Earth's magnetic field on the wrong track

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Our Earth's magnetic field resembles a so called dipole field, this means it has two poles, magnetic north and magnetic south near Earth's rotation axis. The fact that the polarity of the dipole field of the Earth's magnetic field has reversed several times in the past (i.e. the north and south poles have swapped positions) is known to many people outside the scientific community. However, it is less well known that there were also phases in which the dipole field became weaker and the position of the poles moved away from the vicinity of the earth's rotation axis without eventually leading to a pole reversal. When the pole position moves more than 45° away from the Earth's axis of rotation, paleomagnetists call this a geomagnetic field excursion. These are extremely interesting if one wants to understand field reversals. Are excursions cancelled field reversals or are they uüubased on other mechanisms? What determines whether the field is completely reversed or "only" a field excursion occurs? What is the geometry of the Earth's magnetic field during a field excursion (dipole, quadrupole, multipole)? These questions are of particular interest to modellers of geodynamo simulations.

Geomagnetic field excursion in Siberia?

Stephanie is investigating a possible field excursion that was named the "Bolshoye Shchuchye Event" by the authors of another study. Three 24-meter-long drill cores from two Siberian lakes were examined in this study and this deviation of the Earth's magnetic field direction was discovered. However, there are still some unanswered questions in the context of the discovery of the so-called Bolshoye Shchuchye Event, which Stephanie is now investigating. These are:

• Does the recording of the Bolshoye Shchuchye Event show a real deviation of the Earth's magnetic field or is it just a so-called artifact: the recording of the event was disturbed by various factors and thus the event would never have existed in reality?
• How does the strength of the Earth's magnetic field behave during the Bolshoye Shchuchye event?
• Is the Bolshoye Shchuchye event a newly discovered local feature of the Earth's magnetic field or a global event, i.e. possibly an excursion of the Earth's magnetic field that has already been described and named elsewhere in the world?

Magnetic particles in the sediment "remember" the Earth's magnetic field from times long past

To answer these questions, Stephanie first analysed the magnetic particles in the sediment cores in more detail. Only the magnetic particles in the sediment record the Earth's magnetic field, but not all magnetic particles can reliably store this information for thousands and millions of years: The larger the magnetic particles are, the more "forgetful" they are. Stephanie first identified and characterised the magnetic minerals in the laboratory at the University of Cologne. And indeed, the composition and grain size of the magnetic minerals in this area of the drill cores in which the event was recorded are suitable for reliably preserving the magnetic signal to this day. Since the Bolshoye Shchuchye event was also recorded in two independent lakes, Stephanie can assume that it is a real deviation of the Earth's magnetic field.

Next, she investigated whether the intensity of the Earth's magnetic field decreased at the time of the Bolshoye Shchuchye event. It is known that the dipole field weakens during excursions and pole reversals. Stephanie also observed a weakening of the field at the time of the Bolshoye Shchuchye event. However, this does not yet prove that the Boshoye Shchuchye event is a geomagnetic excursion: according to the definition, only the deviation of the pollage from the axis of rotation is decisive for this. This is determined using the magnetic inclination: Inclination is the angle at which the magnetic field lines meet the earth's surface: at the poles, the magnetic field lines emerge perpendicular to the ground (=90°), at the equator they run parallel to the earth's surface (=0°). If the inclination is now lower with respect to what can be observed today at the same location, the position of the pole was different than it is today. Stephanie is still evaluating this, as further analyses are necessary to calculate the exact pole position.

Plant remains, ash, seasons - different methods for age determination

Stephanie has compared the temporal changes in inclination and the strength of the Earth's magnetic field at Lake Bolshoye Shchuchye with those of other sedimentary geoarchives. To do this, she needs to know the age of the sediment at the different depths of the drill core. Most of the ages of the sediment cores from Lake Bolshoye Shuchye were determined using the radiocarbon method (also known as C14 dating). This involves dating plant remains that have been preserved in the sediment. The age of a sediment layer was also determined using tephrochronology. Tephra (also known as volcanic ash) is a deposit formed by explosive volcanism. The method is based on a comparison of the results of precise element analyses with the values in a database. In fact, a tephra of the same composition was found in the database for which the age is already known. This allows Stephie to narrow down the age of her samples more precisely. In the lowest part of the sediment core, the age was also determined by counting the seasonal layers, so-called warves. Using all these different methods, the age of the Bolshoye Shchuchye was narrowed down fairly precisely so that the measured fluctuations in the inclination and intensity of the Earth's magnetic field can be compared with other magnetic records from sedimentary archives. Together with records of the Earth's magnetic field from the Black Sea and off the coast of Yemen, Stephanie now has clear evidence that the Bolshoye Shchuchye event corresponds to a known event that occurred globally: Hilina Pali/Tianchi.

Bolshoye Shchuchye Event → Hilina Pali/Tianchi

Thus, the event found in Siberia does not need a newly introduced name, as it corresponds to Hilina Pali/Tianchi, which some authors refer to as a geomagnetic excursion. However, the drill cores from Lake Bolshoye Shchuchye in Siberia are the most detailed record of this event to date. The existence and characterisation of Hilina Pali/Tianchi has been the subject of much debate among experts because very little evidence has been found globally. With the records from Siberia, it can now be studied in much greater detail in the future.

Abstract

Does the Bolshoye Shchuchye Event exist?

Stephanie Scheidt¹, Norbert Nowaczyk², Jo Brendryen³, Carl Regnéll³, Sædís Ólafsdóttir³, Haflidi Haflidason³, Jon Inge Svensson³

1: Universität zu Köln, Deutschland, 2: GFZ Potsdam, Deutschland, 3: Universität von Bergen, Norwegen

In 2009, drilling campaigns recovered sediment cores from the lakes Bolshoye Shchuchye (cores 506-48 and 506-50) and Maloye Shchuchye (core 506-51), both located in the polar Ural Mountains in Siberian Russia. The cores are 24 meter long and reach back to up to approximately 24 kyr. The age models of the cores are based on radiocarbon dating, a tephra horizon and varve counts, mainly carried out on core 506-48 and applied to the other cores by correlating lithological and inclination features. Based on the inclination data, Haflidason et al. (2022) named a distinct feature of the geomagnetic field the Bolshoye Shchuchye Event. This event is clearly visible in all three cores. According to the age model, it took place between 20.47 ka cal BP and 19.23 ka cal BP. Here we present the first mineral magnetic results from core 506-48 to determine the carriers of the magnetic signal and ensure the primary origin of the directional observations, as well as the suitability of the sediment for relative palaeointensity (RPI) determination. We also present the RPI record of cores 506-48 and 506-50 and compare our findings to other records of the geomagnetic field. We answer the question if the Bolshoye Shchuchye Event is a local feature, an equivalent to another geomagnetic excursions, or a new discovered feature of the geomagnetic field.

Does the Bolshoye Shchuchye Event exist? Stephanie Scheidt, Norbert Nowaczyk, Jo Brendryen, Carl Regnéll, Sædís Ólafsdóttir, Haflidi Haflidason, Jon Inge Svensson, Castle Meeting 2024, Utrecht, NL, June 30 – July 6, 2024, Session B: Archeomagnetism; Geomagnetism; Environmental magnetism, 2024, access poster via http://www.doi.org/10.13140/RG.2.2.25692.19840