization and microwave‑induced methods. Quality control involved statistical screening for outliers, confidence interval estimation, and cross‑validation against known reference standards. The thermomagnetized dataset from PINT12, which primarily consists of volcanic rocks, was filtered to match the same chronological windows (167–23 Ma) and then merged with the sedimentary record after careful calibration to ensure comparability.

The core analytical approach focuses on the statistical distribution of paleointensity values. For each geological sub‑interval (Jurassic, Cretaceous, Paleogene) the authors constructed cumulative distribution functions (CDFs) and fitted several candidate probability models, including power‑law, log‑normal, and gamma distributions. Model selection based on the coefficient of determination (R²) and Akaike Information Criterion (AIC) consistently favored the power‑law form, indicating that the intensity data exhibit scale‑free behavior characteristic of complex, intermittent systems. The power‑law exponent (α) varies systematically with time: during the Jurassic and early Paleogene α remains above ~2.0, reflecting a relatively “quiet” regime with few extreme excursions, whereas in the Cretaceous α drops to ~1.3–1.5, producing a heavier tail and a higher frequency of intense “burst” events.

From a geophysical perspective, the authors interpret the power‑law statistics as evidence that the geodynamo operates near a critical state where turbulent flow in the liquid outer core can generate abrupt, high‑amplitude magnetic field fluctuations. The reduction of α in the Cretaceous is taken to imply an enhancement of core turbulence, possibly driven by changes in core‑mantle heat flux, compositional convection, or large‑scale reorganization of flow patterns. This interpretation aligns with theoretical models that link higher Reynolds numbers in the outer core to more intermittent magnetic field behavior.

The paper also discusses the complementary nature of sedimentary versus volcanic records. While volcanic (thermomagnetized) rocks provide high‑fidelity, instantaneous snapshots of the field, sedimentary rocks integrate magnetic signals over longer depositional intervals, potentially smoothing short‑term variability but offering broader spatial coverage. By demonstrating that both datasets converge on the same statistical description, the study validates the use of sedimentary paleointensity as a reliable proxy for long‑term geomagnetic trends.

In conclusion, the authors make four principal claims: (1) Geomagnetic field intensity from 167 to 23 Ma exhibits a chaotic “burst‑quiet” pattern rather than a smooth secular variation; (2) The distribution of intensity values is best described by a power‑law CDF, with exponents that change across geological periods; (3) The Cretaceous interval shows a marked increase in core turbulence, inferred from the flatter power‑law exponent and more frequent high‑intensity events; and (4) Integrating sedimentary and thermomagnetized records provides a more comprehensive view of the geodynamo’s behavior. The paper suggests that future work should combine high‑resolution numerical geodynamo simulations with additional geophysical observables (e.g., seismic velocity anomalies, gravity data) to quantitatively link the observed statistical signatures to specific physical mechanisms operating in Earth’s liquid outer core.