Sea Change

The foraminifera Acarinina soldadoensis allowed researchers to track changing ocean oxygen levels.


Only certain organisms can convert the atmosphere’s abundant nitrogen into a “fixed” form usable by living things. The historical ebb and flow of this essential and limited element through the environment hints at changing conditions for life. In the ocean, cyanobacteria at the sea surface are the primary source of fixed nitrogen, which then flows through the marine food web. Now, an international team of geoscientists has assembled a new record of ancient ocean conditions, tying the nitrogen cycle to major planetary events.

Princeton doctoral student Emma R. Kast and her supervisor Daniel M. Sigman used a highly sensitive technique for measuring minute differences in the ratio of two nitrogen isotopes, 15N and 14N, preserved in the shells of tiny, plentiful sea creatures known as foraminifera. The fossil record of foraminifera, pulled from seafloor drilling cores in the North Atlantic, North Pacific, and South Atlantic, reveals how nitrogen moved through ancient oceans and serves as an indirect way of tracking ocean oxygen levels as well.

In oxygen-poor conditions, marine decomposition uses nitrate, a form of fixed nitrogen, to break down organic matter. Because the decomposition process preferentially uses nitrate with 14N, less 14N is available for foraminifera to use in their shells, and their 15N to 14N ratio goes up when oxygen is low. When dissolved oxygen is plentiful, marine decomposition uses oxygen instead, which frees up more 14N for foraminifera and decreases their 15N to 14N ratio. Thus, fluctuations in foraminifera’s nitrogen isotope ratio allow researchers to track changing ocean oxygen levels, and, by extension, the productivity of marine life, over millions of years.

Up until fifty-seven million years ago, the 15N to 14N ratio was high, suggesting large areas of ocean with low oxygen. But the ratio dropped significantly, especially in the North Pacific, between fifty-seven and fifty million years ago. Oxygen levels can vary based on water temperature, but Earth’s climate was continuously warm at the time. From the data, the researchers hypothesize that the increase in oxygen was caused by a tectonic shift: the collision of the Indian subcontinent with Asia, which closed the relatively shallow Tethys Sea. The event changed ocean circulation, likely pulling waters that were cooler and more oxygen-rich deeper into the ocean. The increased oxygen would have altered conditions for marine life, making the oceans more favorable for most living organisms. (Science)