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Geology

Receding seas on Trindade intrigue researchers

The Atlantic has receded by 3 meters over the last 6,000 years on the shores of Brazil’s eastern-most island

Sea levels have fallen by 3 meters in the last 6,000 years on Trindade, an oceanic island standing 1,200 kilometers off the coast of Espírito Santo State that marks the east boundary of Brazilian territorial waters. Perhaps surprising given the current evidence of rising ocean levels, these findings are from a study by Brazilian researchers recently published in the Journal of South American Earth Sciences. The authors collected fossil samples from ancient coral reefs and observed hedgehog burrows on the island’s beaches to come to their findings. When found above the current sea level, these fossils indicate that sea levels were once higher and later receded. “Many of these samples were found on the beaches of Tartarugas and Paredão,” says geologist Maria Cristina de Souza, who led the study alongside Rodolfo José Angulo, a fellow geologist at the Federal University of Paraná (UFPR). Sampled fossils were dated by Carbon 14 methods. This provided an understanding of how the sea has advanced and retreated on the island over time.

The falling sea levels observed on Trindade, an island under the administration of the Brazilian Navy, are thought to be linked to global climate change over previous millennia. The melting of the polar ice caps around 18,000 years ago is believed to have caused the sea to advance on Trindade and in other parts of the world. This continued until about 6,000 years ago, when the seas began to retreat until reaching current levels. During glacial or colder periods, more water builds up at the poles in the form of ice, and sea levels then fall. The reverse occurs when temperatures rise again during interglacial periods like the present. Satellite observations indicate that, globally, sea levels have risen by an average of 20 centimeters over the last 100 years. “The primary causes of sea level rise today are the melting of glacial ice and thermal expansion of ocean waters due to global warming from greenhouse gases,” explains oceanographer Lauro Julio Calliari of the Federal University of Rio Grande (FURG), who was not part of the study team but has done previous research on the beaches of Trindade.

Erosion
The global trend of sea level rise has already been felt along much of Brazil’s coast and on other oceanic islands, such as Fernando de Noronha. But in Trindade, recent data indicates the opposite is occurring. The apparent retreat of the sea is all the more surprising given that local conditions, alongside global warming, would be expected to cause sea levels to rise on the island. With its total area of only 9 square kilometers, Trindade is highly exposed to the unpredictable action of ocean waves. Erosion corrodes its beaches and winds reduce its vegetation cover. “The erosion occurring on Trindade is due to deforestation caused by livestock introduced by Azorean settlers in the eighteenth century,” says João Wagner Alencar Castro of the Laboratory of Coastal Geology, Sedimentology, and the Environment of the Federal University of Rio de Janeiro’s (UFRJ) National Museum.

Another factor making the sea-level retreat on the island intriguing is that there is evidence that Trindade gradually sank over a period of time hundreds of thousands of years ago. Formed by lava flows more than 2 million years in the past, the island is the tip of an extinct volcano belonging to the Vitória-Trindade mountain range, a submarine ridge rising up to 5,500 meters above the ocean floor and extending from the mid-Atlantic to a point near the capital of Espírito Santo. The magma from these submarine volcanoes cools rapidly and becomes increasingly dense, pushing the mountains (and Trindade) toward the bottom. According to the new study, significant natural subsidence ceased to occur about 5,000 years ago. “But when an island, like Trindade, is in a region where tectonic activity is high, it can either rise or fall,” says Calliari. “This movement makes it difficult to determine exactly what is causing sea-level change.”

The discrepancy between the data for Trindade, which describes episodes occurring over the last 6,000 years, and the current global trend of sea-level rise, which began in the last century, could be caused by the use of two different time scales. There is currently insufficient data on sea-level change on the island over the last 100 years. “Further research is needed,” says de Souza, whose paper was coauthored by colleagues from the Federal University of Ceará and the Federal University of Rio Grande do Sul.

Scientific article
ANGULO, R. J. et al. Quaternary sealevel changes and coastal evolution of the Island of Trindade, Brazil. Journal of South American Earth Sciences. Vol. 84, pp. 208–22. July 2018.

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