Uranium series dating shows South African midden to be the oldest in the world

Archaeologists have learned a lot about our ancestors by digging through their garbage piles, which contain evidence of their diet and population levels as the local flora and fauna have changed over time.

A common kitchen waste in Africa – ostrich eggshells – now helps unravel the mystery of when these changes took place, providing a timeline for some of the earliest Homo sapiens who settled down to use food resources. navies along the South African coast. over 100,000 years ago.

Geochronologists at the University of California at Berkeley and the Berkeley Geochronology Center (BGC) have developed a technique that uses these ubiquitous releases to accurately date landfills – politely called middens – that are too old to be radiocarbon or carbon dated. – 14 techniques, the standard for materials like bone and wood less than 50,000 years old.

In an article published this month in the journal Proceedings of the National Academy of Sciences, Former UC Berkeley PhD student Elizabeth Niespolo and geochronologist and BGC and associate director Warren Sharp reported using uranium-thorium dating of ostrich eggshells to establish that a midden at outside Cape Town, South Africa, was deposited between 119,900 and 113,100 years ago.

This makes the site, called Ysterfontein 1, the oldest known seashell in the world, and implies that the first humans were fully adapted to coastal life around 120,000 years ago. It also establishes that three hominid teeth found at the site are among the oldest Homo sapiens fossils recovered from southern Africa.

The technique is precise enough that the researchers convincingly state that the 12.5-foot-deep pile of mostly marine shells – mussels, mollusks and limpets – mixed with animal bones and eggshells may have been deposited over a period of as little as 2300. years.

The New Ages are already revising some of the assumptions made by archaeologists about the early Homo sapiens that dumped their trash at the site, including how their population and foraging strategies changed with changing climate and sea levels. .

“The reason it’s exciting is that this site could not have been radiocarbon dated because it is too old,” Niespolo said, noting that there are many more such sites in Africa. , especially in the coastal areas of South Africa.

“Almost every site like this has ostrich eggshells, so now that we have this technique, there is this potential to go and revisit those sites and use this approach to date them more precisely and more precisely, and most importantly, whether they are the same age as Ysterfontein or more or less, and what that tells us about foraging and human behavior in the past, ”she added.

Because ostrich eggshells are ubiquitous in African middens – eggs are a rich source of protein, equivalent to around 20 chicken eggs – they have been an attractive target for geochronologists. But the application of uranium-thorium dating – also known as the uranium series – to ostrich shells has been beset by many uncertainties.

“Previous work to date on eggshells with uranium series has been really hit and miss and for the most part lacking,” Niespolo said.

Precision dating pushed back to 500,000 years

Other methods applicable to sites over 50,000 years old, such as luminescence dating, are less accurate – often by a factor of 3 or more – and cannot be performed on archival material available in museums, Sharp said.

Researchers believe uranium-thorium dating may provide ages for ostrich eggshells as old as 500,000 years, extending the precise dating of middens and other archaeological sites about 10 times farther in. the past.

“This is the first published body of data that shows that we can get really consistent results for things well outside the radiocarbon range, about 120,000 years ago in this case,” said Sharp, who specializes in the use of uranium-thorium dating to solve paleoclimate and tectonic problems as well as archeology. “It shows that these eggshells keep their serial uranium systems intact and give reliable ages later than previously shown.”

“The new dates on the ostrich eggshell and excellent wildlife preservation make Ysterfontein 1 the best dated multi-layered Middle Stone Age shell to date on the west coast of South Africa, “said co-author Graham Avery, a retired archaeozoologist and researcher. Iziko South African Museum. “Further application of the new dating method, where fragments of ostrich eggshell are available, will strengthen chronological control in nearby Middle Stone Age sites, such as Hoedjiespunt and Sea Harvest,” which have similar faunal and lithic assemblages, and others on the south coast of Cape Town. “

The first human settlements?

Ysterfontein 1 is one of a dozen shell middens scattered along the western and eastern coasts of the Western Cape Province, near Cape Town. Excavated in the early 2000s, it is considered a Middle Stone Age site established during a time when Homo sapiens were developing complex behaviors such as territoriality and intergroup competition, as well as cooperation between non-groups. related. These changes may be due to the fact that these groups were moving from hunter-gatherers to settled populations, using stable sources of high-quality protein – shellfish and marine mammals – from the sea.

