Abstract
We use polarity reversal systematics from numerical dynamos to quantify the hypothesis that the modulation of geomagnetic reversal frequency, including geomagnetic superchrons, results from changes in core heat flux related to growth and collapse of lower mantle superplumes. We parameterize the reversal frequency sensitivity from numerical dynamos in terms of average core heat flux normalized by the difference between the present-day core heat flux and the core heat flux at geomagnetic superchron onset. A low-order polynomial fit to the 0-300 Ma Geomagnetic Polarity Time Scale (GPTS) reveals that a decrease in core heat flux relative to present-day of approximately 30% can account for the Cretaceous Normal Polarity and Kiaman Reverse Polarity Superchrons, whereas the hyper-reversing periods in the Jurassic require a core heat flux equal to or higher than present-day. Possible links between GPTS transitions, large igneous provinces (LIPs), and the two lower mantle superplumes are explored. Lower mantle superplume growth and collapse induce GPTS transitions by increasing and decreasing core heat flux, respectively. Age clusters of major LIPs postdate transitions from hyper-reversing to superchron geodynamo states by 30-60 Myr, suggesting that superchron onset may be contemporaneous with LIP-forming instabilities produced during collapses of lower mantle superplumes.
Highlights
The geomagnetic field has reversed polarity ∼1 thousand times within the Phanerozoic (Gradstein et al, 2012) and there is evidence for polarity reversals throughout the Proterozoic (Pavlov and Gallet, 2010) as well as in the late Archean (Layer et al, 1996)
Mantle heat transport is mostly advective, carried by the global-scale circulation associated with surface plate motions and by smaller scale motions such as thermal plumes (Schubert et al, 2001). Both of these elements of the mantle circulation affect the geodynamo by regulating the heat flux on the core-mantle boundary (CMB), and are variable on time scales commensurate with the geomagnetic superchrons and their spacing
Phanerozoic examples of this variability include the change in mantle flow associated with the aggregation and breakup of supercontinent Pangaea (Zhang et al, 2010) and the Cretaceous pulse in Large Igneous Province (LIP) activity (Larson, 1991)
Summary
The geomagnetic field has reversed polarity ∼1 thousand times within the Phanerozoic (Gradstein et al, 2012) and there is evidence for polarity reversals throughout the Proterozoic (Pavlov and Gallet, 2010) as well as in the late Archean (Layer et al, 1996). Mantle heat transport is mostly advective, carried by the global-scale circulation associated with surface plate motions and by smaller scale motions such as thermal plumes (Schubert et al, 2001) Both of these elements of the mantle circulation affect the geodynamo by regulating the heat flux on the CMB, and are variable on time scales commensurate with the geomagnetic superchrons and their spacing. Phanerozoic examples of this variability include the change in mantle flow associated with the aggregation and breakup of supercontinent Pangaea (Zhang et al, 2010) and the Cretaceous pulse in Large Igneous Province (LIP) activity (Larson, 1991). We argue that the observed time lag between the decreases in reversal frequency that preceded the CNS and the ages of major Cretaceous and Cenozoic LIPs are broadly consistent with our hypothesized sequence of events, as is the uplift history of the African continent
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