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Biostratigraphy
Geochronology

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Introduction

Biostratigraphy is the branch of stratigraphy which assigns relative ages to sediments using the fossils contained within them.  Information from fossils can also be used to infer environments of deposition.

Mainly because of the small size of samples available, industrial biostratigraphy is usually concerned with very small fossils which for practical convenience are typically placed in three separate disciplines, each requiring its own expert analysis.

Application

Fossils can be used both as indicators of age (using known first appearance and extinction events). However, over short time intervals (in the order of a few tens of thousands of years or less), the presence or absence of species, or their relative abundance, is dictated more by environmental factors rather than the passage of time. Within a small area, changes in environment may have been synchronous so that variations in fossil assemblages may have regional or local correlative value regardless of the long geological ranges of the species involved.  For instance, by recording the abundances of the various pollen and spores in any sedimentary section, it is possible to establish the kind of vegetation present when the sediments were deposited, and in turn to make an interpretation about the climate.

Further information

Micropalaeontology is the study of a wide variety of biological groups usually between 0.1 and 1mm in size, sometimes a little smaller and exceptionally larger.  They are easily extracted from unconsolidated sediments by washing and sieving.  In marine sediments the emphasis is usually on foraminifera (single-celled Protista with a shell) a distinction being made between planktonic (floating in the upper water layers of oceans) and benthonic (bottom dwelling) ones.  Ostracods (arthropods with a carapace consisting of two valves) are also frequently found and are nearly always bottom-dwellers. Occasionally radiolaria (planktonic marine protists with a siliceous skeleton), diatoms (siliceous skeletons of microscopic algae) and other groups are studied.  Non-marine sediments typically yield ostracods, diatoms and charophyte oogonia (fructifications of calcareous algae). 

Nannopalaeontology is the study of the calcite skeletons of single-celled marine planktonic algae, less than 30µm across and usually between 5 and 10µm which is one or two orders of magnitude smaller than that of microfossils. They are very easily extracted from sediments and are very abundant in calcareous marine sediments.

Palynology is the study of organic-walled fossils including pollen, spores and marine microplankton (mainly cysts of dinoflagellates). They are usually between 5 and 100µm in size and in most cases contain sporopollenin, a compound very resistant to decay. Specimens are extracted from sediments using chemical digestion to remove the non-organic fraction and carbonate content. Residual organic matter (e.g. woody detritus and other plant material) can be broken down by brief oxidation using nitric acid or similar reagents. Pollen is produced from seed plants, of which the main types are “gymnosperms” (mainly conifers and related plants such as pine) and “angiosperms” (other flowering plants such as oak, grasses etc.). Spores are produced from ferns, mosses and some fungi. Palynology is a useful tool in that it can be applied in a variety of marine and non-marine settings.