Quick Tips: Archaeological Techniques –Use of Isotopes in Archaeology.

Isotopic analysis is widely used within the worlds of archaeology and anthropology. From analysing isotopes we’re able to uncover a wide range of information regarding the past; ranging from palaeoenvironments to palaeodiets, and even using isotopes to reconstruct trade routes of materials.

But first, what are isotopes?

All of the chemical elements consist of atoms which are specific to the element and the mass of an atom is dictated by the number of protons and neutrons it contains. The identity of the chemical element depends on the number of protons found within the atom’s nucleus, but the number of neutrons within the atom can vary. Atoms of the same chemical element (same number of protons), but with different masses, which is from the varying amount of neutrons, are called isotopes.

Stone Circle at Drombeg

Within nature, most of the elements consist of a number of isotopes. These isotopes can be found within water, livestock, crops and plants, which can then be used to reconstruct palaeodiets and palaeoenvironments.

Within nature, most of the elements consist of a number of isotopes. For a great majority of elements these relative proportions of isotopes are fixed, but there are a group of elements which either due to chemical or biochemical processes are of variable isotopic composition. These elements are oxygen, carbon, nitrogen and sulphur. Another group of isotopes that are used for analysis are strontium, lead and neodymium. These are formed by elements which contain stable but radiogenic isotopes, which are formed by radioactive decay of another element. Carbon and nitrogen isotope composition are primarily used to reconstruct diets, and oxygen isotopes are used to determine geographic origin. Strontium and lead isotopes found within teeth and bone can sometimes be used to reconstruct migration patterns in human populations and cultural affinity

Isotopes Table

A table of the various elemental isotopes that are valuable in archaeological and anthropological research.

But how do isotopes get into skeletal remains?

Carbon isotopes are taken up through the diet of animals during their lifetime and these isotopes are deposited into teeth and bones of humans when they are consumed and digested. By studying animal bones and examining the 12C and 13C isotope ratio, it is possible to determine whether the animals ate predominately 3C or 4C plants. Oxygen isotopes are constantly being taken up and deposited into the body through the water a population drinks. This process ends with the organism’s death, from this point on isotopes no longer accumulate in the body, but do undergo degradation. For best result the researcher would need to know the original levels, or estimation thereof, of isotopes in the organism at the time of its death.

By creating a map of these natural occurring isotopes in different environments, rivers and areas, it is possible to identify where in an area the population lived, sourced their water or where the livestock grazed, by comparing the levels of isotopes that were obtained from skeletal remains to the environmental map. This mapping can also help identify trade routes that once existed and can also identify the migration patterns of populations.

References:

Balme, J., Paterson, A. 2006. Archaeology in Practice: A Student Guide to Archaeological Analayses. Oxford, UK: Blackwell Publishing. Pg 218.

Renfrew, C., Bahn, P. 1991. Archaeology: Theories, Methods and Practice. London, UK: Thames & Hudson. Pg 249-53.

If you’re new to the realm of archaeological, anthropological and forensic sciences (AAFS), or are a student needing sturdy and reliable references, or wondering “what archaeology or anthropology textbooks to buy? Check out our new ‘Useful Literature’ page!

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Quick Tips: Archaeological Techniques – Aerial Photography.

Aerial photography is a surveying technique that involves taking a photographic record from satellites, aircrafts and balloons, to aid with the detection of buried archaeological remains and features, which may be difficult to identify at ground level.

There are two types of aerial photography;

  • Oblique – Oblique aerial photography involves taking a photograph from lower altitudes at an angle. This gives a better perspective and a pictorial effect, and allows for identification of earthworks.
  • Vertical – Vertical aerial photography involves taking a photograph from great heights directly above an area. This gives a bird’s eye view of an area, allowing for easier map making and identification of crop marks.
aerialphoto

Fig. 1: There are two types of aerial photography; oblique and vertical.

These aerial photographs can show numerous phenomena, some of which are sometimes not from archaeological origins. These phenomena include:

  • Crop marks – These types of marks develop when a buried wall or ditch increases or decreases crop growth; this is due to the feature affecting the availability of moisture and nutrients in the soil.
crop mark

Fig 2. An example of a crop mark. You can see in the excavation site the ditch that is affecting the crop’s growth.

  • Soil marks – These marks are caused by changes in the subsoil colour, when a plough brings part of the buried feature to the surface.
Fig. 3: After this field was ploughed, it has exposed the feature which has had parts brought to the surface.

Fig. 3: An example of soil marks. After this field had been ploughed, this buried feature had parts brought to the surface which has caused discolouration in the soil.

  • Earthworks – This phrase is used to describe any features seen in relief. These are also known as shadow marks when viewed from the air.
Fig. 4: This is an example of an earthwork. This particular archaeological site is an abandoned Medieval settlement.

Fig. 4: This is an example of an earthwork. This particular archaeological site is an abandoned Medieval settlement.

It is from these phenomena that we’re able to identify whether there is buried archaeology in an area which can then allow for an in-depth investigation.

References:

Balme, J., Paterson, A. 2006. Archaeology in Practice: A Student Guide to Archaeological Analayses. Oxford, UK: Blackwell Publishing. Pg 218.

Renfrew, C., Bahn, P. 1991. Archaeology: Theories, Methods and Practice. London, UK: Thames & Hudson. Pg 249-53.

