A tooth fragment from the lower jaw was submitted for radiocarbon dating to the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Lab in California. Kena Fox-Dobbs, graduate student of Dr. Paul Koch, Geology Professor and Director of the Stable Isotope Laboratory, Departments of Earth and Ocean Sciences, University of California, Santa Cruz, prepared the samples. We are grateful to a Principia parent who underwrote some of the expenses for this work and to these labs that contributed time and supplies to the project as well.
An initial sample of tooth dentine was analyzed and yielded a date of 17,810 +/- 4300 radiocarbon years before present. This corresponds well to our estimate of 17,500 years that was based on stratigraphic control (the mammoth’s position in the Peoria Silt it is buried in), but it has a large margin of error. A second sample of tooth enamel was analyzed and yielded a date of 15,380 +/- 60 radiocarbon years before present. It is known that enamel is more susceptible to alteration by weathering than the dentine, which is on the interior of the tooth, and therefore an enamel date is less reliable and likely to be younger than reality.
Optically Stimulated Luminescence Date
In the spring of 2006, two samples of the loess matrix entombing the mammoth were collected by Dr. David Grimley of the Illinois State Geological Survey. Samples were ten feet apart and directly above the ancient soil (paleosol) on which the bones have been found. This loess should have been deposited shortly after the mammoth’s demise so as to preserve the bones as well as they have been preserved.
These loess samples were dated by Dr. Paul Hanson in the School of Natural Resources at the University of Nebraska in Lincoln using optically stimulated luminescence. The luminescence age tells us when the samples were last exposed to daylight. For this reason, the samples were not exposed to light until light was applied in the lab, at which point energy accumulated in crystal lattices of minerals during burial was released and an age determined.
The dates on our two samples were 1600 years apart, which is within the standard error for this method, and averaged at 17,300 +/-1800 radiocarbon years. This date corresponds well with our date of 17,500 radiocarbon years using the mammoth’s stratigraphic position within the Peoria loess and is close to the radiocarbon age determination on poorly preserved collagen from the tooth dentine of 17,810 +/- 4300 radiocarbon years. So we are now more confident in stating that our mammoth lived about 17,500 radiocarbon years ago.
Each day, the class tackles a multitude of tasks around the site, at the surface as well as in the pit. These include:
- Careful digging with trowels and bamboo skewers and paint brushes in order to find bone and remove matrix from around bone
- Pedestaling bones in preparation for removal
- Jacketing fully uncovered and pedestaled bone with plaster and burlap to protect it while removing it to the lab
- Hauling excavated dirt
- Collecting samples of the matrix for detailed study
- Wet-sieving the matrix samples
- Studying the sieved material under the microscope to detect any clues about the paleoecology of the site or the demise of Benny
- Mapping fully uncovered bone
- Maintaining troughs for proper handling of site runoff
- Placing sand bags and concrete blocks in strategic locations for crowd control and pit protection
How’s poking around in the dirt with a bamboo skewer for a glamorous job? When we reach bone, tusks, or teeth, we switch from trowels to bamboo skewers and brushes. Shovels are used in removing the upper layers of dirt above known bone. Even then, we carefully shave off a thin layer at a time. As we approach the depths where bone has been found, we use trowels. The dirt is removed layer at a time, keeping a level surface as we work down. Diggers are responsible for cleaning up their work area and making sure their dirt is hauled to the dirt pile.
Representative sediment(loess) samples are removed from the pit and wet-sieved to isolate small material that might give us important clues about the climate, habitat, and time period in which our mammoth lived. This material might include bone fragments of mammoth or other animals, snail shells or other microfossils, and possible artifacts (object showing human modification). The loess samples are placed in nested sieve boxes with coarse (18 mesh) and fine (30 mesh) screens and swished through water on the site in order to remove the fine clay and silt in the soil (loess). The sieved material is dried and placed in containers labeled with the sample’s quadrant location and depth. They are described in the lab using a microscope.
A grid system has been laid out at our site to record the location and depth of every bone and soil sample we remove. This is critical in the forensic part of the project – how and why did our mammoth die here and what happened to his bones between the time he died and our finding him? This study is called taphonomy.
This has varied through time. Initially the site was covered daily with a large tarp that was supported by rafters that straddled the pit. During the summer of 2002, students, supervised by Prin’s Facilities staff, constructed a shelter over the pit. Now we are not as affected by the weather and we can stand up tall everywhere in the pit! We love our shelter – “Benny’s Bone Hut.” Since the pit is on a gentle slope, drainage is a key concern. We have dug trenches and lined problem areas with sand bags to redirect runoff away from the pit.