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Geology

The weather guardians

Caves reveal how the climate of the southern hemisphere has been for the last 100,000 years

Ivo Karmann entered onto the geology course at the University of Sao Paulo (USP), in 1977, to study in depth the caverns that he loved to explore since his days as a young boy. But at that time the caverns were still seen as an alternative for adventure tourism – and in Brazil there was little interest from the geological point of view. Karmann insisted, conquered the attention of a few scarce professors who showed themselves disposed to supervising his research, and has now also become an USP professor, has formed a team, and almost thirty years later, has proven that caverns house formidable raw material for geology, capable of registering climate and vegetation changes over thousands of years.

After associating a series of data collected inside and outside of a cavern in Santa Catarina state and another in Sao Paulo State, Karmann and his group of students, almost all of them speleologists, as they are called, have concluded that over the last 116,000 years there has been intense variation in the amount of rainfall in the south and southeast regions of Brazil: the heavy rains of the past or, the reverse situation of a shortage of rainfall, were principally due to a greater or lesser intensity of solar radiation – insolation -, which varies in accordance with the inclination of the Earth’s rotational axis, in cycles of around 23,000 years. Depending on the inclination of the Earth’s axis there can be more shadow in the northern hemisphere and greater solar cover in the southern hemisphere – or vice versa.

These discoveries about climate behavior in the past are helping to refine the models of climate simulation, which are going to provide more reliable forecasts while they have reflected the past with greater precision. “Insolation during summer has been increasing over the last 4,000 years in the southern region, making the summers on the Santa Catarina coastline become gradually more rainy”, explains Francisco William da Cruz Jr., ex-master’s and doctorate degree student of Karmann and the first named author of a study published in March in Nature with these findings. “This is a natural tendency that should be maintained over the next few thousand years, without considering any human intervention on climate.”

A greater insolation, as this USP group has verified, intensifies the circulation of humid winds that arrive from the Atlantic to the east coast and feed the Amazon Rainforest. Known as the Trade Winds, these winds circulate in the lower atmosphere, close to the surface, move off to the south and enter into the Amazon and bring humidity to the south and southeast regions of Brazil. They also reach the north of Argentina and Paraguay, favoring the formation of clouds and rainfall. In times of lesser insolation, the Trade Winds calm down and the humidity that brings about the rains in the south and southeast regions provides rain mainly in the Southern Atlantic.

Forest and shrublands
The displacement of humidity in the proximity of the equator results in more rainfall in the semi-arid northeast of Brazil. “In the northeast the climate and environmental changes were radical”, says Augusto Auler, a geologist at the Geosciences Institute of the Federal University of Minas Gerais (UFMG), and the co-author of an article about the ancient climate of the northeast, published in December in the magazine Nature, the first named author of another, also produced at the end of last year in the Journal of Quaternary Science. These works have shown that the so called Zone of Convergence Inter Tropical (ZCIT), a region of the atmosphere filled with humidity, which moves in a north to south direction above the ocean, close to the equator,  had previously been more to the south ‘ closer to the Brazilian northeast ‘ and brought rains that fed the tropical forest. Afterwards, when the ZCIT moved off to the north, without approximating itself so much to the continent, it had been directed to reign over a countryside similar to that of today. Studies by other groups had already raised the remains of ancient humid forests in the northeast of Brazil, but what was lacking was to show exactly in which eras of the past the present shrublands had been tropical forest.

The last time that there had been in this location high, green and lush forest, so vast that it probably linked the Amazon to the Atlantic Rainforest, was around 15,000 years ago. But before in the interior of the northeast a dry and sparsely vegetated region, similar to that of today, had reigned. The forest, fed by humid winds, flourished for brief periods of a few thousands years – around about 39,000, 48,000 and 60,000 years ago, to cite only the closest intervals of time – which correspond to the phases of most intense and constant rains. The longest period in which a semi-deciduous forest – one that loses its leaves for a few months of the year – populated the today dry lands of the northeast, lasted almost 5,000 years, from 68,000 to 63,000 years ago, according to the studies carried out by geologist Auler in conjunction with researchers from the universities of Minnesota, in the United States, Bristol, in England, and of Taiwan, in China. In Brazil, he was able to count upon the collaboration of the biologist Patrícia Cristalli, from the University of Mogi das Cruzes and USP.

