Master Naturalists: Harvesting mesquite trunks for science
By Paul and Mary Meredith
July 26, 2014 at 2:26 a.m.
Many folks have heard about archeologists reporting how old artifacts are by taking charcoal found at the same layer of an undisturbed dig and doing an analysis called carbon-14 dating.
That's done by accepting the assumption - it's a pretty good one - that the percentage of a radioactive isotope of C-14 to carbon-12 in the atmosphere today is pretty much the same as in the past. C-12 is stable. C-14 decays to nitrogen at a known rate of one-half every 5,730 years.
Living plants and other stuff absorb fresh C-14 and C-12. Once plants die, that stops, and C-14 decay begins. Using some cute and improving tools, we can now date carbon to 40,000 years ago and maybe further by measuring the percent of C-14 in a sample.
What was the weather like back then?
C-14 tells experts little about climatic conditions people lived in at a prehistoric time. Things like tree-ring analysis can help some but not that far back. Looking at the academic literature, you run into more ideas for techniques to figure it out; all have problems documenting climates.
For a Texas prehistoric climate estimate, a technique being used in a dig north of Victoria is to sort out shells of prehistoric freshwater snails from dig dirt. Some studies of the same snail show that if it is warm and wet, more snails live; if it is dry and cold, fewer live. That, plus chemical analysis of uncontaminated dig shells, can help explain historic climate.
Phil Dering, an archaeobotanist - a profession studying plants and prehistory peoples' interactions - has developed a new charcoal technique to assess Texas' prehistoric climate. Based on work done in South Africa, he found that the structure in a tree trunk that carries water to the tree varies in size and density with the weather conditions at the time.
These water tubes, xylem, are preserved in charcoal remains. It's not true for all trees, but it is for mesquite. Dering proved it.
What do the archeologists need from us?
Dering's technique needs one thing to work in understanding Texas' climate history: a measuring stick to calibrate the variation in those water tubes to actual weather conditions. That means collecting lots of mesquite tree trunk pieces from all over the state; carefully documenting their location, terrain, etc., and then having someone make them into charcoal to compare the tubes to detailed modern weather records for each location. Using that ruler, prehistoric weather can be found in archaeological charcoal.
The job we just described is huge. Collection of samples means lots of boots on the ground. Fortunately, J. Kevin Hanselka, an archeologist in the archeological studies branch of the Texas Department of Transportation, knows how to do the analysis, but he needs Texans' help to get the samples. He's already asked Texas Native Plant Society folks to help. TxDOT and the Texas Archaeology Society are supporting the project. You can, too.
If you have property or access to property with mesquite on it, collect samples and send them to Hanselka. Just follow the provided collection guidelines.
It'll be worth your time and the small mailing expense. Got kids or grandkids? Get them involved; just be safe. If you have any questions, contact Hanselka at 512-416-2639 or email Kevin.Hanselka@txdot.gov.
txarch.org; nde-ed.org; academia.edu
Paul and Mary Meredith are master naturalists. Contact them at firstname.lastname@example.org.