Calcareous Nannofossils

The term "calcareous nannofossils" comprises the heterogeneous group of planktic calcareous fossils smaller than 30 µm, of which the coccolithophores make out by far the major share. Coccolithophores are marine, unicellular algae that form µm-sized calcitic plates (coccoliths). Also the other calcareous nannofossils are assumed to have been produced by calcareous phytoplankton.

Light microscopic images (from left to right): Watznaueria fossacincta, Rhagodiscus asper, Tripinnalithus shetlandensis, Conusphaera rothii (all images: Valanginium/Hauterivium boundary interval, Lower Saxony Basin) Source: BGR

Coccolithophores are at the base of the marine ecosystems and represent one of the major phytoplankton groups in the world's oceans. Their abundance and assemblage composition depends not only on biotic factors, but also on the physical and chemical properties of seawater, thus, coccoliths are widely used to reconstruct palaeoceanographic conditions. Besides their important role as calcifying primary producers for the global carbon cycle, coccolithophores contribute to cloud formation via the metabolic product dimethyl sulfide. Thus, by way of the global biogeochemical cycle, a potential link exists between coccolithophore abundance and the climate system.

Calcareous nannofossils have been occurring in marine sediments since the Triassic. Despite their small size they are significant pelagic carbonate producers and in some time intervals of rock-forming importance. Calcareous nannofossils have proven to be extremely valuable in biostratigraphy since the Jurassic. For this reason, they are mainly used for biostratigraphic purposes at the BGR. For this application, standard light microscopic examination will be carried out at 1000 to 1600x magnification under crossed-polars. In addition, scanning electron microscopy (REM) is used for examinations of Quaternary and recent assemblages as well as in special cases.

Scanning electron microscope images (REM; from left to right): Stradnerlithus geometricus, Truncatoscaphus pauciramosus; Holococcolith (all images: Valanginium/Hauterivium boundary interval, Lower Saxony Basin) Source: BGR