Calcareous nannofossils
Calcareous nannofossils are a class of tiny microfossils that are similar to coccoliths deposited by the modern-day coccolithophores. The nannofossils are a convenient source of geochronological data due to the abundance and rapid evolution of the single-cell organisms forming them and ease of handling of the sediment samples. The practical applications of calcareous nannofossils in the areas of biostratigraphy and paleoecology became clear once the deepwater drilling took off in 1968 with the Deep Sea Drilling Project, and they have been extensively studied ever since. Nannofossils provide one of the most important paleontological records with the contiguous length of 220 million years.
History of research
, while examining the chalk from Ruegen, recorded in 1836 an observation of what was later termed "coccolith" and had pictured the coccoliths and Discoasters in his Mikrogeologie, erroneously classifying these discs as a kind of complex spheric concretion. T. H. Huxley coined the term coccoliths in 1858, while agreeing with their inorganic nature. In 1861 George Charles Wallich and, independently, Henry Clifton Sorby, figured out the organic nature of coccoliths after observing their aggregations, coccospheres. Huxley then changed his views and declared that coccoliths are skeletal elements of an unknown organism, Bathybius haeckelii, a primordial form of organic life. One of the goals of the Challenger expedition was to understand the nature of the Bathybius, but the scientists aboard the ship reached the conclusion that the gel-like substance apparently holding the disks in a coccosphere together was a result of processing the samples and later declared the coccoliths to constitute the defensive armor of tiny nannoplankton algae.Research of the nannoplankton systematics in the early 20th century enabled M. N. Bramlette and W. R. Riedel to successfully use the nannofossils for biostratigraphy. The Deep Sea Drilling Project revealed the power of the technique: stratigraphic positions were found within minutes after the drilling core was hauled aboard the ship. At the same time, the continuous DSDP cores provided a solid foundation for setting up the nannofossil biozones. It took decades to establish comprehensive chronological schemes.
The researchers started to use the transmission electron microscopes in the mid-1950s, switching to scanning electron microscopes in the 1960s and 1970s. Optical microscopes with cross-polarization and phase-contrast illumination, techniques introduced in 1952 by Kamptner and Braarud & Nordli respectively, are still used for routine field work.
Terminology
The terminology in the field evolved over time and nannofossils are also sometimes called "nannoplankton" and "coccoliths" as well as some other names, especially in the literature published in 1950s and 1960s. The term "calcareous nannofossil" was chosen in the DSDP publications and gained popularity afterwards, in the early 1970s. "Calcareous" is derived from, "lime", and means "containing lime".Siesser & Haq describe the general use as follows:
- coccolith is restricted by some authors to designate round-shaped elements similar to the ones produced by the living coccolithophores. For differently-shaped objects, nanolith is being used. Some other authors, however, use coccolith in a broader sense for all calcareous nanofossils;
- nannoplankton is sometimes used to identify the living organisms, with nannofossils referring to the now-extinct species. Other researchers use the nannoplankton for all forms, both living and extinct arguing that even though the true taxonomy of the extinct ones might never be known, their planktic way of living is subject to little doubt.
Biostratigraphy
Multiple characteristics of the calcareous nannofossils make them a valuable tool of biostratigraphy and biochronology:- continuous record from 220 million years ago to present;
- abundance in marine sediments;
- worldwide distribution due to the planktonic nature;
- rapid evolution that provides hundreds of points of appearance and extinction;
- tiny size allows the work to be performed with small samples.
The boundaries of the biozones in stratigraphy are defined by the biohorizons, points in strata where significant changes in fossil content and distribution occur. Typical events used for biohorizons are: first occurrence, last occurrence, change in abundance of taxons. A combination of biozones arranged in stratigraphical order results in a zonation.
The first Cenozoic biozonation with 21 biozones for Neogene and 25 biozones for Palaeogene was published in 1971 by Martini, it used alphanumeric notation starting with NN for the Neogene and NP for the Palaeogene, the enumeration increased from the deepest stratigraphic layer. Okada & Bukry introduced their schemes in 1980 with zones code-numbered with letters CN and CP. Agnini et al. in 2017 had combined the scales, reintroducing the new biohorizons for the unreliable ones, resulting in schemes coded with CNP for Palaeocene, CNE for Eocene, CNO for Oligocene, CNM for Miocene, CNPL for Pliocene/Pleistocene.
The agreed upon stratification reference is codified as Global Boundary Stratotype Section and Point by the International Commission on Stratigraphy. The calcareous nannofossils, with very few exceptions, provide clear biohorizons indicating the positions of the GSSP boundaries in Cenozoic.