During the last ice age, around 40,000 – 41,000 years ago, there was a very rapid and complete reversal of the Earth’s geomagnetic field, according to new research. There was already localized evidence of polarity reversals during this time, but with the new research, the theory that it was a global event is now strongly supported. And very interestingly, it is one that nearly coincided with the very fast, short-term climate variability of the last ice age and the largest volcanic eruption in the northern hemisphere during the last 100,000 years.
Magnetic studies using sediment cores taken from the Black Sea, done by the GFZ German Research Centre for Geosciences, have clearly shown that if you had a compass at the Black Sea during that time, it would have pointed towards the south, not the north.
But more importantly, new data gathered by the researchers when it’s combined with additional data from previous studies in the North Atlantic, the South Pacific, and Hawaii, strongly supports the theory that this polarity reversal was truly global.
What’s considered so remarkable about this reversal is the speed that it’s now thought to have occurred: “The field geometry of reversed polarity, with field lines pointing into the opposite direction when compared to today’s configuration, lasted for only about 440 years, and it was associated with a field strength that was only one quarter of today’s field,” explains Norbert Nowaczyk. “The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast.”
The magnetic field at the time would have only had about 5% of the strength of the field today. This would have resulted in the Earth almost completely losing its protective shield, allowing hard cosmic rays to hit the surface, and greatly increasing radiation exposure.
“This is documented by peaks of radioactive beryllium (10Be) in ice cores from this time, recovered from the Greenland ice sheet. 10Be as well as radioactive carbon (14C) is caused by the collision of high-energy protons from space with atoms of the atmosphere.”
The polarity reversal observed in the magnetisation of Black Sea sediments isn’t new knowledge, though — it’s been known of for at least 45 years. First discovered in “the analysis of the magnetisation of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central, which differed significantly from today’s direction of the geomagnetic field.” This geomagnetic feature has since then been called the Laschamp event.
That’s limited data though, representing only some specific point readings of the geomagnetic field during that time, so there had been debate about how to interpret it. When combined with all the new data though, it creates a convincing image of “geomagnetic field variability at a high temporal resolution.”
The cores analyzed also provided insight into “numerous abrupt climate changes during the last ice age” that occurred at the same time — these climate changes had already been observed in ice cores taken from Greenland. “This ultimately allowed a high precision synchronization of the two data records from the Black Sea and Greenland.”
“The largest volcanic eruption on the Northern hemisphere in the past 100,000 years, namely the eruption of the super volcano 39,400 years ago in the area of today’s Phlegraean Fields near Naples, Italy, is also documented within the studied sediments from the Black Sea.”
This volcanic eruption released about 350 cubic kilometers of ash, rock, and lava over all the eastern Mediterranean, and parts of Russia and Asia.
Evidence shows that the eruption was a single event lasting 2 to 4 days. It was triggered by abrupt changes in composition, properties and physical state in the melt or overpressure in the magma chamber.
The eruption started with phreatomagmatic explosions, followed by a Plinian eruption column, fed by simultaneous extraction of two magma layers.
The resulting ash plume is estimated to have been 70 km (43 mi) high. As gradually an unstable pulsating column formed, fed only by the most evolved magma due to upward migration of the fragmentation surface, reduced magma eruption rate, and/or activation of fractures, the Plinian phase ended.
Emissions consisted of pumice and dark colored volcanic rock (scoria). The mafic minerals cover smaller areas than the more acidic members, also indicating a decrease of explosivity over the course of the eruption. The eruption column caused a large pumice-fall deposit to the east of the source area.
All three of these global disasters; a rapid and complete reversal of the Earth’s magnetic field, the drastic short-term climate variability of the last ice age, and one of the largest volcanic eruptions in the last million years; have now been tied together and placed into a precise and connected chronological order.
The new research was just published in the journal Earth and Planetary Science Letters.
Source: Helmholtz Centre Potsdam – GFZ German Research Centre for Geosciences