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

A foundational assumption in paleomagnetism is that the Earth's magnetic field behaves as a geocentric axial dipole (GAD) when averaged over sufficient timescales. Compilations of directional data averaged over the past 5 Ma yield a distribution largely compatible with GAD, but the distribution of paleointensity data over this timescale is incompatible. Reasons for the failure of GAD include: (a) Arbitrary selection criteria to eliminate unreliable data vary among studies, so the paleointensity database may include biased results. (b) The age distribution of existing paleointensity data varies with latitude, so different latitudinal averages represent different time periods. (c) The time-averaged field could be truly non-dipolar. Here, we present a consistent methodology for analyzing paleointensity results and comparing time-averaged paleointensities from different studies. We apply it to data from Plio/Pleistocene Hawai'ian igneous rocks, sampled from fine-grained, quickly cooled material (lava flow tops, dike margins and scoria cones) and subjected to the IZZI-Thellier technique; the data were analyzed using the Bias Corrected Estimation of Paleointensity method of Cych et al. (2021, https://doi.org/10.1029/2021GC009755), which produces accurate paleointensity estimates without arbitrarily excluding specimens from the analysis. We constructed a paleointensity curve for Hawai'i over the Plio/Pleistocene using the method of Livermore et al. (2018, https:// doi.org/10.1093/gji/ggy383), which accounts for the age distribution of data. We demonstrate that even with the large uncertainties associated with obtaining a mean field from temporally sparse data, our average paleointensities obtained from Hawai'i and Antarctica (reanalyzed from Asefaw et al., 2021, https://doi. org/10.1029/2020JB020834) are not GAD-like from 0 to 1.5 Ma but may be prior to that.Plain Language Summary Paleomagnetists make the assumption that the Earth's magnetic field behaves like a bar magnet centered at the spin axis, known as a Geocentric Axial dipole or GAD. Compilations of the magnetic field's direction are largely consistent with this assumption, but compilations of its strength (paleointensity) are not. A number of causes for this could be: (a) The different experimental methods and the criteria used to pass or exclude paleointensity data might cause differences in records. (b) The ages of records differ between locations. (c) The field really doesn't behave like a bar magnet. To test this, we performed paleointensity experiments on rocks collected in Hawai'i and compared our results to results of similar age from other locations analyzed using the same methodology. The three locations analyzed in this study do not produce time-averaged paleointensities consistent with a GAD field for the most recent 1.5 million years, but a GAD field cannot be ruled out before this time. This indicates that differences in time-averaged field strength in global records can be unrelated to differences in methodology or age between studies.

Automated summary

Paleomagnetists assume that the Earth's magnetic field behaves like a Geocentric Axial Dipole (GAD). Directional data compiled over the past 5 million years support this assumption, but paleointensity data do not. The failure of GAD can be attributed to arbitrary selection criteria, varying age distribution of data, and the possibility of a non-dipolar field. This study presents a consistent methodology for analyzing paleointensity data and compares results from different studies. The findings suggest that the time-averaged paleointensities from Hawaii and Antarctica are not GAD-like for the past 1.5 million years. However, GAD cannot be ruled out before this time. Differences in global records of field strength can be unrelated to methodology or age.