by Mary Caperton Morton Friday, August 29, 2014
When Francisco Pizarro landed in Peru in 1532, his band of Spanish conquistadors set off a chain of far-reaching consequences for the people and economics of western South America. A new study has found that the Spanish invasion also changed the shoreline of northern Peru, by actually ending a several-thousand-year cycle of anthropogenic alteration.
The Chira Beach-Ridge Plain in northwestern Peru is rippled by a set of nine ridges — several meters tall by up to 300 meters wide and 40 kilometers long, and large enough to be visible from space — running parallel to the shoreline. The pattern, observed along at least five other Peruvian beaches, was thought to have formed over the past 5,000 years in part due to the combination of major earthquakes loosening large quantities of sediment upstream followed by El Niño cycles, which deposit sand and gravel on the beach above the tidal zone.
The sediment transported down the Chira River is mostly sand, which is easily blown away by persistent winds coming off the ocean. Left on its own, however, Chira Beach does not form ridges. But during pre-colonial times, coastal communities harvested huge quantities of mollusks and left their shells behind on the beach, which created a kind of scaffolding for the ridges to form around by protecting the beach sand from wind erosion.
“These ridges are completely covered with shells for miles,” says Daniel Belknap, a geologist at the University of Maine in Orono and lead author of the new study, published in Proceedings of the National Academy of Sciences. This sort of man-made protection was different from the natural process at the other ridged beaches, where more gravelly sediment keeps the ridges from being blown away.
“Once the Spanish arrived, there was a complete reorganization of the region’s economic system, and the archaeological sites along the coast were abandoned by 1600,” says James Richardson, a geoarchaeologist at the Carnegie Museum of Natural History in Pittsburgh, who was not involved in the new study. Radiocarbon dating of the shells confirms that the most recent well-preserved ridges were built up prior to the arrival of the Spanish. Once shell fishing declined, new ridges no longer formed, and that continues to this day. “Peruvian archaeologists may have suggested this connection before, but this study is the first to make the connection in the field,” Richardson says.
A similar record of pre-Spanish ridges is unlikely to be found at the other ridged beaches in Peru because those are protected by gravel, not by shells, Belknap says. At the other beaches, the ridge-building process is still ongoing, but more investigation is needed to suss out the relationship between earthquakes, El Niño cycles and the distinctive pattern of beach ridges.
“It’s an interesting philosophic argument whether the Spanish invasion restored this section of beach to a more natural geologic system, since these ridges seem to have been created by the native people,” Belknap says. Mollusk harvesting at Chira Beach has resumed in modern times, he says, but now the shellfish are hauled away in refrigerated trucks to be shucked elsewhere.
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