Coccolith contribution to South Atlantic carbonate sedimentation

Abstract

In order to assess the significance of calcifying organisms for the carbonate budget it is necessary to have reliable estimates of the relative proportion of carbonate production of the different organism groups. In this chapter we firstly review the carbonate distribution patterns of both bulk coccoliths and planktic foraminifera, estimated by means of different carbonate calculation techniques. These studies clearly reveal a variable pattern of carbonate sedimentation and accumulation for planktic foraminifera and coccolithophores, respectively. Whilst coccolith carbonate dominates the oligotrophic gyres of the South Atlantic, carbonate produced by planktic foraminifera is more important in more fertile, mesotrophic to eutrophic areas, such as the equatorial divergence zone. Mass estimates of coccolith carbonate burial in surface sediments of the South Atlantic document coccoliths to be major carbonate contributors in most mid-Atlantic Ridge sediments, exceeding up to 70 wt.-%. In contrast to these oligotrophic areas, they are of lesser importance in sediments accumulating on the continental margins, where they account for only a fifth of the carbonate fraction. Based on estimates of mean coccolith masses, species-specific coccolith carbonate contributions were calculated for the South Atlantic Ocean. Even though absolute numbers of most species, particularly of Oolithotus fragilis, Rhabdosphaera clavigera, Coccolithus pelagicus, and Helicosphaera carteri are many times lower than total numbers of Emiliania huxleyi, these subordinate massively calcifying species contribute most to the coccolith carbonate. Despite the large cell numbers generated by E. huxleyi and F. profunda in the photic zone, as single species they are of minor importance to coccolithophore carbonate production. While absolute accumulation of coccolith carbonate in upwelling regions is estimated to be about three times higher than in the oligotrophic gyres, the latter areas are about 5-10 times larger globally. This means that total accumulation of coccolith-carbonate in oligotrophic areas exceeds by far that of the upwelling regimes and is therefore of prime importance in global carbonate burial budgets.