Why does magnetostratigraphy not work in the Devonian?

Abstract:

The global polarity time scale (GPTS) is relatively unconstrained for the Paleozoic, particularly the Devonian. Constraining the GPTS and reversal frequency for the Devonian is crucial for the understanding of the behaviour of Earth’s magnetic field. Furthermore, construction of a GPTS for the Paleozoic could provide a valuable tool for age determination in other studies. However, most paleomagnetic data from the Devonian is problematic. The data are difficult to interpret and don’t have a single easy to resolve (partial or full) overprint. Paleointensity studies suggest that the field was much weaker than the field of today, which could have been accompanied by many reversals (a hyperreversing field). We sampled middle and late Devonian sections in Germany, Poland and Canada which show low conodont alteration indices, implying that they were not heated significantly. We show in this study that there are significant issues with these data, and they are not straightforward to interpret. We compare our data to other magnetostratigraphic studies from the Devonian and review the polarity pattern as presented in the Geologic Time Scale. Combined with estimates for the strength of the magnetic field during the Devonian, we tentatively suggest that the field during the Devonian might have been so weak, that obtaining reliable paleomagnetic data from Devonian rocks is essentially impossible. In order to push forward the understanding of the Devonian field, we emphasise the need for studies to provide fully accessible data down to specimen level demagnetisation diagrams. Incorporating all data, no matter how weird or bad they might seem, is the only way to advance the understanding of the Devonian magnetic field. Paleointensity studies show that the field during the Devonian had a similar strength to the field during the Ediacaran. A fundamentally weak and perhaps unusual field during the Devonian might have had an influence on evolution and extinctions during the Devonian. There is a large number of biological crises in the Devonian, and we here pose the question whether the Earth’s magnetic field had an influence on those.