Micropaleontology

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Approximately 12,000 year old sample with microfossils from the Antarctic continental slope ( Weddell Sea ): radiolarians (translucent small spheres, Ø ≈ 0.5 mm), sponge needles (glassy), planktic foraminifera (white, small), benthic foraminifera (large white shell and yellowish shells made of grains of sand)

Micropaleontology is the branch of paleontology that deals with the study of microfossils . Microfossils are fossils of microorganisms and microscopic fossil remains of larger living beings. The study of macrofossils is called macropalaeontology .

Preparation of the fossil-bearing sample

Microfossils can often not be seen in the field with the naked eye or a magnifying glass. In order to determine whether a rock contains microfossils, thin sections (a few tenths to hundredths of a mm thick rock slices ) are made from a rock sample , in which cross sections of microfossils can be seen under the microscope. The microfossils can be obtained by processing in the laboratory. To do this, the handpiece must first be mechanically shredded.

Depending on the chemical composition of the rock and the presumed microfossils, various physical- mechanical or wet- chemical processing methods can then be applied. Here are some examples (the rock fragments are in suitable sample vessels):

method Microfossil: skeletal matter Sedimentary rock type Mode of action
Acid etching with dilute formic acid or acetic acid Silica , phosphate or organic carbonate the (more) acid-soluble carbonate in the rock is dissolved away / the fossils and other insoluble components are exposed
Acid etching with hydrogen fluoride organic all (including silicate) all rock components except for the organic-walled fossils are dissolved away
Treatment with hydrogen peroxide all weakly to moderately consolidated clays and marls catalytic reaction with carbon compounds; the released oxygen bursts the rock
Crystallization burst with Glauber's salt all all with pores the solution penetrates into the pores and as a result of an increase or decrease in temperature, Glauber's salt crystals form, which, due to their larger volume, break up the rock

After the remaining sample ( sludge residue ) has dried , grain preparations are present in which the microfossils are more or less large, depending on the method. If the fossils and the residual rock differ in density , enrichment of the fossils is possible through density separation .

To read out the fossils from a grain preparation, standard reading dishes (perforated or unperforated) with 5 × 9 fields of 1 cm² each are used.

In the last step before the scientific processing, the microfossils are sorted out from the grain preparation under the stereo microscope and placed in a suitable storage medium such as a Franke cell. A Franke cell is a cardboard or plastic carrier in the format of the usual microscope slide with a recessed storage recess that can be closed by a transparent sliding lid made of glass or plastic.

Scientific processing

The morphology of individual microfossils is documented through drawings and photographs. The stereomicroscope, with which one can see objects in three dimensions, is the most important tool. A suitable adapter component is required for the camera / stereo microscope interface for taking pictures. In the case of particularly small and filigree microfossils, the scanning electron microscope , which also depicts objects spatially, is used for documentation.

Finally, the microfossils are measured , scientifically described and classified . With a large number of specimens, the data of the measurements and descriptions can be statistically evaluated.

Microfossil groups

Geological importance of microfossils

The special importance of microfossils lies in their often abundant occurrence in the smallest sample quantities. Usually hundreds to tens of thousands of specimens of a species are available for further investigations. This means that extensive specimen suites can be obtained even with closely leveled sampling. The important work on the foraminifera genera Gaudryina and Spiroplectinata , for example, is based on around 18,000 specimens from a period of around 14 million years. Due to the small size of the fossils, investigable specimens are also available in the so-called cuttings , the rock debris transported from a deep borehole with the drilling fluid . This makes micropalaeontology, together with geophysics, an indispensable scientific discipline in petroleum geology .

In most of the branches of geology and palaeontology mentioned below and the methods used in them, macrofossils can in principle serve the same purpose as microfossils. However, microfossils have the advantage already mentioned that they are also often present in large numbers in macrofossil-poor facies (e.g. in hemipelagic sediments) and there also in small samples.

