Ancient seawater’s chemical mysteries come into focus

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 δ¹⁸O—a measure of change in the abundance of ¹⁶O and ¹⁸O in seawater—has remained relatively constant over the last 540 million years. Others contend that δ¹⁸O 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 δ¹⁸O of seawater over the last 540 million years. They explain that this increase may have contributed to the long-term increases that scientists noted previously in δ¹⁸O of fossilized carbonates (Proc. Natl. Acad. Sci. U.S.A. 2024, DOI: 10.1073/pnas.2400434121).

The researchers point out that two geological processes can affect δ¹⁸O values for seawater. The dominant process—hydrothermal alteration—results from hot water stimulating reactions in rocks, which leads to the formation of new minerals, especially in underwater mountains formed by volcanic activity. Geological weathering of continental crusts also affects δ¹⁸O values for seawater but to a lower extent, the team says.

The findings are a big deal, says Kasting, who was not involved in the study but reviewed the paper. “If the new measurements are correct, then seawater isotopic composition has undergone change.” Thus, for example, seemingly implausible hot ocean temperatures estimated for geological periods such as the Archean (between 2.5 billion and 4 billion years ago) may be too high, and the actual temperatures may have been “much more like today,” he adds.

For the study, the researchers analyzed extremely well-preserved rocks from the Ordovician period (485–444 million years ago), an era that experienced a significant marine biodiversity boom. The samples came from above-ground outcrops and a drill core the researchers extracted from Estonia’s Baltic basin. They used a technique called clumped isotope thermometry, which measures the abundance of the isotope ¹³C that’s bound to ¹⁸O in carbonate minerals. Because this binding process is temperature-dependent, scientists increasingly favor this thermometry technique to infer past temperatures. The benefit is that “you don’t need to know or assume the oxygen isotope composition of seawater” to determine past seawater temperature, says Nithya Thiagarajan, a geochemist at the California Institute of Technology and coauthor of the study.

Her team then used these new estimates of past temperatures to calculate the δ¹⁸O of seawater. They found that in the Ordovician period the seawater had lower δ¹⁸O values and was cooler than previously estimated. The researchers noted similar conditions when assessing the paleoclimatic records from the same period from Laurentia—an old continent largely comprising present-day North America. The team believes that their findings highlight the need to reevaluate climate records that are based on oxygen isotopes.

Meanwhile, other scientists have measured δ¹⁸O in ancient rocks called ophiolites, which are pieces of oceanic crust that were lifted above sea level and may retain imprints of the ocean’s isotopic composition. Those scientists believe “they’ve got direct evidence that seawater isotopic composition has not changed,” Kasting says.

But it’s not so straightforward, argues John Eiler, a geochemist at the California Institute of Technology and coauthor of the study. These metamorphic rocks have “undergone further processes; they have seen other fluids; they have experienced other conditions,” he says. That means that the δ¹⁸O measurements may not always reflect the ocean’s original isotopic signatures. He also argues that if the abundance of oxygen isotopes in seawater remained constant over time, “then the best fossils available to us suggest that the ocean was much hotter than any macroscopic animal can live in.”