The authors have declared that no competing interests exist.
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.
Pottery vessels and fragments thereof are mainstays of archaeological analyses worldwide. Because of uncertain chronological associations between these artifacts and spatially associated charred plant material and animal bone, the ability to directly date these vessels is important. 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 [
Concern about the accuracy of such age estimates was prominently raised in the early 2000s [
Experiments with water-based cooking have contributed to understanding how the contributions of C from varying resources affect residue formation [
In this article, we provide preliminary assessments of both issues. These were accomplished through cooking experiments with proportional mixes of fish and maize. AMS dates were obtained on fish of varying species from three lakes and one stream in New York. We used bulk isotope analyses to assess the proportion of C from fish that contributed to samples obtained from proportionally prepared mixes of dried fish and maize. We used subsamples for radiocarbon dating to determine what proportions of fish C in a residue resulted in significant FROs. We also extracted fatty acids from proportional mixes of maize and fish powders to determine when biomarkers for aquatic resources became evident. Our results provide important new insights into issues surrounding FROs from directly radiocarbon dating charred cooking residues.
Twenty-three fish and two maize samples were subjected to radiocarbon dating (
UCIAMS | Year | Species | Common Name | Location | δ13C |
FMC | D14C | 14C age (BP) | FRO |
FDC |
---|---|---|---|---|---|---|---|---|---|---|
153202 |
2014 | Lake Whitefish | Lake Ontario | -21.9 | 1.0167±0.0018 | 8.8±1.8 | modern | 26±14 | 0.0000 | |
166335 |
2015 | Common Rudd | Seneca Lake | -17.2 | 1.0215±0.0020 | 21.5±2.0 | modern | -7±16 | 0.0055 | |
166336 |
2015 | Common Rudd | Seneca Lake | -17.0 | 1.0264±0.0017 | 26.4±1.7 | modern | -40±13 | 0.0218 | |
166337 |
2015 | Common Rudd | Seneca Lake | -16.0 | 1.0199±0.0016 | 19.9±1.6 | modern | 6±12 | 0.0246 | |
179650 | 2016 | Rainbow Trout | Catherine Creek | -22.0 | 1.0197±0.0016 | 19.7±1.6 | modern | -12±12 | 0.0185 | |
179651 | 2016 | Rainbow Trout | Catherine Creek | -19.1 | 1.0126±0.0012 | 12.6±1.2 | modern | 44±9 | 0.0256 | |
179652 |
2016 | Lake Trout (1) | Cayuga Lake | -26.5 | 0.9960±0.0012 | -4.0±1.2 | 30±15 | 177±10 | 0.0238 | |
179653 | 2016 | Lake Trout (2) | Cayuga Lake | -27.7 | 0.9931±0.0012 | -6.9±1.2 | 55±15 | 200±9 | 0.0217 | |
179654 |
2016 | Lake Trout (3) | Cayuga Lake | -24.2 | 0.9994±0.0012 | -0.6±1.2 | 5±15 | 150±9 | 0.0265 | |
179655 |
2016 | Lake Trout (4) | Cayuga Lake | -24.9 | 0.9922±0.0012 | -7.8±1.2 | 65±15 | 208±10 | 0.0224 | |
179656 |
2016 | Lake Trout (5) | Cayuga Lake | -26.8 | 0.9939±0.0013 | -6.1±1.3 | 50±15 | 194±11 | 0.0209 | |
179657 |
2016 | Largemouth Bass | Cayuga Lake | -23.7 | 0.9967±0.0013 | -3.9±1.3 | 30±15 | 176±10 | 0.0275 | |
179658 | 2016 | Atlantic Salmon | Cayuga Lake | -27.5 | 0.9913±0.0012 | -8.7±1.2 | 70±15 | 215±10 | 0.028 | |
179659 | 2016 | Atlantic Salmon | Cayuga Lake | -25.2 | 0.9954±0.0012 | -4.6±1.2 | 35±15 | 182±9 | 0.0059 | |
179660 | 2016 | Atlantic Salmon | Cayuga Lake | -24.1 | 0.9969±0.0012 | -3.1±1.2 | 25±15 | 170±9 | 0.0000 | |
179661 | 2016 | Atlantic Salmon | Cayuga Lake | -24.9 | 0.9902±0.0012 | -9.8±1.2 | 80±15 | 224±10 | 0.0074 | |
179662 |
2016 | Chain Pickerel | Cayuga Lake | -20.9 | 0.9897±0.0013 | -10.3±1.3 | 85±15 | 228±11 | 0.0111 | |
179663 | 2016 | Rainbow Trout | Catherine Creek | -19.2 | 1.0122±0.0012 | 12.2±1.2 | modern | 47±9 | 0.006 | |
179664 | 2016 | White Sucker | Catherine Creek | -21.9 | 1.0288±0.0013 | 28.8±1.3 | modern | -83±10 | 0.0123 | |
179665 | 2016 | White Sucker | Catherine Creek | -20.6 | 1.0107±0.0013 | 10.7±1.3 | modern | 60±10 | 0.0000 | |
179666 | 2016 | White Sucker | Catherine Creek | -21.8 | 1.0069±0.0012 | 6.9±1.2 | modern | 90±10 | 0.0055 | |
179667 | 2016 | White Sucker | Catherine Creek | -23.6 | 1.0121±0.0012 | 12.1±1.2 | modern | 49±10 | 0.0218 | |
179668 | 2016 | White Sucker | Catherine Creek | -21.5 | 1.0057±0.0012 | 5.7±1.2 | modern | 99±10 | 0.0246 | |
166338 | 2015 | Maize | Washington Co. | -11.9 | 1.0205±0.0016 | 20.6±1.6 | modern | 0 | 0 | |
180884 | 2016 | Maize | Washington Co. | -11.9 | 1.0181±0.0016 | 18.2±1.6 | modern | 0 | 0 |
aUsed in 2014 proportional cooking experiments with maize meal.
