New technique shows old temperatures were much warmer than expected

Image of long tubes containing layered deposits.
Enlarge / Isotopes in sediment cores like these can provide clues to past temperatures.

In a recent article Posted in Science, Professor Nele Meckler of the University of Bergen and his colleagues claim that the climate around 35 to 60 million years ago could have been considerably warmer than we thought. Their finding suggests that a given level of CO2 could produce more warming than previous work indicated, and this suggests that the ocean circulated differently during this warm, ice-free climate.

Their findings 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. Previous work with similar samples had estimated temperatures using oxygen isotopes– a technique that could be confounded by changes in the amount of water locked up in the ice at the poles and, to a lesser extent, variations in the salinity of the oceans. The new study used a technique that records temperatures more reliably and produced much hotter numbers.

A newer and clearer thermometer

Benthic foram oxygen isotopes have been a mainstay of ancient global climate studies, with the most detailed last record dating back to 60 million years. Deep ocean temperatures reflect surface ocean temperatures on time scales greater than about 1,000 years, because the “conveyor belt” of the ocean circulation turns around on this time scale. Oxygen Isotopes in that the water reflects the surface temperature of the ocean and, by extension, the global climate, since water containing the heavier isotope oxygen-18 is somewhat more difficult to evaporate than water containing oxygen-16; when the sea is warmer and there is more evaporation, oxygen-18 builds up in the oceans.

This accumulation of isotopes is calibrated according to temperature, but this calibration requires knowing the salinity of the oceans and the amount of water locked up in the ice caps. “The Global [oxygen isotope] curve…has always had this semi-hidden uncertainty due to the dual influence of temperature and ice volume that we can now solve using clumped isotopes,” said Sierra Petersen of the University of Michigan, who doesn’t did not participate in the Meckler study.

The clumped isotope method eliminates the need to make this assumption about the amount of water locked up in the ice because it simultaneously measures the carbon-13 levels found in the same sample of calcium carbonate in a foram shell. Thermodynamics promote the “clumping” of heavier isotopes into calcium carbonate in cold water, but as the water warms, entropy increasingly exerts its influence and heavier isotopes become more dispersed in the shell material. The degree of isotope agglutination is calibrated to temperature in the laboratory for a variety of materials, allowing clumped isotope measurements to produce far-time temperature measurements.

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

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