A revised chronology of the lowest occupation layer of Pedra Furada Rock Shelter, Piauí, Brazil: The Pleistocene peopling of the Americas

The present work revisits the chronology of the archaeologically controversial Pedra Furada Rock Shelter of Southeast Piauí, Brazil, using an improved radiocarbon laboratory pre-treatment and measurements on charcoal samples. The procedure, known as ABOX-SC (acid-base-wet oxidation followed by stepped combustion), has previously been used to secure radiocarbon dates of >40 ka for the antiquity of human occupation of Australia and South Africa, and now has been applied to charcoal from the previously dated oldest occupation layer of the Pedra Furada site. Previous radiocarbon dating had obtained only lower limits of 40-45 ka BP for the Pedra Furada basal layer. Nine charcoal samples from well-structured hearths were subjected to the ABOX-SC procedure and their radiocarbon content determined by accelerator mass spectrometry. Measurements on five of the samples returned ages of greater than 56 ka BP, from graphites produced from ABOX pre-treated charcoal combusted at 910°C. Two other samples were greater than 50 ka BP. The remaining two samples were essentially completely combusted at 650°C, with no material surviving to make a 910°C CO₂ fraction. Their ages were 41.3 and 47.2ka BP. Ages obtained from graphites generated from the 650°C combusted fraction are considered minimum ages.