This team searched through the interior and the surroundings of two caverns in the interior of the State of Bahia, named Toca da Barriguda and Toca da Boa Vista, the largest in the southern hemisphere with some 108 kilometers in extension. Geologist Auler and his team collected and analyzed stalagmites from the caverns the same as the USP group, but went beyond that and also studied the fossils of leaves found in the deposits of calcite – the mineral composed of calcium carbonate- accumulated under an open sky. Thus it was possible to reconstruct, as well as the vegetation formation of up to 210,000 years ago, the animals that also lived there. There were no lack of examples of large size animals, such as sloths and giant armadillos. There was also a monkey, the Caipora bambuiorum, of close to 40 kilograms, and double the size of the current largest Brazilian monkey, the woolly spider monkey. The UFMG team that described in an article published in 1996 in the Proceedings of the National Academy of Science knew that they were dealing with a tree dwelling animal, but they had no idea as to when it could have lived. Now it can be confirmed that the Caipora bambuiorum probably lived some 15,000 years ago, in the middle of the forest that would later give way to the dry corridor of almost 3,000 kilometers in extension, which separates the Amazon Forest from the Atlantic Rainforest.

Together the two research groups conferred a continental dimension to the study of paleoclimate in Brazil, showing that the intense rains that fell – and still fall – in the south and southeast regions could have originated in the humid Amazonian basin, transported for thousands of kilometers. They have also suggested that the reduction of humidity in the Amazon Rainforest, currently caused by deforestation, can alter the ritual of rains in the southern region. That was what happened last year, remembered Pedro Leite da Silva Dias, from the Astronomy, Geophysics and Atmospheric Sciences Institute (IAG) of USP, who participated in the work conducted by professors Karmann and Cruz, with researchers from the University of Massachusetts and from the Berkeley Geochronology Center, both in the United States, and from the Geology Institute of Sao Paulo. During last summer the humid winds from the Amazon basin did not move down as much south of the equator. They only reached the States of Minas Gerais and Bahia, causing intense rains. The Rio Grande do Sul State, without the humidity that it normally comes from the Amazon basin, suffered a drought.

Also the team provided evidence on the versatility of one of the methods used for checking their conclusions: The analysis of the proportion of the isotopes of oxygen, 18O and 16O, which correspond to the heaviest and lightest forms of oxygen atoms, in the minerals that make up the stalagmites – the calcite columns that grow up from the ground in the direction of the ceiling of the caverns. The reason is that the proportion between the two forms of oxygen in the stalagmites depends principally on the composition of the rainwater and can reflect the atmospheric changes of up to 500,000 years ago, as the USP researchers attested to in a recently accepted article for publication in the magazine Chemical Geology. The biologist Marie-Pierre Ledru, a specialist in pollens from the University of Montpellier II, in France, used the information obtained by the USP team from the Santana cavern to complete those that she herself had managed by analyzing pollens, whose method of dating can reach back to the maximum of 40,000 years. Her objective was to date and to recompose the ancient vegetation of the Colonia, a circular depression of almost four kilometers in diameter to the south of the city of Sao Paulo, possibly the result of a meteorite impact or a comet fragment. The results of Marie-Pierre’s study, in conjunction with the USP team, published this month in the magazine Quaternary Research, indicated that the periods of expansion of the forests in this area that reminds one of the crater of a volcano, coincide approximately with the variations in the atmospheric circulation of the last 110,000 years.

When the theory is insufficient
But there were many stones along the pathway of these discoveries that have managed to relate the proportionality of the isotopic oxygen atoms in the stalagmites over the thousands of years with changes in the global atmospheric circulation. In 1999, Karmann and Cruz, along with Oduvaldo Viana Jr., one of Karmann’s students for mater degree, began a monthly visit to the Santana caverns, located in the Ribeira Valley, in the southeast of the state of Sao Paulo, and those of Botuverá, in the east of Santa Catarina state. They didn’t go there just to appreciate the caverns, but as researchers, equipped to collect the water that dropped from the ceiling and which with time would form the stalagmites. They also brought back samples of the stalagmites themselves for analysis in the laboratory.

After two years of work the team was joined by the geographer José Ferrari, from the Geology Institute of Sao Paulo, having already discovered the age of two stalagmites. One of them, 1.6 meters in length that had been retrieved from a point some 300 meters below the ground and some 1,500 meters from the mouth of the Santana cavern, through which a subterranean river runs. The other, some 70 centimeters in length, was collected at a depth of 110 meters and some 300 meters from the mouth of the Botuvera cavern.  Cruz and Warren Sharp, from the Berkley Geochronology Center (BGC), in the United States, carried out the dating of the stalagmites and concluded that both were between 111,000 and 116,000 years old. This was an excellent beginning, since they were probably dealing with the oldest records obtained at the end of the last ice age in South America when the masses of ice from the north and south poles expanded and the coast line shrunk, as if the sea had lowered considerably – around 100 meters on average.

