The last complete reversal of the Earth's magnetic field, the so-called Laschamps event,
took place 42,000 years ago.
Radiocarbon analyses of the remains of kauri trees from New Zealand now make it possible
for the first time to precisely time and analyse this event and its associated effects, as
well as to calibrate geological archives such as sediment and ice cores from this period.
Simulations based on this show that the strong reduction of the magnetic field had considerable
effects in the Earth's atmosphere.
This is shown by an international team in a study that appears in the journal Science.
The Earth's magnetic field undergoes permanent fluctuations and occasionally even reversals
of polarity occur.
Their causes, course and effects are not yet fully understood.
Researchers have now investigated the so-called Laschamps event in more detail.
It refers to the last complete reversal of the polarity of the Earth's magnetic field
around 42,000 years ago.
Not only did the magnetic field change direction, it also dramatically lost strength over a
period of several hundred years.
About 42,000 years ago, the magnetic north pole moved south.
Within this process, which lasted about 500 years, the magnetic field weakened to between
six and zero per cent.
During a period of about 500 years, the poles remained reversed, with a field strength that
varied below 28 per cent of today's value, only to reverse again over the course of about
250 years.
This exact chronological classification is now possible by linking different data sets.
Firstly, the researchers used results on the Earth's magnetic field from sediment cores
of the Black Sea by the team from 2013, which were matched with Greenland ice cores via
climate variation documented at the same time.
Secondly, the exact analysis and dating of the events was only made possible by the radiocarbon
(14C) analysis of a sub-fossil kauri tree that grew in the wetlands in northern New
Zealand for around 1700 years during the period in question and was subsequently very well
preserved in the swamps.
With the dwindling of the magnetic field, the Earth is losing an important protective
shield against cosmic radiation, at least in part.
This is also reflected in increased levels of the radioactive carbon isotope 14C in the
trees.
The reason for that is the increased formation of 14C in the Earth's atmosphere during the
bombardment of nitrogen by high-energy, electrically charged cosmic particles.
The sub-fossil kauri trees are an exciting archive of atmospheric composition.
These trees can live for several thousand years and record annual variations in atmospheric
radiocarbon content as they grow, which the research team measured precisely.
These data improve the calibration curve for radiocarbon dating, allowing more accurate
dating of a wide range of climate archives and fossils.
They also allow a direct comparison to ice cores: beryllium isotopes measured there show
similar variations to the radiocarbon in the trees, as the production of both isotopes
in the Earth's atmosphere depends on the intensity of cosmic rays hitting the Earth.
To investigate further effects of the weak Earth's magnetic field on the atmosphere and
thus also on the global climate, the researchers carried out simulations of atmospheric chemistry.
Among other things, they found a decrease in ozone.
It is important to further analyse the effects of the weak magnetic field in this direction
in view of current developments.
Because the Earth's magnetic field has already been weakening for about 2000 years.
Compared to the first direct measurements 170 years ago, a weakening of nine per cent
was observed, in the area of the South Atlantic even thirty per cent.
Whether this means that a pole reversal is in the offing for the next one to two thousand
years is debatable.
However, a collapse of the natural radiation shield would pose a great challenge to our
present-day society, which is very much based on electronics.