Evidence for a very-long-term trend in geomagnetic secular variation

The Earth's inner core is believed to inhibit rapid fluctuations in the geomagnetic field from developing into full polarity reversals(1,2). Consequently, during the Precambrian, the smaller size of the inner core might suggest that polarity reversals could occur more readily. It is therefore surprising that there are indications that reversals were rare during this period(3,4). Here we use new and existing palaeomagnetic data from three continents to examine the stability of the Earth's magnetic field from 2.82 to 2.45 billion years ago. We show that, on average, geomagnetic secular variation (the field variations produced by normal geodynamo action) during the late Archaean and early Proterozoic was different from that of the past 200 million years; specifically, the apparent variability of the geomagnetic pole as viewed at low and mid-latitudes was reduced relative to the past 200 million years. According to both dynamo simulations(4) and more recent palaeomagnetic field observations(5), the observed pattern of secular variation suggests a lower frequency of polarity reversals 2.5 billion years ago. This may imply that the geodynamo is becoming progressively less stable over long timescales, consistent with some numerical simulations(6,7), possibly as a result of changing outer-core geometry that has accompanied inner-core growth.