“How old is The Mammoth Site?” by: Dr. Jim I. Mead
We know The Mammoth Site dates to the Pleistocene (Ice Age) and so far all of our geochemical dating (such as radiocarbon dating) has indicated that the sinkhole was full of warm water and struggling mammoths about 26,000 years ago. We have always accepted the age but we have also wanted to assess that age – basically refine the age to see if we can obtain a more precise date. Steve Holen (one of The Mammoth Site Science Associates right here in Hot Springs) has been collaborating with a colleague (Dr. Shannon A. Mahan) of the US Geological Survey in Denver for a number of years. They have been using OSL dating on some of Steve’s mammoth sites in Nebraska and elsewhere around the region. So, on January 8th a group assembled here at The MS and took a number of samples of sediments to analyze for OSL dating.
OSL is a simpler way to say Optically Stimulated Luminescence. As Shannon tells us, in contrast to isotopic methods (such as radiocarbon dating; 14C) which uses a function of elemental decay or buildup, OSL dating measures the growth of natural environmental radiation that results in trapped electron charged signals within a mineral grain that are eliminated by exposure to light or intense heat. Basically the longer a grain is buried from light, the more a signal is preserved on various grains of sediments.
The January crew included Paul Hanson (University of Nebraska, Lincoln), Liz O’Rourke (USGS), Shannon Mahan (USGS), along with individuals from The Mammoth Site: Justin Wilkins, Monica Bugbee, Steven Holen, Sandy Swift, and me. Samples of sediments were removed using a number of techniques such as chisel and hammer, electric saw, and core. Background radiation was determined using a portable gamma spectrometer.
A large sample was removed by Justin using a chisel and hammer from the sand and clay units at the west end of the site. Here Shannon is spray painting the outside surfaces of the block with a reflective silver coating. The outside surface will be removed in the protective, dark lab because it has been contaminated by light. Inside the block will be the mineral grains ultimately used in the dating.
Here Paul and Justin take a sample of the sand from near a mammoth skull and tusks from the west end. They are using an electric saw that helps cut out a block of sediments.
A short video illustrates how the electric saw is used to remove the block of sediments.
The special gamma spectrometer measures the background radiation. Fragile fossil remains in the same area are covered up with white plaster jackets to protect them while work continues in the ‘log-jam’ of bones and sediments.
A sample is also taken from the high, middle sediment profile at the site. Here Steve (left), Shannon (middle), and Paul (right) confer where is best to sample.
Sometimes a subsample must be taken in the field so a black cloth is used to create a tent to keep contaminating light out while the smaller sample is removed. Here Liz looks on as Shannon, Paul, and Justin remove a subsample.
A third type of sampling the sediments is produced by pounding a steel tube into the wall. Here Shannon pounds the short tube into the sandy sediments. When the tube is removed, sediments and the ultimate sample for dating are encased inside.
Now we wait for the detailed laboratory analyses. The long, tedious work really begins when the samples get back to the lab. Shannon and Paul will be analyzing different mineral components carefully removed from each sample. As soon as we hear the exciting news, we will let you know.
Is The Mammoth Site really 26,000 years old? Possible! BUT… maybe it is younger, having been active during the height of the most recent full glacial period about 22,000 to 18,000 years ago. Alternatively it could be older, 40,000, 50,000 or maybe even 60,000+!
Until the samples just removed are analyzed – it is just patience. Sigh.