Tufa deposits can be thought of as springs, waterfalls, and ponds that have turned into rock. As water evaporates, it can leave behind a calcium carbonate precipitate, forming a porous rock we call tufa.
We can find out how long-ago tufa formed using a method called uranium-thorium dating. Tufas in general have been considered too porous and ‘dirty’ for this method to be applied effectively. However, this study is adding to the growing body of research that proves otherwise.
PhD candidate Jessica von der Meden at the University of Cape Town, and first-author of the study published in PLOS ONE, carried out the extensive survey program across the landscape at Ga-Mohana Hill, documenting and sampling the variety of tufa deposits.
Ga-Mohana Hill has spiritual significance for the local communities. Out of respect for this, Jessica adopted a low-impact sampling approach, with targeted samples carefully chosen in inconspicuous locations.
Jessica discovered she could reliably date the tufas at Ga-Mohana Hill by specifically targeting ‘cleaner’ layers within the tufa formation. The method used for this is called laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), which in a nutshell uses laser technology to measure the relative amounts of uranium and thorium along a small transect. Layers in the tufa that have high uranium levels and low thorium levels are better for dating, and so once those layers are identified they are then sampled for dating analysis.
The Isotope Geochemistry Group at the University of Melbourne and Dr Robyn Pickering (University of Cape Town) helped develop the laser ablation and uranium-thorium approach, and trained Jessica on these procedures. The approach used here is a significant methodological advance for the science of dating geological formations, and to the best of our knowledge, this is the first application of its kind to tufa samples.