bUsed in 2015 proportional cooking experiments with maize meal.
cUsed in 2016 proportional powder mixes with maize.
dUsed in 2016 proportional cooking experiments with maize kernels.
eMeasured to a precision <0.1‰.
fEquations from [
FRO = ‒8033*ln(FMCsample/FMCatmosphere)
FRO 1σ = ‒8033*ln((FMCsample)+(FMC1σ))+(8033*ln(FMCsample))
gFDC = (FMCmaize- FMCsample)/FMCmaize. Negative values round to 0
All fish were kept frozen until muscle tissue was sampled. The sampled muscle tissue was freeze dried before submission for AMS dating. Commercial cornmeal was used in the 2014 cooking experiments. Ears of Dent maize (
Forty-five 25g resource mixes using whole kernels or meal with fish muscle tissue (
Lab No. | Fish | Maize Form | Source | % Raw Fish | Residue |
% Fish C in residue |
---|---|---|---|---|---|---|
UCIAMS 185316 | Lake Trout 5 | kernel | liquid | 90 | -22.9 | 70.38 |
UCIAMS 185315 | Lake Trout 3 | kernel | liquid | 80 | -21.5 | 65.67 |
UCIAMS 185314 | Lake Trout 3 | kernel | liquid | 70 | -20.0 | 55.31 |
UCIAMS 185313 | Lake Trout 3 | kernel | liquid | 60 | -19.6 | 20.06 |
UCIAMS 185312 | Lake Trout 3 | kernel | liquid | 50 | -17.5 | 38.62 |
UCIAMS 185311 | Lake Trout 3 | kernel | liquid | 40 | -17.6 | 38.74 |
UCIAMS 185310 | Lake Trout 3 | kernel | liquid | 30 | -15.2 | 22.71 |
UCIAMS 185309 | Lake Trout 3 | kernel | liquid | 20 | -13.8 | 12.96 |
UCIAMS 185308 | Lake Trout 5 | kernel | liquid | 10 | -13.2 | 8.12 |
UCIAMS 185325 | Largemouth Bass | kernel | liquid | 90 | -22.9 | 92.92 |
UCIAMS 185324 | Largemouth Bass | kernel | liquid | 80 | -20.2 | 69.76 |
UCIAMS 185323 | Largemouth Bass | kernel | liquid | 70 | -19.3 | 62.75 |
UCIAMS 185322 | Largemouth Bass | kernel | liquid | 60 | -17.5 | 47.55 |
UCIAMS 185321 | Largemouth Bass | kernel | liquid | 50 | -17.4 | 46.79 |
UCIAMS 185320 | Largemouth Bass | kernel | liquid | 40 | -16.4 | 37.68 |
UCIAMS 185319 | Largemouth Bass | kernel | liquid | 30 | -15.1 | 27.21 |
UCIAMS 185318 | Largemouth Bass | kernel | liquid | 20 | -17.4 | 46.11 |
UCIAMS 185317 | Largemouth Bass | kernel | liquid | 10 | -13.9 | 16.90 |
UCB ISO-17-04_C5 | Lake Trout 5 | kernel | wall | 90 | -25.4 | 92.60 |
UCB ISO-17-04 rerun_A3 | Lake Trout 3 | kernel | wall | 80 | -26.0 | 96.40 |
UCB ISO-17-04_A6 | Lake Trout 3 | kernel | wall | 70 | -25.5 | 93.25 |
UCB ISO-17-04_C2 | Lake Trout 3 | kernel | wall | 60 | -24.8 | 84.83 |
UCB ISO-17-04_B1 | Lake Trout 3 | kernel | wall | 50 | -24.3 | 73.77 |
UCB ISO-17-04_C4 | Lake Trout 3 | kernel | wall | 40 | -22.7 | 76.75 |
UCB ISO-17-04_A9 | Lake Trout 3 | kernel | wall | 30 | -23.1 | 60.18 |
UCB ISO-17-04_C12 | Lake Trout 3 | kernel | wall | 20 | -20.7 | 90.96 |
UCB ISO-17-04_B5 | Lake Trout 5 | kernel | wall | 10 | -25.2 | 92.60 |
UCB ISO-17-04_A2 | Largemouth Bass | kernel | wall | 90 | -24.0 | 102.55 |
UCB ISO-17-04_C9 | Largemouth Bass | kernel | wall | 80 | -23.5 | 97.63 |
UCB ISO-17-04_A10 | Largemouth Bass | kernel | wall | 70 | -23.6 | 98.93 |
UCB ISO-17-04_A4 | Largemouth Bass | kernel | wall | 60 | -23.1 | 94.27 |
UCB ISO-17-04_A3 | Largemouth Bass | kernel | wall | 50 | -22.3 | 87.53 |
