Archaeologists use sediment and sunlight to date important site on South African Coast
Pinnacle Point South Africa Site Complex, a UNESCO World Heritage site. Caves and rock shelters line the cliffs where Middle Stone Age people lived between 170,000 and 40,000 years ago. Photo by Patrick Fahey.
Along the coast of South Africa, near the town of Mossel Bay, caves and rock shelters tell the story of modern humans who lived about 160,000 to 50,000 years ago. The archaeological site, Pinnacle Point, is a designated World Heritage Site and contains the earliest evidence of humans eating seafood, using paint, collecting seashells, producing micro-lithic tools and using fire to modify stone for making stone tools.
Arizona State University archaeologist Curtis Marean has led teams of scientists and students at Pinnacle Point for excavations and research for over 20 years. Now, scientists at ASU, Australia, Greece and Portugal are the first to make a very detailed, high-resolution timeline of one of the rock shelters at Pinnacle Point.
“An age model is a chronological description of how the sediments accumulated over time,” said Marean, research scientist at the Institute of Human Origins and Foundation Professor at the School of Human Evolution and Social Change. “Pinnacle Point 5-6 (PP5-6) dates from about 110,000 years ago to 50,000 years ago; that's the full span. But, the site is made up of about 135 individual layers from bottom to top with human occupation.”
“The goal is always to make our estimates of the timing of human occupation more and more precise and accurate," said Marean. “We are constantly pushing that forward and ultimately bring it to the point where we’re talking about the lifespan of a human. We are near that resolution now at PP5-6.”
Figuring out the exact age of sediment layers at very old archaeological sites like this one is hard, especially because the material was too old for carbon dating. Instead, the scientists used a method called optically stimulated luminescence (OSL) dating, which estimates when individual grains of sand were last exposed to sunlight or heat. In this study, they used a refined approach where single grains of sand are measured, called single-grain OSL.
When working to create a timeline for sites, archaeologists typically do not use more than 20 to 40 OSL dates. This is due to the high cost of sampling and the labor-intensive task of collecting the samples.
Running an individual sample in an OSL machine costs around $1,000 per sample. Gathering samples requires excavation during the day, then carefully collecting the samples at night so the sediments are not exposed to sunlight — damaging the dating process. Excavation for this project at PP5-6 took 12 years.
However, at Pinnacle Point 5-6 — the team ran 169 sample dates, and for the wider project, around 400.
How were they able to do it?
Marean purchased an OSL machine costing $100,000 in 2007 from a National Science Foundation Grant he received. He installed the machine at University of Wollongong, Australia where Professor Zenobia Jacobs ran the lab, and samples, for over a decade. The team was able to get around $400,000 worth of dating for $100,000.
At PP5-6, the 169 dates were then fitted to a Bayesian statistical model to create an age model that, for some layers, is at 100-year resolution. Such a high resolution has never been attained before and thus the PP5-6 sequence can act as a ‘Rosetta Stone’ for studying fine-grained change in human behavior and culture, said Marean.
This timeline enables scientists to see how human behaviors and environmental conditions changed over time and how those changes relate to regional and global climate shifts, explained Patrick Fahey, an affiliated graduate student at the Institute of Human Origins and graduate student at the School of Human Evolution and Social Change.
“My PhD work focuses on the human ecology at PP5-6, such as what environments humans were living in and what animals they hunted,” said Fahey. “With this new, fine-scale age model, I can now dig into these patterns in much more detail. Instead of grouping the archaeological materials into layers that might represent thousands—or even tens of thousands—of years, I can zoom in and study changes that happened on much finer timescales, sometimes at the century scale. That's really impressive, considering the deposits date to 100,000 to 50,000 years ago!
“A high-resolution chronology for the archaeological deposits at Pinnacle Point 5–6, Western Cape Province, South Africa,” was published recently in Quaternary Science Reviews.