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Howard Lee - Aug 30, 2022 4:53 pm UTC

In a paper recently published in Science, Professor Nele Meckler of the University of Bergen and colleagues argue that the climate between around 35 and 60 million years ago may have been considerably warmer than we thought. Their finding suggests that a given level of CO2 might produce more warming than prior work indicated, and it hints that the ocean circulated differently during that warm, ice-free climate.

Their conclusions come from new measurements of carbon and oxygen isotopes found in the shells of tiny creatures, called benthic foraminifera or “forams,” that lived on the seafloor at the time. Earlier work with similar samples had estimated temperatures using oxygen isotopes—a technique that could be confused by changes in how much water was locked away in ice at the poles and, to a lesser extent, variations in ocean salinity. The new study used a technique that registers temperatures more reliably and produced much warmer numbers.

Benthic foram oxygen isotopes have been a mainstay of ancient global climate studies, with the latest most detailed record extending back 60 million years. Deep ocean temperatures reflect ocean surface temperatures over timescales longer than about 1,000 years because the global “conveyor belt” of ocean circulation turns over on that timescale. Oxygen isotopes in that water reflect ocean surface temperature, and by extension global climate, because water with the heavier isotope oxygen-18 is a bit harder to evaporate than water with oxygen-16; when the sea is warmer and there’s more evaporation, oxygen-18 builds up in the oceans.

This isotope buildup is calibrated to temperature, but that calibration requires knowing ocean salinity and how much water is locked up in ice caps. “The global [oxygen isotope] curve… has always had this semi-hidden uncertainty due to the dual influences of temperature and ice volume that we can now resolve using clumped isotopes,” said Sierra Petersen of the University of Michigan, who was not involved in Meckler’s study.

The clumped isotope method removes the need to make that assumption about how much water is locked away in ice because it simultaneously measures the levels of carbon-13 found in the same sample of calcium carbonate in a foram shell. Thermodynamics favors “clumping” of heavier isotopes in calcium carbonate in cold water, but as the water gets warmer, entropy increasingly exerts its influence, and the heavier isotopes become more scattered in the shell material. The degree of isotope clumping is calibrated to temperature in the lab for a variety of materials, enabling clumped isotope measurements to yield temperature measurements in deep time.

The new method indicates that between 57 and 52 million years ago, the North Atlantic abyss was about 20°C. That’s a big difference from the oxygen isotope data, which yielded temperatures of 12–14°C. “That's a whole lot warmer,” said Meckler. For comparison, today’s equivalent is around 1–2°C.

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