Determining past ocean temperatures with old micro-organisms

5-10-2011   Sea water temperatures in the past can be established on the basis of the chemical composition of micro-organisms. Ulrike Fallet, a NIOZ PhD-student, developed a series of ‘paleothermometers’ based on the calcareous skeletons of Foraminifera and on the organic remains of micro-organisms, and she used these to deduce sea water temperatures in the Mozambique Channel in the past. Ms Fallet will defend her thesis at the VU University Amsterdam on 7 October.

Global warming currently seems to be the cause of rapid climate changes, deeply affecting our society. Most warmth is spread across the globe by the great ocean currents that link the various oceans and exchange vast amounts of energy. This exchange of energy is driven by an intricate network of surface and deep ocean currents that keeps the global circulation of seawater going. The southern tip of Africa plays an important role in this, because this is where the exchange between the warm surface water from the Indian Ocean (the Agulhas Current) and the cold water from the Atlantic deep sea takes place.  Producing realistic scenarios of future climate change requires sea temperature data from a period that started much earlier than when temperatures began to be measured with modern thermometers. It is therefore necessary to develop a method for reconstructing ocean temperatures in the past (using so-called ‘paleothermometers’).

As a PhD-student, Ulrike Fallet developed various paleothermometers and tested them in the Mozambique Channel, the strait between Madagascar and the African east coast. The paleothermometers were developed on the basis of the calcareous skeletons and organic molecules of various types of unicellular micro-organisms living near the sea surface (Foraminifera, Haptophytes and Archaea). The composition of the calcareous skeletons and organic molecules varies depending on the season and temperatures, and this has now for the first time been calibrated on the basis of the current surface temperature of the sea water, measured by satellites. It then proved possible to deduce the sea water temperatures over the past 2000 years from old sediment samples as well. The temperatures reconstructed from the calcareous skeletons were more variable but on average higher along the coast than in the middle of the Mozambique Channel. In addition, current temperatures were 1-3 oC higher than the temperatures deduced from the Foraminifera in the sediment samples, which points to a rise in temperature over the past 2000 years. Surprisingly, temperatures deduced from organic molecules do not show this result. Fallet explained: ‘The Foraminifera’s calcareous skeletons are relatively heavy, compared to the organic remains of micro-organisms. That’s why they sink more easily and give sharper indications of temperatures’.

This PhD project was funded by the Netherlands Organisation for Scientific Research (Nederlandse Organisatie voor Wetenschappelijk Onderzoek, NWO).

Thesis title
Fallet, U. Dynamic Oceanography and Paleothermometry in the Mozambique Channel, upstream of the Agulhas Current, 158 pp.