For four years, physicist Gelvam Hartmann collected and examined nearly 600 fragments of brick from churches and old houses in Bahia, São Paulo, Rio de Janeiro and Espírito Santo to investigate variation in Brazil’s geomagnetic field over the last 500 years, a period for which there is virtually no geophysical information. His work revealed an unexpected drop in the strength of the magnetic field in the Northeast and Southeast regions of the country, and from there he developed a method for analyzing archaeological materials which confirmed or defined the probable dates of old buildings, some of which have no historical documentation whatsoever.
Working alongside archaeologists, architects and geologists, Hartmann sampled small chips of brick from colonial houses and churches in Pelourinho, in the historic center of Salvador, using a hammer and chisel whenever possible or, when hand tools were not adequate, a water-cooled drill. Gradually, while examining the material at the Global Physics Institute of Paris (GPIP) the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), and University of São Paulo (USP), he constructed the magnetic history of Brazil, to confirm dates of construction and associate them with their respective magnetic intensities. It is in this way that new information has emerged – the magnetic field intensity of 36.2 microteslas (tesla is the unit of measurement for magnetic flux density) – for one of the oldest buildings in Brazil, the São Salvador Cathedral, erected by Jesuits between 1561 and 1591 with money from the third governor general of Brazil, Mem de Sá, and adorned with a bell brought from Portugal.
There were hardly any problems with the majority of samples taken from the foundations and walls of Salvador’s churches, but, strangely, the analysis of a sample from the home of poet Gregório de Matos, known as the Mouth of Hell because of the sarcasm with which he treated the authorities of Salvador, indicated that the building had been erected in 1830, not between 1695 and 1700, as the documents for the house indicated. Hartmann later determined that the date from his analysis only applied to the third floor – which was built later – where he had collected brick samples when that part of the house was undergoing restoration.
“Geophysicists are helping us tell the story of the settlement of Brazil” notes Marisa Afonso, a professor of archeology and deputy director of the Museum of Archaeology and Ethnology (MAE) at USP. In April 2004, she had spent a long rainy day in the regional center of MAE in Piraju, located in the interior of the state of São Paulo, when she received an email from Ricardo Trindade, a professor from IAG and Hartmann’s doctoral adviser From Paris, Trindidade invited her to help construct a dating curve for archaeological materials, something that had not been done yet in Brazil, using magnetic field measurements, similar to what he had seen there. “The more methods of dating you can use, the better, because the most widely used techniques, such as carbon-14 and thermoluminescence, do not always work in all cases,” she says. “As luck would have it, both Gelvem and Ricardo like archeology and know how to talk about what they do in a simple manner.”
At the same time, Hartmann and other researchers at IAG are detailing variation in the Earth’s magnetic field, principally in regions where it is less intense. The magnetic field is generated by the movement of liquid iron in Earth’s core, which is expressed at the surface of the planet, orienting compasses, and forms an invisible barrier 30,000 km above the surface of the Earth that hinders the entry of particles from the Sun into the atmosphere. It is now clear that in the region where the magnetic field is weakest across the entire surface, known as the South Atlantic Magnetic Anomaly, it is actually moving and expanding. Previously restricted to southern Africa, this area currently covers the southern part of South America and nearly all of the South Atlantic The point of least intensity of this blur is shifting to the west: it was in southern Africa, and then in the middle of the South Atlantic, halfway between Brazil and South Africa By 1930 it was close to the city of Rio de Janeiro, moved south and parked on the state of Santa Catarina, before moving to its current position in Paraguay, where it has a magnetic intensity of about 22 microteslas. Some consequences of this shift in the magnetic field are known, precisely in those areas where the field is weakest, telecommunications satellites and space craft may suffer more magnetic interference that can damage equipment, just the way that a magnet can demagnetize a computer and cause information to be lost on a smaller scale.
The results emerged after a series of surprises, not all of them pleasant. Hartmann says that he felt overwhelmed in May 2008, at the beginning of a six-month internship in the paleomagnetism laboratory at the Institute of Global Physics in Paris. His goal was to characterize the magnetic field of the material that he had sampled in Brazil- ceramic fragments from the past 2000 years – but things started to go wrong. “Yves Gallet, the head of the laboratory, said that I couldn’t analyze my samples because they had not been fired enough to be thoroughly heated all the way through. Ceramics, bricks, tiles or any other material that has been subjected to intense heat can keep a record of the Earth’s magnetic field at the time of firing, but for this imprint to remain, the material must be uniformly heated. So Yves made me an offer: ‘Go back to Brazil, stay there 20 days and collect historical samples from material that is no more than 500 years old, and I will pay for your ticket’,” says Hartmann.
