New paleointensity results from Hawaii indicate age sampling may be responsible for the “missing dipole”

Abstract:

Paleomagnetists assume that the Earth’s magnetic field can be approximated by a Geocentric Axial Dipole (GAD) over sufficiently long time scales. Paleointensity data from the last 5 million years challenge this assumption, with data at high latitudes giving surprisingly similar values to those found at low latitudes (impossible for a GAD field, which would predict a doubling in field strength from equator to poles). Independence of intensity on latitude could be caused by biases in sampled ages or methodology.

To examine the role of methodology, studies in Antartica (Asefaw et al., in prep) and Iceland (Cromwell et al., 2015a) sampled rapidly cooled parts of lava flows in an attempt to obtain material containing single domain magnetic carriers. These studies suggest pre-Holocene average dipole moments of ~44 and 47 ZAm^2 respectively which would predict an average field strength at Hawaii of ~20 uT (far less than the average field strength from sites in the MagIC data base).

For this study, we sampled rapidly cooled material from lava flow tops, cinder and spatter cones and from the chilled margins of dikes with ages between 0 and 4 Ma from the Hawaiian islands. Specimens were subjected to the IZZI-Thellier protocol (Yu et al., 2004) at Scripps paleomagnetic laboratory. Different specimens from the same site were given a TRM in different lab fields to determine whether this affected the success rate of the experiment.

The average field value for the 18 sites passing the same criteria as in the high latitude studies (CCRIT, Cromwell et al, 2015b) was approximately 27 µT. Specimens collected from sites with ages ranging from 2-4 Ma recorded lower intensities (average ~22 µT) than those collected from Brunhes age specimens (average ~40 µT), demonstrating the strong effect that biases in the distribution of ages can have on the intensity record. Our new data are therefore consistent with those of similar quality from high latitude of similar age and also those from Wang et al. (2015) from Galapagos.

Cromwell et al. , 2015a (doi: 10.1002/2014JB011828)

Cromwell et al. , 2015b (10.1016/j.pepi.2014.12.007)

Yu et al. , 2004(10.1029/2003GC000630)

Wang et al. , 2015 (10.1073/pnas.1505450112)