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New technique shows earlier temperatures were much higher than previously thought

Enlarge / Isotopes in sediment cores like these can provide indications of past temperatures.

In a recent article published in Sciences, Professor Nele Meckler of the University of Bergen and colleagues argue that the climate between about 35 and 60 million years ago may have been considerably warmer than we thought. Her finding suggests that a given level of COtwo could produce more warming than previous work indicated, and suggests 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 “foraminifera,” that lived on the seafloor at the time. Previous work with similar samples had temperatures estimated using oxygen isotopes—a technique that could be confounded by changes in the amount of water locked up in ice at the poles and, to a lesser extent, variations in ocean salinity. The new study used a technique that records temperatures more reliably and produced much warmer numbers.

A newer and clearer thermometer

Oxygen isotopes from benthic foraminifera have been a mainstay of ancient global climate studies, with the last most detailed record going back to 60 million years. Deep ocean temperatures reflect ocean surface temperatures on time scales of more than 1,000 years because global temperature “conveyer belt”of ocean circulation rotates on that time scale. oxygen isotopes in which water reflects ocean surface temperature and, by extension, global climate, because water with the heavier isotope oxygen-18 is slightly harder to evaporate than water with oxygen-16; when the sea is warmer and there is more evaporation, oxygen-18 accumulates in the oceans.

This accumulation of isotopes is calibrated to temperature, but that calibration requires knowing the salinity of the ocean and how much water is locked up in the polar 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 clustered isotopes,” said Sierra Petersen of the University of Michigan, who was not involved in Meckler’s study.

The clustered isotope method eliminates the need to make that assumption about the amount of water locked up in the ice because it simultaneously measures the levels of carbon-13 found in the same sample of calcium carbonate in a foraminiferal layer. Thermodynamics favors the “clustering” of heavier isotopes into calcium carbonate in cold water, but as the water warms, entropy exerts more and more of its influence, and the heavier isotopes disperse further into the cover material. The degree of isotope agglomeration is calibrated at temperature in the laboratory for a variety of materials, allowing clustered isotope measurements to produce deep-time temperature measurements.

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 returned temperatures of 12 to 14°C. “That’s a lot warmer,” Meckler said. For comparison, the current equivalent is around 1 to 2 °C.

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