A comparison of detailed equatorial red bed records of secular variation during the Permo-Carboniferous Reversed Superchron

Abstract

S U M M A R YDetailed secular variation records during a superchron may provide information on the behaviour of the geodynamo during periods that the field does not reverse. The Permian red beds in Dôme de Barrot (southern France)-deposited during the Permo-Carboniferous Reversed Superchron (PCRS, 317-265 Ma) -were previously argued to accurately record palaeosecular variation (PSV). This result is particularly valuable because the red beds were deposited near the palaeo-equator and therefore, according to one model of secular variation, determine the contribution of the even (symmetric) spherical harmonics of the field.We have extended the Dôme de Barrot sequence, and we have established a new record from the Permian red beds from the Lodève basin (southern France), also deposited at low latitudes. Additionally, we critically review published data from low latitude Permian red beds from the same basins. We focus on records with a sufficient number of samples, and acquire, whenever possible, the original data, or-as a second choice-parametrically sampled published site means. We test the distributions of these directions using a statistical secular variation field model (TK03.GAD). The angular standard deviation (ASD) of the virtual geomagnetic pole (VGP) distribution is a measure of (palaeo)secular variation, and we compare our values of ASD from the Permian red beds to those from lavas erupted in the last 5 Myr. Contrary to our previous studies, we now conclude that secular variation during the Permo-Carboniferous Reversed Superchron is slightly, but significantly, reduced at low latitudes compared with the recent field, as based on a latest compilation that supercedes earlier compilations. It also agrees with a new study, which found that PSV, at low palaeolatitudes during the Cretaceous Normal Superchron, was also significantly lower than in periods with frequent reversals.

Paleomagnetic directional groups and paleointensity from the flood basalt in the Tarim large igneous province: implications for eruption frequency

“…In terms of PSV magnitude, the Cretaceous normal Superchron has been characterized by a slightly smaller variation at low latitudes compared to the previous 5 Myr (Tarduno et al 2002;Biggin et al 2008). A similar tendency was reported for the PCRS (Halden et al 2009). However, for latitudes higher than ~30º, the PSV magnitude during the PCRS is statistically indistinguishable from data for the last 5 Myr (Franco et al 2012).…”

The Magnetic Field of Planet Earth

“…First, the few numerical dynamos displaying such a strong imbalance (with a weak "Quadrupole" family field) are particularly stable with respect to reversals (Coe and Glatzmaier 2006). Second, the VGP scatter during the previous Kiaman Reverse Superchron was quite similar to the one observed during the Cretaceous Normal Superchron (Haldan et al 2009). Taken together the above results suggest that during superchrons the geodynamo operated in a very different way than at other times, with no reversals and a significantly depleted "Quadrupole" family field.…”

Late Permian paleomagnetic results from the Lodève, Le Luc, and Bas-Argens Basins (southern France): Magnetostratigraphy and geomagnetic field morphology

“…The Lodève Basin redbeds have been studied by several authors (Kruseman, 1962;Merabet and Guillaume, 1988;Cogné et al, 1990;Maillol and Evans, 1993;Haldan et al, 2009) who all conclude that the entire sequence carries reversed polarity and thus falls within the PCRS. But Evans (2012) found evidence-from a single sample-of a return to normal polarity at the very top of a section that is stratigraphically higher than all previous studies.…”

“…Results from a further 88 samples (cut from six large blocks collected from different layers in the lower part of the section studied by Maillol and Evans) were added by Haldan et al (2009). These latter authors make the important observation that the directional results from the entire 1200 m section fall into two groups, a lower one (9), which they assign to the Sakmarian-Artinskian Stages, and an upper one (10), which they assign to the Kungurian-Wordian Stages.…”

“…Second, the pole obtained from igneous rocks (16) is compromised by ambiguous conglomerate tests(Vlag et al, 1997). Pole 17 is from the Dôme de Barrot compilation ofHaldan et al (2009) who add new data to the earlier work of van den Ende (1970) andKruiver et al (2000). We include…”