  • Stratigraphy :
    • Microfossils as index fossils are an essential pillar of modern biostratigraphy , especially in the Paleozoic and Cenozoic
    • Especially in the history of bed load , the separate micropalaeontological processing of individual bed load (= individual ice age pebbles ) allows the sediment body in question to be stratigraphically classified and the delivery area determined or severely restricted. In some cases delivery areas are represented that have meanwhile been removed and therefore no longer exist “ in situ ”.
  • Sedimentology - Biofacies / Palecology :
    • The occurrence or accumulation of certain microfossils provides information about the living conditions in the relevant sedimentation area (facies indicator).
  • Geochemistry - Isotope Stratigraphy / Paleoclimatology :
    • The ratios of the stable isotopes of oxygen , carbon or other light elements preserved in the shells of microfossils provide information about certain biotic / abiotic environmental factors such as e.g. B. temperature, salinity , biological activity and / or the global ice volume. Since climatic processes such. B. the evaporation of water, set certain isotope ratios, they can be used as a paleoclimatic indicator . A time series of isotope data obtained from microfossil samples of successive layers can provide the chronological classification of the sedimentary rock sequence examined by comparison with a global isotope curve.
  • Petroleum geology - thermal maturity of sediments
    • The elements of the feeding apparatus of the conodonts show an alternation of skeletal phosphate and organic matter inside. By increasing temperatures during diagenesis (sediment solidification) due to the increasing depth of burial, or thermal stress by a near igneous body it comes to the carbonization of the organic matter and the original cream-colored elements discolor increasingly brown and black at about 300 ° C. Even higher temperatures lead to a lightening through various shades of gray and at 700 ° C the elements are completely white and often transparent. A seven-step color scale, the so-called Conodont Alterations Index (CAI) , is based on this temperature-dependent discoloration , which is a measure of the thermal overprinting and the degree of metamorphosis of the rock. At high CAI values, no hydrocarbons were stable in the sediments for a certain period of time , so they cannot be storage rocks for crude oil . The discoloration of conodonts is therefore an important indicator when prospecting for crude oil and natural gas .

Web links

  • Micropaleontology ( memento of October 24, 2008 in the Internet Archive ) - archived subpage of the website of the Federal Institute for Geosciences and Natural Resources (BGR)
  • Micropaleontology Press - Website of the editor of the journals Micropaleontology and Stratigraphy (English)
  • NAMS - Internet presence of the North American Micropaleontology Section (NAMS) of the Society for Sedimentary Geology (SEPM) (English)

literature

  • Howard A. Armstrong, Martin D. Brasier: Microfossils. 2nd ed. Blackwell Science, Malden, MA 2005, ISBN 0-632-05279-1 .
  • M. Dan Georgescu: Microfossils through Time: An Introduction First Steps in Micropaleontology. Schweizerbart Science Publishers, Stuttgart 2018, ISBN 978-3-510-65413-0 .
  • Arno H. Müller: Textbook of Palaeozoology, Vol. 1: General Basics. 5th edition Gustav Fischer Verlag, Jena 1992, ISBN 3-334-60378-4 .
  • Jörg Mutterlose, Bernhard Ziegler: Introduction to Paleobiology I: General Paleontology. 6th edition. Schweizerbart, Stuttgart 2018, ISBN 978-3-510-65415-4
  • Fritz-Nielsen Wissing, Ekkehard Herrig: Working techniques in micropalaeontology. An introduction. Enke Verlag, Stuttgart 1999, ISBN 3-432-29641-X (EA Stuttgart 1998).

swell

  1. ↑ Arno H. Müller 1992, pp. 450-451.
  2. ↑ Brunhilde Grabert: Phylogenetic investigations on Gaudryina and Spiroplectinata (Foram.) Especially from the northwest German Apt and Alb. Treatises of the Senckenberg Natural Research Society, Vol. 498. Frankfurt / M. 1959, ISBN 978-3-510-61301-4 (also dissertation, FU Berlin 1959).