Carbonate fluxes and calcareous nannoplankton

Abstract

Coccolithophores first became significant participants in the carbonate cycle in the Jurassic, but throughout the Jurassic they were largely restricted to shelf and epeiric sea environments. They spread into the open ocean in the Cretaceous, and with this became a major factor in governing the carbonate cycle in the sea. With the development of dissolution-resistant forms, such as Watznaueria barnesae, the coccolithophores perturbed the carbonate system and switched the major site of carbonate deposition from shallow seas to the deep ocean. Several major evolutionary steps in the development of the coccolithophores have forced further changes in the carbon cycle, favoring the deep sea as a site of carbonate deposition. Samples of recent coccolith assemblages from bottom sediments differ from those of living coccolithophores in surface waters. Many of the coccolitbs of more delicate species, particularly holococcoliths, are dissolved in the water column or at the sediment surface and are only rarely preserved as fossils. They, along with the pteropods, form an important part of the shallow carbonate cycle. There appears to be a continuous gradation in the level of susceptibility of coccoliths to dissolution, from forms that dissolve in the near-saturated waters of the surface ocean to those that are among the most dissolution-resistant forms of calcite. This continuous dissolution spectrum is in contrast to the planktic foraminifera, in which dissolution of the tests also occurs in a sequence, but through a much more restricted depth range, the lysocline. Whereas the order of dissolution of planktic foraminifera follows their habitat, with warm-water species being most susceptible and cold-water forms most resistant to dissolution, the order of dissolution of coccoliths appears to be related to phylogeny. The steepness of the coccolith carbonate dissolution gradient appears to have changed over time. In the Oligocene almost pure nannofossil carbonate oozes devoid of terrigenous material were widespread, perhaps reflecting unusual climatic conditions on land. The overall effect of coccolithophore evolution has been to move carbonate deposition to the deep sea, where coccolith oozes accumulate on ocean crust and will ultimately be subducted. Only a fraction of the carbon in the subducted carbonate is returned to the surface through volcanic activity. If their activity were to continue for several hundreds of millions of years the coccolithophores would remove much of the carbon from the surface of the Earth to be emplaced in the mantle.