Cooper’s Ferry in Idaho: authors challenge ASI index inference of vertical mixing at the site
Authors argue that ASI, a statistical index built on binned artifact counts, does not support claims of large-scale mixing at the Idaho site, dated to ∼16,000 calibrated years BP.
A paper in Science Advances re-examines the Apparent Stratigraphic Integrity Index (ASI), a statistical measure used to infer postdepositional mixing at Late Pleistocene sites and to support a relatively late occupation of temperate North America. The authors tested it on Cooper’s Ferry/Nipéhe in Idaho, where the occupation is dated to ∼16,000 calibrated years before the present science.org
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ASI describes vertical discontinuity in artifact-frequency profiles split into bins. According to the authors, it does not directly measure artifact movement or stratigraphic integrity. At Cooper’s Ferry, the 5-centimeter absolute-elevation bins used for the calculation do not match the actual stratigraphic positions of artifacts across the irregular LU2 surface, on which LU3 loess accumulated.
The authors conclude that the resulting ASI value cannot support claims of large-scale vertical mixing. They cite independent chronological, stratigraphic, pedogenic, archaeological and excavation evidence for the integrity of the occupation. In their view, the site provides early inland evidence of the kind expected under coastal-migration models, and models should be evaluated against archaeological evidence rather than used to override it.
