Dating the Past

Dating techniques are carried out in two basic ways for a site or an artefact ­relative dating techniques, and absolute dating techniques. Dating the past implies chronological sequencing of archaeological entities and past events. Dating is different from time. Time has various cultural interpretations and is therefore a social construct. Time is a construct to establish a chronological framework some of which is absolute and some relative. This means the imposition of time to establish a chronology or linear sequence of events that is therefore continuous. Dating is also a categorisation. Chronology is 1 of 3 main axes of analysis that has 3 conceptual axes of space, time, and form.

Modern techniques include fission track dating. radiocarbon methods and accelerated mass spectrometry (AMS), potassium argon, obsidian-hydration dating, thermoluminescence, and archaeomagnetic techniques. All techniques can be used in conjunction with cross-dating. The cross-dating method determines stratigraphic or assemblage similarities between sites within a region. This extends known dates from one or more sites where chronometric techniques might not work, and therefore develops cohesive chronologies to explore regional social and cultural evolution over time.

Relative chronology states the simple stratographic principle that older materials are found lower than newer and this is the law of superposition. The method is used to establish the relative sequence of depositional events. It is used to determine temporal relationships between occupation events in behavioural sites. With relative methods the increase in fieldwork, coupled with rigorous techniques, led to data increase and then the rapid increase of the empirical database. Relative dates are rarely exact and show non-conformity with fixed periods of time or particular years. It only provides a sequence of objects from a site earlier than a later site. Relative dates are based on stratigraphy, association, and typology. Stratigraphy states that a succession of layers will run bottom to top, latest at the top, and earliest at the bottom. Association states that objects found in layers can be placed chronologically. Thus objects of known date those of unknown date. Items by association are placed thus in a sealed context. Therefore a flint knife of uncertain age in a grave with known late Neolithic pottery is of contemporary use if not in manufacture.

Typology was developed by Montelius (Sweden). Typology sees physical characteristics in evolutionary sequence from lower to higher, and simpler to more complex. With typology therefore there is a relative sequence that is therefore random. Typology is not much use in the British Iron Age (though a preferred method), as seen with the hierarchical classification of Danebury pottery as types to forms to varieties. Typology depends on the assumption that one object evolved out of another. Usually, not universally, there is a sequencefrom the technologically simple to the complex.Thus, by placing objects in developmental order, a relative sequence of ages is established.

Seriation is another relative dating technique. Its principle is that artefactschangein decorative style and form over time. Thus a sequence runs from: (1) early limited use to; (2) acceptance; (3) increased popularity; (4) decline; followed by (5) disuse. However, this trajectory does not provide actual dates. Seriation thereforeattempts to reconstruct typological or stylistic changes in material culture through time. it examines typological or stylistic shifts from different strata and changes placed in chronological order. For example ceramic cross-dating is used to place other sites into regional temporal ordering. Relative dating therefore provides a working sequence but, however. a range of absolute dating techniques is also available.

Absolute dating allows the assignment of specific calendar dates to deposits within sites and also within regions. The simplest technique uses artefacts of known age and is only applicable to certain areas. Early attempts were cross-dating, which was tied in with diffusionism,the mechanism of cultural change, spread, and technological export, and therefore date transference. This created a network of cross-transferred dates. For example the coastal spread of megalithic tombs with the earliest in Brittany.

The most commonly used form of absolute dating is radiocarbon dating, discovered by Willard F. Libby in 1949. Radiocarbon dating thus developed during the 1950’s and 1960’s. Radiocarbon dating determines actual age of carbonised wood and bone for accurate determination of date events. Radiocarbon dating is based on the isotopic or nuclear decay method of inferring age for organic materials. Carbon 14 or C14 provides a common chronometric time scale worldwide and has applicability for the Late Pleistocene and Holocene. The method measures the decay of the radioactive isotope C14. C’4 is absorbed by all living things through CO2. When a plant or animal dies it ceases to take part in the absorption of this isotope. C14 then disintegrates via radioactive decay by half every 5730 years. Measurement of the remaining C14 determines when the organism died.

All organic material is suitable for radiocarbon dating. There is a wide spectrum of carbon containing samples, such as charcoal, wood, marine and freshwater shell, bone, antler, marl is a soil consisting of clay and lime. Tufa is a porous rock formed round volcanic springs.Caliche is a natural form of calcium carbonate encrusting dry, stony soil.Carbon 14 decays to Carbon 12 at a fixed rate. Its half-life equals half the amount left or changed back after 5730 years (originally calculated at 5568 years), therefore half is left after 5730 years. Radiocarbon dating fixes time of death of the sample it does not date context. The range for carbon dating is from 300 to 40-50,000 years and samples of 1 to 10 grams use conventional decay or beta counting. The method is not wholly accurate because of counting errors and contamination from background radiation. For this reason all dates are quoted with a standard deviation(SD) of plus or minus so many years. For accuracy C’4 dates have to be calibrated to calendar dates. This is done by the C14 determination of tree-ringsof known age with the production of a calibration curvefrom which dates can be corrected. It is standard practice for uncorrected C14 dates to be called BP (before present) and corrected C14 dates as cal BCor cal AD.

