Researchers from Germany and Switzerland did an analysis on sediment cores from the Black Sea and concluded that, for a brief period during the last ice age, a compass at the Black Sea would have pointed south instead of north. And that wasn't the worst thing going on around the same time.

41,000 years ago, say the researchers, a complete and rapid reversal of the geomagnetic field occurred. Along with the Black Sea sediment cores, they look at other studies in the North Atlantic, the South Pacific and Hawaii, and say it proves that this polarity reversal was a global event. 

If accurate, what is remarkable is the speed of the reversal: "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 Dr. Norbert Nowaczyk
at the Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences. "The actual polarity changes lasted only 250 years. In terms of geological time scales, that is very fast." 

During that period, the field was even weaker, with only 5% of today's field strength. As a consequence, the Earth nearly lost its protective shield against hard cosmic rays, leading to  significantly increased radiation exposure. They made the determination using 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 Laschamp event

The polarity reversal now found with the magnetization of Black Sea sediments
was first postulated after analysis of the magnetization of several lava flows near the village Laschamp near Clermont-Ferrand in the Massif Central decades ago. That geographic feature is known as the 'Laschamp event'. However, the data of the Massif Central represent only some point readings of the geomagnetic field during the last ice age, whereas the new data from the Black Sea give a more complete image of geomagnetic field variability at a high temporal resolution.

Yet that is not all, they say. Besides giving evidence for a geomagnetic field reversal 41,000 years ago, the researchers say they discovered numerous abrupt climate changes during the last ice age in the analyzed cores from the Black Sea, as was already known from the Greenland ice cores.

They synchronized the two data records from the Black Sea and Greenland and say the largest volcanic eruption on the Northern hemisphere in the past 100,000 years, 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 sediments from the Black Sea. 

The ashes of this eruption, during which about 350 cubic kilometers of rock and lava were ejected, were distributed over the entire eastern Mediterranean and up to central Russia. These three extreme scenarios, a short and fast reversal of the Earth's magnetic field, short-term climate variability of the last ice age and the volcanic eruption in Italy, have been investigated for the first time in a single geological archive and placed in chronological order.

Citation: Nowaczyk, N. R., Arz, H. W., Frank, U., Kind, J. and Plessen, B. (2012), 'Dynamics of the Laschamp geomagnetic excursion from Black Sea sediments', Earth Planet. Sci. Lett. Volumes 351–352, October 15th, 2012, Pages 54–69 doi:10.1016/j.epsl.2012.06.050