Correlated Relative and Absolute Geomagnetic Paleointensities From a Pliocene N‐R Polarity Reversal Record in Basaltic Lava Flows on Kauai, Hawaii

New paleomagnetic results from basaltic lava flows erupted 4Myr ago on Kauai (Hawaii) reveal how the geomagnetic intensity changed as the field switched from normal to reverse (N-R) polarity. Following stepwise alternating field demagnetization of each sample's natural remanent magnetization, an anhysteretic remanent magnetization (ARM) was imparted in a 50 T bias field and then alternating field demagnetized. Very similar behavior of the 35 to 90mT coercivity fractions suggest that the natural remanent magnetization (interpreted to be a primary thermoremanent magnetization, TRM) and ARM reside in the same subpopulation of magnetic grains. With the ARM intensity serving as an analog for TRM susceptibility, the ratio TRM/ARM provides a measure of relative paleointensity. Flow-averaged TRM/ARM ratios correlate with a small set of double-heating paleointensities determined on companion samples, allowing a scaling of the ratios to absolute ancient field intensities. During the N-R transition, the field strength dropped by 91% compared to the maximum recorded by the flows. This decrease is comparable to that observed in other paleomagnetic studies and numerical simulations. During an interval of NE down directional stasis recorded by 10 flows in the lower half of the section, the geomagnetic intensity rose to over 20T and then decreased to near 5T. This distinctive behavior could mark the waxing and waning of a stationary, inward directed flux patch on the core-mantle boundary centered NE of Kauai. Plain Language Summary The Earth's magnetic field reverses its orientation every few hundred thousand years (on average), but little is known about what goes on during the transitional interval. This paper reports research on a stack of lava flows in Kauai (Hawaii) that happened to erupt while the geomagnetic field was switching to reverse (south pointing) polarity 4million years ago. The faint and ancient magnetism of the lava flows preserves a record of how the field varied during the flip. As has been found in several previous studies, the geomagnetic field weakened considerablyby approximately 90%during the reversal. There was also a distinctive interval during which the ancient field at Kauai first strengthened and then weakened (by nearly 80%), pointing all the while in a constant northeast and down direction. This patternsteady direction but changing strengthmay have resulted from the blossoming and demise of geomagnetic field feature fixed to a spot northeast of Kauai.