A Measuring Technique – OSL

I mentioned in the last post a recently developed technique that “measures” how long soil has not been in direct sunlight, called OSL.  It was used to measure the age of the Uffington White Horse (UWH), or at least, measure how long the soil that had been used to build the chalk trenches that are the outline of the UWH has not been in direct sunlight, with the inference that that amount of time represents the age of the UWH.

I supposed that it is a tool or technique that can be used by archeologists in other digs, and a recent article in the New York Times Science section, by-line is John Noble Wilford, touches on one site where OSL might have used for its dating.  While OSL is not named, the techniques seem to indicate that something very like OSL must have been used.

http://www.nytimes.com/2010/02/16/science/16archeo.html?_r=1&ref=rock_and_stone

The article: Some stone tools have been found on Crete which reflect several interlocking time frames based on various measurements.  First, Crete has been an island for the last five million years or more.  Second, previous artifacts have been dated as being 10,000 to 12,000 years old.  The timeline of Homo sapiens, modern man, is about 200,000 years.  The oldest sea voyaging that is pretty well established is the migration of Homo sapiens to Australia, roughly 60,000 years ago.  The stone tools were found in cliffs on the southern coast of Crete, the tectonic aging of which has been “…well studied and dated.”  The next sentence is the one which made me think of the OSL dating technique: “…the team analyzed the layer bearing the tools and determined that the soil had been on the surface 130,000 to 190,000 years ago.”

As an aside, the style of the artifacts match those produced by much earlier hominids, approximately 700,000 years.  One story/explanation is that they were brought over the water by humans who then left them where they covered surfaces for 130,000 to 190,000 years.  Not sure that the story/explanation really works: I probably need to go to the principal source to understand it better, but once again, it looks as if the OSL method was used to determine how long it had been since the soil had received direct sunlight.

I thought it would be useful to look more closely at OSL.  In an incredibly thorough book that I found, Uffington White Horse and Its Landscape, there is a description of what OSL is and how it works, in theory.  Then, the book goes on to describe how the procedure was actually done in the research at the Uffington White Horse.  The following is the description of the theory:

Luminescence is the light given out by crystals on exposure to heat (thermoluminescence) or light (optically stimulated luminescence), due to the release of energy stored in the crystal lattice following exposure to ionising radiation. The luminescence signal of a crystal is proportional to the radiation dose the crystal has received since the zeroing either by heat or by exposure to light and can therefore be used for the dating if the dose rate is known (Aitken 1985).  Sediments can be dated because the optically stimulated luminescence (OSL) of constituent quartz and feldspar crystals will be zeroed when the sediment is exposed to the sunlight at deposition.  The sediment then becomes buried and is kept dark.  During burial the sediment is exposed to naturally occurring radiation from isotopes in the ground and cosmic radiation; thus the luminescence signal grows. Samples taken for dating are kept dark and the luminescence signal is measured in the laboratory enabling an optical date to be calculated using the formula given below. For further details of the method see Aitken (1998).1

There are two books by Aitken cited, which are:

Aitken, M.J., Thermoluminescence Dating, London, 1985;

and

Aitken, M.J., An Introduction To Optical Dating, Oxford, 1998.

I looked at both of these: very technical, and very complete.  M.J. Aitken is one of the leaders in using OSL for archaeology, but his information is more technical that I would like to get in this blog.  I mention them so that you can find them if you are interested in more info about OSL.  What is clear, though, is that the beginning of using this sort of measurement is relatively recent, beginning in the early 1980s.

The book as a whole reports on the investigation at the Uffington White Horse and several sites apparently related to it, at least nearby in geography, if not actually of the same date or group of builders.  The investigation and report are very thorough: each of the various sites receives a team to focus on it, as well as work being coordinated centrally: the UWH; Uffington Hillfort; round barrow and long mound; Dragon’s Hill; The Manger; and several White Horse Hill cemeteries.  The report places White Horse Hill into the context of the “Ridgeway”, an ancient path, and ranges from the geology to the soil composition, from myths to documents, to discussions of the late Bronze Age and Iron Age life.  Excavations were performed to try to understand the construction of the White Horse, and samples of soil were taken for dating by OSL.  Before the excavations, however, a survey of the area around the UWH was done to test “resistivity”, which measures soil composition and compaction.  The picture that resulted was suggestive of possible shape changes in the UWH.

Six soil samples were taken and processed.  Of these, only four were usable, but the dates apparently agreed sufficiently to give an approximate amount of time since the White Horse was created, with some level of uncertainty: between 1380 and 550 BC, or approximately 3390 to 2560 years ago.

