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Biostratigraphy


The biostratigraphy is concerned with the recognition of fossils and the relative position of their occurrences in space and time. The limited stratigraphic range and the wide geographic dispersal  of many fossil groups are used for dating and correlation. The biostratigraphers typically subdivide and organize sedimentary successions by means of biostratigraphic units, the biozones, i.e. rock intervals characterized by their own fossil content.

Calcareous Nannofossils biostratigraphy is studied for combined analyses of biostratigraphy and magnetostratigraphy, and paleomagnetism applied to geodynamic and tectonic. Moreover we study the calcareous nannofossils species abundance and diversity as a proxy of paleoenvironmental changes in such parameters as temperatures.

Calcareous Nannofossils

Calcareous nannofossils are the tiny calcite plates, 2 to 30 µms in size, that formed (or likely formed) the covering of marine unicellular algae. The extant organisms that produce these thin plates are the Coccolitophores. They are  planktonic, photosynthesising, algae that inhabit the photic zone (especially the first 50-100 meters of water) with a concentration ranging from about 100 to 100,000 cells per litre, that during a bloom is millions of cells per litre. Coccolithophores have a widespread oceanic distribution and are reported from most latitudes, but not those higher than 70°. They produce a calcareous exoskeleton, called Coccosphera, made up of  thin disc-like plates, the Coccoliths. Their calcareous exoskeletons, especially the single coccoliths, are found in marine deposits often in vast numbers, sometimes making up the major component of rocks, such as the chalk of England (Fig.1).

Figure 1 From left to right: Coccosphera of Emiliania huxleyi, dominant living specie (7 µm); Coccolith of E. huxleyi (4µm); satellite image of  coccolithophores “bloom”; the cliffs of Dover: made up by the precipitation of  billion fossil coccoliths.

The fossil assemblages contain many types of coccoliths and variously-shaped extinct nannofossils, called nannoliths, which are of unknown biological classification (Fig. 2).

Figure 2 From left to right: Quadrum trifidum (Campanian, Southern Apennines, Italy. 8µm); Istmolithus recurvus (Late Eocene, southern Pyrenean foreland basin, Cataluna. 7µm); Sphenolithus heteromorfus (Langhian, Southern Apennines, Italy. 9µm ); Discoaster pentaradiatus (Tortonian, Southern Apennines, Italy. 10µm).

The rapid evolution of this group, their global distribution, controlled strongly by the nature of the surface water mass they inhabit, make them a powerful tool in dating and correlating the Late Triassic through Recent marine sediments  (biostratigraphy), and in  paleoenvironmental reconstructions (paleoclimatology). Biostratigraphic zonal schemes based on calcareous nannofossils have been established. Some of these,  recognizable over wide areas, are regarded as standard. Whereas “regional” schemes, based on standard and additional biohorizons, have been established to improve the biostratigraphic resolution for areas where standard biohorizons are rare or absent, such as in the Mediterranean region during the Miocene Epoch (Fig. 3). 

Figure 3 Oligocene through Middle Miocene calcareous nannofossils standard Zonations of Martini (1971) and Okada e Bukry (1980) (1 and 2, respectively) compared  with zonations proposed for the Mediterranean area (3) by Fornaciari et al. (1996; from MNN6 to MNN4b), Fornaciari and Rio (1996, from MNN4a to MNP25a). GPTS from Cande and Kent (1995).

The calcareous nannofossils  are usual abundant in marls and chalks, but fine grained muddy sands and silts could contain good assemblages. The analyses of the assemblages for biostratigraphic studies require a small amount of sediment (3 cm;) and not laborious processing of  the samples.

Samples preparation technique

A fast and effective preparation technique is the so called “smear-slide”. Smear slides are produced by first cleaning the outer surfaces of a hand specimen. A fine "dust" of material is then scraped off onto a cover slip. This is then moistened with distilled water and spread across the cover slip with a suitable utensil such as a flat wooden tooth pick. The cover slip is placed under an infrared lamp (or on a hotplate) to dry. Once dry the cover slip is inverted and affix to a slide using an optical adhesive which is cured under U.V. light. The observation of the assemblages for routine studies is performed by a polarized light microscope at 1250x magnification, both transmitted light and crossed nicols (Fig. 4).

Figure 4 Phases of  smear-slide preparation. The last picture shows an assemblage viewed by a polarized light microscope, crossed nicols, at 1250x magnification. (Patrizia Pantani, INGV of Pisa, is thanked for helping in the preparation of figures).

The electron microscope (SEM) is used for detailed studies of  the nannofossil morphology and structure.


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