How well can we know the ancient geomagnetic field? - Wyn Williams

The structural, chemical, and thermal evolution of the deep Earth over the billions of years of its existence has modified both the power and shape of the geomagnetic field over time, and left its trace in the paleomagnetic recordings accessible in rocks on the surface of the Earth. This vast record holds the story of some of the most dramatic geological changes in the young and adolescent Earth. However, reliable palaeomagnetic records of these events have proven difficult to obtain and paleointensity surveys are often plagued by large errors and high failure rates, and where frequently the origin of the poor recording fidelity remains undefined.
In recent years we have begun to explore the more complex types of magnetic structures that dominate many paleomagnetic samples. We have been able to show that pseudo-single-domain (PSD) grains often occupy a single vortex (SV) domain structure, especially for grains just above the maximum single-domain grain size. The remanence of such grains is carried in the vortex core, and shares many magnetic recording characteristics with single domain (SD) grains, such as high magnetic remanence and high thermal stability. However, we have now also discovered a more intriguing aspect of SV states, where they can take on many of the features of multidomain (MD) grains, with low stability and multiplicity of allowed domain states.
In this presentation we will review the similarities and differences between SD, SV and MD domain states and explore how predictable SV behaviour is as a function of time and temperature. We will take a detailed look at the reciprocity of magnetic blocking and unblocking in PSD grains, and demonstrate that we are now able to ask the question: How well can we know the ancient geomagnetic field?