The technique is complicated.  The samples were extracted and placed in steel cylinders that were immediately closed and covered with black-out plastic.  The samples were then processed in a lab by a series of chemical baths to isolate the quartz and feldspar grains, done in low light to insure that no light “contaminated” the samples.  While reading about the process, I had my reoccurring nightmare of trying to perform qualitative analysis in my sophomore chemistry class in high school, in broad daylight.  I can only say how glad I am that there are those who are able to do this sort of analysis.

Ultimately, some quartz and feldspar grains remained after the cleansing process and were used for the luminescence testing. Luminescence testing is also a multi-step process, using, for separate samples, infrared light, and green light from an argon ion laser.  By shining these lights on the samples, the infrared causes the feldspar to luminesce: produce a glow which is measured by a sensitive photomultiplier, and the green light has the same effect on the quartz.  The amounts of luminescence are captured by the photomultiplier.  This effectively “zeros” the samples, which are then exposed to calibrated ionizing radiation for known periods of time, shined on at the different levels of ionization, and so forth until the artificial ionization results in luminescense that matches what the sample originally put forth.

Measurements back on site with radiometers were made to calibrate the rate that the ionizing radiation would have affected the grains in the sample.  Based on that rate and the amount of ionization required, the date is figured out.  Exhausting!

What is clear from the description is that this sort of measurement, while it involves an extensive set of steps, all of which must be performed carefully, is a repeatable measurement that will give similar or the same results whenever it is performed on objects from the same source.  The methodology provides a framework for deriving results that are meaningful.  Calibration could change (it might be discovered that the rate of ionizing differed due to some not yet discovered condition, for instance), additional factors might need to be considered, but this method of measuring is clear about how to perform the measurements and in its implications.

To step back a little and try to gain some perspective on this method, and on the Uffington White Horse (UWH), dating using OSL narrows the range of time during which the White Horse was created, thereby narrowing the kinds of stories that can be developed about who created it, and how.  It does not answer some other questions, though.  There is no way to attribute the creation to a specific person or even a specific tribe, other than a generic late bronze age group.  This is similar to Stonehenge, which in the past has had a number of wild stories told about its creation (Merlin magically levitated the stones from Wales…), but is considerably older than the UWH.  For Stonehenge, there is at least the contemporary tale that it was set up as an observatory with specific view lines to rising and setting stars and the rising and setting of the sun on specific days of the year.  There is no such story about the UWH: just who was the genius who laid out the shape and gathered his friends, neighbors and family to create the original UWH?  And, how long did it take them?

Figuring out how the UWH was constructed is another question of interest.  About all that can be said is that in the areas where the excavations were made, it was discovered that there were several layers of “compacted chalk” above the chalk bedrock, which indicates that beyond the maintenance was that performed periodically, substantial re-working was done.  The reworking was more than just refreshing the chalk and cutting back the turf around the edges.  While there had been initial indications, based on the resistivity survey, that the shape might have changed over the years, the final judgment is that the shape has remained stable and constant since the original version.

These are all contemporary measurements, or at least quite recent.  Since we have been talking about where measurement comes from, what sort of evidence does the UWH provide of measurement and measurement tools in the late Bronze age in the Wiltshire Downs of England?  Although some late Bronze age tools such as axes that were found at Tower Hill are mentioned in the book, none have been directly associated with the UWH, and the report does not mention any measuring tools or techniques that would be required to construct the UWH.

Based on the curvilinear shapes of the UWH, perhaps they had Bezier curves?  No, of course not.  The style of the UWH has few analogs – possibly some very old coins, though they are iron age coins.  One archeologist was convinced the design was Celtic, another that it is a Gaulish representation of the horse goddess Epona.  While the dating makes these stories unsupportable, the UWH does not seem to provide any help in our understanding of early measuring and measuring methods.  The only answers that can be provided by the best scientific measurement at this time relate to determining the age of the structure.

I have not found any study that describes how to build something like the UWH – though build might be the wrong verb – create? construct? dig?  It is late enough in its construction that all of the measurement elements listed at the beginning of this post (except standards – mentioned in the Magna Carta in 1215 AD) could have been available, from language to describe it to tools, techniques and concepts, to methods of recording measurements.

The main question is why was it created, and for that there really is no definitive answer.  The report provides some tentative thinking about it: for unspecified rituals, or ostentatious display of wealth/power.  No one has suggested, so I am, that the designer was an aesthete who wanted to make something of beauty, nor has it been suggested that this was done as a trick to mess with some archeologists’ minds, several thousand years in the future.  Seriously, though, there is no reason for the creation of the Uffington White Horse that can be proved.

