LAUSR

24 AUGUST 2018 • VOL 361 ISSUE 6404 739 SCIENCE sciencemag.org G R A P H IC : L . L IS IE C K I, M . R A Y M O , P A L E O C E A N O G R A P H Y A N D P A L E O C L IM A T O L O G Y , 2 0 , (2 0 0 5 ), A D A P T E D B Y J . Y O U / S C IE N C E A bout 1 million years ago, one of Earth’s most important metronomes mysteriously shifted: Ice ages went from occurring every 40,000 years to every 100,000 years. At the same time, the “conveyor belt” of warming currents in the North Atlantic Ocean slowed sharply. Last week, scientists here at the Goldschmidt Conference presented a clue to these twin mysteries: evidence that glaciers in the Northern Hemisphere suddenly began to stick to their beds. Growing thicker, they might have triggered a cooling that disrupted the conveyor belt and allowed the 100,000-year cycle that we see today to take root. “The system basically crashed,” says Steve Goldstein, an ocean geochemist at Columbia University who led the study. Other scientists welcome the new clues to the transition. “This is really exciting new evidence,” says Henrieka Detlef, a paleoclimatologist at Cardiff University in the United Kingdom . But she and others aren’t sold yet on the long causal chain that Goldstein’s team posits. Scientists have long known that tiny changes in Earth’s orbit around the sun, called Milankovitch cycles, drive the planet in and out of ice ages. But nothing changed in those orbital patterns 1 million years ago. Recently, Goldstein and his colleagues found signs of a possible contributor to the ice age transition: a near-collapse of the Atlantic meridional overturning circulation (AMOC). The AMOC shepherds shallow warm water to the North Atlantic, where it cools and sinks before returning south along the sea floor to the Southern Ocean to meet Pacific Ocean waters. Goldstein’s group deduces the overall strength of the AMOC from geochemical markers in ocean sediment cores. The researchers take advantage of a ratio between two isotopes of neodymium that varies with the age of their source rocks: ancient crust runs negative, whereas younger rocks are more positive. As it happens, the North Atlantic is surrounded by ancient crust, whereas the Pacific, thanks to its volcanic Ring of Fire, tilts younger. The neodymiumcarrying grit ends up incorporated into the shells of single-celled foraminifera or fish teeth, both of which accumulate over time on the sea floor. Changes in the isotope ratio record the wax and wane of intruding North Atlantic or Pacific waters. Earlier this decade, the Columbia group tested its approach on two archived sediment cores from the South Atlantic. About 950,000 years ago, they saw the isotopic signals shoot up, reflecting an incursion of Pacific waters, with little evidence of returning North Atlantic waters—suggesting a stark “AMOC crisis.” The slowdown could have sharply cooled the North Atlantic region—and might have lengthened the ice age rhythm.