UCB ISO-17-04_A11 | Largemouth Bass | kernel | wall | 40 | -22.2 | 86.76 |
UCB ISO-17-04_A5 | Largemouth Bass | kernel | wall | 30 | -21.1 | 77.56 |
UCB ISO-17-04_B10 | Largemouth Bass | kernel | wall | 20 | -23.7 | 92.14 |
UCB ISO-17-04_B4 | Largemouth Bass | kernel | wall | 10 | -22.8 | 102.55 |
UCIAMS 166316 | Common Rudd 1 | meal | liquid | 90 | -13.3 | 27.11 |
UCIAMS 166317 | Common Rudd 1 | meal | liquid | 70 | -12.1 | 4.85 |
UCIAMS 166318 | Common Rudd 1 | meal | liquid | 50 | -12.1 | 4.45 |
UCIAMS 166319 | Common Rudd 1 | meal | liquid | 30 | -12.1 | 3.55 |
UCIAMS 166320 | Common Rudd 1 | meal | liquid | 10 | 12.0 | 2.28 |
UCIAMS 166321 | Common Rudd 2 | meal | liquid | 90 | -13.2 | 26.99 |
UCIAMS 166322 | Common Rudd 2 | meal | liquid | 80 | -12.6 | 14.86 |
UCIAMS 166323 | Common Rudd 2 | meal | liquid | 70 | -12.3 | 8.73 |
UCIAMS 166324 | Common Rudd 2 | meal | liquid | 60 | -12.2 | 6.00 |
UCIAMS 166325 | Common Rudd 2 | meal | liquid | 50 | -12.0 | 2.27 |
UCIAMS 166326 | Common Rudd 2 | meal | liquid | 40 | -12.1 | 4.03 |
UCIAMS 166327 | Common Rudd 2 | meal | liquid | 30 | -12.1 | 3.85 |
UCIAMS 166328 | Common Rudd 2 | meal | liquid | 20 | -12.1 | 3.88 |
UCIAMS 166329 | Common Rudd 2 | meal | liquid | 10 | -11.9 | 0.00 |
UCIAMS 166330 | Common Rudd 3 | meal | liquid | 90 | -13.3 | 35.36 |
UCIAMS 166331 | Common Rudd 3 | meal | liquid | 70 | -12.2 | 7.69 |
UCIAMS 166332 | Common Rudd 3 | meal | liquid | 50 | -12.1 | 5.39 |
UCIAMS 166333 | Common Rudd 3 | meal | liquid | 30 | -12.1 | 5.65 |
UCIAMS 166334 | Common Rudd 3 | meal | liquid | 10 | -11.8 | 0.00 |
UCB ISO-16-03_Tray2_C2 | Common Rudd 2 | meal | wall | 90 | -13.3 | 27.13 |
UCB ISO-16-03_Tray2_C4 | Common Rudd 2 | meal | wall | 80 | -13.7 | 35.55 |
UCB ISO-16-03_Tray2_C3 | Common Rudd 2 | meal | wall | 70 | -13.0 | 21.79 |
UCB ISO-16-03_Tray2_B10 | Common Rudd 2 | meal | wall | 60 | -12.3 | 7.62 |
UCB ISO-16-03_Tray2_B8 | Common Rudd 2 | meal | wall | 50 | -12.2 | 5.60 |
UCB ISO-16-03_Tray2_C1 | Common Rudd 2 | meal | wall | 40 | -12.5 | 11.57 |
UCB ISO-16-03_Tray2_B9 | Common Rudd 2 | meal | wall | 30 | -12.3 | 7.68 |
UCB ISO-16-03_Tray2_B11 | Common Rudd 2 | meal | wall | 20 | -12.1 | 3.08 |
UCB ISO-16-03_Tray2_B12 | Common Rudd 2 | meal | wall | 10 | -12.4 | 10.16 |
UCIAMS 153203 | Whitefish | meal | wall | 80 | -15.3 | 39.27 |
UCIAMS 153204 | Whitefish | meal | wall | 70 | -12.4 | 12.58 |
UCIAMS 153205 | Whitefish | meal | wall | 60 | -14.2 | 29.12 |
UCIAMS 153206 | Whitefish | meal | wall | 50 | -14.5 | 32.15 |
UCIAMS 153207 | Whitefish | meal | wall | 40 | -13.1 | 19.71 |
UCIAMS 153208 | Whitefish | meal | wall | 30 | -12.0 | 9.36 |
UCIAMS 153209 | Whitefish | meal | wall | 20 | -11.5 | 4.34 |
UCIAMS 153210 | Whitefish | meal | wall | 10 | -10.9 | 0.00 |
aMass Balance:(δ13Csample ‒ δ13Cmaize)/(δ13Cfish ‒ δ13Cmaize))*100
Freeze-dried Lake Trout and Chain Pickerel muscle tissue and dried whole maize kernels were ground into 0.5 mm powders. These were mixed in 10% increments in 1g samples (10% and 90% maize to 90% fish and 10% maize by weight). Each thoroughly mixed sample (~0.025 g) was subsampled for AMS dating, and the remainder was subjected to lipid analyses.
