Dating Methods Used

There have been significant advances in dating techniques since the conflicting dating results from the archaeological excavations at the Valsequillo Basin during the 1960s and 1970s.

 

The current team of researchers used the following techniques to date the human footprints and the sediments above and below these trace fossils:

 

• Accelerator Mass Spectrometer radiocarbon dating (AMS) was carried out on mollusc shells and organic balls at the Oxford Radiocarbon Accelerator Unit (ORAU) by Drs Tom Higham and Chris Bronk Ramsey of Oxford University.

Radiocarbon dating is one of the most widely used dating methods in archaeology and environmental science. It can be applied to most organic materials and can produce dates from a few hundred years ago to about 50,000 years ago. For radiocarbon dating to be possible, the material must once have been part of a living organism.

The term “radiocarbon” is used to denote C¹⁴, a radioactive isotope of carbon with a half-life of about 5730 years. C¹⁴ is produced by the sun’s rays in the atmosphere and is absorbed by plants during photosynthesis. All animals in the food chain, including carnivores, get their carbon indirectly from plant material, even if it is by eating animals which themselves eat plants. When a plant or animal dies, it stops absorbing carbon and so after 5730 years, about half of its C¹⁴ will have radioactively decayed.

Put simply, scientists can use the C¹⁴ concentration found in archaeological samples, in this case the mollusc shells and organic balls, to calculate their age.

• Electron Spin Resonance (ESR) on a mammoth molar found in the Barranca Caulapan (Valsequillo Basin), was carried out by Professor Rainer Grün, Australian National University.

Electron Spin Resonance Dating is a non-destructive dating method usually applied to teeth, bone, heat-treated flint, ceramics and sediments. It enables electrons, displaced by natural radiation and then trapped in the structure, to be measured; their number indicates the age of the specimen.

ESR dating is done primarily on the teeth of large animals, in this case on a mammoth’s molar. For this research, four enamel and four dentine samples were cut from the centre of the mammoth molar, see figure below:

Mammoth molar from the Barranca Caulapan
dated by ESR to 27.8 ± 3.8 K BP.

 

• Optically Stimulated Luminescence Dating (OSL) on sediments and the Xalnene Ash was carried out by Dr Jean-Luc Schwenninger from Oxford University

Optically Stimulated Luminescence or OSL Dating is used to establish the age of soil sediments that cannot be dated using the radiocarbon method. It is reliable over a longer period than C14 and can be used on deposits up to 200,000 years old. OSL calculates the time elapsed since luminescent minerals in the soil, such as quartz, were last exposed to sunlight and so researchers can calculate the time period since the sediment was buried by later sediments.

 

• Argon-Argon dating on the Xalnene Ash and overlying lava was carried out by Dr Simon Kelley, Open University.

Argon-Argon dating is used to determine the age of igneous or volcanic rocks, which all contain radioactive potassium-40, which has a half life of about 1.3 billion years and decays to argon-40. The technique is similar to Carbon Dating but calculations are based on the amount of argon-40 contained in the rock sample, in this case the Xalnene Ash and lava.

 

 

• Uranium series dating on animal bones from the Valsequillo Gravels was carried out by Dr Alistair Pike, Bristol University.

 

Uranium series dating is based on measurement of the radioactivity of short-lived daughter isotopes of uranium formed in samples which initially contained only the parent uranium. Materials suitable for U-series dating are found in many prehistoric archaeological sites, and include stalagmitic layers (flowstones), travertines, marls and calcretes. The method is more problematic on molluscan shells, bones and teeth.

 

Dr Pike is in particular investigating the application of models of Uranium uptake, bone diagenesis, burial environment hydrology and geochemistry to the problem of dating bones ( a notoriously “open system”) by uranium series dating. He uses analysis of bone by thermal ionisation mass spectrometry (TIMS).

 

Sampling of bone for Uranium Series dating by Dr. Alistair Pike, watched by Professor David Huddart, José Concepcion Jiménez-Lopez and José Antonio Pompa y Padilla.	Sampling of Paleoamerican bone for Uranium Series dating by Dr. Alistair Pike.

 

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