By Jason D. Moser
This summer during the height of the University of South Florida/Ohio State University field school at Crystal River Archaeological State Park, I had the opportunity to speak with Dr. Jack Rink about a new technique that he using to determine the age of the Crystal River archaeological site. Dr. Rink is a professor and researcher at McMaster University, McMaster Institute of Applied Radiation Sciences. He began his education in Florida where he received his Ph.D. in Geology at Florida State University. After working on projects in Africa, Europe and Asia, Dr. Rink returned to Florida several years ago to work on the Salt Springs site near Palatka. He has since worked at sites around the state including several shell middens on St. Joseph’s Bay in Florida.
Dr. Rink and his associates specialize in a special type of geochronology called Optical Stimulated Luminescence—or OSL for short–that is used to date archaeological sites and geological features. OSL dating is a system of sampling and measuring the amount of energy that is trapped within soils. Quartz and quartzite accumulate energy in them through time. This energy comes from the breakdown of very small quantities of radioactive materials that are locally present in the earth’s crust. The rate of breakdown and energy release is relatively constant. However, some environmental factors such as moisture can affect the accumulation of this energy.
Interestingly, both quartz and quartzite lose their accumulated energy whenever they are exposed to sunlight. When you measure the amount of energy that is present within individual sand grains, it serves as a proxy measurement for the amount of time that the quartz grains have been buried since they were last exposed to sunlight. Currently, quartz is the only material that can easily be dated through this technique although some other types of materials may be used in the future such as feldspar.
The science behind this dating technique is interesting; quartz that has been exposed to sunlight experiences an atomic level energy change that causes electrons to become un-trapped from the crystals. The crystals serve as a geologic stop watch. Whenever the quartz grain is exposed to direct sunlight, electrons are released—effectively setting the elapsed time clock back to zero. When the sand grain becomes buried, by events such as floods carrying sediment across a site, or when the ground becomes covered by a midden, or prehistoric people create a sand mound, the grains become buried in the dark, and then begin to accumulate electrons. Once you know the rate of energy accumulation in these quartz grains over a given time period, then, by measuring the amount of energy the grain still contains, you can calculate the amount of time that has elapsed since it was buried. The measurement process is actually much more complicated…but, for now, I’ll will just leave it at that.
OSL dating is potentially very useful for archaeologists. It can identify soils exposed to sunlight as recently as 5-10 years ago to between 150,000 to 200,000 years ago. Dr. Rink’s lab is also currently developing new techniques that will allow them to date sediments ranging from 2 to 5 million years ago. It will be a major breakthrough; however, dating such ancient soils will require an extremely long exposure to sunlight in order for this technique to be useful.
One interesting thing about collecting OSL samples is that they have to be collected so that they aren’t exposed to sunlight. How can you do this—you ask? Do you work at night? Well, you could collect your samples at night, but then it becomes difficult to see your work and write your notes! The Crystal River field school used an opaque container to capture samples collected from the Geoprobe. On some projects I’ve seen the samples collected by pounding an opaque PVC tube into the wall of an excavation unit. The ends of the PVC tube are capped, preventing the sample from exposure to light. Theoretically, only one end of the tube is exposed to sunlight—therefore the scientist analyzing the OSL sample can then select grains that were from the middle of the tube. The greatest drawback to this method is the issue of bioturbation.
Bioturbation is the process through which living things move the soil around in the ground. Roots, ants, worms, gopher tortoises, tree falls all continuously move soil up and down through the soil column. Other process such as wind, rain, frost heave, erosion and deposition can all work to expose soil to sunlight and then cover it back up.
These factors make the use of OSL dating both a science and an art form. Although, it is relatively easy to distinguish some types of disturbances to soil stratigraphy–it is much more difficult to see the types of disturbances created by insects and worms. These organisms are constantly burrowing through the soils and continually moving grains of sand up and down. This movement of soils is one of the reasons why Dr. Rink is interested in the Crystal River site. Many of the mounds and midden that are located at the site are composed of alternating layers of shell and soil. This combination provides an advantage in OSL dating. The thick layers of shell tend to prevent natural processes such as erosion and re-deposition from churning up the soil. The shell also prevents organisms and roots from smaller plants from moving the soil around quite as much.
To take advantage of all the shell, the soil cores from the Crystal River site were collected with a specialized mechanical coring device called a Geoprobe (see previous blog article). The Geoprobe was necessary in order to penetrate the thick layers of shell that were present on the site. The Geoprobe team comprised of Dr. Rink, Dr. Glen Doran (Florida State University) and Grayal Farr, collected cores from across the site. Each core was collected using opaque black tubes to prevent its exposure to sunlight. Using this technique, the investigators will use the OSL dates to develop a chronological framework for different parts of the site. This information will really help the archaeologists to understand how and when the site was formed. These dates can help identify when a mound was first built, when it was expanded or rebuilt, and when it was abandoned.
Hopefully, when the results of the investigation are completed this information, combined with the information collected by the USF/OSU field school will provide a detailed history of the Crystal River site and its prehistoric inhabitants. Tune in again, as the investigators begin to release the results of these investigations.