All samples for AMS dating were submitted to the W. M. Keck Carbon Cycle Accelerator Mass Spectrometry Laboratory at the University of California-Irvine. These included samples from the 23 fish, two maize kernels, 18 cooking experiments, and 18 resource powder mixes. Protocols for AMS dating modern samples are documented on the laboratory’s website (
Fish-maize powder mixes were subjected to lipid analysis to determine when biomarkers for aquatic resources become evident. Liquid samples obtained after the cooking experiment were also analyzed but lipid yields obtained were too low to be interpretable (<0.5μg/mg of lipids per mg of residue sample). As described above, the samples consisted of freeze-dried fish muscle tissue and dried whole maize kernels ground into 0.5 mm powders. These were mixed in 10-percent increments in 1 g samples (10% and 90% maize to 10% fish and 90% maize by weight). To generate ω-(o-alkylphenyl)alkanoic acids, sterile clay powder was added to the fish-maize mixes [
Lipids were extracted from the 21 samples by direct methylation with acidified methanol to maximize recovery [
Results of the AMS dating of the fish samples are presented in
Twelve of the samples returned modern ages. FROs for four of these ages are negative, while the others range from 6±12 to 99±10 14Cyr. Fish obtained from Cayuga Lake during September of 2016 were the only ones to produce non-modern 14C ages. The largest offsets for these fish ranged from 150±9 to 228±11 14Cyr. In total, the variation in FROs on fish were consistent with results obtained in other parts of the world, but which can range to >1,000 14Cyr [
Previous experiments have investigated the relationship of resource mixes to bulk δ13C values on residues to determine if those values can be used to detect if maize was cooked in a given pot [
Mass balance using δ13C values on charred residues cannot be used to determine the percentage of raw maize cooked in a pot as suggested by Morton and Schwarcz [
The mass balance formula was used here with δ13C and FMC values to determine the percent of fish C in the 72 experimental residues and 18 fish-maize powder mixtures (Tables
UCIAMS# | % Raw Fish | δ13C (‰) | FMC | D14C (‰) | 14C age (BP) | FRO | % Fish C δ13C |
% Fish C FMC |
FDC |
---|---|---|---|---|---|---|---|---|---|
179657 | 100 | -23.7±.01 | 0.9967±0.0013 | -3.9±1.3 | 30±15 | 176±10 | 100 | 100 | 0.0275 |
185325 | 90 | -22.9±.01 | 0.9962±0.0012 | -3.8±1.2 | 30±15 | 176±10 | 92.92 | 99.05 | 0.0216 |
185324 | 80 | -20.2±.01 | 0.9980±0.0016 | -2.0±1.6 | 15±15 | 161±13 | 69.76 | 90.66 | 0.0198 |
185323 | 70 | -19.3±.01 | 1.0015±0.0013 | 1.5±1.3 | >Modern | 133±10 | 62.75 | 75.06 | 0.0164 |
185322 | 60 | -17.5±.01 | 1.0042±0.0013 | 4.2±1.3 | >Modern | 111±10 | 47.55 | 62.81 | 0.0137 |
185321 | 50 | -17.4±.01 | 1.0043±0.0013 | 4.3±1.3 | >Modern | 110±10 | 46.79 | 62.52 | 0.0137 |
185320 | 40 | -16.4±.01 | 1.0077±0.0013 | 7.7±1.3 | >Modern | 83±10 | 37.68 | 47.02 | 0.1030 |
185319 | 30 | -15.1±.01 | 1.0084±0.0014 | 8.4±1.4 | >Modern | 78±11 | 27.21 | 44.11 | 0.0096 |
185318 | 20 | -17.4±.01 | 1.0016±0.0019 | 1.6±1.9 | >Modern | 132±15 | 46.11 | 74.67 | 0.0163 |
185317 | 10 | -13.9±.01 | 1.0130±0.0016 | 13.0±1.6 | >Modern | 41±12 | 16.90 | 23.50 | 0.0051 |
180884 | 0 | -11.9±.01 | 1.0182±0.0016 | 18.2±1.6 | >Modern | 0 | 0 | 0 | 0 |
179654c | 100 | -24.2±.01 | 0.9994±0.0012 | -0.6±1.2 | 5±15 | 150±9 | 100 | 100 | 0.0265 |
179656d | 100 | -26.8±.01 | 0.9939±0.0013 | -6.1±1.3 | 50±15 | 194±11 | 100 | 100 | 0.0209 |
185316 d | 90 | -22.9±.01 | 0.9996±0.0013 | -0.4±1.3 | 5±15 | 148±10 | 70.38 | 83.49 | 0.0182 |
185315c | 80 | -21.5±.01 | 1.0007±0.0013 | 0.7±1.3 | 0±15 | 139±10 | 65.67 | 78.54 | 0.0172 |
185314c | 70 | -20.0±.01 | 1.0031±0.0013 | 3.1±1.3 | >Modern | 120±10 | 55.31 | 67.88 | 0.0148 |
185313c | 60 | -19.6±.01 | 0.9932±0.0013 | -6.8±.1.3 | 55±15 | 200±10 | 20.06 | 112.57 | 0.0246 |
185312c | 50 | -17.5±.01 | 1.0074±0.0013 | 7.4±1.3 | >Modern | 86±10 | 38.62 | 48.77 | 0.0107 |
185311c | 40 | -17.6±.01 | 1.0081±0.0013 | 8.1±1.3 | >Modern | 80±11 | 38.74 | 45.33 | 0.0099 |
185310c | 30 | -15.2±.01 | 1.0110±0.0013 | 11.0±1.3 | >Modern | 57±10 | 22.71 | 32.14 | 0.0070 |
185309c | 20 | -13.8±.01 | 1.0160±0.0013 | 16.0±1.3 | >Modern | 17±10 | 12.96 | 9.68 | 0.0021 |
185308d | 10 | -13.2±.01 | 1.0159±0.0013 | 15.9±1.3 | >Modern | 18±11 | 8.12 | 10.40 | 0.0023 |
180884 | 0 | -11.9±.01 | 1.0182±0.0016 | 18.2±1.6 | >Modern | 0 | 0 | 0 | 0 |
Mass balance: a(δ13Csample ‒ δ13Cmaize)/(δ13Cfish ‒ δ13Cmaize))*100
b(FMCsample ‒ FMCmaize)/(FMCfish ‒ FMCmaize))*100
cLake Trout 3. dLake Trout 5.
