Ancient plankton fossils show warmer oceans had lower nitrogen fixation

An international team used sediment cores and fossil foraminifera shells to reconstruct nitrogen fixation rates in the tropical Atlantic over the past 3 million years. They found that nitrogen fixation was significantly reduced during the warm late Pliocene and increased as the climate cooled into the Pleistocene. This suggests that warmer oceans may have lower natural nitrogen fixation, affecting marine productivity.

The study relied on nitrogen isotope analysis of foraminifera shells recovered from ocean sediment cores in the tropical North Atlantic. These microscopic organisms incorporate nitrogen into their calcareous skeletons during growth, preserving a chemical record of past ocean conditions spanning the Pliocene and early Pleistocene epochs.

The researchers connected their Atlantic nitrogen fixation record with a previously published Pacific denitrification record. Both weakened during the warm late Pliocene, reflecting a phosphorus-driven coupling: reduced denitrification leaves less phosphorus available to stimulate nitrogen-fixing cyanobacteria in nutrient-poor surface waters.