Once the dating of the stalagmites was been carried out, the USP group began preparing a graph about the variation, over time, of the ratio between the two oxygen isotopes accumulated along the axis of growth of the stalagmites. In the tropical regions of the planet, the variation between them is accustomed to indicating the eras in which it rained more or rained less, because the heavier isotope 18O, is lost with the first rains, leaving the water with more and more of the 16O isotope. The proportionality curve between the two forms of oxygen had accompanies the periods of insolation, but the only theory at hand – according to which the ratio between 18O and 16O should respond directly to the volume of rain – had not been able to explain the results.

“We remained in the dark”, says Karmann, reminding himself of the times in which he was in the caves and suddenly the carbide charge that generated the light to illuminate the way forward had ended. They thought that there could have been more than one origin of the rainwater – the problem was that there was no way of proving it. But they were certain. “The major merit of this work is exactly in showing that the humidity of the south and southeast could have origins completely different”, comments Pedro Dias, a professor at USP’s IAG to whom the geologists turned for help after they had suffered a year in silence. And the solution was relatively simple. According to Dias and his IAG team, the intensity of the rains in the past – and those of today as well – in the south and southeast regions result from humidity that arrives from the Amazon basin or from the Atlantic Ocean. Stirred up by the insolation, the Trade Winds coming from the Amazon basin help to compact the atmospheric region known as the South Atlantic Convergence Zone (SACZ) with even more humidity. Fed as well by the humidity that it receives from the ocean south of the equator, the SACZ is a vast gathering of clouds, at times with up to 5,000 kilometers in extension, directed in a northeast to southwest direction, which crosses the Brazilian coastline between the latitudes of 18o to 25o south.

When the insolation is greater, there is more heat and humidity in the tropics. The SACZ intensifies and displaces itself more in a southerly direction, resulting in more rain in this region. This is when the lighter isotope of oxygen, the 16O, predominates – the heavier isotope stays behind, washed out by the rain that falls along the way. In the reverse situation, when the insolation is lower in the southern hemisphere (and greater in the northern hemisphere), there is a lowering of the level of heat and of the Trade Winds – and hardly any humidity arrives from the Amazon basin. “When it rains less in summer”, comments Andréa Cardoso, from the IAG team, “the SACZ weakens or remains more to the north”. The humidity that reaches the south and southeast regions then comes from a much closer source, the Southern Atlantic. Brought by the winds that blow from the ocean to the continent and by the masses of polar air, this humidity results in a drier climate that can be currently be observed in the winters of the southern region. This is when the heavier isotope of oxygen, the 18O, which had no time to fall as precipitation, predominates since the distance to the continent is relatively short.

Islands of green – This coming and going of the atmosphere’s humid air to the south of the equator also explains how the humid forests in the territory today occupied by the Caatinga (semi-arid desert), according to studies coordinated by Auler. In the semi-arid northeast the intensification of humidity brought about an expansion of the jungles that covered the higher grounds and of the forests of the Cerrado (wooded savanna), an environment accustomed to intense climatic variations. When the humidity moved away, the soil dried up and the forest retreated. Currently there still are islands of green and dense vegetation – the so-called stands – in the middle of the torrid Caatinga. They grow at the foot of sierras, such as the plateaus of Borborema, Araripe and Ibiapaba, at altitudes that vary between 500 and 1,000 meters, favored by the current climate.

“This work opened my eyes to the value of information removed from caverns”, recognizes Pedro Dias, a mathematics graduate who made his way towards the study of climate some three decades ago “I had only been in caves a few times, and”, he stated, “I hadn’t enjoyed it much. Certainly lots of things have still to come out”. Some months ago, while Franscisco Cruz had been examining stalagmites from other caverns in Brazil at the University of Massachusetts, in Amherst, a town close to New York city, Ivo Karmann had been sweating away in a laboratory working on a stalagmite of a meter and a half in length. “I believe that this one will reach back to 150,000 years”, he comments while  preparing to saw it up, that is, to open up and saw off yet another slice of the rock column extracted from the Santana cavern, which as yet has not been studied as much as that of Santa Catarina.

Projects
1. Paleo-Environment Registration of the Quaternary Period in Carstic Relief systems (nº 99/10351-6); Modality Regular Line of research Assistance; Coordinator Ivo Karmann – IG/USP; Investment
R$ 103,316.08 (FAPESP)
2. The Late Quaternary Period in Continental Areas; Modality Post Doctoral Grant; Coordinator Augusto Auler- IGC/UFMG; Investment R$ 45,000.00 (CNPq)

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