Sand-sized fossils hold secrets to history of climate change

Between 18,000 and 11,000 years ago, the amount of carbon dioxide in the atmosphere suddenly shot up. This caused rapid global warming, the mass melting of glaciers, and the end of the last ice age.

Much of this sudden influx of atmospheric CO2 came from the Southern Ocean around Antarctica, highlighting the key role this body of water plays in regulating the global climate.

However, we have a poor understanding of how and why CO2 release from this region changed during periods such as the end of the last ice age. But our new study, published in Nature Communications, reveals how much CO2 was released to the atmosphere from the polar Southern Ocean during this period – and what factors were responsible.

We reached these conclusions by examining the chemistry of sand-sized fossils, called foraminifera, from the seafloor south of Tasmania.

Foraminifera are tiny single-celled organisms, either floating on the ocean surface or living on the seabed. Most of them build shells made of calcium carbonate to protect themselves. After death, these foraminifera shells are preserved in the mud on the seabed.

By looking at trace elements in these foraminifera shells found in the sequence of mud on the seabed, we can decipher mysteries about the past seawater condition in the book left by foraminifera on the seabed.

Foraminifera have lived almost everywhere in the ocean for millions of years. Based on their chemical composition, scientists have reconstructed a continuous record of seawater temperature during the past 66 million years in great detail.

Among a few places in the ocean where you cannot find foraminifera is the polar Southern Ocean. Although some foraminifera live there, seawater in this region is often too corrosive for their shells to preserve on the seabed. The lack of foraminifera in the polar Southern Ocean brings a huge challenge for scientists eager to understand past changes in CO2 exchanges between the ocean and the atmosphere.

Seawater at that depth near Tasmania is ideal for studying the chemistry of the polar Southern Ocean. That's because seawater from the polar Southern Ocean sinks to the bottom of the ocean, moves northwards, and eventually occupies the seabed south of Tasmania.

Seawater chemistry – including concentrations of carbon, phosphate and oxygen – does change along its way at the bottom of the ocean.

Using the chemistry of foraminifera, we reconstructed changes in concentrations of carbonate ion, phosphate and oxygen at the bottom of the ocean near Tasmania during the end of the last ice age roughly 20,000–10,000 years ago. This period is known as the last deglaciation.

Based on these reconstructions, we calculated the amount of CO2 released from the polar Southern Ocean during the last deglaciation. We found that biological processes were more important for CO2 releases during the earlier stages of the deglaciation, while the physical processes contributed more during the later stages.

Scientists use climate models to predict future climate and to reproduce past atmospheric CO2 changes.

Our results provide testing targets for climate models to reproduce.

Better reproduction of past changes will improve climate model design for predicting future changes.

This will help us understand how future changes in the polar Southern Ocean can affect atmospheric CO2, contributing to making effective plans to mitigate CO2 emissions.