He arrived in Salvador, the first capital city in Brazil, and immediately sought out Carlos Etchevarne, a professor of archeology at the Federal University of Bahia (UFBA) whom he had met at a conference three years earlier, and Rosana Najjar, an archaeologist at the National Institute of Historical and Artistic Heritage (Iphan) and Project coordinator of the Pelourinho Archaeology Project (Monumenta/Iphan). Etchevarne and Rosana introduced Hartmann to other archaeologists, who helped him collect fragments from the brick foundations, walls or ceilings of 20 historical buildings in Pelourinho. “We had never before worked with physicists before,” says Etchevarne, “but we were quickly able to have a very good dialogue with common goals.”
They selected buildings for which the construction dates were already known through historical records and archaeological research. The reason for this approach is simple: Hartmann needed an independent baseline reference in order to assign dates to his other samples using his own method of measuring the intensity of the magnetic field recorded in ferrous minerals, such as magnetite and hematite, which are components of the clay bricks used to erect these old buildings. As much as he was interested in dating the samples, he was also curious about the intensity of the magnetic field at the time of firing. “The Earth’s magnetic field fluctuates constantly, at different time scales, from milliseconds to billions of years, such that ceramic fragments from buildings of different ages also record distinct values of the magnetic field,” he says.
Back in Paris, Hartmann says that he worked “16 hours a day, including Saturdays and Sundays,” for two months in order to determine the age and magnetic field strength of the materials he had collected. With these and other samples obtained during another trip to Salvador, he applied his own independent methods and confirmed the dates of these historic buildings, and fine tuned the technical aspects of his method. “These data serve as a tool for dating historical buildings,” attests Trinidade, who accompanied Hartmann on his second collecting expedition to Salvador in December 2008. This really worked. As he mastered the technique and created a strong association between the dates and variations in the intensity of the magnetic field, Hartmann was able to estimate the date of construction – between 1675 and 1725 – of a house in Pelourinho, number 27, for which the archaeologists had no historical documentation.
Working at the institute in Paris and at IAG, Hartmann prepared the samples he had taken from 295 houses and 14 churches in Salvador. Then, in the Southeast region of Brazil, he took samples from farmhouses, churches and other buildings in São Paulo, along with archaeologist Paul Zanettini, as well as in Espírito Santo and Rio de Janeiro, with archaeologist Rosana Najjar, obtaining a total of more than 289 samples from 11 different localities. Hartmann left each sample in the shape of a cube with one inch sides. Then he placed the samples in a paleomagnetic oven, which, after repeated cycles of heating and cooling, recovers the intensity and orientation of the magnetic field as it was when the clay was initially fired. This method is time consuming and, for the time being, not very efficient, Hartmann obtained good results from 56% of the samples from the Northeast and only 38% from those acquired in the Southeast.
After heating, cooling and then using a magnetometer to analyze the samples from each place he visited, Hartmann was able to construct variation curves of the magnetic field strength for each region. The Northeast showed decreasing values – around 40 microteslas in 1560 to just 25 in 1920 – with an average decrease of nearly five microteslas per century. “It’s enough,” he says. The values for the samples in the Southeast showed an even sharper decline, as detailed in a paper published this year in the journal Earth and Planetary Science Letters, where the data for samples from the Northeast had come out in 2010. “These two publications represent a fundamental contribution to our understanding of how the Earth’s magnetic field has changed over the past 500 years,” according to Trinidade. The geophysicist, Igor Pacca, professor at IAG and one of the pioneers in research on the Earth’s magnetic field in Brazil, compiled the information about the magnetic signals imprinted in rocks a million years ago. The most recent data, from the beginning of last century to present, are being collected by ground-based observatories and satellites.
At least in the first attempts, this technique was not deemed useful for dating rock paintings or clay pots, which have typically lost their original magnetic field signal after having come into contact with intense heat many times. Nor was the technique useful for analyzing samples from the houses of bandeirantes (Portuguese colonial scouts) in São Paulo, which were made of crushed and pressed clay. Etchevarne believes that the technique may serve in clarifying the origins of ceramic pots used for holding water, which are only subjected to high temperatures when they are initially fired. “One of the next challenges is figuring out how to date materials that are over 500 years old and were not repeatedly subjected to heat,” says Marisa. “I asked Gelvam not to give up. We have ceramic pieces that are up to 7000 years old to date.” Hartmann has already started working on samples collected in Missões and intends to examine samples from churches in Minas Gerais as soon as possible, to supplement his analyses of variations in the magnetic field in regions of Brazil.
According to Trinidade, these regional analyses showed that the magnetic field in Brazil is far from presenting an ideal pattern, which could be compared to the magnetic field of a bar magnet. In both regions, the magnetic field is complex and influenced by strong multipole components – or rather, it is not dipole, as geophysicists say. “In these cases,” says Hartmann, “the needle of a compass is strongly deflected by as much or more than 20º with respect to due North.” In France, he said, which is dominated by a dipole field, it’s as if the earth was a nearly perfect bar magnet, and deflections from due North do not not exceed 5º.
