Steady increase in geomagnetic reversal frequency after the Cretaceous Normal Superchron

The Earth's core is constantly and efficiently cooled by mantle convection. The heat flux transferred from the core to the mantle through the core-mantle boundary (CMB) is critical for understanding the dynamics of solid Earth. Although it is difficult to estimate the CMB heat flux, its history could be reconstructed from geomagnetic reversal frequency. However, overlooked short geomagnetic reversals may exist in the geomagnetic polarity time scale (GPTS), which affects the estimation of the heat flux history. Here, we report four new high-precision 40Ar/39Ar ages of the Oligocene Ethiopian traps. The traps may contain undiscovered reversals in marine magnetic anomaly. Based on the ages, we identified new reversals in Chron C12n, which was not found in marine magnetic anomalies. Our non-parametric analysis of GPTS suggests four potential periods of missing geomagnetic reversals, which correspond to long polarity intervals in GPTS. We found that C12n correspond to one of the periods. This indicates that several undetected reversals may exist within or near the edge of long polarity intervals after the Cretaceous Normal Superchron (prolonged stable polarity period). Considering the undetected reversals, we conclude that the CMB heat flux increased more slowly and monotonically after the Superchron than that ever estimated.

The Paraná magmatism (Brazil) as the villain regarding the remagnetization of the Paleozoic sediments in the basin – half-truth!

<p>The sedimentation in the Paraná Basin started during the Ordovician and ended with the Early Cretaceous magmatic event. Thick lava pilescovered the entire basin, and voluminous dikes and sills occur in the sedimentary sequences, mainly in the northeastern border of the basin. Despite the thermal effect, some Paleozoic sedimentary formations preserved their primary magnetization. The paleomagnetic results from the glacial Aquidauana Formation, an equivalent to the Itararé group in the north-western portion of the basin, indicated that the magnetization is compatible with the Middle-Late Permian age assigned in literature. A detailed investigation of the magnetic mineralogy and the magnetization of other Paleozoic sedimentary rocks led to the conclusion that the intrusive rocks were more effective than the lavas in disturbing the primary magnetization, especially in the low-clay content rocks. The secondary magnetizations identified in the different areas of the basin are not always compatible with the Early Cretaceous magnetization imprinted by the Paraná magmatism. This component prevails in the northeastern area, while a Permo-Triassic magnetization was identified in other areas. The results obtained so far are coherent with the geomagnetic reversal scale for the considered time interval, and the paleomagnetic poles agree with the APWP for South America.</p>

A Robust Proxy for Geomagnetic Reversal Rates in Deep Time

The strength of Earth’s magnetic field in the distant past can tell scientists whether the planet’s magnetic poles were steady or prone to frequent reversals.

Correction to: A full sequence of the Matuyama–Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

An amendment to this paper has been published and can be accessed via the original article.

A full sequence of the Matuyama–Brunhes geomagnetic reversal in the Chiba composite section, Central Japan

Geological records of the Matuyama–Brunhes (M–B) geomagnetic reversal facilitate the development of an age model for sedimentary and volcanic sequences and help decipher the dynamics of the Earth’s magnetic field. However, the structure of the geomagnetic field during the M–B geomagnetic reversal remains controversial due to its complex field behavior. In this study, we conducted paleo- and rock-magnetic analyses of samples from the Chiba composite section (CbCS), a continuous and expanded marine succession in Central Japan, to reconstruct the full sequence of the M–B geomagnetic reversal. We define an average stratigraphic position of the M–B boundary and estimate its age based on three sections in the CbCS and a neighboring drill core, TB-2. The average stratigraphic position of the M–B boundary in the CbCS is established at 1.1 ± 0.3 m above a widespread volcanic ash bed (the Byk-E tephra). Assuming a chronological error associated with orbital tuning of 5 kyr and stratigraphic uncertainty of 0.4 kyr, the M–B boundary in CbCS is at 772.9 ± 5.4 ka (1σ). The virtual geomagnetic pole, which is calculated from the paleomagnetic directions, shows several short fluctuations between 783 and 763 ka, with concomitant decreases in geomagnetic field intensity index. After termination of the field instabilities, the field intensity recovered and became higher than before the M–B boundary, with a stable normal polarity direction. The paleomagnetic records in the CbCS exhibit a field asymmetry between the axial dipole decay and field recovery, providing a full sequence of the M–B reversal, suggesting that the non-axial dipole field dominated several times during periods ca. 20 kyr long across the M–B boundary, due to depletion in the main axial dipole component. Our results provide probably the most detailed sedimentary record of the M–B geomagnetic reversal and offer valuable information to further understand the mechanism and dynamics of geomagnetic reversals. Graphical abstract