Scientists understand that Earth's magnetic field has flipped its polarity many times over the millennia. In other words, if you were alive about 800,000 years ago, and facing what we call north with a magnetic compass in your hand, the needle would point to 'south.' Reversals are the rule, not the exception. Earth has settled in the last 20 million years into a pattern of a pole reversal about every 200,000 to 300,000 years, although it has been more than twice that long since the last reversal.
But a new study published in PNAS from Tel Aviv University, Hebrew University of Jerusalem, and University of California San Diego researchers finds there is no reason for alarm: The Earth's geomagnetic field has been undulating for thousands of years. Data obtained from the analysis of well-dated Judean jar handles provide information on changes in the strength of the geomagnetic field between the 8th and 2nd centuries BCE, indicating a fluctuating field that peaked during the 8th century BCE.
Credits: Peter Reid, The University of Edinburgh
"The field strength of the 8th century BCE corroborates previous observations of our group, first published in 2009, of an unusually strong field in the early Iron Age. We call it the 'Iron Age Spike,' and it is the strongest field recorded in the last 100,000 years," says Dr. Erez Ben-Yosef of TAU's Institute of Archaeology, the study's lead investigator. "This new finding puts the recent decline in the field's strength into context. Apparently, this is not a unique phenomenon -- the field has often weakened and recovered over the last millennia."
Additional researchers included Prof. Oded Lipschits and Michael Millman of TAU, Dr. Ron Shaar of Hebrew University, and Prof. Lisa Tauxe of UC San Diego.
Delving into the inner structure of the planet
"We can gain a clearer picture of the planet and its inner structure by better understanding proxies like the magnetic field, which reaches more than 1,800 miles deep into the liquid part of the Earth's outer core," Dr. Ben-Yosef observes.
The new research is based on a set of 67 ancient, heat-impacted Judean ceramic storage jar handles, which bear royal stamp impressions from the 8th to 2nd century BCE, providing accurate age estimates.
Computer simulation of the Earth's field in a period of normal polarity between reversals. The lines represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core
To accurately measure the geomagnetic intensity, the researchers conducted experiments at the Paleomagnetic Laboratory of Scripps Institution of Oceanography (SIO), University of California San Diego, using laboratory-built paleomagnetic ovens and a superconducting magnetometer.
"Ceramics, baked clay, burned mud bricks, copper slag -- almost anything that was heated and then cooled can become a recorder of the components of the magnetic field at the time of the event," said Dr. Ben-Yosef. "Ceramics have tiny minerals -- magnetic 'recorders' -- that save information about the magnetic field of the time the clay was in the kiln. The behavior of the magnetic field in the past can be studied by examining archaeological artifacts or geological material that were heated then cooled, such as lava."
Advanced dating method
Observed changes in the geomagnetic field can, in turn, be used as an advanced dating method complementary to the radiocarbon dating, according to Dr. Ben-Yosef. "The improved Levantine archaeomagnetic record can be used to date pottery and other heat-impacted archaeological materials whose date is unknown.
"Both archaeologists and Earth scientists benefit from this. The new data can improve geophysical models -- core-mantle interactions, cosmogenic processes and more -- as well as provide an excellent, accurate dating reference for archaeological artefacts," says Dr. Ben-Yosef.
The researchers are currently working on enhancing the archaeomagnetic database for the Levant, one of the most archaeologically-rich regions on the planet, to better understand the geomagnetic field and establish a robust dating reference.
The changes are a natural variation due to processes in the deep interior of the Earth, explained Nils Olsen, a Swarm team member from the Technical University of Denmark. The movement of molten iron in the core creates electric currents, and electric currents create a magnetic field. So every change in the flow of the core means changes in the magnetic field.
“The magnetic field changes in a chaotic manner, and we do not know why it changes in the way it does nor how it will evolve in the future,” said Olsen. “There is no periodic behavior, and it is therefore rather difficult, if not impossible, to predict how the magnetic field evolves over time. We can just observe how it has changed in past and what it looks like today.”
Contacts and sources:
Tel Aviv University (TAU)
George Hunka, American Friends of Tel Aviv University (AFTAU)