Less intense field
To geophysicists, this continuous decline in magnetic field intensity and the fact that samples from the Northeast and Southeast regions have vastly different values, suggests a link to the South Atlantic Magnetic Anomaly (SAMA) . Governed by non-dipole fields, the SAMA is a widespread region that is characterized by having a lower magnetic field intensity – about 28 microteslas (the average magnetic field intensity for the planet Earth is 40 microteslas and the maximum is 60 microteslas). “Because of geographical proximity, the influence of this anomaly is greater in the Southeast than it is in the Northeast regions of Brazil,” says Hartmann. “The anomaly represents an area where the magnetic field shielding against cosmic rays and solar particles is more fragile.”
Pacca sees the SAMA as a “window” for high-energy particles, known as cosmic rays, which may pass more easily to the Earth’s surface through this area where the magnetic field is less intense. He and Everton Frigo, also from the IAG, believe that these cosmic rays could, on the other hand, also facilitate cloud formation, causing it to rain more and lower the temperature, especially over expanses of land covered by a less intense magnetic field.
It has long been known that sunspots affect the climate, but we never knew exactly how,” says Pacca. When more sunspots occur, the Sun’s activity is greater – and it’s magnetic field becomes more intense. At such times, the Sun’s magnetic field acts in conjunction with the Earth’s magnetic field, further hindering the entry of cosmic rays. During periods of less intense solar activity, there are fewer sun spots and the sun’s magnetic field is weaker.
“When the magnetic fields of the Sun and the Earth are at a minimum level of intensity, cosmic rays pass more easily to the Earth, where they collide with particles in the atmosphere and generate a huge amount of electrons and other particles,” says Pacca. “All of the energy that is created by these collisions is an ionization event, which can promote the condensation of water vapor. Cosmic rays could be like triggers that catalyze reactions leading to the formation of rain clouds,” he theorizes.
Researchers in the United Kingdom and Denmark also support this possibility, but there is still ample room for other points of view. “At the moment,” says physicist Paulo Artaxo of USP, based on studies of the Intergovernmental Panel on Climate Change (IPCC), to which he belongs, “there is no solid evidence, either for or against, that cosmic rays have any affect on the process of cloud formation.”
How is this region with a less intense magnetic field formed and how can it reduce the intensity of the magnetic field signal recorded in rocks or bricks? Nobody knows. What else might occur as a result of this drop in magnetic field strength, in addition to interference with telecommunications systems? This is another mystery. “In 1905, Einstein had already said that the origin and evolution of the Earth’s magnetic field is one of the most difficult problems in physics, since it does not follow any sort of pattern,” Hartmann argues.
The behavior of the Earth’s magnetic field is even complex enough to have already experienced complete reversals of the magnetic poles – in which the North Pole becomes the South Pole – the most recent such event occurring some 780,000 years ago. And there is a possibility that the polarity of the Earth may change yet again: “We have observed a magnetic anomaly in Siberia, which is expanding and is more intense than the magnetic field at the North Pole,” says Pacca. “For the time being, it’s as if the Earth has two North Poles, but the current North Pole is losing strength and there might be another, stronger, North Pole in thousands of years.”
Pacca assembled one of the first paleomagnetism laboratories in Brazil in 1971, in the Physics Institute at USP. Two years later he reinstalled his lab equipment at the IAG, where he had accepted an invitation to become a visiting professor and establish a research group in the area of geophysics. As there were no other sorts of materials available to study for many years, the research group mostly focused their analyses on rock samples.
One of their most ambitious projects was to analyze the intensity and direction of the magnetic field signal present in 10,000 rock samples from Brazil and Africa. The results revealed new details about the position of the continents on Earth a billion years ago, which was very different than it is now, and suggested that what corresponds to the current territory of Brazil was once a series of large and widely separated islands and that the block of rocks that is modern day Amazonia was separated from the state of Goias and the Northeast region of Brazil by sea and was actually further South than it is today (see Pesquisa FAPESP nº 75). Today, research groups in 24 countries – in South America, only Argentina and Brazil – are working on questions involving geomagnetism and paleomagnetism.
Pacca recently found what he believes to be the oldest Portuguese study of magnetism in rocks, the Roteiro do Goa a Diu, published in 1538 (Goa and Diu were the Portuguese domains in what is the Southwestern part of modern day India). The author is Dom João de Castro, a Portuguese nobleman who ended his life, at the age of 48, as viceroy of India. During the course of his explorations, he showed how navigators could find their positions when traveling on the high seas, by using the positions of stars and simple tools, like the magnetic compass. “If there were no magnetic field, there would be no such compass,” he says. “And without the compass, there would not have been the great ocean voyages, which made many merchants wealthy and allowed the conquest of new lands, such as Brazil.”
Evolution of the Earth’s magnetic field in South America over the past 500 years (nº 2000/10754-4); Modality Doctoral Scholarship; Coordinator Gelvam André Hartmann – IAG/USP; Investment R$ 145,801.14
Hartmann, G.A. et al. New historical archeointensity data from Brazil: Evidence for a large regional non-dipole field contribution over the past few centuries. Earth and Planetary Science Letters. v. 306, p. 66-76. 2011