Carbon Dioxide Mystery in the Southern Ocean May Finally be Solved

There's a mystery in a Southern Ocean. Twenty thousand years ago, when humans were still nomadic hunters and gatherers, low concentrations of carbon dioxide in the atmosphere allowed the Earth to move into an ice age. Despite decades of research, though, the reasons why levels of the greenhouse gas were so low have been unclear-until now.

In this latest study, the researchers took a closer look at what may actually be causing this loss of carbon dioxide and found that the answer may lie at the bottom of the world. The researchers took sediment samples from the seafloor near Antarctica. This supported the idea that more carbon dioxide was dissolved in the deep Southern Ocean at times when levels in the atmosphere were low.

The study shows that during the ice age, the deep Southern Ocean carried much smaller amounts of oxygen than today. This indicated that photosynthetic algae, or phytoplankton, were taking up large amounts of carbon dioxide near the surface. As dead algae sank to the depths, they were consumed by other microbes, which used up the oxygen there in the process. The scientists actually found chemical fingerprints of the oxygen level by measuring trace metals in the sediments.

The study also shows that variations in carbon-dioxide storage in the Southern Ocean were probably behind a series of natural "wobbles" in atmospheric levels of about 20 parts per million that took place over thousands of years. These wobbles were probably caused by changes in the amount of iron-rich dust, which fertilizes phytoplankton, being blown from land onto the ocean surface.

The findings reveal a bit more about carbon cycling, and may also tell us a bit more about how emissions may be affected today.

The findings are published in the journal Nature.

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