Weak paleointensities from 1.6 Ga Greenland dykes: Further evidence for a billion-year period of paleomagnetic dipole low during the Paleoproterozoic

The Paleoproterozoic era is the longest in Earth's history, with significant changes hypothesised to have occurred in the deep Earth's physical and chemical conditions at this time. It has been suggested that the paleomagnetic field became weaker at this time (∼2.4 Ga) and remained weak for the next billion years. Paleomagnetism is intrinsically linked to, and is able to inform on, ancient deep Earth processes; a weak dipole strength sustained over this time period may have implications for both core and mantle evolution.

We test this hypothesis here in a two-fold approach: (1) A paleointensity study on the widespread ca. 1.6 Ga diabase/dolerite Melville Bugt dyke swarm. The swarm extends along the west coast of Greenland for more than 1000 km and intruded over ∼13 million years, capturing polarity reversals of Earth's magnetic field. (2) A detailed statistical analysis on the long-term trend in average dipole moment from an improved paleointensity dataset (PINT.org) that has recently undergone a major update.

Five of the Greenland dykes produce paleointensity results ranging from 1.4 μT to 5.1 μT (virtual dipole moment range 0.3–1.2 × 10²² Am²) during the mid-point of this extended period of ‘dipole low’. Our statistical study robustly confirms that this one-billion-year period was indeed associated with an anomalously weak dipole moment (2.7 × 10²² Am²) relative to 500-million-year intervals before and after, which were almost twice as strong. Sampling of more geographically diverse rocks from this time is needed to yield a clear picture of the long-term time evolution of the dipole moment.