Climate's carbonate cypher

For the best part of 50 years, palaeoceanographers have been exhuming marine sediments and meticulously decoding the isotopic signals locked into the ancient carbonate shells of buried animals, such as the microscopic foraminifer, Orbulina universa, pictured here. The fruits of this cryptographic endeavour are detailed pictures of past climates and ocean conditions stretching back hundreds of millions of years. But have the right codes been used? Not always, according to new experiments on O. universa and another foraminifer, Globigerina bulloides, reported elsewhere in this issue (H. J. Spero et al. Nature 390, 497–500; 1997

Pioneers in this field worked out that the oxygen-isotope ratio, 18O/16O, of shell carbonate depends not only on the ratio in the surrounding sea water, but also on the temperature at which the shell formed. A shell's carbon-isotope ratio, ¹³C/¹²C, seems to reflect that of the ambient sea water, overprinted with the effects of physiological processes, such as respiration and the photosynthesis of symbiotic organisms.

It is no secret that other processes affect isotope fractionation, but their identity and extent of influence have proved largely elusive. Spero et al. show that the concentration of dissolved carbonate ions in sea water has a marked effect on the fractionation, and consequently argue that present understanding of some past climate changes will need to be revised. For example, the glacial-interglacial shift in shell ¹³C/¹²C — thought to be due to a large transfer of terrestrial carbon to the ocean — could simply reflect known changes in the surface ocean's carbonate ion concentration; and oxygen-isotope-based estimates of tropical sea-surface temperature during the last glacial period might need to be lowered.