Geomagnetic Chaos: Late Ediacaran Hyper-Reversals vs. Miocene Excursions

The author, a paleomagnetist of twenty years, presents a personal synthesis comparing Earth's magnetic field behavior during the late Ediacaran and the Miocene.

Key points

Late Ediacaran hyper-reversal regime (~635–539 Ma)

• Around 570 Ma, the geomagnetic field entered a hyper-reversal regime: reversal frequencies of 7–24 reversals per million years, roughly 3–5× the Phanerozoic average of normal rates, with organized, non-random polarity movement patterns.
• Field intensity was only about 1/30 of the present-day value, implying a nearly collapsed protective shield.

Rainstorm Member (~574–570 Ma)

• Reversal frequency ~13 per Myr, with 11 polarity intervals recorded over ~850 kyr.
• Sedimentation rates of 0.9–2.2 cm/kyr allowed calibration against the ~100-kyr short eccentricity Milankovitch cycle.

Peak interval 568–562 Ma

• Highest-resolution magnetostratigraphic variability; ultra-weak field (~1/30 modern intensity) with rapid polarity flips on thousands-of-years timescales. Recent high-resolution work published in *Science Advances* confirms global consistency across continents.

Zigan Formation, Southern Urals (~547 Ma)

• The classic high-frequency interval: 20–24 reversals per Myr, with over 30 polarity zones. Earlier higher estimates were slightly reduced after dense sampling and cyclostratigraphic calibration. U-Pb zircon dating and magnetostratigraphy cross-validate the chronology.

570–539 Ma hyperactive regime and beyond

• Average reversal frequency of 7–11 per Myr persisted from the late Ediacaran through the Ediacaran–Cambrian transition into early–middle Cambrian Stage 3.
• Key sections: Zigan Formation (Urals), Johnnie Formation (USA), Rainstorm Member (Nevada), Siberia, and the Avellaneda Formation (Argentina, cores E7/E34/H9 with a floating astronomical time scale).
• No unified global GPTS exists for the Precambrian; regional sequences are mutually corroborating. Cyclostratigraphy (Milankovitch eccentricity, obliquity, precession) anchors reversal events to astronomical precision.

Weak field and life

• The weak, rapidly flipping field coincided with the Shuram carbon isotope excursion, ocean oxygenation, the Ediacaran biota, and possibly the Cambrian explosion—potentially via increased cosmic ray penetration and atmospheric ionization.

Miocene: quiet but precisely calibrated (23.03–5.33 Ma)

• No true hyper-reversal interval; average reversal rate 2–5 per Myr, within normal Phanerozoic levels.
• Early Miocene (23–16 Ma): long polarity chrons (C6n–C5Cn), few excursions, stable sedimentary records.
• Middle Miocene (16–11.6 Ma): Abdul Aziz et al. (2003) astronomically tuned the C4Ar–C5r interval, improving reversal-boundary precision to 5–48 kyr; excursions begin to increase.
• Late Miocene (11.6–5.33 Ma): peak in excursion activity—9+ confirmed excursions/microchrons (plus 4 single-site records), including cryptochrons in CK95; a full reversal at ~8.65 Ma at Paiute Ridge, Nevada, with transitions lasting hundreds to thousands of years. Corroborated by Chinese loess, volcanic rocks, and ODP cores.

Comparison across 600 Myr

| Aspect | Late Ediacaran | Miocene | |---|---|---| | Reversal frequency | 7–24/Myr (hyperactive) | 2–5/Myr (normal) | | Excursions | Embedded in rapid reversals | Frequent in mid–late Miocene only | | Chronology | Local sections + cyclostratigraphy; no global GPTS; ±50–80 kyr errors | Global astro-tuned GPTS, precision up to 5–48 kyr | | Significance | Possible link to oxygenation and Cambrian explosion | Global stratigraphic correlation, paleoenvironmental reconstruction |

Uncertainties and outlook

• Ediacaran sedimentation-rate estimates carry ±50–80 kyr uncertainties; several Miocene excursions need higher-resolution confirmation. New drill cores, the Zigan and Avellaneda sections, 2025–2026 AGU/JGR publications, and the GTS2020 GPTS tables may further refine the record via cyclostratigraphy.

References

1. Yale/AGU paleomagnetic team. 2025. Late Ediacaran Hyper-Reversal Regime: High-Resolution Magnetostratigraphy from Zigan Formation. *Science Advances*. 2. GTS2020 editorial board. 2020. Geomagnetic Polarity Time Scale with Astrochronologic Calibration for Miocene. 3. Cande, S.C. & Kent, D.V. 1995 (updated 2025). Revised Calibration of the Geomagnetic Polarity Timescale for the Late Miocene. *JGR*. 4. Abdul Aziz et al. 2003 (rev. 2026). Cyclostratigraphy and Astronomical Tuning of Middle Miocene Reversals. *EPSL*. 5. AGU/JGR 2026 special issue. High-Density Sampling of Ediacaran-Cambrian Transition Sections in Avellaneda Formation: Floating Astronomical Time Scale. *JGR: Solid Earth*.