Tropical oceans were overheated during prehistoric greenhouse effect

Biogeochemists from the Royal Netherlands Institute for Sea Research have shown that prehistoric tropical oceans were no less than five to eight degrees warmer than they are now. Their findings have been published in the December issue of the renowned American journal Geology.

During the mid-Cretaceous period, some 90 to 120 million years ago, the seawater around the equator had a temperature of 30 to 37 degrees Celsius, which is five to eight degrees higher than the temperature now. This was revealed in research that used a new method to determine the temperatures of oceans in the distant past.

The finding concurs with recently developed climate models, which indicate that higher carbon dioxide concentrations in the greenhouse climate of 90 to 120 million years ago resulted in warmer tropical oceans. The biogeochemists’ findings reveal how seawater temperatures changed when large quantities of greenhouse gases entered the atmosphere. Scientists had suspected that seawater temperatures were significantly higher then, but no method had been available to precisely determine these.

The new method is based on the chemical structure of membrane lipids in archaea, which are also referred to as archeabacteria. These organisms live in the surface waters of oceans. The composition of the membrane lipids was found to depend on the seawater temperature. At higher temperatures, the organisms produce greater quantities of lipid molecules containing rings.

The researchers examined fossil remains taken from the floors of the Atlantic and Pacific Oceans. The remains were located in deposits from the mid-Cretaceous period, which is known for its very high concentrations of greenhouse gases.

Earlier research by the biogeochemists had demonstrated that the membrane lipids of archaea remained conserved in sediments. In a paper published in Science two years ago, scientists described the discovery of fossil remains containing membrane lipids 112 million years old. Since then the researchers have found remains as old as 140 million years.