Separation of monospecific and restricted coccolith assemblages from sediments using differential settling velocity

Techniques are described to allow concentration of individual coccolith species from sediment samples, for geochemical analysis. Simple repeated decanting and density-stratified settling columns permit separation of sediment fractions whose carbonate is highly dominated (> 70% but in most cases > 90%) by coccoliths from a single species. In each of six different core top sediments from different oceanographic settings, we were able to separate two or more fractions dominated by different species, although the species which can be effectively separated is highly dependent on the species makeup of the original sample. Isolates were obtained of Calcidiscus leptoporus, Helicosphaera carterae, Gephyrocapsa oceanica, Emiliania huxleyi, and Florisphaera profunda. The deep photic zone dweller F. profunda was isolated from all but one site, even where its initial abundance is very low. Separated fractions show a wide range of stable isotope ratios, up to 1.5 ‰ in δ¹⁸O and 2.5 ‰ in δ¹³C, consistent with a range of non-equilibrium effects in culture, and offsets among species are generally constant. While these non-equilibrium effects were originally believed to hamper paleoceanographic studies, new work suggests that these effects can be used to trace different physiological responses of different species. Sr/Ca ratios of separated fractions vary by as much as 75% from the same site, but offsets between species tend to be variable rather than constant, consistent with other studies indicating that coccolith Sr/Ca varies with productivity. The larger coccoliths C. leptoporus and Helicosphaera show a more dynamic range in Sr/Ca variation among different sites. The ability to separate out productivity responses of individual species should greatly expand the use of coccolith Sr/Ca proxies.