Ancient shellfish provide clues to El Niño history

New insight into the El Niño Southern Oscillation has been provided by 25-foot piles of ancient shellfish remains. Researchers analysing the shells have found El Niños 10,000 years ago were as strong and frequent as the ones experienced today. Their results call into question the accuracy of computer models to reproduce historic El Niño cycles, and predict how they might change under future climates. “We thought we understood what influences the El Niño mode of climate variation, and we’ve been able to show that we actually don’t understand it very well,” said Julian Sachs, professor of oceanography at the University of Washington (UW). “Our data contradicts the hypothesis that El Niño activity was very reduced 10,000 years ago, and then slowly increased since then,” said Matthieu Carré from the University of Montpellier, who carried out the research while a postdoctoral researcher at UW. Samples taken from 25-foot piles of shells from Mesodesma donacium clams eaten and discarded over centuries into pile called middens revealed one to three-year-long records of monthly temperature of the Pacific Ocean along the coast of Peru. Analysis was carried out using a technique developed by Carré. Researchers used carbon dating of charcoal from fires to get the year, and the ratio of oxygen isotopes in the growth layer to get the water temperature as the shell was forming. [caption id="attachment_39464" align="alignright" width="200"] This image shows a magnified cross-section of a shell. The lines in the outer layer come from growth during low tides, and help to put a time on the temperature measurements. Credit: M. Carré / Univ. of Montpellier[/caption] The resulting record shows that 10,000 years ago, El Niño cycles were strong, while roughly 7,000 years ago there was a shift to the central Pacific of the most severe El Niño impacts, followed by a lull in strength and occurrence between 6,000 and 4,000 years ago. Researchers believe another factor was compensating for the dampening effects expected from cyclical changes in Earth’s orbit around the sun 10,000 years ago. “The best candidate is the polar ice sheet, which was melting very fast in this period and may have increased El Niño activity by changing ocean currents,” said Carré. Around 6,000 years ago, this melting would have finished, so the effect of the Earth’s orbital geometry could have caused a weakening of El Niños. The study has been published in Science. Holocene history of ENSO variance and asymmetry in the eastern tropical Pacific