Bones, which are laid close to the surface are younger than those, which are laid deeper; law of superposition
There are different techniques for dating fossilised bones. In general, you can separate them into relative- and absolute techniques.
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| Fig 2.2.7: Bones, which are laid close to the surface are younger than those, which are laid deeper; law of superposition |
Relative techniques simply mean that you can establish whether one specimen is younger or older than another one. One relative technique for dating fossilised bones is stratigraphy. Stratigraphy is a part of geology, which studies identification, the naming and dating of stratigraphisised stones; stones, which are found in layers ('strata').
Following the 'law of superposition' the younger stones are laid above the older. So fossils near the surface are more recent than those in older layers. Unfortunately, this is not always true. Through earthquakes, lines of fracture, mountain- forming and vulcanic eruptions the layers become bent, raised or even turn around.
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| Fig 2.2.8: Different layers of earth can raise or even turn over. Because of this fossils can be laid deeper or higher. Through erosion fossils can come to light. |
Nevertheless palaeontologists have mapped out fossils, which are typical for each layer. This work has produced some fossil orders, so that newly found fossils can be dated relatively judging from other fossils of the same layer or sort of stones they were situated in.
Another relative dating technique is the dating with the help of fluor; fluordating. The bones, which are laid under ground, slowly transform to stone. In some places, the naturally present substance fluor is absorpted by the groundwater during that process. The amount of absorbed fluor is dependent on the amount present in the groundwater and, consequently, varies a lot. So different habitats are hard to compare, but different bones in the same habitat are easily compared. The higher the amount of fluor in the bones, the older they are. This method has helped to unmask the Piltdown- fake.
There are also absolute techniques of dating bones. These methods are based on radio-activity. During the formation of stones, chemical substances, some of which are radio-active by nature, are absorbed. The stones can transform after some time into a completely different element. This is called radioactive decline: the original substance is the 'parent' and the new one the 'daughter'. The decline is regular and has a characteristic pattern for each parent substance. Normally the semivalue time is measured; this is the period that is required for the disintegration of half of the total parent-atoms in the daughters.
An absolute dating technique is the carbonide-14 dating. This is a method to establish the age of organic material, based on the fact that the level of radio- active carbon isotope 14 (14C) is constant approximately, but diminishes through radio-active decline in dead material. By measuring the 14C- level in the material and comparing it with a standard measuring resulting from research, the age can be calculated. Another word for this method is radiocarbonidedating. Een absolute dateringstechniek is de koolstof-14 datering. Dit is een methode om de ouderdom van organisch materiaal te bepalen, gebaseerd op het gegeven dat in de lucht het gehalte aan de radioactieve koolstofisotoop 14 (14C) ongeveer constant is, maar in dood materiaal afneemt door radioactief verval. Door het 14C-gehalte in het materiaal te bepalen en te vergelijken met een via onderzoek opgestelde ijkschaal, berekent men de ouderdom. Een ander woord voor deze methode is radiokoolstofdatering.