UCIAMS# | % Raw Fish | δ13C (‰) | FMC | D14C (‰) | 14C age (BP) | FRO | % Fish C δ13C |
% Fish C FMC |
FDC |
---|---|---|---|---|---|---|---|---|---|
179662 | 100 | -20.9±.01 | 0.9897±0.0013 | -10.3±1.3 | 85±15 | 228±11 | 100 | 100 | 0.0280 |
180867 | 90 | -20.1±.01 | 0.9904±0.0016 | -9.6±1.6 | 75±15 | 222±13 | 91.64 | 97.64 | 0.0273 |
180868 | 80 | -20.0±.01 | 0.9938±0.0017 | -6.2±1.7 | 50±15 | 195±14 | 90.39 | 85.77 | 0.0240 |
180996 | 70 | -18.8±.01 | 0.9982±0.0016 | -1.8±1.6 | 15±15 | 160±13 | 77.25 | 70.41 | 0.0197 |
180869 | 60 | -18.3±.01 | 0.9972±0.0016 | -2.8±1.6 | 25±15 | 167±13 | 70.87 | 73.78 | 0.0206 |
180870 | 50 | -17.2±.01 | 1.0018±0.0020 | 1.8±2.0 | modern | 130±16 | 59.15 | 57.65 | 0.0161 |
180871 | 40 | -17.0±.01 | 0.9988±0.0017 | -1.2±1.7 | 10±15 | 154±14 | 56.61 | 68.145 | 0.0191 |
180872 | 30 | -15.0±.01 | 1.0084±0.0016 | 8.4±1.6 | modern | 77±13 | 34.83 | 34.31 | 0.0096 |
180873 | 20 | -15.1±.01 | 1.0092±0.0018 | 9.2±1.8 | modern | 71±15 | 35.80 | 31.60 | 0.0088 |
180874 | 10 | -13.0±.01 | 1.0154±0.0016 | 15.4±1.6 | modern | 22±13 | 12.65 | 9.75 | 0.0027 |
180884 | 0 | -11.9±.01 | 1.0182±0.0016 | 18.2±1.6 | modern | 0 | 0 | 0 | 0 |
179656 | 100 | -24.9±.01 | 0.9922±0.0012 | -7.8±1.2 | 65±15 | 208±10 | 100 | 100 | 0.0255 |
180875 | 90 | -24.4±.01 | 0.9925±0.0019 | -7.5±1.9 | 60±20 | 205±16 | 96.30 | 98.67 | 0.0252 |
180876 | 80 | -23.6±.01 | 0.9923±0.0016 | -7.7±1.6 | 60±15 | 207±13 | 90.06 | 99.63 | 0.0254 |
180877 | 70 | -22.3±.01 | 0.9984±0.0016 | -1.6±1.6 | 15±15 | 158±13 | 80.03 | 76.19 | 0.0195 |
180878 | 60 | -21.3±.01 | 0.9991±0.0018 | -0.9±1.8 | 5±15 | 152±15 | 71.96 | 73.46 | 0.0188 |
180879 | 50 | -19.4±.01 | 1.0036±0.0017 | 3.6±1.7 | modern | 116±14 | 57.73 | 55.96 | 0.0143 |
180880 | 40 | -18.0±.01 | 1.0065±0.0016 | 6.5±1.6 | modern | 93±13 | 46.61 | 45.05 | 0.0115 |
180881 | 30 | -16.7±.01 | 1.0082±0.0016 | 8.2±1.6 | modern | 79±13 | 36.96 | 38.39 | 0.0098 |
180882 | 20 | -18.1±.01 | 1.0032±0.0016 | 3.2±1.6 | modern | 119±13 | 47.36 | 57.58 | 0.0147 |
180883 | 10 | -14.2±.01 | 1.0135±0.0016 | 13.5±1.6 | modern | 37±13 | 17.35 | 17.96 | 0.0046 |
180884 | 0 | -11.9±.01 | 1.0182±0.0016 | 18.2±1.6 | modern | 0 | 0 | 0 | 0 |
Mass balance: a(δ13Csample ‒ δ13Cmaize)/(δ13Cfish ‒ δ13Cmaize))*100
b(FMCsample ‒ FMCmaize)/(FMCfish ‒ FMCmaize))*100.
