Low Geomagnetic Paleointensity in the Mid-Part of the Kiaman Superchron

Abstract

The Kiaman Reversed Superchron (∼260–318 Ma) is the longest known period of single geomagnetic polarity in Earth history (∼55 million years). It is associated with anomalously low dispersion of virtual geomagnetic poles and some high estimates of Earth's dipole moment. However, many of these strong paleointensity data are of poor or unknown quality. Here we report full-vector paleomagnetic measurements from a series of mid-Kiaman (∼282–302 Ma) lamprophyre dykes from Orkney, Scotland. A total of 258 paleointensity experiments were performed alongside rock magnetic experiments and scanning electron microscopy. Eleven dykes produced virtual dipole moment estimates indicating that the field was weak (between 0.1 and 2.9 × 10²² Am²) at 302 Ma and only moderately stronger (between 2.7 and 7.1 × 10²² Am²) at 282 Ma. These new data challenge the paradigm of a uniquely strong field in the Kiaman superchron and are especially intriguing when considered alongside recent studies of geomagnetic field behavior during the later Cretaceous Normal Superchron (∼84–121 Ma). Average dipole moment may be marginally elevated and paleosecular variation moderately suppressed during the superchrons but, in other respects, the field can appear similar to that encountered during other times. The deep-Earth conditions allowing for the generation of a geomagnetic field that is capable of weak, unstable behavior and transitory polarity inversions, while nevertheless maintaining a dominant single polarity for tens of millions of years, is not yet clear. The challenge of explaining superchrons and their geodynamic origin motivates further study integrating paleomagnetic observations with predictions from geodynamo simulations.