Climate History Rewritten: Arctic Ice an Early Arrival
For the first time, scientists have pulled up prehistoric geologic records from the frigid vault of the Arctic Ocean. One of the findings, evidence of glacial Arctic ice from 45 million years ago, recasts a critical chapter of global climate history.
The evidence, pea-sized pebbles locked inside a nearly 1,411 foot-long sediment core, shows that glaciers formed in the Arctic Ocean about 14 million years earlier than geologists thought. This means that the immense sheets of ice at the Earth's poles formed simultaneously, something researchers call “bipolar symmetry” in one of three reports on Artic ice highlighted on the cover of Nature.
Previously, geologists believed glaciers formed in Antarctica long before they appeared in the Arctic. The new evidence clears up this climate mystery and underscores the role that carbon dioxide and other greenhouse gases play in climate change, according to Steven Clemens, an associate professor of research in the Department of Geological Sciences at Brown University and a co-author of the Nature papers.
“In the past, scientists thought shifting tectonic plates and changes in ocean circulation patterns in the Southern Hemisphere may have prompted ice to form earlier in Antarctica,” Clemens says. “But there was other data that contradicted this theory. Now much of what we know about the evolution of ice on Earth makes more sense. And the evidence underscores the importance of greenhouse gases as a driver of climate change. If you inject, or remove, large amounts of carbon in the atmosphere, you get truly global climate change.”
Earth shifted from a “greenhouse” climate to an “icehouse” climate roughly 45 million years ago, a dramatic cooling that changed wind patterns, ocean currents and land and ocean temperatures and had a profound impact on plant and animal life. How this change played out in the Arctic, however, never has been decisively documented; that history, written in sediment on the sea floor, sits locked under thick, dense ice.
But the Arctic Coring Expedition (ACEX), supported by the Integrated Ocean Drilling Program, an international marine research program, successfully penetrated sea ice up to 16 feet thick. This feat took two ice-breakers and a drill ship that hauled up sediment gathered 1,411 feet below the seafloor along the Lomonsov Ridge, an undersea sliver of the Eurasian continent that broke away and sunk as the Arctic Ocean opened to the Atlantic and Pacific oceans.
Clemens was a member of ACEX, led by Kathryn Moran, University of Rhode Island and Jan Backman at Stockholm University, Sweden. A paleoceanographer, Clemens was brought in to help analyze the sediment cores, which contained a stunning 54 million years of climate history. Working in a German laboratory, Clemens and other members of the science team studied the samples to determine their physical, mineralogical and biological makeup.
The team found:
- Sediment from the most recent era, stretching from the present to about 25 million years ago, was marked by clay with course sand. The sand, along with small pebbles, comes from continents - blown by wind or carried by icebergs that deposited these bits onto the seafloor. These samples contained plankton, clear signs of salt water.
- Sediment dating from 25 to 44 million years ago was marked by condensed silt containing no discernible signs of biological life. This era is a mystery, a hole in the historical record.
- Sediment dating back 44 million to 49 million years ago contained remnants of both fresh and saltwater algae in the clay, including a surprise - spores of Azolla, a freshwater fern. Scientists believe that fresh water may have sat on the surface, with denser saltwater sitting toward the bottom. The evidence suggests that the ocean was in transition, moving from the prehistoric “greenhouse” world to the “icehouse” world that exists today.
- Sediment from the earliest era, dating from 49 to 54 million years ago, was silty clay containing Apectodinium, microfossils that are strong markers of an abrupt warming event. This evidence suggests that Artic was as warm as the subtropics about 55 million years ago - much warmer than previously estimated.
Clemens says the major finding - evidence of ice-deposited debris 45 million years ago - came in the form of pebbles. Why? Glaciers scrape across the land as they grow, carrying ground rock with them. When glaciers reach the sea, some of that ice shears off and forms icebergs, which shed pebbles as they melt.
This evidence of synchronous ice formation in the Arctic and Antarctic in the past may help bolster the evidence of “bipolar symmetry” today. In recent months, scientists have reported rapid melting of the world's ice, from the Antarctic ice sheet to Greenland glaciers. Many scientists believe the shrinking ice is linked to a sharp increase in greenhouse gases.
“Bipolar ice forms together and melts together,” Clemens says. “Carbon levels are a huge driver of ice volume, and global climate change, at all time scales.”
Ocean Drilling through Multi-year IceTo recover the sediments that yielded the prehistoric climate records, the research team needed to manage three icebreakers, one of which was equipped with a drill rig. The sediment records were recovered in water about 3,280 feet deep.
Expedition co-chief scientist Kathryn Moran notes that the overall age span of the sediments recovered was a few million years longer than was expected to retrieve. Such successful recovery was possible, in large part, due to strategic planning in anticipation of strenuous ice management. Planners had predicted that the three-vessel fleet could maintain the drilling vessel's station for up to two full days, yet the station-keeping achievements went far beyond this expectation. The Vidar Viking was kept on location in multi-year ice for nine days - a landmark feat that empowered ACEX scientists to sustain their ocean-drilling explorations for a significant period. “The scientific community benefited from our extraordinary ice management efforts - we are the first to study full geologic records from the Arctic Ocean," says Moran.
Moran's colleague, co-chief scientist Jan Backman, also recalls the challenging research conditions - the drill site, at times, was covered with ice several feet thick. “We encountered an ice flow of multi-year ice, harder and denser than ice from just one Arctic winter; it was like driving into a brick wall." The upper sections (525 feet) of retrieved sediments indicate prevailing ice conditions during the last 14 million to 16 million years.