Consistent with previous results [
Fish | Maize Form | Sample | Percent Raw Fish (wt) | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | |||
Largemouth Bass | Kernel | Liquid | 20.61 | 43.23 | 29.63 | 39.45 | 50.31 | 59.22 | 66.67 | 73.82 | 93.73 |
Lake Trout | Kernel | Liquid | 8.92 | 14.57 | 24.12 | 39.96 | 40.25 | 53.82 | 58.89 | 66.27 | 83.30 |
Common Rudd 1 | Meal | Liquid | 2.28 | — | 3.55 | — | 4.45 | — | 4.85 | — | 27.11 |
Common Rudd 2 | Meal | Liquid | 0.00 | 3.88 | 3.85 | 4.03 | 2.27 | 6.00 | 8.73 | 14.86 | 26.99 |
Common Rudd 3 | Meal | Liquid | 0.00 | — | 5.65 | — | 5.39 | — | 7.69 | — | 35.36 |
Largemouth Bass | Kernel | Wall | 92.14 | 99.50 | 77.56 | 86.76 | 87.53 | 94.27 | 98.93 | 97.63 | 100.00 |
Lake Trout | Kernel | Wall | 90.96 | 60.18 | 76.75 | 73.77 | 84.83 | 88.30 | 93.25 | 96.40 | 92.10 |
Common Rudd 2 | Meal | Wall | 10.16 | 3.089 | 7.68 | 11.57 | 5.60 | 7.62 | 21.79 | 33.55 | 27.13 |
Lake Whitefish | Meal | Wall | — | 0.00 | 4.34 | 9.36 | 19.71 | 32.15 | 29.12 | 12.58 | 39.27 |
AMS dates were obtained on the 18 liquid residue samples and 18 maize-fish powder mixes to assess when the contribution of fish C to residue formation may result in significant FROs (Tables
There was a very high positive correlation (r = 0.989) between the fraction of fish C in the residues and Fraction Dead Carbon (FDC;
Error (±yr) | Significant FRO (yr) | FDC | % Residue Fish C | Percent Raw Fish (wt) | |||
---|---|---|---|---|---|---|---|
Wall-Meal | Wall-Kernel | Liquid-Meal | Liquid-Kernel | ||||
15 | 42 | 0.00519 | 18.18 | 46.33 | 0.94 | 46.62 | 19.38 |
20 | 56 | 0.00690 | 28.84 | 55.74 | 1.66 | 55.03 | 30.58 |
35 | 97 | 0.01190 | 44.19 | 78.27 | 2.86 | 64.56 | 46.70 |
50 | 139 | 0.01705 | 64.15 | 88.72 | 4.97 | 73.37 | 67.65 |
65 | 181 | 0.02220 | 84.20 | 93.11 | 9.07 | 79.42 | 88.74 |
As is evident from the data in
For wall scraped samples an age estimate with a typical 15-yr error term requires 18.18% fish C in the residue. This is realized when there is between 0.94% and 46.33% raw fish content in the mix for wall-scraped residues dependent on maize in whole kernel and meal form, respectively.
Wall scraped samples with a less-typical 65-yr error require 84.20% fish C in a residue. Depending on the form of maize in the mix this is realized with between 9.07% and 91.11% raw fish in the mix, respectively.
Results for the liquid-derived samples, representing potential C contribution to residue formation, can be understood as follows:
For a 15-yr error, the mix would require between 46.62% and 19.38% raw fish for maize in meal and kernel form, respectively, to result in a statistically significant FRO.
For a 65-yr error term, the mix would require from 79.42% to 88.74% raw fish, with maize meal and kernels, respectively, to result in a statistically significant FRO.
These results demonstrate that it is possible for resource mixes including very little fish C, as low as 0.94%, to result in a statistically significant FRO depending on the form of the other resources in the mix and the radiocarbon date error term. It is also possible that fish must constitute the bulk of the resources being cooked depending on the same variables.
Certain trends in fatty acid ratios are evident as the proportions of fish in unheated samples diminish, such as a general decrease in C14:0, C16:1, C16:0, C18:0 and C18:1, and an increase in C18:2 (
Gas chromatograms of lipid extracts from unheated maize-fish powder mixes consisting of 90% Lake Trout and 10% maize (A) and 10% Lake Trout and 90% maize (B). Cn:x are fatty acids with carbon length n and number of unsaturations x; br are branched-chain acids; IS is internal standard (n-hexatriacontane).
Gas chromatograms of lipid extracts from heated maize-fish powder mixes consisting of 90% Chain Pickerel and 10% maize (A) and 10% Chain Pickerel and 90% maize (B). Cn:x are fatty acids with carbon length n and number of unsaturations x; br are branched-chain acids; APFA Cx are ω‐(o‐alkylphenyl) alkanoic acids with carbon length x.
Cx:y = fatty acids with carbon length x and number of unsaturations y (C18:1s and C18:2s are the sum of all isomers); br = branched chain acids; TMTD = 4,8,12‐trimethyltridecanoic acid, chol = cholesterol, stig = stigmasterol.
