New analysis of flowstones from Cradle reveals no evidence of intrusion
New research from a team of researchers at at the University of Cape Town (UCT), led by Georgina Luti from the Human Evolution Research Institute (HERI), provides the most comprehensive analysis to date of flowstones found in caves from South Africa’s Cradle of Humankind, adding support to their role in accurately dating fossils from the region.
Published in the December issue of the journal, Earth Sciences, the study analysed 67 rock samples from 12 caves across the Cradle to reveal no evidence of intrusion. Instead, researchers identify distinct variations that fall under five newly proposed categories of classification.
“Our work counter assumptions that flowstones are uniform, dispute claims of intrusion and provide strong evidence in favour of flowstone chronology as the most robust way of dating both the caves and fossils of the Cradle,” says Luti, a geologist at UCT.
Flowstones are geological formations that form in horizontal layers on cave floors and walls. They are found in all the caves across the Cradle, forming sequentially over millions of years as layers that capture clues about the past.
Because the Cradle is the world’s richest early hominin site, and home to nearly 40% of all known human ancestor fossils, certainty about the age of flowstones found there is essential for accurately dating these remains.
Using petrographic analysis, or microscopic examination of thin sections of flowstone, the researchers found no evidence of flowstones intruding into older sediments. Instead, they suggest that fossil-bearing sediments and flowstones form in sequential layers, and that flowstones can indeed be dated to provide precise ages for the fossils found above and below them.
“Our findings push back against arguments that flowstones are intrusive and cannot be used to provide bracketing ages for fossils, and suggest more evidence and larger sample analysis from the Cradle is needed to substantiate such claims,” says Luti.
In addition to supporting flowstone chronology for dating caves and fossils of the Cradle, the study also reveals the important role petrographic analysis can have for strengthening other research findings.
“Studying the microscopic makeup of flowstones isn’t commonly done, but we show it can provide a better understanding how rocks form, how well they are preserved, support the quality of dating processes, and ensure accuracy when identifying the age of fossils,” says Luti.
Luti is a 2024 Francis H. Brown African Scholar, and the first African woman to date these rocks from South Africa as part of a doctorate in geochemistry that sheds light on human evolution. This is her first publication as lead author, and the paper represents an all female team of UCT researchers from HERI.
“Georgina is championing innovative research that provides solid evidence about the material we find in the Cradle. She is an example of the breadth of African talent and scientific excellence we have studying human origins,” says HERI Co-director Associate Professor Robyn Pickering.
Flowstones are geological formations that form in horizontal layers on cave floors and walls. They are found in all the caves across the Cradle, forming sequentially over millions of years as layers that capture clues about the past.