Biometry of Emiliania huxleyi and its biostratigraphic significance in the Eastern North Atlantic Ocean and Western Mediterranean Sea in the last 20 000 years

A detailed biometric study of coccoliths of Emiliania huxleyi has been performed on 34 samples from three sediment cores of the North Atlantic Ocean and the Western Mediterranean Sea (SU90-08, M39029-7 and MD95-2043). All three cores contain the last glacial–interglacial transition (marine isotopic stages 1–2), enabling us to study in detail the morphology of this taxon during this period of change. One hundred coccoliths of E. huxleyi were randomly chosen in each sample; distal shield length and width measurements were performed on each of the individuals selected. The data show that E. huxleyi specimens larger than 4 μm are frequent in glacial samples and that these larger forms decreased sharply in abundance during the deglaciation and Holocene; smaller forms are more abundant in this latter group of samples. The decrease in larger forms seems to be time-transgressive, since it is recorded between 12 and 11 kyr cal. BP in southern locations (Alboran Sea and Gulf of Cadiz) and around 8.4 kyr cal. BP in the central North Atlantic. Scanning electron microscope analyses indicated that the two forms have the same degree of calcification and hence this parameter should not be used in taxonomic classifications. We suggest that the larger coccoliths belong to a cold-water variety of E. huxleyi, which can be distinguished from the small-coccolith variety in light-microscope analyses by its distal shield length. The decrease in the abundance of this larger variety during the deglaciation period could be used as a biostratigraphic event in the North Atlantic and Mediterranean areas.