Percentages of Lake Trout | ||||||||||
Raw resource based on weight | 90.0 | 80.0 | 70.0 | 60.0 | 50.0 | 40.0 | 30.0 | 20.0 | 10.0 | |
Residue based on δ13C |
96.3 | 90.1 | 80.0 | 72.0 | 47.4 | 57.7 | 46.6 | 37.0 | 17.3 | |
Residue based on FMC |
98.7 | 99.6 | 76.2 | 73.5 | 57.6 | 55.6 | 45.1 | 38.4 | 18.0 | |
Fatty Acids (relative %) | C14:0 | 4.96 | 4.13 | 3.40 | 2.64 | 4.09 | 1.73 | 1.39 | 0.93 | 0.47 |
C15br | 0.57 | 0.67 | 0.55 | 0.31 | 0.45 | 0.21 | 0.16 | 0.10 | 0.04 | |
C15:0 | 0.60 | 0.52 | 0.43 | 0.32 | 0.43 | 0.21 | 0.17 | 0.11 | 0.06 | |
C16br | 0.34 | 0.31 | 0.31 | 0.23 | trace | 0.17 | 0.15 | 0.09 | 0.00 | |
C16:1 | 10.52 | 9.04 | 7.50 | 5.84 | 8.76 | 4.08 | 3.13 | 2.28 | 1.12 | |
C16:0 | 28.20 | 25.39 | 21.45 | 20.74 | 22.56 | 17.09 | 16.86 | 16.30 | 15.84 | |
C17br | 1.03 | 0.63 | 0.48 | 1.10 | 1.77 | 0.73 | 0.20 | 0.41 | 0.21 | |
C17:1 | 0.79 | 1.13 | 0.92 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | |
C17:0 | 0.59 | 0.56 | 0.43 | 0.36 | 0.52 | 0.24 | 0.20 | 0.16 | 0.09 | |
C18:3 | 0.78 | 1.01 | 1.19 | 3.15 | 4.13 | 1.25 | 1.29 | 1.37 | 1.83 | |
C18:2s | 6.22 | 9.46 | 14.44 | 19.65 | 3.39 | 28.93 | 35.29 | 40.99 | 47.68 | |
C18:1s | 33.77 | 34.12 | 30.86 | 30.57 | 32.18 | 27.30 | 26.98 | 25.76 | 24.57 | |
C18:0 | 4.79 | 4.63 | 3.45 | 3.19 | 4.59 | 2.38 | 2.24 | 1.95 | 1.62 | |
C19:0 | trace | trace | trace | trace | trace | trace | trace | 0.05 | 0.00 | |
C20:3 | 1.93 | 2.23 | 4.37 | 3.05 | 4.56 | 4.27 | 4.12 | 3.96 | 3.20 | |
C20:2 | 0.59 | 0.87 | 1.14 | 1.02 | 1.45 | 0.96 | 0.88 | 0.10 | trace | |
C20:1 | 1.02 | 1.79 | 1.59 | 0.87 | 1.27 | 0.91 | 0.76 | 0.56 | 0.45 | |
C20:0 | 0.71 | 0.16 | 0.15 | trace | trace | 0.17 | 0.18 | 0.16 | 0.21 | |
C22:2 | 0.27 | 1.33 | 4.81 | 5.93 | 8.67 | 8.38 | 5.12 | 4.03 | 2.11 | |
C22:1 | trace | 0.20 | 0.24 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | |
C22:0 | trace | trace | trace | trace | 0.00 | trace | trace | trace | trace | |
C24:1 | trace | 0.17 | 0.15 | trace | 0.00 | trace | trace | trace | trace | |
C24:0 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 | trace | trace | trace | trace | |
Biomarkers | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | |
chol | chol | chol | chol | chol | chol | chol | chol | chol | ||
stig | stig | stig | stig |
aCalculated with mass balance equation.
Cx:y = fatty acids with carbon length x and number of unsaturations y (C18:1s and C18:2s are the sum of all isomers); br = branched chain acids; APFA Cx = ω‐(o‐alkylphenyl) alkanoic acids with carbon length x; chol = cholesterol.
Percentages of Chain Pickerel | ||||||||||
Raw resource based on weight | 90.0 | 80.0 | 70.0 | 60.0 | 50.0 | 40.0 | 30.0 | 20.0 | 10.0 | |
Residue based on δ13C |
91.6 | 90.4 | 77.3 | 70.9 | 59.2 | 56.6 | 34.8 | 35.8 | 12.7 | |
Residue based on FMC |
97.6 | 85.8 | 70.4 | 73.8 | 57.6 | 68.1 | 34.3 | 31.6 | 9.7 | |
Fatty Acids (relative %) | C14:0 | 2.66 | 2.65 | 1.94 | 2.02 | 2.01 | 1.72 | 1.65 | 1.81 | 2.15 |
C15:0 | 0.89 | 0.72 | 0.59 | 0.51 | trace | trace | trace | trace | trace | |
C16:1 | 2.32 | 2.12 | 1.99 | 0.93 | 0.62 | 0.63 | 0.00 | 0.00 | 0.00 | |
C16:0 | 33.40 | 33.15 | 31.73 | 32.67 | 29.73 | 32.16 | 33.66 | 35.51 | 33.53 | |
C17br | 1.84 | 1.58 | 1.31 | 1.27 | 1.31 | 1.09 | 0.44 | trace | 0.59 | |
C17:0 | 1.48 | 1.28 | 0.94 | 1.17 | 1.43 | 1.10 | 1.15 | 0.96 | 1.40 | |
C18:2s | 0.00 | 0.00 | 0.00 | 0.39 | 1.05 | 0.99 | 0.90 | 0.89 | 1.07 | |
C18:1s | 18.57 | 21.35 | 26.75 | 24.47 | 21.62 | 23.39 | 22.96 | 25.75 | 21.27 | |
C18:0 | 23.16 | 20.27 | 14.29 | 22.59 | 33.10 | 24.82 | 28.08 | 24.40 | 34.56 | |
C20:1 | 0.75 | 0.79 | 0.77 | 0.63 | 0.46 | 0.73 | 0.66 | 0.82 | 0.82 | |
C20:0 | 1.13 | 1.14 | 1.03 | 1.26 | 1.00 | 1.52 | 1.55 | 1.71 | 1.92 | |
C22:0 | 0.36 | 0.38 | 0.49 | 0.42 | 0.34 | 0.48 | 0.51 | 0.59 | 0.43 | |
C23:0 | trace | trace | trace | trace | trace | 0.32 | trace | trace | trace | |
C24:1 | 1.38 | 1.20 | 1.52 | 0.91 | trace | trace | trace | 0.00 | 0.00 | |
C24:0 | 0.63 | 0.47 | 0.80 | 0.84 | trace | 0.66 | 0.58 | 0.58 | 0.00 | |
Biomarkers | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | TMTD | |
phytanic | phytanic | phytanic | phytanic | phytanic | phytanic | phytanic | phytanic | phytanic | ||
APFA C18 | APFA C18 | APFA C18 | APFA C18 | APFA C18 | APFA C18 | APFA C18 | APFA C18 | APFA C18 | ||
APFA C20 | APFA C20 | APFA C20 | APFA C20 | APFA C20 | APFA C20 | APFA C20 | APFA C20 | APFA C20 | ||
chol | chol | chol | chol | chol | chol | chol |
aCalculated with mass balance equation.
