Skip to 0 minutes and 12 secondsFor laser ablation U-series dating, we can analyse relatively large samples, such as a complete tooth, without any chemical pretreatment.

Skip to 0 minutes and 27 secondsHere, a tooth is pressed into the sample holder so that the top is flush with the holder, which is the focal plane for the laser. We add some standards to holder.

Skip to 0 minutes and 51 secondsThe holder is then inserted into the laser ablation cell.

Skip to 1 minute and 6 secondsAll air needs to be removed from the laser ablation cell. Otherwise, the plasma from the mass spectrometer will extinguish. The cell is fully computer controlled allowing us to program where exactly we want to analyse.

Skip to 1 minute and 37 secondsThe laser is fixed and the cell moves according to the program sequence. This sequence can involve more than 100 distinct measurements.

Skip to 1 minute and 48 secondsThe laser has a camera attached so that we can cross-check where the sample is ablated.

Skip to 1 minute and 55 secondsA few seconds after the laser is switched on, we can see the traces of the isotopes we are interested in on the computer screen of the mass spectrometer. Ages are calculated by comparing the measured isotopes of the sample with those of the standard. Here we see the result of a sample that was analysed with 30 distinct spot analyses.

Uranium-series (U-series) dating method

Modern Uranium-series methods use decay chains and lasers to allow dating calculations to around 500,000 years.

Uranium-series (U-series) dating is another type of radiometric dating. You will remember from our consideration of C-14 dating that radiometric dating uses the known rate of decay of radioactive isotopes to date an object. Each radioactive isotope has a known, fixed rate of decay.

As its name suggests, uranium-series dating uses the radioactive decay of uranium to calculate an age.

When uranium decays, it goes through a series of decays until it eventually reaches a stable isotope. So, for example, uranium 238 will decay to uranium 234, which will decay to thorium 230. Thorium will then decay to another isotope, radium, which will in turn decay to radon and so on down the chain until it becomes a stable lead isotope. This is called a decay chain. The first isotope is called a “parent isotope”. The isotopes that occur along the decay chain are called “daughter isotopes”.

Natural uranium consists of two parent isotopes. These are uranium 238 (99.28% in abundance) and uranium 235 (only 0.72%). These two parent isotopes have different decay chains. From the first decay chain, we are interested in the decay of uranium 234 (U-234) to thorium (Th-230).

Remember the cave popcorn

U-series dating was principally used for dating the formation of stalagmitic calcite – like our “cave popcorn”. And it is probably simplest to first explain the dating principles of this method from this perspective. Stalagmites grow because of the formation of calcite crystals from ground water. As the water flows through – say a crack in a cave roof – it leaves behind deposits of the calcite crystals, which build up over time to form different shapes, such as stalactites and stalagmites. These mineral deposits commonly found in cave environments are called speleothems.

The water that carries these calcite crystals also contains traces of the naturally occurring uranium, because uranium is soluble – it is able to be dissolved in water. However, thorium (the daughter isotope) is not soluble, so it is not present in the water. This means that while the water that is creating the speleothem is also depositing traces of uranium in the calcite, it is not depositing thorium. Which in turn means that any thorium in the speleothem has been formed by the gradual decay of uranium to thorium (U-234 to Th-230). The thorium is growing inside the speleothem.

Thorium itself is radioactive and begins its own process of decay in the chain. Eventually the rate that the thorium is decaying will become equal to the rate that the uranium is producing it. Until that state of equilibrium is reached, measurement of the ratio between U-234 and Th-230 allows us to calculate the time that has passed since crystal formation began. Th-230 has a half-life of 75,700 years, allowing dating up to around 500,000 years ago.

This same principle can be used to date corals, as again, the presence of thorium in the corals will be the result of uranium decay – not because the thorium has been deposited there by the sea water.

Some assumptions

So what’s the challenge? Well, here are our assumptions. We assume that at crystal formation the thorium content is zero. We also assume that over the thousands of years, uranium and thorium have not been moved into or out of the material we are now testing. This is called a closed system assumption.

The closed system assumption is particularly relevant to applying U-series dating to human fossils, as bones and teeth do exchange uranium with the environment. This is unlike speleothem, that usually remain closed to any subsequent migration after they have been formed. Fossils can contain hundreds of times more uranium than modern bones, due to exposure to ground water. When a bone is buried in sediment, it acts a bit like a sponge for uranium. Uranium can migrate into the bone (a phenomenon known as ‘incorporation’ or ‘uptake’). Uranium can also move out of the bone (leaching).

This has an effect on our process. If uranium (the parent isotope) has been leached from a bone - we may face a situation where there is more thorium (daughter isotope) than uranium. When this happens - well, we can’t actually calculate a U-series age. One of the main challenges of U-series dating of fossil bones is identifying samples that haven’t experienced uranium leaching.

So… what are we actually dating?

U-series analysis of fossils dates the moment when uranium migrates into the bones, not the moment of the death of an organism. It is possible that the uranium entered the bone a long time after the death of the organism (called a ‘delayed uptake’). This means that any U-series age that is calculated will always provide a minimum age possible for the bone. The age could be similar to the age of the death of the fossil - if the uptake occurred right after the death of the organism. In the case of a delayed uptake, the fossil will be older than the calculated U-series date.

We do attempt to reconstruct the uptake of uranium into a fossil sample. We use a model to do this (called a diffusion-adsorption, or DA model), which predicts the distribution of uranium across a bone or tooth enamel section. This adds a margin of error that is difficult to calculate.

We asked Mathieu to give us a quick summary of U-series dating.

This is an additional video, hosted on YouTube.

In the early days of the U-series dating method, samples were required to be dissolved for analysis. Modern techniques for U-series dating use laser ablation sampling combined with inductively coupled plasma mass spectrometry analysis (ICP-MS). This method allows us to project a laser onto the flat surface of a sample and atomise the material in a tiny circle that is hardly visible to the naked eye. This gives us high-resolution measurements with minimum sample destruction. Ages are then calculated by comparing the measured isotopes with those of a standard. A standard is a reference sample of known U-series age.

In our laboratory video, we show you the preparation of the sample and the standards, the use of the laser and the resulting analyses on the mass spectrometer.

Your task

How convincing do you find U-series dating to be?

We’ve already briefly mentioned U-series dating with the work of Maxime on the rock art of Sulawesi. What do you think of its application to human fossil specimens?

Select the comments link below and share your thoughts.

References

Aitken, M.J. (1990). Science-based dating in Archaeology. Longman Inc., New York.

Duval, M., Aubert, M., Hellstrom, J. and Grün, R. (2011). “High resolution LA-ICP-MS mapping of U and Th isotopes in an early Pleistocene equid tooth from Fuente Nueva-3 (Orce, Andalusia, Spain).” Quaternary Geochronology 6(5): 458-467.

Grün, R. (2006). “Direct Dating of Human Fossils”. Yearbook of Physical Anthropology, 49: 2-48.

Grün, R., Aubert, M., Hellstrom, H. and Duval, M. (2010) “The challenge of direct dating old human fossils”. Quaternary International, vols 223-224, 87-93.

Grün, R., Aubert, M., Joannes-Boyau, R. C. & Moncel, M. (2008). High resolution analysis of uranium and thorium concentration as well as U-series isotope distributions in a Neanderthal tooth from Payre (Ardeche, France) using laser ablation ICP-MS. Geochimica et Cosmochimica Acta, 72 (21), 5278-5290.

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A Question of Time: How We Date Human Evolution

Griffith University