Can the Stable Hydrogen Isotopic Composition of Dinosterol Be Used to Reconstruct Paleosalinity?

Culture studies of Emiliania huxleyi and Gephyrocapsa oceanica showed a strong correlation of the fractionation factor α_{alkenones-growth water} with salinity for both E. huxleyi and G. oceanica, but with an offset of ca 30‰ between the two species. These results have been used to estimate paleosalinities using the δD of alkenones during deposition of the S5 sapropel in the Aegean Sea and for the last ±3000 years in the Black Sea.

In this study we have investigated the relationship between the hydrogen isotopic composition of dinosterol from dinoflagellate cultures grown at different salinities. The hydrogen isotope fractionation factor α for dinosterol from Heterocapsa triquetra showed an excellent correlation with salinity where α = 0.0033*S+0.591 (r²=0.98, n=5). The slope of the correlation is similar to that of the C₃₇ alkenones from E. huxleyi, the intercept is, however, much smaller.

We applied this α_{dinosterol-water} salinity relationship determined for H. triquetra to the same set of samples that was used to estimate sea surface salinities in the Black Sea for the last ±3000 years. The salinity estimates varied from 33 in the middle of Unit I to 61 at the first occurrence of coccoliths, between Unit II and the transition sapropel. These reconstructed salinities are clearly incorrect and suggest that the a-salinity relationship established for H. triquetra does not hold for the Black Sea. Indeed, analysis of the dinocyst distribution did not reveal the presence of this dinoflagellate. Our results thus suggest large difference in isotopic fractionation between different dinoflagellate species. We are currently investigating the α-salinity relationship for other dinoflagellate species. If this species-dependent variation of the α-salinity relationship is confirmed than paleosalinity reconstructions based on the hydrogen isotopic composition of dinosterol will become difficult if not impossible unless the major contributor of dinosterol is known (e.g. by fossil DNA analysis).