Abstract
Ascertaining the cause of variations in the frequency of geomagnetic polarity reversals through the Phanerozoic has remained a primary research question straddling paleomagnetism and geodynamics for decades. Numerical models suggest the primary control on geomagnetic reversal rates on 10 to 100 Ma timescales is the changing heat flux across the core-mantle boundary and that this is itself expected to be strongly influenced by variations in the flux of lithosphere subducted into the mantle. A positive relationship between the time-dependent global subduction flux and magnetic reversal rate is expected, with a time delay to transmit the thermal imprint into the lowermost mantle. We perform the first test of this hypothesis using subduction flux estimates and geomagnetic reversal rate data back to the early Paleozoic. Subduction area flux estimates are derived from global, full-plate tectonic models, and are evaluated against independent subduction flux proxies based on the global age distribution of detrital zircons and strontium isotopes. A continuous Phanerozoic reversal rate model is built from pre-existing compilations back to ~320 Ma plus a new reversal rate model in the data-sparse mid-to-early Paleozoic. Cross-correlation of the time-dependent subduction flux and geomagnetic reversal rate series reveals a significant correlation with a time delay of ~120 Ma (with reversals trailing the subduction flux). This time delay represents a value intermediate between the seismologically constrained time expected for a subducted slab to transit from the surface to the core-mantle boundary (~150–300 Ma), and the much shorter lag time predicted by some numerical models of mantle flow (~30–60 Ma). While the reason for this large discrepancy remains unclear, it is encouraging that our novel estimate of lag time represents a compromise between them. Although important uncertainties in our proposed relationship remain, these results cast new light on the dynamic connections between the surface and deep Earth, and will help to constrain new models linking mantle convection, the thermal evolution of the lowermost mantle and the geodynamo.
Highlights
Earth contains two great heat engines that meet at the core-mantle boundary (CMB) and interact thermally across it
Because sinking slabs are the primary driver of mantle convection, we expect that the subduction flux has played a dominating role in modulating the CMB heat flux, and that the positive correlation between the subduction area flux (SAF) and reversal rate time series is foremost driven by a subduction flux lead
With the aforementioned caveats in mind, we return to the question: what could the delay times between the SAF and reversal rate mean? According to the contention that the subduction flux lead is the most likely to be physically meaningful, we here focus on the ~120 Ma time delay with the reversal rates following the subduction flux (Fig. 10)
Summary
Earth contains two great heat engines that meet at the core-mantle boundary (CMB) and interact thermally across it. Whatever the transit time for a slab (and the thermal anomaly associated with it) to arrive in the lowermost mantle, the response of the geodynamo may be subject to a further lag of several tens of Ma, as stratified layers in the uppermost core change thickness in response to the changing conditions of heat flow across the CMB (Buffett, 2015). Before the thermal effects of descending slabs even reach the CMB, the shift in mass that they present may well modify Earth's moment of inertia tensor, causing true polar wander to occur (Gold, 1955; Steinberger and Torsvik, 2010) This could rotate the entire CMB heat flux pattern in the reference frame of the geodynamo, potentially causing changes in geomagnetic reversal rate but with a much shorter time lag (Biggin et al, 2012).
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