Biogeography of selected Holocene coccoliths in the Atlantic Ocean

Summary

In this chapter we present a revision of the biogeographical distribution of five coccolithophorid species (Coccolithus pelagicus, Calcidiscus leptoporus, Heli-cosphaera carteri, Syracosphaera pulchra and Umbilicosphaera sibogae) and the genus Gephyrocapsa in the Atlantic Ocean. The mapping is based on surface sediment samples. Each of the taxa considered here constitutes an unambiguous morphological group ideal for rapid low taxonomic resolution analysis of assemblages, which is a tempting strategy for ecological and paleoecological analysis of assemblages. However, in each case recent research has indicated that these broad taxa are in fact composed of several discrete species, or sub-species. The clearest example is C. pelagicus, with discrete morphotypes in sub-Arctic and temperate upwelling areas. For Gephyrocapsa and Umbilicosphaera the separation is less obvious but still unambiguous. Species separation is manifestly essential to understanding the biogeography of these taxa. For H. carteri and S. pulchra the mapped distributions are relatively straightforward and we do not yet know how they relate to the recently proven genotypic variation within the taxa.

At high latitudes temperature and productivity belts parallel each other and the effects are difficult to distinguish. At lower latitudes however, the effects are more clearly separable – it is for instance obvious that S. pulchra shows a warm water low productivity preference whilst H. carteri shows a warm water higher productivity distribution. In particular there are several cases where distribution patterns in the North and South Atlantic are strikingly different. These include the absence of C. pelagicus in the sub-Antarctic; the much higher abundance of C. leptoporus in temperate South Atlantic than North Atlantic; much higher abundance of U. sibogae var. sibogae in the oligotrophic South Atlantic than the North Atlantic. The Calcidiscus and Umbilicosphaera patterns are more symmetric, since the North and South Atlantic show broadly similar sets of environments in terms of temperature, salinity, productivity and macronutrients (nitrate, phosphate and silicate). Obvious possible hypotheses are that the populations in the two oceans are sufficiently separated to have evolved slightly different ecological tolerances or that an additional factor, such as a trace element is responsible for the distribution contrasts. More generally we suspect that the comparably broad coccolithophorid bio-geographic zones in all oceans and the absence of obvious vicariance in coccolith species distributions may have prevented recognition of significant contrasts between oceans, although such contrasts may provide key clues for interpreting past temporal shifts in assemblages.