Devonian magnetostratigraphy: new data and old problems

2021 ◽  
Author(s):  
Annique van der Boon ◽  
Andy Biggin ◽  
Daniele Thallner ◽  
Mark Hounslow ◽  
Jerzy Nawrocki ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Time Scale ◽  
Age Determination ◽  
Paleomagnetic Data ◽  
Show Evidence ◽  
Reversal Frequency ◽  
Geomagnetic Polarity ◽  
Geomagnetic Polarity Time Scale

<p>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). In order to improve the geomagnetic polarity time scale in the Devonian, and quantify the number of reversals in this time, we sampled three Devonian sections in Germany, Poland and Canada. These sections show evidence that the rocks were not significantly heated, and they show little evidence for remineralisation. This minimises the chance the rocks were remagnetised after the Devonian. Our data show that even when rocks are well qualified to have reliably recorded the Devonian field, the interpretation is not straightforward. We also discuss problems with the Devonian GPTS as presented in the geologic timescale.</p>

Synthesis of palaeomagnetic datasets suggests that the geomagnetic field was persistently non-uniformitarian in the Devonian

2021 ◽  
Author(s):  
Annique van der Boon ◽  
Andy Biggin ◽  
Daniele Thallner ◽  
Mark Hounslow ◽  
Jerzy Nawrocki ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Weak Field ◽  
Paleomagnetic Data ◽  
Independent Evidence ◽  
Tectonic Plates ◽  
Polar Wander ◽  
Push Forward ◽  
Geomagnetic Polarity ◽  
Life On Earth ◽  
Geomagnetic Polarity Time Scale

<p>The Devonian has long been a problematic era for paleomagnetism. Devonian data are generally difficult to interpret and have complex partial or full overprints. These problems arise from paleomagnetic data obtained from both sedimentary and igneous rocks. As a result, the reconstruction of motions of tectonic plates is often troubling, as these rely on apparent polar wander paths constructed from Devonian paleomagnetic poles. Also the geomagnetic polarity time scale for this time period is poorly constrained. Paleointensity studies suggest that the field was much weaker than the field of today, and it has been hypothesised that this was accompanied by many polarity reversals (a hyperreversing field). We review studies on Devonian paleopoles, magnetostratigraphy and paleointensity. We tentatively suggest that the field during the Devonian might have been so weak and perhaps of a non-dipolar configuration, that obtaining reliable paleomagnetic data from Devonian rocks is extremely difficult.  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 complex or bad they might seem, is the only way to advance the understanding of the Devonian magnetic field. Recent paleointensity studies appear to suggest that the Devonian and Ediacaran were both extreme weak field intervals. For the Ediacaran, it has been hypothesised that the field had an impact on life on earth. A fundamentally weak and perhaps non-dipolar field during the Devonian might have had an influence on evolution and extinctions. As there is a large number of biological crises in the Devonian, we here pose the question whether the Earth’s magnetic field was a contributing factor to these crises. New independent evidence from the Devonian-Carboniferous boundary suggests that the Hangenberg event was caused by increased UV-B radiation, which is in line with a weak magnetic field.</p>

A new, early Puercan (earliest Paleocene) species ofPurgatorius(Plesiadapiformes, Primates) from Saskatchewan, Canada

2011 ◽  
Vol 85 (3) ◽  
pp. 537-548 ◽  
Author(s):  
Richard C. Fox ◽  
Craig S. Scott
Keyword(s):  
North America ◽  
North Dakota ◽  
Time Scale ◽  
Taxonomic Composition ◽  
Williston Basin ◽  
Evolutionary Diversification ◽  
Early Paleocene ◽  
Brick And Tile ◽  
Geomagnetic Polarity ◽  
Geomagnetic Polarity Time Scale

The early PaleocenePurgatoriusVan Valen and Sloan is the most primitive plesiadapiform primate yet discovered, mostly known from middle to late Puercan strata in Montana, deposited during the interval C29N of the geomagnetic polarity time scale. Here we describePurgatorius coracisn. sp. from the Ravenscrag Formation, at the Rav W-1 horizon, Medicine Hat Brick and Tile Quarry, southwestern Saskatchewan. This horizon occurs within C29R, makingP. coracisthe earliest known primate, while strengthening the evidence that plesiadapiforms, and hence primates, originated and underwent their initial evolutionary diversification in North America. Most North American mammalian local faunas correlating with C29R have been assigned to the Pul (earliest Puercan) interval zone, but the taxonomic composition of the mammals accompanyingP. coracisat Rav W-1 more resembles local faunas of Pu2 age. The occurrence at Rav W-1 of Pu2 aspect mammals within C29R agrees with similar occurrences at the Hiatt and PITA Flats localities in Montana and North Dakota, also possibly correlated with C29R. The evidence from these three sites, all in the Williston Basin, suggests that in some areas of the Western Interior Pu2 aspect local faunas were coeval with those of latest Pu1 age, having evolved earlier than has commonly been assumed.

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