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Geology

A carpet on the sand

As a result of humid weather, vegetation is covering the Rio Grande do Sul dunes

josé stolz/ufrgsIn the next few decades, the sand dunes on the Rio Grande do Sul coast, which is almost 500 km long, are expected to diminish sharply. They may even disappear altogether under a layer of soil and plants that will hide the sand. This is the prediction of geologist Caroline Martinho from the University of Brasilia (UnB), who for the last five years has been studying the geological changes in the coastal plane of the state of Rio Grande do Sul over the last four thousand years. She detected cycles of about one thousand years in which the dunes spread inland during dryer spells and then practically vanish when rainfall increases. During these wet times – such as the present – the clumps of grass, the deep shocking pink fuchsias, the yellow daisies and other typical crawling plants of the region extend their stalks and end up by covering the sand. According to the researcher, the sea level, which has not fluctuated by more than two meters in the last 6,500 thousand years, did not play the chief role in this process.

During her doctorate, which she completed in 2008 at the Federal University of Rio Grande do Sul (UFRGS), Caroline analyzed nine dune fields along the 250 km from the town of Torres, on the border with the state of Santa Catarina, to Mostarda, near the Lagoa dos Patos lagoon, in the middle of the Rio Grande do Sul coast. At the collection points, with the sometimes poetic and sometimes grandiose names that are typical of the region’s beaches (Rondinha, Capão Novo, Atlântida Sul, Jardim do Éden, Magistério, Dunas Altas, Solidão, São Simão and Mostardas), the geologist perforated the soil down to as much as five meters and collected samples that were previously on the surface, but that later became covered in sand. “The soil  only forms when there’s vegetation”, she explains. That is why its presence indicates an entire set of environmental circumstances. She then estimated the age of these samples using carbon 14 dating and identified three main periods in the last five thousand years in which the dunes became plant-covered fields: from 4,820 to 3,970, from 2,760 to 2,460 and from 1,570 to 710 years ago – periods that other studies had also suggested were somewhat more humid, as she explains in an article in the journal Marine Geology. Her work reconstructs environmental processes that took place over the course of thousands of years, but whose history is hidden meters under the surface and can only be recovered via a geological approach.

In another not yet published segment of her thesis, Caroline analyzed the most recent changes in the dunes, over the last 50 years. She compared aerial photos that were representative of different moments in this period  and compared them with information on rains and winds, confirming what she had already inferred in regard to bygone times. “When it rains more, the dunes shrink and tend to disappear”, she described. That is what is currently happening: from 1948 to 2003, average rainfall rose by 20 mm. The winds are equally important because they carry  sand from the beach inland. When they are weak, they stop feeding the dunes, which then become unable to resist the plants’ invasion. And from 1964 and 1988, wind strength dropped continuously, which Caroline believes was crucial for the dunes’ current configuration.

The Rio Grande do Sul coast is not uniform and the dunes are vanishing faster in the north, in the stretch from Torres to Atlântida Sul, where the Geral mountain range almost reaches the sea, blocking the wind and concentrating the rain in a strip of dunes no more than two kilometers wide. This moister climate is perfect for a type of vegetation full of carrapichos (Acicarpha spathelata), which prick the feet of those who try to walk to the beach. “Still, there used to be well-developed dunes there, as aerial photos from 1948 show”, Caroline tells us. Today, she says, this region only has vegetation-covered dunes that are undergoing fixation. As for the southern part of the area, the dunes extend up to six kilometers inland. In this region there are also hills of naked sand, separated by moister hollows that may hold some vegetation and even small swamps.

Caroline Martinho/UnBPlants and tuco-tuco rodents (left) are part of the landscape on Brazil’s southern coastCaroline Martinho/UnB

The variation in the extension of the dunes is also recorded in the genetic base of the dune tuco-tucos (Ctenomys flamarioni), only found in the Rio Grande do Sul dunes and under risk of extinction according to the Environment Ministry’s “red book” published in 2008. For these animals, the tendency of these sandy fields to disappear is bad news. They are sand-colored rodents that spend their lives in tunnels they excavate in the dunes. “A lot of vegetation or none whatsoever excludes the species”, says biologist Gabriela Fernández, from UFRGS. The studies she pursued during her doctorate, completed in 2007, indicate low genetic diversity in the 500 kilometers of the coastal plane in which they live, as compared to other kinds of tuco-tuco rodents. “This can be explained by the coastal region’s high instability, due to climate and current human processes as well as  the geological scale processes”, she concludes.

In the last few decades, human intervention has appeared in the wake of natural changes – and enhanced them. “The stabilized dunes encourage urban development”, says Caroline, who has witnessed land-parcel developments spreading along with the vegetation. The weekend homes work like barriers to the sand carried by the wind and fix the soil even more permanently than grass roots, furthering the dunes’ disappearance. If everything remains as it is now, even if climate fluctuations remain the way they used to be and in a few centuries the weather becomes dryer in that area again, the geologist believes that the dunes will no longer spread over long fields where the tuco-tucos peek out of their burrows.

Scientific article
MARTINHO, C.T. et al. Mid to late Holocene evolution of transgressive dunefields from Rio Grande do Sul coast, Southern Brazil. Marine Geology. v. 256, n. 1/4, p. 49-64, Dec. 2008.

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