A new C14 technique is accelerator mass spectrometry(AMS) which directly detects C14 relative to C’3 and C12. This method allows small samples of the material to be used. AMS is direct or ion countingof CH with a routine sample of 1­2 mg of carbon. It measures age ranges from 40 to 50 thousand years. Future estimations are assumed to be 80 to 90 thousand years. C14 is the secondary effect of cosmic ray bombardment in the upper atmosphere where C14 is oxidised to C1402 and distributed throughout the atmosphere. A small percentage becomes part of the terrestrial biosphere by means of photosynthesis. Living organisms maintain their C14 content in equilibrium with atmospheric C14. AMSdetermines age by measuring residual C14 content. For most periods conventional C14 ages deviate from the real, e.g., calendar, historical and sidereal (measured by the stars) time. Calibrated C14 age considers C14 activity in living organisms not content and is therefore calibrated with dendrochronology. AMS is used in Oxford using small samples of about 500 milligrams and can measure C’4 levels of around 0.0001%. For example a 50 mg human bone sample has its 25% collagen protein isolated. Carbonate is then removed to leave gelatine which is burnt to drive off Coe to derive graphite. Carbon dioxide and liquid nitrogen then convert the CO2 to 2mg of graphite which is placed in the target head for testing. The particle accelerator, using high voltage and electromagnets, takes one hour to measure the sample.

Potassium Argon dating dates rocks and depends on the breakdown of the potassium isotope Ku. It is a viable method for sites earlier than 100,000 years, and therefore can develop outline chronology for early human evolution and origins. Based on the decay of potassium to argon the isotope has a half-life of 1.3 billion (1,330,000,000) years. It is spectrometer measured using volcanic rocks and ble to measure the greater part of geological time. It is used only on rocks rich in Kand has been successful on certain sites e.g., Olduvai Gorge in Tanzania to date the remains of Zinjanthropus or ‘Nutcracker Man’, some 1,750,000 years ago.

Thermoluminesence or OSLis optically stimulated thermoluminescencethat works through firing on quartz fragments and dates when the mineral was last exposed to sunlight. For example, pottery is crushed, fired rapidly, and then the luminescence emitted measured. The result relates to the original firing and the clay crystal lattice. Thermoluminescenceis used to date inorganic materials such as pottery or burntflint. Materials with crystalline structure contain small amounts of radioactive elements which decay at a known rate. These isotopes emit radiation which displaces electrons that then become trapped in the crystal lattice of the material. Trapped electrons are released only when the material is heated to over 500°C. The emission of light is called thermoluminescence. For example, in pottery or the clay lining of a kiln, the process of electron displacement begins when it is fired. Measurement of the released emission when the sample is reheated enables its age to be established. It is a candidate method for samples too old for Carbon 14 dating. Thus at Pontnewydd Cave in north Wales early human remains associated with stone tools were dated at 200,000 years by this technique.

800px-Thermolumine

Thermoluminescence of fluorite after heating on a hot plate.

Archaeomagnetic dating is little used. It is based on the observation that the magnetic field of earth is continuously changing in direction and intensity. Baked clay structures which contain iron oxide include ovens and kilns. These retain the magnetisation present at the time of firing. it is therefore possible to measure the deviation of the ancient magnetic field from that of today and thus date the object.

The simple technique of dendrochronologyhas great precision. The method allows for wood to be dated by using tree-ringsas measurement of time. The annual growth of tree-rings varies in thickness from year to year depending on rainfall. Similar thickness variations in all trees in a given region. The pattern of broad and thin rings in one tree closely match the pattern in another. Therefore, with sections of wood of overlapping age, it is possible to correlate rings and build up a scale of dates stretching back into prehistory. It is only usable with large surviving wood pieces, generally from waterlogged environments. For example — The Sweet Track, which is a Neolithic timber walkway in the Somerset Levels dated to the winter of 3807 to 3806 BC.

Dendrochronology is the scientific study of chronological and environmental information contained in annual growth layers of trees. The method uses accurately dated tree-ring sequences, and therefore places past events in time and reconstructs environmental conditions. It is useful in bog and marsh conditions plus dry, arid regions. Created in the early 20th century tree-rings were found to correlate with winter precipitation. The fundamental principle of dendrochronology is cross-dating, or the matching of identical patterns of variation in ring morphology among trees of a particular area.

Archaeological tree-ring collection provides 3 kinds of information ­chronological, behavioural, and environmental. Tree-ring dates, chronologically, have two notable attributes and these are (1) accuracy to calendar year, and (2) no associated statistical error, with the final ring meaning the death of the tree. Behavioural information views tree-rings as artefacts and analyses historical treatment of trees as a natural resource and raw material. Hence analysis of tree-cutting, tree-transport, tool-use, and timber re-use. Environmental information analyses aspects of environmental variability and changes shown by ring widths and species assemblages. Thus dendroclimatology reconstructs past climates and analyses climate sensitive archaeological tree-ring chronologies. Dendrochronology depends on a regional master sequence for certain species of tree, and does not work for non-seasonal growth. On its own it is accurate to the year. The method is calibrated against C14 dates for the Bristlewood pines (known date) which produces a calibration curve and can be tested against Egyptian king lists.

Essay contribution to University of Oxford Undergraduate Certificate in Archaeology (2004).

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