The Earth’s inner core grows by the freezing of liquid iron at its surface. The point in history at which this process initiated marks a step-change in the thermal evolution of the planet. Recent computational and experimental studies1,2,3,4,5 have presented radically differing estimates of the thermal conductivity of the Earth’s core, resulting in estimates of the timing of inner-core nucleation ranging from less than half a billion to nearly two billion years ago. Recent inner-core nucleation (high thermal conductivity) requires high outer-core temperatures in the early Earth that complicate models of thermal evolution. The nucleation of the core leads to a different convective regime6 and potentially different magnetic field structures that produce an observable signal in the palaeomagnetic record and allow the date of inner-core nucleation to be estimated directly. Previous studies searching for this signature have been hampered by the paucity of palaeomagnetic intensity measurements, by the lack of an effective means of assessing their reliability, and by shorter-timescale geomagnetic variations. Here we examine results from an expanded Precambrian database of palaeomagnetic intensity measurements7 selected using a new set of reliability criteria8. Our analysis provides intensity-based support for the dominant dipolarity of the time-averaged Precambrian field, a crucial requirement for palaeomagnetic reconstructions of continents. We also present firm evidence for the existence of very long-term variations in geomagnetic strength. The most prominent and robust transition in the record is an increase in both average field strength and variability that is observed to occur between a billion and 1.5 billion years ago. This observation is most readily explained by the nucleation of the inner core occurring during this interval9; the timing would tend to favour a modest value of core thermal conductivity and supports a simple thermal evolution model for the Earth.
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We thank T. Torsvik for organising the 7th Nordic Supercontinents Meeting and acknowledge financial support for this from the European Research Council (ERC Advanced Grant 267631) and the Research Council of Norway through its Centres of Excellence funding scheme (CEED 223272). We also thank J. Rees and L. Waszek for discussions. A.J.B. acknowledges funding from a NERC standard grant (NE/H021043/1). G.A.P. acknowledges funding from an NSFC grant (41374072). L.T. acknowledges funding from an NSF grant (EAR 1345003).
Extended data figures
Supplementary Table 1 shows the raw dataset used in this study. The format is identical to the PINT database (see http://earth.liv.ac.uk/pint/ for full details) with QPI criteria and values appended in the final column.
Supplementary Table 2 contains a TAA summary of data with QPI values of at least 3 showing the Mantle Groups and Secular Variation (SV) Groups used in the new likelihood test (see Methods for details). For definition of Interval, refer to the main text. Ref No. refers to the Reference Number within the PINT database (http://earth.liv.ac.uk/pint/); NData refers to the number of site mean V(A)DM estimates; V(A)DMMed and V(A)DMIQR refer to the median and interquartile range of the V(A)DM estimates respectively.
Supplementary Table 3 contains summary data for time intervals referred to in the main text and Figure 3 using minimum QPI values in the range 1-5. AgeMin and AgeMax refer to the minimum and maximum age estimates of the data within each interval. V(A)DMMed and V(A)DMIQR V(A)DMMed and V(A)DMIQR refer, respectively, to the median and interquartile range of the V(A)DM estimates within each interval and V(A)DM+95 and V(A)DM-95 refer to the 95% confidence limits on the median calculated using 10,000 bootstraps. NData refers to the number of V(A)DM estimates in each interval and NRef refers to the number of published studies that these are drawn from. In the final column, the P values produced by a series of Kolmogorv-Smirnov tests for equality of probability distribution are shown. The results are colour-coded according to the confidence with which the null hypothesis of identical distributions can be rejected as follows: red >99% confidence, orange >95% confidence, yellow > 90% confidence, green < 90% confidence. *Data in the RECENT interval were not assigned QPI values but rather selected on the basis of passing the criteria described in the Methods section.