Seafloor cores suggest sticky thick glaciers caused mysterious shift in ice age

first_img 2.5 1.4 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 neodymium-carrying 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.Now, the team has analyzed five other ocean cores that also show signs of a weak AMOC. Two of the cores, from the North Atlantic, suggest a possible trigger for the AMOC crisis. In the millennia leading up to it, the neodymium signal sharply trended negative before abating—a sign that an influx of older and older grit from the North Atlantic region had suddenly stopped.The only plausible explanation, they say, is a long-standing hypothesis advanced by Peter Clark, a glaciologist at Oregon State University in Corvallis, and several others: that the northern ice sheets had finally ground their way to bedrock. Before Earth’s current ice age cycles began 3 million years ago, a long warm period had allowed a thick soil layer to build up on northern landmasses. At first, the soil acted as a grease that caused early ice sheets to collapse before they could thicken much. But repeated glaciations gradually scoured this grit away, and meltwater swept it into the ocean. As the glaciers dug deeper into older rock, the neodymium signal in ocean sediment became more negative. Eventually, the glaciers reached bedrock and began to stick to their base, allowing them to grow thicker—leading to a more profound and persistent cooling that somehow caused the AMOC to crash and the glacial cycle to lengthen. “We think we’re seeing the trigger,” says Maayan Yehudai, the Columbia graduate student who presented the work. (Scientists believe pronounced global warming—like the warming underway now—could also disrupt the AMOC.) 100,000-yearcycles BOSTON—About 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. Sign up for our daily newsletter Get more great content like this delivered right to you! Country 4 Email 0.4 Click to view the privacy policy. Required fields are indicated by an asterisk (*) The Russell Glacier in Greenland. After ancient glaciers scoured away soil and reached bedrock, they may have grown thicker—triggering a global cooling that shifted the ice age cycles. 1 By Paul VoosenAug. 22, 2018 , 1:30 PM 40,000-yearcycles Country * Afghanistan Aland Islands Albania Algeria Andorra Angola Anguilla Antarctica Antigua and Barbuda Argentina Armenia Aruba Australia Austria Azerbaijan Bahamas Bahrain Bangladesh Barbados Belarus Belgium Belize Benin Bermuda Bhutan Bolivia, Plurinational State of Bonaire, Sint Eustatius and Saba Bosnia and Herzegovina Botswana Bouvet Island Brazil British Indian Ocean Territory Brunei Darussalam Bulgaria Burkina Faso Burundi Cambodia Cameroon Canada Cape Verde Cayman Islands Central African Republic Chad Chile China Christmas Island Cocos (Keeling) Islands Colombia Comoros Congo Congo, the Democratic Republic of the Cook Islands Costa Rica Cote d’Ivoire Croatia Cuba Curaçao Cyprus Czech Republic Denmark Djibouti Dominica Dominican Republic Ecuador Egypt El Salvador Equatorial Guinea Eritrea Estonia Ethiopia Falkland Islands (Malvinas) Faroe Islands Fiji Finland France French Guiana French Polynesia French Southern Territories Gabon Gambia Georgia Germany Ghana Gibraltar Greece Greenland Grenada Guadeloupe Guatemala Guernsey Guinea Guinea-Bissau Guyana Haiti Heard Island and McDonald Islands Holy See (Vatican City State) Honduras Hungary Iceland India Indonesia Iran, Islamic Republic of Iraq Ireland Isle of Man Israel Italy Jamaica Japan Jersey Jordan Kazakhstan Kenya Kiribati Korea, Democratic People’s Republic of Korea, Republic of Kuwait Kyrgyzstan Lao People’s Democratic Republic Latvia Lebanon Lesotho Liberia Libyan Arab Jamahiriya Liechtenstein Lithuania Luxembourg Macao Macedonia, the former Yugoslav Republic of Madagascar Malawi Malaysia Maldives Mali Malta Martinique Mauritania Mauritius Mayotte Mexico Moldova, Republic of Monaco Mongolia Montenegro Montserrat Morocco Mozambique Myanmar Namibia Nauru Nepal Netherlands New Caledonia New Zealand Nicaragua Niger Nigeria Niue Norfolk Island Norway Oman Pakistan Palestine Panama Papua New Guinea Paraguay Peru Philippines Pitcairn Poland Portugal Qatar Reunion Romania Russian Federation Rwanda Saint Barthélemy Saint Helena, Ascension and Tristan da Cunha Saint Kitts and Nevis Saint Lucia Saint Martin (French part) Saint Pierre and Miquelon Saint Vincent and the Grenadines Samoa San Marino Sao Tome and Principe Saudi Arabia Senegal Serbia Seychelles Sierra Leone Singapore Sint Maarten (Dutch part) Slovakia Slovenia Solomon Islands Somalia South Africa South Georgia and the South Sandwich Islands South Sudan Spain Sri Lanka Sudan Suriname Svalbard and Jan Mayen Swaziland Sweden Switzerland Syrian Arab Republic Taiwan Tajikistan Tanzania, United Republic of Thailand Timor-Leste Togo Tokelau Tonga Trinidad and Tobago Tunisia Turkey Turkmenistan Turks and Caicos Islands Tuvalu Uganda Ukraine United Arab Emirates United Kingdom United States Uruguay Uzbekistan Vanuatu Venezuela, Bolivarian Republic of Vietnam Virgin Islands, British Wallis and Futuna Western Sahara Yemen Zambia Zimbabwe 4.5 A slowing pulse Earth’s orbital patterns cannot explain a shift in the ice ages’ frequency that began about 1 million years ago. Thickening glaciers and crippled ocean currents could be to blame. 1.6center_img The neodymium evidence supports this geological story, Clark says. “It’s a pretty clear signal that you should see.” Detlef notes, however, that there is no conclusive evidence that northern ice sheets were increasing in thickness prior to the AMOC slowdown. But she accepts that something important happened in the North Atlantic leading up to the AMOC crisis.One hypothesis that does seem ruled out, however, is the notion that the growth of Antarctic ice sheets 900,000 years ago played a pivotal role in the tempo change. “Everything that’s happening in the North Atlantic is happening before [that],” Yehudai says.The AMOC and the glaciers may not have been the only factors in the transition, however. Some scientists have suggested that a small drawdown of carbon dioxide (CO2), perhaps driven by a dust-fertilized plankton bloom in the Southern Ocean, would have been enough to shift the ice age rhythm. Yair Rosenthal, a paleo-oceanographer at Rutgers University in New Brunswick, New Jersey, thinks a CO2 drop, thickening ice sheets, and a weak AMOC could have all played a role. “I’m not a fan of single triggers of anything.” 5.5 0.2 Age (million years ago) Jason Edwards/National Geographic/Getty Images Seafloor cores suggest sticky, thick glaciers caused mysterious shift in ice age rhythms 3 0.6 present 1.2 L. LISIECKI, M. RAYMO, PALEOCEANOGRAPHY AND PALEOCLIMATOLOGY, 20, (2005), ADAPTED BY J. YOU/SCIENCE 5 3.5 Temperature proxy 0.8 Mid-Pleistocenetransitionlast_img