Discharges from western North America disrupted climate during last ice age

During the last ice age, Earth's climate was characterized by dramatic, hemispheric-scale shifts.

New research -- published this week in the journal Science -- suggests this shiftiness was, at least in part, caused by successive discharges from the ice sheets that spread across western North America into the Pacific.

Researchers suggest their discovery can help climate scientists better understand how rapidly melting Arctic ice will influence climate patterns in the coming decades.

"Understanding how the ocean has interacted with glacial ice in the past helps us predict what could happen next," lead study author Maureen Walczak, paleoclimatologist at Oregon State University, said in a news release.

By analyzing ancient layers of marine sediment, scientists were able to determine the timing of outflows from the Cordilleran ice sheet, which once covered large swaths of western North America, stretching from Alaska south to Washington state and east to western Montana.

When researchers compared the record of discharges with ancient climate disruptions during the last ice age, they found a strong correlation between Cordilleran discharges and hemispheric-scale disruptions to the climate.

The research showed pronounced outflows from the Cordilleran ice sheet were followed by changes to deep ocean circulation patterns and the retreat of ice sheets.

Previously, scientists have hypothesized that ice losses from theLaurentide ice sheet, which spread across much of Canada, the Midwest and New England, altered climate dynamics in the North Atlantic, triggering abrupt climate shifts -- dubbed Heinrich Events.

The latest findings paint a more complicated picture.

"The outcome of this research was unexpected. The data irrevocably says that the Pacific ice goes first, with Heinrich Events and other changes following in a rhythm. The Pacific Ocean sets the drum beat," Walczak said. "This is a paradigm shift in our thinking about how these events are connected."

The sediment cores that helped scientists gain new insights into the timing of Cordilleran outflows were recovered from the seafloor of the northern Gulf of Alaska.

"Getting these new insights took years of work. We first mapped the seafloor and recovered short sediment cores in 2004, drilled longer cores in 2013 and had 16 years of painstaking laboratory work involving several Ph.D. students," said study co-author Alan Mix, the project's principal investigator.

"This was a virtually unknown area when we started, and now it offers among the most detailed and best-dated long records of ocean change on the planet during the ice age," said Mix, a distinguished professor of ocean and atmospheric science at Oregon State.

To establish the chronology of ancient icy discharges into the Pacific, researchers measured the ratios of radioactive isotopes of carbon in the different sediment core layers.

The presence of large concentrations of small stones deposited by drifting, called ice-rafted debris, denoted the arrival of surging ice streams in the northern Pacific.

Researchers dubbed these Alaskan iceberg dumps "Siku Events," named for the Inuit word for ice. Their analysis showed Siku Events regularly preceded Heinrich Events.

The discovery suggests changes in the Pacific were a driving force in a chain reaction of events that led to significant climate disruptions during the last ice age, researchers said.

"The Pacific Ocean is the largest exchangeable reservoir of heat and water and carbon dioxide on Earth, simply because of its massive size," Mix said. "It really is the 800-pound gorilla in the zoo of climate beasts."

Scientists expect the rapidly retreating ice along the Alaskan coast to continue flowing into the northern Pacific in the coming decades, altering the balance of more buoyant freshwater and denser saltwater. The phenomenon could lead to similar disruptions to ocean circulation patterns.

"The new findings are likely to fuel increased interest in the North Pacific, an area that has not been as well-studied as other parts of the planet," Walczak said.

Researchers hope future investigations will offer more detailed insights into the relationship between Pacific discharges and icy outflows elsewhere.

"Why did the other ice sheets respond to the retreat of the Cordilleran?" Walczak said. "How fast do the dominoes fall in this sequence of events?"

Answering these questions, researchers said, will be key to understanding how current ice melting trends will influence future shifts in climate patterns.

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Beyond the Ice Age

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