Around 950,000 years ago, northern hemisphere ice sheets must have reached a critical point that favored the collapse of North Atlantic deep water formation (Credit: NASA)
For millions of years, ice ages ran like clockwork in 41,000-year cycles. But around a million years ago, an unknown force lengthened the cycle to 100,000 years and caused markedly colder glacial periods.
“The whole climatic system of our planet changed completely,” said Leopoldo Pena, paleoceanographer and assistant research professor at Columbia University. “This has been a puzzle in our profession for the past couple of decades.”
The solution for that puzzle may be soon at hand, as a new study, led by Pena, suggests deep ocean currents might be responsible for the cycle shift. The currents, which Pena compared to baggage claim conveyor belts, move warm shallow water and cold deep water between the Atlantic and Pacific oceans in a 1,000-year cycle called thermohaline circulation.
The researchers sampled sediments off the South African coast to measure isotopes of neodymium, an element that reflects the unique composition of continental crusts, and reveals signs of North Atlantic seawater. The shells of long-gone plankton provided the perfect medium for neodymium to affix itself.
“We used microfossils that are deposited in the sediments over thousands and thousands of years,” Pena said. “They’re like little time capsules.”
Learning from the fossilized shells wasn’t easy. Only a few nanograms of neodymium could be extracted from hundreds of minuscule shells in each sample. The researchers then measured the isotopes in the sample.
The sediment samples revealed that starting over 1 million years ago, the deep sea currents between the Atlantic and Pacific began to slow and weaken, remaining crippled for the next 100,000 years.
As the slowing currents circulated less warm water around the globe, ice sheets around the poles may have grown larger instead of melting, allowing the 100,000-year cycle to occur. However, another hypothesis suggests a role-reversal: spreading ice sheets could have slowed deep currents and reduced CO2 release from the oceans, resulting in further ice growth.
“Isn’t this like the chicken and the egg?” Pena asked. “The answer is that we don’t really know what happened but it seems that in the past ice sheets might have reached a critical state… that we believed might have triggered this cool down in the Atlantic ocean, leading to larger ice sheets.”
But what does ancient climate change have to do with modern humanity? Potentially quite a lot.
Pena said the oceanic and atmospheric cycles of the past could provide insights into climate change issues today. Analogues between past and present can help scientists develop better climate models.
“Over the last 100 years, we’ve pumped into the atmosphere the same amount of CO2 that would be produced coming out from a glacial period,” Pena said. “What naturally occurs over tens of thousands of years, we’ve done in 100 years, and we don’t know if the system can accommodate that.”
“In the end everything is connected,” he said. “You have to look at the atmosphere and the oceans together. You always have to look at the big picture.”