Source: Paleoceanography and Paleoclimatology
Understanding ancient ocean temperatures—particularly from the Cenozoic era (the past 66 million years), in which Earth experienced dramatic climate shifts—helps scientists reveal more about the planet’s past climates.
To reconstruct ocean temperatures, researchers look at ratios of stable oxygen isotopes in deep-sea waters by measuring oxygen held in seafloor carbonate. The ratio of the “heavy” oxygen-18 isotope, which has eight protons and 10 neutrons in its nucleus, is measured relative to more common “light” oxygen-16, which contains the same number of protons but only eight neutrons. Waters enriched in heavy oxygen reflect colder climates and ice ages, whereas enrichment of light oxygen indicates higher temperatures and reduced global ice volumes.
Isotope ratios are measured using mass spectrometry, which separates out different isotopes and finds their relative abundance. Clumped isotope thermometry is a newer method that focuses on the rate at which rare heavier isotopes bond to one another rather than to lighter isotopes.
Both methods can produce Cenozoic ocean temperature reconstructions. However, the two techniques indicate different water temperature progressions through time, and the discrepancy is clouding our understanding of past ocean conditions.
Rohling et al. compared reconstructions from published studies using the two approaches to better understand why their results vary so drastically. The authors evaluated a range of potential reasons, such as whether changes in ice sheet morphology and salinity levels affect the results of each method to different degrees.
None of the individual mechanisms the researchers explored explained the discrepancies between the results. But they did find that they could significantly reduce the discrepancies when both methods accounted for the effects of pH and warm saline deep water on isotope variance. The reconciliation between the methods was further improved by revising the temperature calibration for the clumped isotope thermometry method, resulting in reconstructed temperatures about 2.2°C lower.
The authors note that their results require further refinement but suggest that they lay a foundation for advancing ocean temperature reconstructions. In particular, given that most sediment samples analyzed with the two isotopic methods to date have come from the Atlantic Ocean, the authors’ new model also best represents past Atlantic temperatures. Considering samples from other regions is needed to improve the representation of global average conditions, the authors write. (Paleoceanography and Paleoclimatology, https://doi.org/10.1029/2024PA004872, 2024)
—Aaron Sidder, Science Writer
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