In a recently published article in Science, Professor Nele Meckler of the University of Bergen and colleagues argue that between about 35 and 60 million years ago, the climate may have been significantly warmer than we thought. Their finding suggests that a certain level of CO2 could cause more warming than previous work indicated, and it suggests 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. Previous work with similar samples had estimated temperatures using oxygen isotopes — a technique that could be confused by changes in the amount of water trapped 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 yields much warmer numbers.
A newer, clearer thermometer
Benthic foram oxygen isotopes have been a mainstay of ancient global climate studies, with the latest, most detailed data going back 60 million years. Deep ocean temperatures reflect ocean surface temperatures over time scales longer than about 1000 years, because the global “conveyor belt” of ocean circulation flips on that time scale. Oxygen isotopes in that water reflect ocean surface temperature, and by extension global climate, because water with the heavier isotope oxygen-18 is slightly more difficult to vaporize than water with oxygen-16; when the sea is warmer and there is more evaporation, oxygen-18 accumulates in the oceans.
This isotope buildup is calibrated for temperature, but that calibration requires knowledge of the ocean’s salinity and how much water is locked up in ice sheets. “The global [oxygen isotope] curve … has always had this half-hidden uncertainty because of 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 research.
The clumped isotope method eliminates the need to make that assumption about how much water is trapped in ice because it simultaneously measures the levels of carbon-13 found in the same sample of calcium carbonate in a foram dish. Thermodynamics prefers the “clumping” of heavier isotopes into calcium carbonate in cold water, but as the water warms, entropy increasingly exerts its influence, and the heavier isotopes become more dispersed in the shell material. The degree of clumping of isotopes is temperature calibrated in the lab for a variety of materials, allowing clotted isotope measurements to yield temperature readings in a deep time.
The new method indicates that the North Atlantic abyss was about 20°C between 57 and 52 million years ago. That is a big difference from the oxygen isotope data, which yielded temperatures of 12-14°C. “That’s a lot warmer,” Meckler said. In comparison, today’s equivalent is about 1-2°C.