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Quick Tips: Archaeological Techniques – Ground Penetrating Radar.

Ground-penetrating or probing radar (GPR) is a non-destructive, geophysical method that uses radar pulses to image the subsurface. The principles of ground-penetrating radar are similar to reflection seismology, except that electromagnetic energy is used instead of acoustic energy, and reflections appear at boundaries with different dielectric constants instead of acoustic impedances.

Ground-penetrating radar was applied in the 1940’s after the use of radar to detect enemy aircraft’s during WW2. In 1960’s, due to the progression of this surveying technique, it was primarily used to probe and explore the polar ice. By using GPR in relation to these two applications, a P-38 lightening fighter plane was pinpointed within the ice surrounding Greenland in 1992. The P-38 was originally part of a squadron of six fighters and two B17 Flying Fortresses that ditched just over Greenland in 1942. The P-38 fighter plane was later recovered from a depth of 75m.

How does Ground-penetrating radar work? 

GPR works by emitting high frequency, usually polarized, radio waves via antennas, into the ground. If the area being surveyed contains artefacts or hidden archaeology; these electromagnetic waves are reflected back. When the wave hits a buried object or a boundary with different di-electric constants, the receiving antenna records the variations in the reflected return signal. These returned signals are then collected and interpreted to identify any hidden archaeology within the surveyed area.

N.B. Higher frequencies do not penetrate the ground as far as lower frequencies do, but these higher frequencies give a better resolution. Also the radar emitting antennas are usually in contact with the ground for the strongest signal strength; however, GPR air launched antennas can be used above the ground.

Advantages of Ground-penetrating Radar:

  • GPR is non-destructive and not invasive – helping to preserve the archaeology/landscape.
  • GPR can be used in a variety of media/sediments including; rock, soil, ice, fresh water, pavements and structures.
  • It can detect objects, changes in material, and voids/cracks in the ground.

Disadvantages of Ground-penetrating Radar:

  • The depth range of GPR is limited by the electrical conductivity of the ground. As conductivity increases, the penetration depth decreases. This is because the electromagnetic energy is more quickly dissipated into heat, causing a loss in signal strength at depth.
  • In moist and/or clay-laden soils and soils with high electrical conductivity, penetration is sometimes only a few centimetres.
  • Metal can interfere with the electromagnetic radiation – this can give false results.

References:

Balme, J., Paterson, A. 2006. Archaeology in Practice: A Student Guide to Archaeological Analayses. Oxford, UK: Blackwell Publishing. Pg 218.

Renfrew, C., Bahn, P. 1991. Archaeology: Theories, Methods and Practice. London, UK: Thames & Hudson. Pg 249-53.

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Quick Tips: Use of Phytoliths in Archaeology.

Phytoliths are a very important identification tool in identifying plants within ancient environments, often even classifying down to the species of the plant.

But firstly, what are phytoliths? As the name phytolith suggests, coming from the Greek phyto- meaning plants and lith– meaning stone, they are tiny (less than 50µm) siliceous particles which plants produce. These phytoliths are commonly found within sediments, and can last hundreds of years as they are made of inorganic substances that do not decay when the other organic parts of the plant decay. Phytoliths can also be extracted from residue left on many different artefacts such as teeth (within the dental calculus), tools (such as rocks, worked lithics, scrapers, flakes, etc.) and pottery.

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Table 1 & 2: Examples of the descriptors found within the International Code for Phytolith Nomenclature (ICPN), 2005, for use of naming phytoliths.
Figure 1: A bulliform phytolith under a microscope, ©Henri-Georges Nation.

Phytoliths can form numerous striking shapes within the plant cells (figure 1), which gives them a characteristic shape, thus aiding the identification of plants. Due to the vast number of shapes and sizes that phytoliths can come in, researchers compiled the International Code for Phytolith Nomenclature (ICPN), 2005. The ICPN was developed to create a standard protocol which is to be used during the process of naming and describing a new or known phytolith type, as well as a glossary of descriptors to help aid with the naming.

To observe phytoliths, a sediment sample needs to be collected preferably away from any human settlements, as the use of agriculture may have introduced non-native plants to the area. The soil sample is then observed under microscope or even scanning electron microscope (SEM). On the discovery of a phytolith after observation it needs to be named using a maximum of three descriptors, the ICPN (2005) can be used to correctly identify what descriptors should be used.

The first descriptor should be of the shape, either using 3D or 2D descriptor (whichever is more indicative/shows the phytoliths symmetry). The orientation of the phytolith should also be noted. The second descriptor should describe the texture and/or ornamentation, if characteristic or diagnostic and not an artefact of weathering.
The third descriptor should be the anatomical origin, but only when this information is clear and beyond doubt (Madella et al, 2005).

Phytoliths are very important and useful if the sediment they are taken from is hostile to the preservation of fossil pollen, so may be the only evidence available for paleoenvironment or vegetation change.

References:

Balme, J., Paterson, A. 2006. Archaeology in Practice: A Student Guide to Archaeological Analayses. Oxford, UK: Blackwell Publishing. Pg 218.

Madella, M., Alexandre, A., Ball, T. 2005. International Code for Phytolith Nomenclature 1.0. Annals of Botany, 96: 253-260. A .pdf of this paper available here.  

Renfrew, C., Bahn, P. 1991. Archaeology: Theories, Methods and Practice. London, UK: Thames & Hudson. Pg 249-53.

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