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

John P Hart,William A Lovis,Karine Taché,Juan J Loor

doi:10.1371/journal.pone.0196407

John P Hart, William A Lovis + Show 2 more

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https://doi.org/10.1371/journal.pone.0196407

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Abstract

Freshwater reservoir offsets (FROs) occur when AMS dates on charred, encrusted food residues on pottery predate a pot’s chronological context because of the presence of ancient carbon from aquatic resources such as fish. Research over the past two decades has demonstrated that FROs vary widely within and between water bodies and between fish in those water bodies. Lipid analyses have identified aquatic biomarkers that can be extracted from cooking residues as potential evidence for FROs. However, lacking has been efforts to determine empirically how much fish with FROs needs to be cooked in a pot with other resources to result in significant FRO on encrusted cooking residue and what percentage of fish C in a residue is needed to result in the recovery of aquatic biomarkers. Here we provide preliminary assessments of both issues. Our results indicate that in historically-contingent, high alkalinity environments <20% C from fish may result in a statistically significant FRO, but that biomarkers for aquatic resources may be present in the absence of a significant FRO.

Highlights

Pottery vessels and fragments thereof are mainstays of archaeological analyses worldwide
Fraction of modern carbon (FMC) measures on maize were used in the Freshwater reservoir offsets (FROs) formula for the fish caught in 2015 and 2016
Potential problems with 14C ages on charred cooking residues encrusted on pottery resulting from the presence of ancient carbon from aquatic resources has drawn considerable attention over the last decade and a half

Summary

Introduction

Pottery vessels and fragments thereof are mainstays of archaeological analyses worldwide. With the development of accelerator mass spectrometry (AMS) radiocarbon dating, the direct dating of charred cooking residues adhering to the interior surfaces of pots and sherds became a common method of obtaining direct age estimates [1,2]. Concern about the accuracy of such age estimates was prominently raised in the early 2000s [3,4,5]. This concern arose because of the potential for ancient carbon present in freshwater bodies to be metabolized by aquatic organisms and contribute to cooking residue formation when those organisms are subjected to water-based cooking. The presence of ancient carbon in the residues results in radiocarbon ages that are older than the pottery in question.

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