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

Detailed 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.