Amazing stuff! How exact a science is stratigraphy?
"It is among the most hotly contested topics in archaeology: When did people first arrive in the Americas? A few decades ago, there was consensus that people began populating North America no earlier than about 13,000 years ago, as evidenced by well-dated spearheads with characteristic fluted bases known as Clovis points, named for the city in New Mexico near the archaeological site where they were first identified in 1929.
Then, in the 1970s, an archaeologist named Tom Dillehay claimed to have dated stone tools and other artifacts found at a site in southern Chile to 14,500 years ago—a time when much of North America would have still be covered in impassable glaciers.
Few gave the claims from that site, known as Monte Verde, much credence until an independent cadre of researchers visited it in 1997 and confirmed the dates . The result sent shockwaves through the archaeological community and overturned the so-called “Clovis first” paradigm in favor of a new “pre-Clovis” one. This interpretation spurred scientists to retool their hypotheses for how and when people entered the continent; a coastal migration hypothesis has since gained popularity in its wake. Several other sites throughout the Americas have since been dated to before 13,000 years, and the matter appeared settled. ...
A study published last week in Science saw one such “pre-Clovis” skeptic revisiting Monte Verde—or sites close to it, as the original dig site has since been destroyed—and redating the sediment layers from which the supposedly human made artifacts were said to have come. ...
In the authors’ analysis, the key layers aren’t 14,500 years old, but instead only 8200 to 4200 years old. The confusion, they argued in the paper, was caused by younger sedimentary material being deposited below older layers by a river’s flow, which scrambled the region’s stratigraphy.
Why did it take so long to uncover this? “The simple answer is that the archaeological site is quite remote and was not visited by many archeologists, geomorphologists, or Quaternary geologists (studying the past 2.6 million years) who could have identified the prominent volcanic ash, the exposed wood bed that is the source for the 14,000-year-old radiocarbon ages, and identified that the site is situated on an abandoned floodplain that had to be younger than the landscape surface,” ...
Critically, these dates from Monte Verde have no impact on the other pre-Clovis sites that have been independently dated, but the brouhaha could lead to enhanced scrutiny and calls for their dates to be replicated. ..."
From the editor's summary and abstract:
"Editor’s summary
There is much debate surrounding the dates of human occupation in the Americas. One site that places South American timing in the pre-Clovis period is Monte Verde II in Chile. This site, however, was mostly only studied by a single group, meaning that replication of the dating has been lacking. Surovell et al. independently examined the site using current approaches and a broader scope and concluded that the site could not be older than 8200 years, significantly younger than the previously determined 14,500 years ...
Abstract
Our understanding of the timing of the human colonization of South America has been anchored by the Monte Verde II site in Chile, reported to date to ~14,500 years before the present (B.P.) and regarded as one of the most secure pre-Clovis archeological sites.
We report the first independent investigation of Monte Verde in the nearly 50 years since initial excavations. We argue that radiocarbon and luminescence dates from alluvial exposures, in combination with the identification of a tephra dated to 11,000 years B.P. stratigraphically underlying the archaeological component, suggest that Monte Verde cannot be older than the Middle Holocene (8200 to 4200 years B.P.). With colonization no longer anchored by Monte Verde, our revised chronology supports a more recent date of human arrival to South America."
A later debut for humans (Perspective, open access) "Stratigraphic analysis resets the time of human arrival in Monte Verde, Chile"
A mid-Holocene age for Monte Verde challenges the timeline of human colonization of South America (open access)
Fig. 1. Map of the study area.
Fig. 2. Ages of geologic deposits and the Monte Verde site.
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