Ancient seawater’s chemical mysteries

The debate goes back to the 1950s. “I’ve seen the argument raging my entire career,” says 71-year-old James Kasting , a geoscientist at the Pennsylvania State University. At the heart of this long-lasting feud is the oxygen isotope composition of ancient seawater. Many scientists have argued that δ 18 O —a measure of change in the abundance of 16 O and 18 O in seawater—has remained relatively constant over the last 540 million years. Others contend that δ 18 O has increased over this period. Settling this debate is important because it would allow scientists to more accurately reconstruct past ocean temperatures, which are derived, in part, from measurements of isotope ratios in fossilized minerals. In a new study, an international team of researchers makes the case for an uptick in δ 18 O of seawater over the last 540 million years. They explain that this increase may have contributed