Do changes in geomagnetic secular variation, dipole moment and polarity reversal frequency correlate over the past 155 Myr?

Abstract

SUMMARY Changes in palaeosecular variation, dipole moment and polarity reversal frequency are salient features of the Earth’s magnetic field over the geological past, yet how these changes are linked by the geodynamo remains controversial. To further understand this issue, we provide new absolute (API) and relative (RPI) palaeointensities from the ∼1-km-thick basaltic sequence of Waja (North Ethiopia) emplaced around 31 Ma, yielding an instantaneous virtual dipole moment of 57 ± 9 ZAm2 (1σ, N = 18) and a relative variability in intensity εF = 0.39 ± 0.07 (1σ, N = 19). Our analysis of the API database with strict selection criteria (inclusion of Thellier-style determinations with pTRM checks only, at least five determinations per cooling unit, and within-unit relative standard error lower than 10 per cent) fails to identify any robust correlation between changes in dipole moment and reversal frequency over the past 155 Myr. More convincingly, the available RPI results are consistent with an increase of the palaeosecular-variation proxy εF with reversal rate, as predicted by numerical dynamo simulations. We also find that the API-based estimate εF = 0.40 ± 0.03 (1σ, N = 104), computed from the filtered version of the World Palaeointensity Database for the 0.77–31 Ma interval, is consistent with the scaling rule, suggesting that the API record has been sufficiently sampled over the past 31 Ma. We thus speculate that the absence of negative correlation between changes in dipole moment and reversal frequency in the API database over the past 155 Myr may be the result of insufficient sampling prior to 31 Ma rather than the signature of an intrinsic geomagnetic feature.

Detecting dipolarity of the geomagnetic field in the paleomagnetic record

“…We based our analysis on the latest version of the BM absolute paleointensity database ( 14 , 15 ), which contains all known results (5,536 from 427 publications) obtained from igneous or baked rocks, irrespective of the chosen API method. We discarded records during transitional times according to the author’s prescriptions as well as the records that are not associated with paleodirectional data in the database.…”

“…2 B ). To get a first insight into the correlation between B and throughout geological time, we applied to the Borok–Munich (BM) API database ( 14 , 15 ) a rolling window of 75 points (each point corresponding to an independent cooling unit), over which we determined the Pearson correlation coefficient R and its P value P ( Fig. 2 C and D ; see Methods for a description of the database and the parameters).…”

“…In this case, an increase in R directly reflects the efficacy of the selection strategy. Considering the most robust Thellier-style determinations with pTRM checks and increasing the number of determinations per cooling unit together with the relative SE ( 14 , 48 , 49 ) prove to be the most fruitful filtering strategy for the 0.773 to 130 Ma interval. On the contrary, increasing the QPI score ( 17 , 19 ) was minimally effective (even counterproductive when increasing the QPI score from four to five; SI Appendix , Table S3 ) to improve the quality of the considered datasets.…”

High‐Latitude Geomagnetic Secular Variation at the End of the Cretaceous Normal Superchron Recorded by Volcanic Flows From the Okhotsk‐Chukotka Volcanic Belt

“…Single‐crystal API determinations thus supports the existence of a correlation between chron duration and dipole moment over the past 160 Myr (Tarduno & Cottrell, 2005). In constrast, most of the recent statistical analyses of the whole‐rock API determinations tend to refute the existence of such a correlation over this time interval (e.g., Bobrovnikova et al., 2022; Di Chiara et al., 2021; Eid et al., 2022; Ingham et al., 2014; Lhuillier, Lebedev, et al., 2023, Lhuillier, Shcherbakov, & Sycheva, 2023), with the exception of Kulakov et al. (2019).…”

“…Generally, the geomagnetic field can also experience fluctuations in its average intensity F and its spatiotemporal variability during stable periods (termed hereafter paleosecular variation, PSV). Whether changes in f rev , F and PSV occur in a coupled or decoupled way is a vivid matter of debate among paleomagnetists (e.g., Cox, 1968; Doubrovine et al., 2019; Eid et al., 2022; Kulakov et al., 2019; Lhuillier, Lebedev, et al., 2023, Lhuillier, Shcherbakov, & Sycheva, 2023; McFadden et al., 1991; Tarduno et al., 2002). Conceptually, numerical dynamo simulations tend to support an increase in F with decreasing f rev or the amplitude of PSV, with a coupling controlled in first approximation by the vigor of convection (e.g., Driscoll & Olson, 2009; Lhuillier & Gilder, 2013).…”