UC San Diego

The structure of the Earth’s magnetic field today is constantly changing, driven by convection currents in the Earth’s outer core. The dominant force in these convection currents is the rotation of the Earth, and so many paleomagnetists assume that when averaged over long enough timescales, the field behaves like a bar magnet centered on the Earth’s spin axis. This is known as the Geocentric Axial Dipole (GAD) hypothesis.

Igneous rocks and other materials can preserve a record of the direction and strength of the Earth’s magnetic field at the time that they formed. Global records of Earth’s magnetic field direction (paleodirection) from igneous rocks over the last 10 million years require only a small deviation from a GAD field, whereas global records of the field strength (paleointensity) display a latitudinal distribution which is highly inconsistent with a dipole.

One problematic aspect of paleointensity records is that some rocks violate the assumptions of the paleointensity experiment, leading to inaccurate estimates of paleointensity. We describe two statistical methods which deal with different behaviors that are related in some way to these inaccurate results. The Bias Corrected Estimation of Paleointensity (BiCEP) method tries to find a relationship between non ideal behavior in the experiment and bias in the results, allowing us to obtain accurate paleointensities without excluding specimens from our analyses. The Thermal Resolution of Unblocking Temperatures (TROUT) method allows us to isolate a range of measurement temperatures that are appropriate to use in a paleointensity analysis.

We present a set of high quality paleointensity data from Hawaii, which we analyze using the aforementioned BiCEP method. We also reanalyze data from paleointensity studies targeting similar lithologies from northern Israel and Antarctica using the same methodology. We account for the different temporal distributions of these studies by taking the time average of a paleointensity curve constructed for each location. Despite having large uncertainties, our time averages show that the Earth’s magnetic field cannot be fit by a dipole when averaged over the last 1.5 Ma, but may have been dipolar before this time.