The hydrogen isotope fractionation in alkenones produced by haptophyte algae is a promising new proxy for paleosalinity reconstructions. To constrain and further develop this proxy the coastal haptophyte Isochrysis galbana and the open ocean haptophyte alga Emiliania huxleyi were cultured at different salinities. The fractionation factor, alpha(alkenones-water), ranged between 0.853 and 0.902 for I. galbana and 0.789 and 0.822 for E. huxleyi. The results show a strong linear correlation between the fractionation factor alpha and salinity for E. huxleyi, in agreement with earlier studies, but also for I. galbana. Both haptophytes show the same response to changes in salinity, represented by the slopes of the alpha-salinity relationship (similar to 0.002 per salinity unit). This suggests that the same process, in both coastal as well as open ocean haptophytes, is responsible for reducing fractionation with increasing salinity. However, there is a significant difference in absolute isotope fractionation between E. huxleyi and I. galbana, i.e. E. huxleyi produces alkenones which are 90 parts per thousand more depleted in D under the same culturing conditions than I. galbana. Our data suggest that the delta D of alkenones can be used to reconstruct relative shifts in paleosalinity in coastal as well as open ocean environments with careful consideration of species composition and other complicating factors especially in coastal regions.
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