The last ice age was a period of major climate transition for our planet, and coincided with a massive drop in global CO2 levels. Scientists haven’t been able to determine where all that carbon went - until now.
By looking some 70,000 years into the past, climate researchers have discovered that most of the atmospheric carbon lost back then was sequestered at the bottom of the Atlantic Ocean.
The study was published today in Nature Geoscience, and was lead by researchers from the Australian National University (ANU) in collaboration with an international team of ocean and geology scientists.
The results add fresh knowledge to our understanding of the carbon cycle - the carbon exchange between various systems on our planet, and one of the key cycles that make Earth capable of sustaining life.
Cracking the carbon cycle
According to lead author of the study, ANU marine biochemist Dr Jimin Yu, ocean currents changed over 10,000 years, leading to extra storage of 50 gigatonnes of carbon in the deep Atlantic.
“Within 10,000 years the sea level dropped 60 metres, and the atmosphere lost 60 gigatonnes of carbon, equivalent to a drop of about 30 parts per million of carbon dioxide,” explained Yu. “This research helps us understand the fate and the impact of that carbon.”
To find out where all that carbon went, researchers looked at ocean sediment cores from ten widely-ranging geographic locations in the Atlantic. By studying the chemical makeup of tiny fossilised shells from ocean bottom dwellers - also known as benthic foraminifera - the researchers found a tell-tale sign.
As the ice age progressed, the shells contained increasingly less boron and more calcium, which shows their marine environment over three kilometres under the surface was receiving more and more carbon dioxide.
As the Atlantic ocean currents changed over thousands of years, the carbon was sequestered into the depths.
“The deep ocean water originating from the South Atlantic stayed at the bottom of the ocean for hundreds of years accumulating carbon from the dead plankton sinking from the surface,” said Yu.
Natural carbon sequestration relevant to climate change today
Climate scientist from the University of New South Wales Dr Laurie Menviel co-authored the study. Her research focuses on the role ocean water masses can play in dealing with excess carbon in the atmosphere.
“What we call the instrumental period is when we started to make direct measurements, and this was just a few decades go,” said Menviel. “If you just look at, let's say, the last 50 years, you're going to have very little data and the climate is changing too much, so you get a limited understanding of what can happen.”
“In the future we want to look at other oceans, such as the Indian Ocean and the Pacific, perhaps over a longer time period, or further in the past,” added Yu.
According to Dr Menviel, understanding our planet’s carbon cycles over millennia could provide insights on the future of our atmosphere during a time when atmospheric carbon dioxide has been rising rapidly.
“Most likely there could be more carbon stored in the deep ocean - on the one hand that is 'good news', because if you store more carbon in the deep ocean, there's less carbon in the atmosphere,” said Menviel. “The problem is that we're putting so much carbon in the atmosphere that it's not going to matter that much.”
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