So far, the age of Middle Stone Age sites like Ysterfontein 1 has been uncertain by around 10%, making it difficult to compare between Middle Stone Age sites and with sites from later stone age. The new dates, with an accuracy of around 2% to 3%, place the site in the context of well-documented changes in global climate: it was occupied immediately after the last interglacial period, when sea level was at a level high, perhaps 8 meters (26 feet) higher than today. Sea level dropped rapidly during the occupation of the site – the shoreline retreated up to 2 miles during this time – but the accumulation of seashells continued steadily, implying that locals found ways to adapt to the changing distribution of marine food resources to maintain their preferred diet.

The study also shows that the Ysterfontein 1 shell accumulated rapidly – perhaps around 1 meter (3 feet) every 1000 years – implying that the Middle Stone Age inhabitants along the southern coast -African extensively used marine resources, just as people did during the later Stone Age, and suggesting that effective marine foraging strategies developed early.

For dating eggshells are better

Ages can be linked to certain archaeological sites over 50,000 years old thanks to the argon-argon (40Ar / 39Ar) dating of the volcanic ash. But the ash is not always present. In Africa, however – and before the Holocene, throughout the Middle East and Asia – ostrich eggshells are common. Some sites even contain ostrich eggshell ornaments made by the early Homo sapiens.

Over the past four years, Sharp and Niespolo have conducted an extensive study of ostrich eggshells, including an analysis of modern eggshells obtained from an ostrich farm in Solvang, Calif., And developed a systematic way to avoid the uncertainties of previous analyzes. A key observation was that animals, including ostriches, do not absorb or store uranium, although it is common at parts per billion levels in most waters. They demonstrated that the newly laid ostrich shells do not contain uranium, but that it is absorbed after being buried in the ground.

The same is true of seashells, but their structure in calcium carbonate – a mineral called aragonite – is not as stable when buried in the ground as the calcite form of calcium carbonate found in the shell. of egg. As a result, eggshells better retain the uranium absorbed during the first hundred years or so for them to be buried. Bone, consisting primarily of calcium phosphate, has a mineral structure that also does not remain stable in most soil environments and does not reliably retain absorbed uranium.

Uranium is ideal for dating because it decays at a constant rate over time into an isotope of thorium which can be measured in trace amounts by mass spectrometry. The ratio of this thorium isotope to the uranium still present tells geochronologists how long the uranium has been in the eggshell.

The dating of the uranium series relies on uranium-238, the dominant uranium isotope in nature, which decays to thorium-230. In the protocol developed by Sharp and Niespolo, they used a laser to aerosolize small patches along a cross section of the shell and passed the aerosol through a mass spectrometer to determine its composition. They looked for places rich in uranium and not contaminated with a second isotope of thorium, thorium-232, which also invades eggshells after burial, but not so deeply. They collected more material from these areas, dissolved it in acid, and then analyzed it more precisely for uranium-238 and thorium-230 with “solution” mass spectrometry.

These procedures avoid some of the previous limitations of the technique, providing roughly the same accuracy as carbon 14, but over a 10 times greater time range.

“The key to this dating technique that we have developed that differs from previous attempts to date ostrich eggshells is the fact that we explicitly take into account that ostrich eggshells do not contain of primary uranium, so the uranium that we use for the date when the eggshells actually come from the pore water in the soil and the uranium is taken up by the eggshells during deposition, ”said Niespolo.

Working with Todd Dawson, professor of integrative biology at UC Berkeley, Niespolo also analyzed other isotopes in eggshells – stable isotopes of carbon, nitrogen and oxygen – to establish that the climate rapidly became drier and cooler during the occupation period, consistent with the known climate. changes at that time.

Niespolo, now a postdoctoral fellow at the California Institute of Technology but soon to be an assistant professor at Princeton University, is working with Sharp to meet middens at other sites near Ysterfontein. She is also developing the uranium series technique for use with other types of eggs, such as emus in Australia and rheas in South America, as well as the eggs of flightless birds now extinct. , such as the two-meter (6.6-foot) tall Genyornis, which became extinct about 50,000 years ago in Australia.

The work was supported by the Leakey Foundation, the Ann and Gordon Getty Foundation, and the National Science Foundation (BCS-1727085).

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