This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Nitrogen is a key nutrient because it fuels the growth of plankton, which forms the basis of the marine food web. Two opposing processes determine the global nitrogen cycle: nitrogen fixation and denitrification.
Nitrogen-fixing cyanobacteria convert nitrogen gas (N2) dissolved in water into a bioavailable form. This occurs primarily in warm, nutrient-poor surface waters. In the Atlantic, nearly 90% of nitrogen fixation occurs in the tropical North Atlantic, making it a key region for understanding this process.
In other ocean regions, nitrogen is removed from the ocean and returned to the atmosphere through denitrification, in which nitrate is reduced by denitrifying bacteria to N2 gas. This process occurs particularly in oxygen-depleted zones, which today are most extensive in the Pacific Ocean. In previous studies, researchers from the Max Planck Institute for Chemistry (MPIC) showed that denitrification in the water column—the body of seawater between the surface and the seafloor—decreased globally under past warmer climate conditions.
However, it was unclear whether and how nitrogen fixation also changed during these warmer periods. This is important because the balance between these two processes determines how much nitrogen is available for the global ocean's biological processes. An international research group led by the MPIC has now demonstrated, using drill cores of ocean sediment, that natural nitrogen fixation in the tropical Atlantic was significantly reduced during the warm late Pliocene and then increased into the cooler Pleistocene.
For their new study published in Nature Communications, the scientists analyzed nitrogen isotopes in fossil shells of foraminifera from ocean sediment cores in the tropical North Atlantic. The foraminifera lived during the Pliocene and early Pleistocene. Foraminifera are microscopic fossils ideally suited for reconstructing past climate conditions and ocean currents.
These organisms incorporate small amounts of nitrogen into their calcareous skeletons as they grow, and their fossils can still be found today in marine sediments. The Pliocene is a geological epoch that began about 5.33 million years ago and ended 2.58 million years ago. It was globally warmer than today.
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