The implications of non-suppressed geomagnetic secular variation during the Permo-Carboniferous Reversed Superchron

Abstract

We have investigated variations in the natural remanent magnetisation (NRM) in red beds which were deposited during the Permo-Carboniferous Reversed Superchron (PCRS) near the paleo-equator. These red beds are reliable recorders of the geomagnetic field, because the NRM is carried by detrital hematite. We analysed the angular standard deviation (ASD) and spectral content of the directional records to quantify the amount of paleosecular variation (PSV) during the time of deposition. The ASD of a high-resolution directional record of the Dôme de Barrot which spans approximately 27 kyr is 10°. This is comparable to recent (0–5 Ma) low-latitude lavas from the PSVRL database ( McElhinny and McFadden, 1997) and to recent lake sediments from the SECVR database ( McElhinny and Lock, 1996). The frequency spectra of declination and inclination are also similar to those from the recent lake sediments: spectral power significant at the 95% level is concentrated in periods of ∼1–5 kyr, higher frequencies are not significant. From the ASD and spectral analyses, it appears that the amount of PSV during—at least part of—the PCRS was comparable to that of recent times. Several heuristic and numerical geodynamo models are discussed in the context of Superchrons and PSV. There is a discrepancy between the amount of SV shown by our data and that suggested by numerical models which use lateral heat flux variations at the core–mantle boundary (CMB). However, computational limitations still force some model parameters to have values that are unrealistic for the Earth. For the phenomenological geodynamos, ‘low-energy state’ models which suggest that PSV is suppressed during a Superchron are not supported by the Dôme de Barrot data. Rather, a ‘high-energy state’ of the geodynamo, in which outer-core convection is vigorous, is favoured as a prerequisite for a Superchron to occur.