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.
The geomagnetic field exists as a consequence of a dynamic system and shows variations across a range of timescales (Johnson & McFadden, 2015). Long-term behavior is often described in terms of the intensity, reversal rate, morphology, and stability of the field. As all of these features are a result of the geodynamo, it follows that their variations should be intrinsically linked and that relationships may exist between them (
<p>Palaeomagnetic field behaviour within the Triassic is relatively poorly documented in comparison with other periods from across the last ca 250 Ma. Developing a more complete understanding of the Triassic field has important implications for discussion surrounding the Mesozoic Dipole Low (MDL) and the processes that govern field intensity and reversal regimes. We have conducted the first palaeosecular variation study that incorporates Triassic virtual geomagnetic pole (VGP) data, and analysed this data within the context of the average reversal frequency for the period. We observed remarkably similar VGP dispersion patterns from the late Permian, after the Permo-Carboniferous Reversed Superchron, until the onset of the Cretaceous Normal Superchron, despite fluctuating mean reversal rates. We have also completed palaeointensity experiments on samples with a range of lithologies collected from two localities in Argentina as well as pillow basalts from northern Italy. Previously published radiometric ages place our sampled lithologies across all three Triassic epochs, presenting an opportunity to populate the palaeointensity record at multiple ages across the ~50 Ma lacuna. Results were obtained utilizing a range of methods, the IZZI+ thermal Thellier, Shaw, and pseudo-Thellier. Estimates of virtual dipole moment from these experiments will be presented and discussed in the context of their reliability, and importance in better defining the MDL.</p>
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