MCADAM: A Continuous Paleomagnetic Dipole Moment Model for at Least 3.7 Billion Years

Abstract

The evolution of Earth's deep interior since core formation (Nimmo, 2015) >4 billion years ago (Ga) remains a topic of considerable study. Obtaining information of the deep interior is generally restricted to present-day observations. Alternatively, insights on processes occurring before the modern era require sampling geologic materials that formed at, or were transported to, Earth's surface. However, the geomagnetic field is generated in the liquid fraction of Earth's core through the geodynamo, and changes in the morphology, strength and variability in the geodynamo may reflect the evolution of core processes and the pattern of heat flux across the core-mantle boundary. The geomagnetic field is also a critical component for Earth's habitability (Rodríguez-Mozos & Moya, 2017) due to the protective envelope provided by the magnetosphere against atmospheric erosion by charged solar particles. It is speculated that changes in the paleomagnetosphere may have contributed to substantial changes in the evolution of life (e.g., Meert et al., 2016).Paleomagnetic studies offer the potential to help close this gap: when rocks bearing magnetic carriers form, the geomagnetic field imparts a remanent magnetization that under ideal circumstances can be robustly preserved for billions of years. The strength of the geodynamo can be described by the magnitude of the dipole moment, the first-degree spherical harmonic component of the field, which should reflect ∼90% of the surface field signal. A fundamental question regarding Earth's dynamo is how the dipole moment has changed over long timescales (≫millions of years). Paleointensities measured from the same geologic time (e.g., from the same cooling-unit, referred to as a "site") can be related to paleointensities from other locations by transforming the paleointensity (B) into a virtual (axial) dipole moment (V(A)DM) using the following equation (Smith, 1967):

Salient Changes of Earth's Magnetic Field Toward the End of Cretaceous Normal Superchron (CNS)

“…The green shaded area indicates the uncertainty of the extrapolated RPI. The violet line is median dipole moment from time‐varying model (MCADAM.1b) of dipole strength from PINT v8.1.1 data (Bono, Paterson, & Biggin, 2022).…”

“…The extrapolated RPI curve for the entire CNS shows strong mean field strength in the middle and weak mean field strength at the beginning and the ending of CNS (Figure 9c). In comparison to the time‐averaged dipole strength variations (MCADAM.1b) based on the absolute paleointensity (API) data during CNS (PINT v8.1.1, Bono, Paterson, van der Boon, et al., 2022; Bono, Paterson, & Biggin, 2022) (Figure 9c), the two data sets show by and large similar variations at the beginning and ending of CNS, and the major discrepancy occurs in the middle CNS where the extrapolated RPI curve (green) shows a strong field while the dipole strength curve (violet) indicates a weak field (Figure 9c). The discrepancy might be due to the relatively limited availability of the API data (Figure 9c) upon which the dipole strength curve was constructed for the middle CNS, suggesting a need to acquire more API data for the middle CNS in the future.…”

Changes in Non‐Dipolar Field Structure Over the Plio‐Pleistocene: New Paleointensity Results From Hawai'i Compared to Global Data Sets

“…A similar approach using curve fitting to calculate time‐averaged paleointensities was recently used in Bono et al. (2022).…”

“…We discuss the rationale for using this approach in Section 4.3. A similar approach using curve fitting to calculate time-averaged paleointensities was recently used in Bono et al (2022).…”

Detecting dipolarity of the geomagnetic field in the paleomagnetic record

“…However, it becomes statistically significant (with R values comparable to those obtained for the 0.773 to 130 Ma interval) when strict filters on both intensities and directions are applied, with the caveat that the number of determinations is lower than 50 and P is close to its critical value ( Table 1 ). For the sake of comparison, we also reproduced the analysis with the PINT v8.1.1 database ( 17 , 18 ), filtered using our selection criteria or its own qualitative selection criteria [QPI ( 19 ); SI Appendix , Table S3 ] and noting that the PINT database starts at 50 kyr. We obtained robust correlations for the 0.773 to 130 Ma interval, noting that the highest values of R, albeit lower than with our selection criteria, were reached for the simultaneous fulfilment of QAGE (reliable age), QSTAT (five or more determinations per cooling unit with a relative SD lower or equal to 25%) and QDIR (well-defined paleomagnetic directions).…”