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Luminescence Dating

Luminescence dating methods that seek to determine when mineral grains were last exposed to sunlight or heat.
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Well, when we bring the sediment samples back to the laboratory, the first place we go is actually right next door. That’s like a regular wet chemistry laboratory. We have fume hoods, ovens, and what we’re really trying to do is separate out sand-sized grains of quartz and potassium feldspar. Those are the two minerals that are really used in luminescence dating. Once we’ve purified those, we bring them into this laboratory, which is where all the equipment is stored. What we now want to do is load them up onto a little metal disc. That allows us to put into the machine,
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and the machine has three things to it: A photomultiplier tube, a radiation source, and some kind of stimulating light source. It might be light emitting diodes or lasers. And what we then do we put those quartz grains and feldspar grains into the machine and shine light on it and see how much light of a different wavelength we get off. We then give it a known dose, spin it around again, and then measure the amount of light we get off for each unit dose. So that gives us a measure of how much dose is stored in the grains when they were buried. And now by measuring the radioactivity in the field, we’ve got the two things we need to know.
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How much does was stored, and how much dose it receives every year. So from those two things, we can work out how old the sediment is, how long ago it was buried in the ground, and how old the artefacts and any fossils associated with that sample, how old they are as well. Well, the latest dating results for the Hobbit show that it might have survived up until maybe 50,000 years or so ago. 50,000 to 60,000 years. And the latest set of field samples we collected this field season, were really trying to nail down when did modern humans first arrive at the site.
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So we’re working on those samples now in the instrument to see if we can really nail the first time the modern human stepped foot at Liang Bua.

Luminescence dating encompasses a range of methods that seek to determine when mineral grains were last exposed to sunlight or heat.

Taking the Sediments to the Lab

In the wet chemistry lab, sand-sized grains of two minerals (quartz and potassium feldspar) are separated out from the sediment and purified. The mineral grains are then placed on to metal discs and loaded into a machine that has a photomultiplier tube, a radiation source, and a light source to optically stimulate the grains (e.g. light emitting diodes or lasers). Light is shone onto the quartz and feldspar grains and the photomultiplier tube detects the light given off by the minerals, using filters to select a different colour from the stimulating light source. This helps to identify how much radiation has been stored in the same grains since they were buried in the ground. Together with measurements of how much radiation the grains received each year in the ground, the time of burial of the grains can be determined. This information can answer the following questions:

  • How long ago were the sediment grains buried in the ground?
  • How old are the artefacts and fossils associated with that sample of sediment or sediment layer?
  • Are there any indications of sediment disturbances after burial that might need to be taken into account?

Currently, experts are applying dating techniques to samples from Liang Bua to identify the first time that modern humans stepped foot in the cave.

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Homo Floresiensis Uncovered: The Science of ‘the Hobbit’

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