SUMMARY The time-averaged geomagnetic field is generally purported to be uniformitarian across Earth history—close to a geocentric axial dipole, with average strength within one order of magnitude of that at present. Nevertheless, recent studies have reported that the field was approximately ten times weaker than present in the mid-Palaeozoic (∼410–360 Ma) and late Ediacaran (∼565 Ma). Here we present the first whole-rock palaeointensity determinations of Ediacaran age outside of Laurentia. These were obtained by the Thellier-Coe, Wilson and microwave methods for basaltic rocks of 560–580 Ma age of the Ediacaran traps, southwestern margin of the East European Craton, Ukraine. All four studied sites showed extremely low instantaneous field values of (3–7) μT with corresponding VDMs of (0.4–1) × 1022 Am2. Summarizing all available data, the Ediacaran field appears to be anomalously characterized by ultra-low dipole moment and ultra-high reversal frequency. According to some geodynamo models, this state could indicate a weak dipole field regime prior to the nucleation of the solid inner core. However, given that ultra-low field intensities have also been detected in the Devonian, and that virtually no palaeointensity data exist for the intervening ∼150 Ma, the date of inner core nucleation remains extremely uncertain. Our new evidence of persistent ultra-weak magnetospheric shielding in the Ediacaran may be considered consistent with the recently hypothesized link between enhanced UV-B radiation in this interval and the subsequent Cambrian evolutionary radiation.

Summary Palaeomagnetic field intensity measurements, derived from rocks with ages that span geological time, provide a crucial constraint on the evolution of Earth’s deep interior and its magnetic environment. The palaeointensity database PINT has been updated to version v.8.0.0  and includes palaeointensity site-mean records spanning an interval from 50 ka to 4.2 Ga, compiling efforts from the palaeomagnetic community spanning from 1959 to the end of 2019. Nearly all site-mean palaeointensity records have been assessed using the qualitative reliability of palaeointensity (Quality of Palaeointensity, QPI) framework. This updated database brings together and harmonizes prior QPI and PINT compilation efforts into a unified database referred to as the PINT database, incorporating recent efforts since 2014 to assess QPI. The spatio-temporal distribution of the PINT database is analyzed, revealing substantial biases towards young records (from the Brunhes chron) in the Northern hemisphere, and intervals with little to no palaeointensity data with a duration of 10s to 100s of millions of years in the Paleozoic and Precambrian. General QPI compliance is characterized for the PINT database, which shows that the median QPI scores range from 2 to 3 (out of a total possible score of 10), with a positive trend towards increasing QPI scores in studies published after the year 2000. This illustrates an increasing community awareness of what is required to establish confidence in palaeointensity data and an increasing robustness of the large scale interpretations that can be made with these data. We additionally present a description of the long-term average dipole field strength with descriptive statistics for distinct intervals of Earth history.

No abstract

No abstract

Summary Archaeomagnetic directions of one hundred and forty-one archaeological structures have been studied from 21 sites in Austria, 31 sites in Germany and one site in Switzerland. Characteristic remanent magnetisation directions obtained from alternating field and thermal demagnetisations provided 82 and 78 new or updated (12 and 10 per cent) directions of Austria and Germany, respectively. Nine of the directions are not reliable for certain reasons (e.g. displacement) while three of the features are not well dated. Apart from this some updated age information for the published databases is provided. Rock magnetic experiments revealed magnetite as main magnetic carrier of the remanences. The new data agree well with existing secular variation reference curves. The extended data set covers now the past 3500 years and a lot of progress were made to cover times BC with data. Here enhanced secular variation is observed manifested in declinations with values up to 70°. The new data will allow for recalculation of archaeomagnetic calibration curves for Central Europe from mid Bronze Age until today.

Information about long-term variations of geomagnetic field behavior, derived from paleomagnetic data, allow insight into the evolution of the deep Earth interior over geological timescales (Sprain et al., 2018). Numerical geodynamo simulations used to tie the geomagnetic field behavior to changes in the Earth's core and mantle rely on high-quality paleomagnetic data and their interpretations. However, scarce and/or ambiguous paleomagnetic data lead to controversial and highly discussed interpretations of the field behavior in time periods like the Ediacaran (e.g., Robert et al., 2017) or the Devonian (Torsvik et al., 2012).A recent rise in interest of the Ediacaran period (635-538 Ma, Xiao & Narbonne, 2020) has led to an improvement in data coverage for this time period but the reliability and ambiguity of data remain troubling. Directional data often result in apparent polar wander paths (APWPs) with rapid oscillations between two widely separated sets of poles (Abrajevitch & Van der Voo, 2010), making paleogeographic reconstructions difficult. In addition, paleointensity studies from Laurentia (Bono et al., 2019;Thallner et al., 2021) and Baltica (Shcherbakova et al., 2020) suggest a weak sustained field, fundamentally different from younger fields (Kulakov et al., 2019), with minimum paleointensity values similar to those in short-term intensity

Palaeointensity information enables us to define the strength of Earth’s magnetic field over geological time, providing a window into Earth’s deep interior. The difficulties in acquiring reliable measurements are substantial, particularly from older rocks. Two of the most significant causes of experimental failure are laboratory induced alteration of the magnetic remanence carriers and effects relating to multidomain magnetic carriers. One method that has been claimed to overcome both of these problems is the Shaw method. Here we detail and evaluate the method, comparing various selection criteria in a controlled experiment performed on a large, non-ideal dataset of mainly Precambrian rocks. Monte Carlo analyses are used to determine an optimal set of selection criteria; the end result is a new, improved experimental protocol that lends itself very well to the automated Rapid 2G magnetometer system enabling experiments to be carried out expeditiously and with greater accuracy.

The 2019 field work on Prohibition Creek (SE of Norman Wells), a joint effort of GSC-Calgary and the DEEP research group of the University of Liverpool (UK), aimed to remeasure and sample the Eifelian-Frasnian succession of the Hume Formation and the Horn River Group. The latter is well exposed along Prohibition Creek and recommended as a reference section for the 'Canol shale play' in the adjacent Mackenzie Valley as rocks in studied outcrops occur in the zone of relatively low thermal maturity with high possibility of good preservation of primary paleo-magnetic, organic-matter, and geochemical signals. This report delivers illustrated baseline descriptions, gamma spectrometry logs, and correlation of measured sections (=field stations), and gives startup information on the field sampling based research. Match of sections from 2019 with sections measured in 2015 identifies a few miscorrelations in earlier versions, leading to improved gamma spectrometry empowered correlation with the subsurface.