Time to adjust the compass?

In the high Canadian Arctic, researchers at the University of Rochester have stripped away some of the mystery surrounding the powerhouse that drives the Earth’s magnetic field. The research strongly suggests that several of the characteristics of the field that were long thought to operate independently of one another, such as the field’s polarity and strength, may be linked. If so, then the strength of the field, which has been waning for several thousand years, may herald a pole reversal-a time where compasses all over the Earth would point south instead of north. The findings are being published in today’s issue of Proceedings of the National Academy of Sciences.

John Tarduno, professor of geophysics, took 14 students on four excursions, the most recent in the summer of 2000, far above the Arctic Circle to pitch tents near 95-million-year-old rocks on the snow-covered islands of Ellesmere and Axel Heiberg. The rocks, part of a formation called the Strand Fiord, were spewed forth from ancient volcanoes during a time when the Earth’s magnetic field was particularly stable. As the volcanoes’ lava cooled to become igneous rock, tiny crystals lined up with the Earth’s magnetic field and were solidified in the rock. Tarduno was seeking these crystals and the data they preserved about the magnetic field.

Tarduno wanted to find whether the crystals in this region bore evidence of brief fluctuations in the magnetic field. Several more accessible areas of the globe house such crystals, but Tarduno had to go to the edge of the “tangent cylinder”-a giant, theoretical cylinder that runs through the Earth like a pimento through an olive. This cylinder extends away from the Earth’s solid iron core to the north and south poles and represents an area of possible high turbulence in the molten iron of the core, stirred up by the Earth’s spin. Near the edge of this cylinder of turbulence scientists believe the liquid iron should be the most chaotic, twisting up the magnetic lines of force. Where this edge contacts the Earth’s crust high above the Arctic Circle should lie traces of the twisted magnetic field in the crystals.

But not just any place along this edge would do. Tarduno needed to find rocks around 95 million years old because they were formed in the middle of an ancient time of highly unusual magnetic stability. That time of stability, called a superchron, lasted for tens of millions of years-a rarity when magnetic reversals can happen in as little as a few tens of thousands of years. Tarduno wanted to know how stable or chaotic the magnetic field was during that time along the supposedly turbulent edge of the tangent cylinder. If the field was chaotic during the stable superchron, then there would probably be no correlation between north-south pole reversals and the way molten iron in the core generated that field. On the other hand, if the field near the cylinder’s edge was stable throughout the superchron, then it becomes more likely that turbulence in the liquid outer core was related to making the Earth’s poles reverse. The answer would peel away another layer of mystery about how the Earth generates its magnetic field.

Above the Arctic Circle, just 11 degrees south of the North Pole, Tarduno and his students pitched tents near the volcanic strata of the Strand Fiord Formation to find and retrieve layers from the 95-million-year-old superchron on the edge of the tangent cylinder. Before they could drill into the rock to retrieve samples, however, they had to precisely note which way the North Pole lay so that they could tell if the crystals in their samples showed any sign of a full or partial pole reversal. Compasses were useless because at their latitude they were actually farther north than the epicenter of the magnetic north pole, and though that could have been corrected for, at such high latitudes solar winds can create unpredictable variations in the field. The network of satellites that makes up the Global Positioning System were likewise useless because much of the drilling had to be done in deep, narrow valleys where the satellites’ signals couldn’t penetrate. The team had to use a sun compass, a way to gauge direction using knowledge of where the sun is at a specific time of day. Once they had determined which way the true North Pole lay, Tarduno and his students drilled out several sections of the 95-million-year-old rock, labeled it, and packed it up to be shipped back to the University of Rochester.

Once back at the University, Tarduno used a SQUID magnetometer, a device that can detect extremely minute amounts of magnetism in small samples, to determine the direction and intensity of the magnetic signature sealed in the crystals in the rock. What they found was that there was little deviation in the direction or intensity in the field, even though the molten iron beneath was theoretically very turbulent. This suggests that the fluctuations in the iron of the inner core of the Earth were not contorting the magnetic field but were efficiently creating a stable and intense field.

This study shows a correlation between the stability of the poles and the intensity of the field, meaning there’s likely a single mechanism in the Earth governing the magnetic field. The news comes as a bit of a relief for scientists who would otherwise have to uncover multiple interacting mechanisms to create a working model.

The findings also suggest that humanity is in for a surprise in the not-too-distant future. Since the Earth’s magnetic field has been decreasing in intensity for the last several thousand years, and the intensity and likelihood of pole reversals are linked, in as little as a few centuries we may see the Earth’s magnetic poles flip, sending everyone’s compasses angling toward the South Pole.

Tarduno plans to extend these studies into the very ancient Earth in the hopes of discovering how the Earth came to have a magnetic field at all.

The research was funded by the National Science Foundation and the Canadian Polar Shelf Project.