Weaker axially dipolar time-averaged paleomagnetic field based on multidomain-corrected paleointensities from Galapagos lavas

Abstract

The geomagnetic field is predominantly dipolar today, and highfidelity paleomagnetic mean directions from all over the globe strongly support the geocentric axial dipole (GAD) hypothesis for the past few million years. However, the bulk of paleointensity data fails to coincide with the axial dipole prediction of a factor-of-2 equator-to-pole increase in mean field strength, leaving the core dynamo process an enigma. Here, we obtain a multidomain-corrected Pliocene-Pleistocene average paleointensity of 21.6 ± 11.0 μT recorded by 27 lava flows from the Galapagos Archipelago near the Equator. Our new result in conjunction with a published comprehensive study of single-domain-behaved paleointensities from Antarctica (33.4 ± 13.9 μT) that also correspond to GAD directions suggests that the overall average paleomagnetic field over the past few million years has indeed been dominantly dipolar in intensity yet only ∼60% of the present-day field strength, with a long-term average virtual axial dipole magnetic moment of the Earth of only 4.9 ± 2.4 × 10 22 A·m 2 .geomagnetism | paleomagnetism | Thellier paleointensity experiment | multidomain correction | time-averaged dipole field I n 1600, William Gilbert (1) articulated that Earth's magnetic field was well approximated by a bar magnet centered along its rotation axis. This model has become elaborated for the timeaveraged field as the geocentric axial dipole (GAD) hypothesis, which essentially all paleogeographic reconstructions rely on. Paleomagnetic field directions for the past 5 My for both normal and reverse polarity chrons from around the globe show that the mean inclinations closely correspond to the GAD model (2), which predicts a simple relationship between mean inclination, I, and site latitude, Lat: tan(I) = 2 × tan(Lat). The GAD hypothesis also predicts that the mean field intensity should vary with latitude, as [1 + 3 × cos 2 (90°− Lat)] 1/2 , whereby the intensity at the poles is twice that at the Equator. The modern field [i.e., International Geomagnetic Reference Field (IGRF) (3)] is dominated by an axial dipole with steep field directions at the poles about twice the intensity (∼60 μT) compared with that (∼30 μT) of near-horizontal directions in the equatorial belt and corresponds to a virtual axial dipole moment (VADM) of ∼8 × 10 22 A·m 2 . However, the expected latitudinal dependence has yet to be shown in the current global paleointensity database PINT2014.01 (4) even for the most recent past 5 My. Analyses of these data using various selection criteria tend to show a puzzling near uniformity of mean intensities with latitude and largely because of the dominance of data from 15°t o 30°latitude with ages of 0.03-0.5 Ma, which average close to the present field intensity, would suggest the time-averaged field for normal and reversed polarity chrons over the past 5 My was also near the present VADM of ∼8 × 10 22 A·m 2 ( Fig. 1 and SI Appendix, SI Text and Fig. S1).The factor-of-2 difference between the poles and Equator provides the maximum signal to res...

Paleointensity From Subaerial Basaltic Glasses From the Second Hawaii Scientific Drilling Project (HSDP2) Core and Implications for Possible Bias in Data From Lava Flow Interiors

“…The cause of the discrepancy between the observations of field intensities and the theoretical predictions made from a GAD model remains unclear. Several possibilities are as follows: (1) the average dipole moment fluctuates through time with several authors suggesting that the more recent average moment was higher than the preceding times (e.g., Selkin & Tauxe, ; Ziegler et al, ) and mixing data from different field states would result in apparent non‐GAD behavior; (2) the high latitude results could be suppressed by the so‐called tangent cylinder related to the Earth's inner core (as discussed in Lawrence et al, ); or (3) one or more data sets could suffer from a potential bias in field strength estimates derived from nonideal behavior (e.g., Wang, Kent, & Rochette, ). Support for the latter hypothesis can be found in, for example, Cromwell et al (), who found that intensities derived from nonlinear Arai plots, which show the relationship between remaining natural remanence and laboratory induced remanence (Nagata, Arai, & Momose, ), were lower than those derived from linear plots from the same lava flows.…”

Considerations for Latitudinal Time-Averaged-Field Palaeointensity Analysis of the Last Five Million Years

“…In studies that compare intensity vs. latitude (e.g., Lawrence et al, 2009;Cromwell et al, 2015;Wang et al, 2015;Døssing et al, 2016), it is the geographical latitude, not the palaeomagnetic latitude, that is used; this removes the circular logic of using GAD theory to calculate the palaeomagnetic latitude in a study of the robustness of the GAD hypothesis. Only palaeointensity data from the last 5 Myr are usually considered, because, as a first-order approximation, this removes the need to include any tectonic corrections to the sample locations' geographic latitudes.…”

“…To study the TAF, the palaeointensity data are binned from different time periods over intervals of typically 10-15 • latitude (Wang et al, 2015;Døssing et al, 2016 …”

“…However, in the last 20 years the palaeointensity database has changed significantly, with the inclusion of far more high-latitude data points. Additionally, the selection criteria for drawing samples from the database are far more stringent now than it was in the late 1990s (e.g., Lawrence et al, 2009;Cromwell et al, 2015;Wang et al, 2015;Døssing et al, 2016). As we will TABLE 1 | Parameters for secular variation models considered in this paper.…”

“…In the analysis of palaeointensity data as a function of latitude, it is common to plot palaeointensity data as a function of latitude for latitudinally binned "reliable" absolute palaeointensity data from the last 5 Myr using the PINT database (Biggin et al, 2009) available at http://earth.liv.ac.uk/pint; when binning the data sometimes the average of each bin is taken (e.g., Lawrence et al, 2009;Wang et al, 2015) and sometimes the median (e.g., Cromwell et al, 2015;Døssing et al, 2016). The definition of "reliable" varies between studies, but the same trend is observed: the measured intensity data deviates most from the GAD intensity model B D at high latitudes, where B D varies with the (magnetic) co-latitude θ by.…”