An international team of scientists led by researchers from the Max Planck Institute for Chemistry and Princeton University has discovered that oxygen-deficient zones shrank during long warm periods in the past.

This is contrary to widespread expectations which have assumed that oxygen-deficient zones in the open ocean have increased because of rising global temperatures. 

The scientists showed that denitrification -- a process in which bacteria convert the nutrient nitrate to molecular nitrogen under oxygen-deficient conditions -- in the water column of the eastern tropical North Pacific was greatly reduced during two warm phases about 16 and 50 million years ago.

The researchers made this discovery by studying layers of marine sediment. These sediment layers provide information about the oxygen content of the sea in the past. This is due to plankton such as foraminifera, which once lived at the sea surface and whose skeletons sank to the sea floor where they became part of the sediment.

“We’ve worked for decades to develop the methods that allowed for these findings,” said Daniel Sigman, Dusenbury Professor of Geological and Geophysical Sciences and the co-author of a new paper in the journal Nature that documents these findings. “And right away, the results are altering our view of the relationship between climate and the ocean’s oxygen conditions.”