New Way To Measure Ancient Ocean Temperatures Refined

Real thermometers have been available since the 17^th century. For all periods before this, researchers depend on signs from nature. For such determinations, geochemists resort to molecules from microorganisms whose structure is well preserved in seabeds.

The TEX₈₆ index has recently been developed at Royal Netherlands Institute for Sea Research (NIOZ). It is based on temperature-dependent changes in the lipid composition of the cell walls of certain types of archeabacteria. Their cell membranes are composed from special lipids of which the number of carbon rings in the molecule changes with the temperature of the surrounding seawater. These organisms therefore adjust the degree of fluidity of their membranes to the prevailing conditions. Carme Huguet studied several aspects of this in greater detail and made significant improvements to the determination.

With a new detection method the analytical reproducibility of the TEX₈₆ paleothermometer was brought to ±0.3 °C and the deviation in the results measured was reduced to 5% of the average. The TEX₈₆ values for organic material out of the water column and from the uppermost layer of the floor sediment best match the temperature of the uppermost 100 m of seawater.

However, the small cells of Crenarchaeota cannot sink to the floor by themselves; they are far too light for that. This is, however, achieved more rapidly if the cells of Crenarchaeota are eaten, for example, by crustaceous zooplankton. Fortunately, the time spent in the gastrointestinal tract of the crustaceans does not harm the molecules. Once they have landed on the sea floor, the preservation of the original fat molecules takes place best in anaerobic sediments.

In modern, anaerobic sediments from a side branch of the Oslo fjord, the measured TEX₈₆ values accurately reflected the average spring-autumn air temperature in Oslo. Temperature estimations of the transition from the last ice age to the present interglacial period were made using two cores drilled from the Arabian Sea. The TEX₈₆ temperatures were compared with values from a British index; the Uk37.

The index differences can be explained by differences in the growing season of the archeabacteria and algae that the Uk37 index is dependent on. The upwelling dynamic of the seawater in the Arabian Sea also exerts an influence. This dynamic is strongly dependent on the monsoon season in this area.

Carme Huguet's research makes it clear that climate reconstructions should always be based on comparisons of several types of parallel measurements to prevent unexpected scientific blunders. Determining the surface seawater temperatures in oceans and coastal waters is essential for the reconstruction of historic climate changes and changes in ocean currents. This information is, in turn, vital for perfecting current climate models.

This research was funded by NWO.