Why Radiocarbon Dates on Bulk Sediment from Serpent Mound Are Problematic

Bradley T. Lepper1

In 2011, Edward Herrmann and colleagues (2014:119) collected 18 “continuous solid-earth cores” from various points along Serpent Mound from which they extracted small sediment samples for dating purposes. The samples included material from what they asserted was a buried A horizon as well as material incorporated into the mound fill. The resulting dates on “organic sediment” (actually soil humates) ranged from 2320 + 30 B.P. (Beta-337168) to 2170 + 30 B.P. (Beta-337163) for the supposed paleosol (2014:119). Samples from the mound fill yielded a similar range of from 2530 + 80 B.P. (Beta-337136) to 2180 + 30 B.P. (Beta-337167) (Herrmann et al. 2014:119).

Based on these data, Herrmann and colleagues (2014:124) concluded that Serpent Mound was “initially constructed 2,300 years ago during the Early Woodland (Adena) period.” They cited the “complete lack of post-Adena charcoal” in the soil cores they analyzed as “the strongest evidence for when the submound paleosol was buried and Serpent Mound construction began” (Herrmann et al. 2014:124).

In a series of responses to these claims, Lepper and various colleagues have disputed the conclusion that the dates reported by Herrmann and colleagues provide an accurate determination of the age of Serpent Mound (Lepper 2018, 2020; Lepper et al. 2018; Lepper et al. 2019). These arguments need not be repeated here, but recent research on the difficulties with obtaining accurate radiocarbon dates for burial mounds in Denmark independently validates the arguments of Lepper and colleagues and clarifies the nature of the problems with dates obtained on bulk sediment extracted from soil cores taken from mounds.

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