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GEOLOGY

Evidence of a catastrophe

Sediments collected in Jureia, on the coast of São Paulo State, tell the story of a radical climate change that occurred 8,200 years ago

Winding along the Jureia: Researchers spent two hours by boat on small rivers to reach the collection point

Alethea Sallun / Geological InstituteWinding along the Jureia: Researchers spent two hours by boat on small rivers to reach the collection pointAlethea Sallun / Geological Institute

Analysis of a column of mud a few centimeters in diameter and nearly six meters deep, collected by geologists at the Jureia-Itatins Ecological Station in the Atlantic Forest, on the southern coast of the state of São Paulo, has revealed the first evidence found on the Brazilian coast of the most radical and sudden global climate change that the Earth has seen in the past 10,000 years—the so-called 8.2 kiloyear (ka) event.

According to the study’s coordinator, geologist Alethea Sallun of the Geological Institute of São Paulo, it is quite rare to find signs of a relatively short-lived episode like the 8.2 ka event, which lasted less than 400 years—a mere blink of an eye in the planet’s 4.5-billion-year geological history. “We were very lucky to have obtained a record in the sediments,” she says.

The 8.2 ka event occurred because of a rise in global temperatures at the end of the last ice age 11,000 years ago, when melting of the glaciers that covered North America created immense lakes. About 8,200 years ago—give or take a few hundred years—the glacier that contained two of these lakes broke up, discharging an enormous volume of fresh water into the North Atlantic. In a short period of time, perhaps less than 10 years, the abrupt change in the ocean’s salinity temporarily interrupted the Gulf Steam that transports heat from the tropics to Europe, freezing the continent and forcing human migrations on a massive scale.

Geological records suggest that the effect of the event spread quickly across the globe, causing droughts in Africa and advancing glaciers in New Zealand and in the Andes. The altered ocean currents are thought to have intensified the summer monsoons in South America. In 2009, researchers examining Brazilian cave stalagmites identified evidence of increased rainfall during that period. In addition, simulations suggest that the coast of Brazil saw a sudden rise in sea level, of about one meter.

15_Jureia_204Ana Paula CamposRecords indicate that the sea level along the Brazilian coast has risen and fallen at least six times in the last 10,000 years, registering as much as five meters above the present level about 5,000 years ago. “There are many contradictions about the sea level fluctuations in the past 10,000 years,” explains oceanographer Michel Mahiques of the University of São Paulo (USP), who did not participate in the research.In fact, it was evidence of these ocean retreats that Sallun and her colleagues were searching for on expeditions into the forest of the Jureia-Itatins Ecological Station in 2007. One of the project’s coordinators, geologist Kenitiro Suguio, a full professor now retired from the Geosciences Institute of USP, had pioneered the mapping of the recent geological history of the coasts along São Paulo State and southern Rio de Janeiro State in the 1970s, in partnership with French geologist Louis Martin.

On the southern coast of São Paulo, the two researchers discovered that on the floodplain a few kilometers from the coast and just two meters above the present sea level, between the Itatins and Jureia mountains, there were layers of mud beneath the forest that had been deposited over a period of thousands of years, during the time when the area was dotted with saltwater lagoons.

The column of sediments from the ancient lagoons did not show clear signs of the recent changes in sea level that the researchers were looking for. Instead, they found evidence of something else that was older still. “That took me by surprise,” Suguio recalls.

A convenient record
Determining the age of the mud by the radiocarbon dating method, the geologists discovered that the collected column revealed a continuous history of sediment deposition from 9,400 years ago to the present. But while the first meter contains information on 7,600 years of that history in small deposits of about one millimeter per year, the remainder preserves the details of an enormous quantity of sediments deposited in the previous 2,000 years.

The highest rate of deposition occurred between 8,385 and 8,375 years ago, when a meter of mud settled there. Sallun explains that the data are insufficient to say exactly what happened, but most likely a large amount of rain, combined with the elevation of the sea level, preserved a standing water environment long enough for the deposition to occur and be preserved.

A team of 16 researchers from the Geological Institute, USP, and the University of Guarulhos in Brazil and from Washington State University in the U.S. collaborated on the analysis of the chemical elements found in the column of sediments.  Their findings, published in the journal Quaternary Research in January 2012, indicate that a very sudden environmental and geographic change occurred in Jureia between 8,500 and 7,800 years ago. During that period, the sediment source changed, while the influence of the ocean and the productivity of the plants and animals increased, and later returned to normal.

Suguio is more cautious than his younger colleagues in associating the change in Jureia with the 8.2 ka event. “It may have been a local event, unrelated to what happened in the Northern hemisphere,” he says, although he thinks that “it’s rather coincidental to be just a coincidence.”

It is possible that something similar to the 8.2 ka event will occur again if global warming causes Greenland to thaw, as former U.S. Vice President Al Gore points out in the 2006 documentary film An Inconvenient Truth.  Sallun believes that her study will help lead to a better understanding of how such an event would affect the Brazilian coast.

Project
Jureia-Itatins ecological station (SP): Holocene sea-level oscillations below the present (nº 06/04467-7); Grant mechanism Regular Line of Research Project Award; Coordinator Kenitiro Suguio – IGc/USP; Investment R$206,024.65 (FAPESP).

Scientific article
SALLUN, A.E.M. et al. Geochemical evidence of the 8.2 ka event and other Holocene environmental changes recorded in paleolagoon sediments, southeastern Brazil. Quaternary Research.  v. 77, pp. 31-43. 2012.

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