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An international team of ocean, Earth and marine scientists has found evidence supporting a theory that a certain oxygen isotope ratio in seawater has changed slowly over the past 540 million years. In their study, published in the Proceedings of the National Academy of Sciences, the group analyzed rocks from the Ordovician period.Over the past half-century, earth scientists have debated the possibility of changes in the ratio of 16O to 18O (written as δ18O) in seawater over the past 540 million years. Some have put forth arguments claiming no change has occurred, while others report evidence that it has increased.The...

In the new study (pdf), Pierre Haenecour of Washington University in St. Louis and his colleagues analyzed two meteorites collected in Antarctica in 2003, each named for a geographic feature near the spot where the meteorite fell. (Antarctica makes an ideal hunting ground for dark-colored meteorites, which stand out clearly against the ice fields.) Grove Mountains 021710, found by a Chinese expedition, and LaPaz Icefield 031117, collected by U.S. searchers, each harbor presolar grains of silica (SiO2), the researchers found, as evidenced by the grains’ enrichment in a heavy isotope of oxygen known as oxygen 18. That signature points to...