As a side question, why would OSL be needed to perform dating, since, as discussed in an earlier post, scientists had already developed radiocarbon dating?  The difference between the two is important to understand: radiocarbon dating can only be performed on formerly living matter: remember the way that the theory works – the amount of radioactive carbon, Carbon 14, cannot change once the living matter stops living since the C 14 is breathed in.  Therefore, as a dating method, it is accurate only up to about 60,000 years, which is several multiples of the half-life of C 14.  OSL is performed on the feldspar and quartz grains in soil that luminesce when stimulated.  Both minerals are inorganic matter: they are non-living.  Evidently OSL can be used to date back 500,000 to 600,000 years, though I’ve not actually seen the dating limit as a number, just as a representation in a logarithmic chart, so I’m not sure exactly.

I have always enjoyed the Sherlock Holmes stories by Arthur Conan Doyle.  His character, Sherlock Holmes, is the epitome of the investigative scientist, accepting no supernatural explanations and using what appears to be a scientific method to solve the crimes that are referred to him.  In one of the stories, Doyle has Holmes turn his method into an epigram:

“How came he, then?” I [Dr. Watson] reiterated.  “The door is locked, the window is inaccessible.  Was it through the chimney?”

“The grate is much too small,” he [Sherlock Holmes] answered.  “I had already considered that possibility.”

“How, then?”  I persisted.

“You will not apply my precept,” he said, shaking his head.  “How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?”2

In a funny way, this appears to be the method that humans with their over-active pattern making ability may have used over the millennia: if all of the possible, easy, non-supernatural explanations are found to be wanting, does this mean that something supernatural must be the reason whatever needs to be explained happened?  Holmes apparent definition for “the impossible” is anything without a natural explanation, or so it seems from all of the stories.  When I discussed the “supersense”,  a result of an incomplete ability to measure correctly – the measurements and concepts required to understand a mysterious event have not yet been developed?  Hmmm.

In the case of the Uffington White Horse, if the question is who put it there and why, even the improbable ones are not acceptable.  There is no way to narrow the possibilities, the way there are if this was a Sherlock Holmes story.

I had an explanation that I liked for why I felt moved by the sight of an inscribed White Horse, but it was wrong for the Cherhill White Horse, the one I saw.  So I dropped it.  But then I found the Uffington White Horse, which, by performing some measurement and drawing the inferences, the ability to tell a story that it was created by iron age men was eliminated, but likewise, so were the stories about Hengist and  King Alfred.  Even though the measurements came from a completely different direction, from Earth Sciences rather than from archeology, history or even myth, it put some severe limits on potential explanations, while complete precision is elusive.

This is by no means the only way that measurement has helped with the refining of understanding.

Another book that I recently read is yet another potential game changer or at least a thought-provoker.  The Mind In The Cave, by David Lewis-Williams3, is an attempt to use a modern understanding of the way that humans’ big brains evolved to explain cave paintings, which I have been looking into to see if there is evidence of “measurement” being required to produce them.

Dr. Lewis-Williams describes how altered states of consciousness could be responsible for the people of so many thousands of years ago painting the caves of southern France, northern Spain, and other “primitive” rock paintings.  He approaches both the shamanistic content of the pictures and the altered states of consciousness from a scientific viewpoint, based on the most up-to-date understanding of the biology of the brain and nervous system.  The case that he builds is fascinating, but is it the best explanation?  I’m not sure.  Could there have been something similar that generated the vision of the Uffington White Horse for the shaman of the local tribe, which led to his or her leading the group to actualize his or her vision on the side of White Horse Hill?  I suspect we’ll never know.

However, Lewis-Williams’s book does one thing that I needed to understand, and that is to show how the paintings could have been made without any sort of measurement required.  If he is correct, and perhaps even if he is not correct, there appears to be little in the way of evidence that even the rudiments of “measurement” were used.


1 Miles, David, Palmer, Simon, Lock, Gary, Gosden, Chris, and Cromarty, Anne Marie, Uffington White Horse and Its Landscape: Investigations at White Horse Hill, Uffington, 1989-95, and Tower Hill, Ashbury, 1993-4, Oxford University School of Archaeology, as part of the Thames Valley Landscapes Monograph series, 2003.  p. 269  (Appendix 1: Optically Stimulated Luminescense (OSL) Dating Results from the White Horse and Linear Ditch, by Julie Rees-Jones and Mike Tite.)

2 The Complete Sherlock Holmes, ‘The Sign of Four’, Doyle, Arthur Conan, in 2 volumes, p.111 in V. I.  Doubleday & Company, Inc.

3 Lewis-Williams, David, The Mind In The Cave, Thames & Hudson, London, England, 2002.

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