m 



FUTURE DIRECTIONS IN OCEAN SCIENCES 73 



of E. huxleyi and related algal species increased at lower culturing 

 temperatures, apparently a chemical response to maintain mem- 

 brane fluidity. Brassell et al. (1986) discovered that in natural 

 deposits the average number of carbon double bonds in alkenones 

 from Quaternary sediments of the northeast tropical Atlantic showed 

 a strong inverse correlation, over the past 120,000 years, with the 

 temperature of the near-surface ocean water as inferred from the 

 stable oxygen isotopic composition of coexisting calcium carbon- 

 ate shells of the planktonic foraminifera Globehgerinoides sacculifer. 

 This observation suggests that the relative changes in water tem- 

 perature at the ocean surface during at least the past 100,000 

 years could be inferred from the stratigraphic record of alkenone 

 composition in the underlying sediments. Prahl and Wakeham 

 (1987) calibrated the alkenone paleothermometer for E. huxleyi 

 within ±0.5°C and they provided evidence against the idea of dia- 

 genetic alteration of the molecular temperature record in the ma- 

 rine water column. 



The alkenone paleothermometer has potential applications beyond 

 simple confirmation of stable oxygen isotopic records of sea sur- 

 face temperatures. For example, alkenone measurements can be 

 applied readily to bulk sediment samples. The alkenone paleo- 

 thermometer thus ranks as one of the major contributions in the 

 past decade of marine organic chemistry research to understand- 

 ing paleoceanography and paleoclimatology. 



The application of the alkenone paleothermometer was made 

 possible by the recent development of gas chromatographic ratio 

 mass spectrometers, instruments that measure the stable carbon 

 isotopic compositions of individual organic molecule types sepa- 

 rated during the rapid (approximately one hour) gas chromato- 

 graphic analysis of complex organic mixtures. 



Future Directions 



Analytical Methods 



The chemical processes occurring within the ocean reactor are 

 kinetically controlled except in high-temperature regimes, where 

 thermodynamic equilibria may be inferred. These kinetic pro- 

 cesses are driven largely by organisms; they involve chemical re- 

 actions that are difficult to reproduce in laboratory experiments 

 because of their complexity and variety. Thus, the general strat- 

 egy in studying the marine geochemistry of the present ocean and 

 its variations in the geologic past is to pose questions in which 



