Posts Tagged stratigraphy

Optically Stimulated Luminescence (OSL) Dating Basics

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.

Photograph illustrating the effects of tree falls on soil stratigraphy (the light gray overlying soil filled in the hole that was left by a tree when it fell over).

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.

Tags: , , , , , , , , , , , , , , , , , ,

Geoprobing into the Past

by Jason D. Moser

It’s springtime, and you know what that means—fieldwork time! Yes, that’s right; an archaeological field school has arrived in Citrus County. This year, the University of South Florida (USF) has begun an archaeological field school at Crystal River Archaeological State Park. For those of you who aren’t familiar with the park it is a 61-acre site located just north of the town of Crystal River. The park encompasses a large part of an American Indian ceremonial mound complex located on the banks of the Crystal River. The complex includes two burial mounds, two shell temple/platform mounds, a plaza area, and substantial shell middens. Shell middens were dump areas created from the accumulated waste and debris of a community over the course of many generations.

Eroded and exposed shells on the surface of Mound A.

The Crystal River site was used for 1,600 years making it one of the longest continuously occupied sites in Florida. Perhaps, due to this long occupation, extensive midden deposits are found throughout the park. They are composed mostly of shells, but they also contain broken pottery, discarded, broken shell and stone tools, and the bones from many of the birds, fish, mammals and reptiles that the inhabitants trapped and hunted. The presence of so many-well preserved artifacts make the midden deposits, some of the most interesting places that archaeologists can investigate.

The USF field school is part of a new investigation of the site called the Crystal River Early Village Archaeological Project (CREVAP) which is funded through a National Science Foundation (NSF) grant. The investigation is designed to address previously unresolved questions about the formation of early village societies and the dynamics of relationships between the prehistoric people in the Crystal River area. The project is being carried out under the direction of Dr. Thomas Pluckhahn (USF), Dr. Brent Weisman (USF), and Dr. Victor Thompson (Ohio State University). Additional information about the project can be found at http://www.crystalriverarchaeology.org/

To assist with these investigations USF has brought in several experts to conduct highly specialized investigations that are often beyond the expertise of typical archaeologists. This week I had the opportunity to speak with two of them while they were working at the park and to find out exactly what they were up to!

Grayal Farr (left) and Dr. Glen Doran (right)

First, I spoke with Dr. Glen Doran–an archaeologist at Florida State University (http://www.anthro.fsu.edu/). Dr. Doran and his team brought a specialized piece of equipment that is designed to take soil cores from the ground. These cores are a way to sample the layers of shell and other material, or stratigraphy of the site without large scale excavations.

Geoprobe machine during a rest break.

Site stratigraphy is a layering effect that occurs over time on archaeological sites. To visualize stratigraphy, imagine a multi-layer cake, with alternating layers of icing and cake. Just like making a layer cake—the first layer of the cake is located on the cake plate—and that layer is then covered with icing and subsequent layers of cake and icing are added. On an archaeological site, the process is very similar—the oldest layers of the site along with the oldest artifacts are located at the bottom of the site (Usually!). The layers that are on top of base layer are not as old, and contain more recent artifacts. Once you understand the stratigraphy and the processes that might ruin the stratigraphy—then you can understand exactly when the site formed, the periods in which it was used, and when the site was abandoned. Widespread excavations—which are both time consuming and expensive to undertake would also significantly impact the archaeological site and limit the types of information that might be recovered in the future.

Dr. Doran’s team used a powerful machine to collect soil cores that were approximately 2-inches in diameter. The machine is a custom made 540 RT/D direct push coring system manufactured by Geoprobe (http://geoprobe.com/). Geoprobe has also generously provided technical assistance and additional equipment for this project. The geoprobe machines use a high speed hammer to rapidly pound the steel core into the ground. The device will penetrate shell, clay, or the types of soft rock that are present in Florida. The soil cores are collected within a clear PVC tube which is fitted inside the exterior core sleeve. Once a core section has been pounded into the ground the operators withdraw the core from the ground using a hydraulic system. Getting the sample out of the ground is relatively easy, getting the clear PVC liner out of the sleeve, is actually much more difficult than it sounds!

Students and professors from USF, FSU, and McMaster University and Florida Park Service Staff at work on Mound H.

After the core is out of the ground, the steel core is withdrawn from the machine and opened. A nearly perfect profile of the soil on the site is contained within the interior clear PVC tube. The plastic tube is removed from the machine and capped with color coded caps to mark which end is up and which end is down. Each tube is marked with relevant information about sample such as the location and depth of the test core. The process can be repeated as many times as necessary to create a cross section of a soil that is up to 10 meters in depth. However, most of the cores sampled at Crystal River are only three to four meters in depth. The cores were collected on a grid pattern at locations across the site and on some of the shell mounds.

The clear PVC tubes enable the team to immediately view the results. Because the cores are sealed they can later be submitted to labs for all types of additional processing and analyses. Soil grain size and composition can provide information about how the soil was formed such as whether the soil was formed from human activity, or whether it was blown in by the wind. Pollen and phytoliths samples can help reconstruct what the environmental and climate conditions near the site were during the past. Soil chemistry can indicate how specific parts of the site were used. For example certain activities leave specific types and combinations of chemicals in the soil. Radiocarbon dating of charcoal recovered during the coring can help establish a timeline for individual layers of the site, and provide specific information about the building sequence of the mounds.

As you can see–soil coring can often provide a huge amount of information about the people at Crystal River and how they lived. The cores that Dr. Doran and his team recovered from Mound A (The big mound) surprisingly indicate the presence of shell completely through the mound—with midden soil below the mound, and then clay, sand, limestone below the midden.

Dr. Jack Rink (left) McMaster University capping a soil core.

Once the remainder of the testing is completed—Dr. Doran’s team will hand over the soil cores to USF and move on to work at other sites in the area. USF will begin analyzing the soil cores after the field school is over. This will take quite a long time to complete. Hopefully, we will have some of the preliminary results by March 2012 when Dr. Pluckhahn will speak at an event during next year’s archaeology month. If you are interested in the Geoprobe work of Dr. Doran and his team, check out their website at http://www.anthro.fsu.edu/research/doran/geoprobe/geoprobe.html

Tags: , , , , , , , , , , , , , , , , , ,