The LSU campus mounds, with construction beginning at ∼11,000 BP, are the oldest known extant man-made structures in the Americas

Abstract

Radiocarbon dating of the two LSU Campus Mounds (16EBR6) indicates that the construction of one, Mound B, began at ∼11,000 BP, making Mound B the oldest known and intact manmade structure in the Americas. The age analyses presented here are based on thirty one ¹⁴C dates. The older (deeper) parts of both of the LSU Campus Mounds contain many thin, burned ash lenses, suggesting that the Mounds may have been used for ceremonial or cremation purposes. These ash layers are composed mainly of phytoliths, bio-silicate (SiO₂) structural compounds in plants that remained after burning of these plants. Analysis of the abundant ash lenses indicates that the plants burned were mainly C4 hydrophilic grasses that are dominated by 90-98% reed and cane plants. The ash layers also contain microscopic fragments of burned, large mammal osteons (bone). The layers of reed and cane phytoliths, containing very small numbers of osteons, are indicative of very hot fires. This finding supports the argument that the fires were used for ceremonials or cremations. No ash beds later than 5,000 BP are known from either LSU Campus Mound A or B, although at ∼800 calBP, a wooden post (now charcoal) was planted and burned on the top of Mound B. It appears that construction of Mound B began during the climate amelioration that followed the Younger Dryas climate event, which ended at ∼11,700 BP. Construction of Mound A appears to have begun at ∼9,500 calBP. Building of the LSU Campus Mounds shows a hiatus when climate deteriorated during the 8200 Climate Event, which defined the end of the Holocene Greenlandian Stage and the beginning of the Northgrippian Stage. Construction began again at ∼7,500 BP, when both mounds continued construction until ∼6,000 BP, with one apparently anomalous date in Mound A at ∼5,100 calBP.