A Re-Evaluation of the Reliability of AMS Dates on Pottery Food Residues from the Late Prehistoric Central Plains of North America: Comment on Roper (2013)

Abstract

Ancient carbon reservoirs in freshwater bodies have the potential to introduce ancient carbon into charred cooking residues adhering to pottery wall interiors when aquatic organisms are parts of cooked resource mixes. This ancient carbon results in old apparent ages when these cooking residues are subjected to accelerator mass spectrometry dating, the so-called freshwater reservoir effect (FRE). Roper's (2013) assessment of the FRE on 14 C ages from cooking residue in the Central Plains is only the second such peer-reviewed regional assessment in eastern North America. Roper suggests that 13 of 23 14 C ages on residue are too old as a result of ancient carbon from fish or leached from shell temper or old carbon introduced via maize nixtamalization. Herein, we re-assess Roper's data set of 14 C ages on cooking residues and annual plants and argue that she is mistaken in her assessment of the accuracies of 14 C ages from residues. This outcome is placed in the context of the larger FRE literature.

Freshwater reservoir offsets and food crusts: Isotope, AMS, and lipid analyses of experimental cooking residues

“…The freshwater reservoir effect (FRE) resulting in freshwater reservoir offsets (FROs) is now well established in the literature [ 6 ]. Questions have been raised as to the implication of the FRE in certain site-specific and regional radiocarbon age datasets [ 7 , 8 ] and the use of current hydrological conditions to interpret the past [ 9 ]. However, concern remains as to the accuracy of radiocarbon dates obtained on cooking residues, especially when those dates do not match accepted regional chronologies [ 10 – 12 ].…”

A Response to Hart and Lovis

“…Many offsets are statistically significant; some are not. Hart and Lovis (2014) apparently do not accept the possibility that these offsets result from old carbon possibly incorporated into the food prepared in the pot but in any event incorporated into the residue at some point during or after the vessel's use as a cooking pot. Instead, they would see offsets as a product of context-specific occupation redundancy or other formation processes.…”

“…I also briefly review some work with a collaborator being undertaken to correct the obvious problem with age-offset dates on residue. Hart and Lovis (2014) have written what they describe as a "re-evaluation" of my analysis of the reliability of accelerator mass spectrometry (AMS) age determinations on ceramic residue from late prehistoric sites on the North American Central Plains and come to a different view than my reading of my results (Roper 2013a). Beyond being an isolated comment, it also is one of several papers from these authors and/or collaborators that champion the use of residue for AMS dating in archaeological studies (Schulenberg 2002;Hart and Brumbach 2005;Hart and Lovis 2007a; and is not the first paper in this series that seeks to refute or downplay conclusions that residue is not reliable (Hart and Lovis 2007b).…”

Aquatic Resources in Foodcrusts: Identification and Implication

“…Considerable progress has been made in the determining the presence (see Cramp and Evershed 2014 for a comprehensive review) and absence ) of marine resources in pottery vessels using lipid biomarker and single-compound carbon isotope determinations. In contrast, the processing of freshwater resources in pots has received less attention (see, however, Craig et al 2007;Hart et al 2013).…”

“…In few cases, however, has this been supported by lipid biomarker evidence. Hart et al (2013) have cautioned against the over-reliance on bulk carbon and/or nitrogen isotope data alone as a proxy for a freshwater reservoir effect. Using pottery and other data from the Finger Lakes region of the northeastern USA, the authors argued against a freshwater reservoir effect impacting on the 14 C dates obtained on foodcrusts.…”