The presence of ω-(o-alkylphenyl)alkanoic acids with 18 and 20 carbon atoms, together with at least one of the three isoprenoid fatty acids (phytanic, pristanic or 4,8,12-tetramethyltridecanoic acid) have been established in the literature as the full set of molecular criteria needed for the identification of degraded aquatic products in archaeological residues [
In this experiment, we were able to confirm the formation of ω-(o-alkylphenyl)alkanoic acids with 16–20 carbon atoms when food sources containing unsaturated fatty acids are exposed to prolonged and intense heating (270°C for 17 hours) in the presence of a clay matrix. Analysis of single ingredients (e.g., dried maize, dried Chain Pickerel, and dried Lake Trout) confirmed that degraded aquatic and plant oils cannot be distinguished based on the presence of ω-(o-alkylphenyl)octadecanoic acids alone (
Partial m/z 105 ion chromatograms showing ω-(o-alkylphenyl)alkanoicacids with 16(+), 18(*), and 20(#) carbon atoms in heated samples containing lake trout (A), chain pickerel (B) and maize (C).
Cx:y = fatty acids with carbon length x and number of unsaturations y (C18:1s and C18:2s are the sum of all isomers); br = branched chain acids; TMTD = 4,8,12‐trimethyltridecanoic acid; APFA Cx = ω‐(o‐alkylphenyl) alkanoic acids with carbon length x.
Compounds | Maize | Chain Pickerel | Lake Trout | |
---|---|---|---|---|
Fatty Acids (relative %) | C12 | 0.00 | 0.00 | trace |
C13 | 0.00 | 0.00 | trace | |
C14 | 0.00 | 1.93 | 4.76 | |
C15br | 0.00 | 0.70 | 2.03 | |
C15 | 0.00 | 4.95 | 8.79 | |
C16:1 | 0.00 | 2.60 | 3.11 | |
C16 | 38.41 | 27.87 | 29.43 | |
C17br | 0.00 | 2.72 | 3.82 | |
C17 | 0.00 | 1.49 | 1.86 | |
C18:2s | 1.98 | 0.00 | 0.00 | |
C18:1s | 22.99 | 16.15 | 18.77 | |
C18 | 12.26 | 13.38 | 12.80 | |
C20:1 | 0.00 | 1.31 | 2.53 | |
C20 | 3.74 | 0.75 | 1.15 | |
C22:1 | 0.00 | trace | 0.55 | |
C22 | 3.12 | trace | 0.40 | |
C23 | 0.00 | 0.00 | trace | |
C24:1 | 0.00 | 2.95 | 0.79 | |
C24 | 0.00 | 0.63 | trace | |
Biomarkers | TMTD | TMTD | ||
phytanic | phytanic | |||
APFA C18 | APFA C18 | APFA C18 | ||
APFA C20 | APFA C20 | |||
Chol | Chol |
Significantly, the complete set of aquatic biomarkers, specifically ω-(o-alkylphenyl)alkanoic acids with 18 and 20 carbon atoms and two isoprenoid alkanoic acids (phytanic acid and 4,8,12-tetramethyltridecanoic acid) were detected in all heated maize-fish powder samples. Interestingly, 4,8,12-TMTD was also detected in all unheated Lake Trout-maize samples, even when the raw fish represented as little as 10% of the mixture. Cholesterol, sometimes in combination with its oxidation products, was also identified in all unheated Lake Trout-maize powder mixes, which also contain stigmasterol when maize composed over 60% of the mixture. In heated samples, however, cholesterol bi-products were not detected when raw Chain Pickerel represented less than 30% of the mixture.
In sum, in this experiment the full set of aquatic biomarkers (i.e., ω-(o-alkylphenyl)alkanoic acids with 18 and 20 carbon atoms two isoprenoid fatty acids) was present in all samples, even when raw fish represented as little as 10% of the mixture. Of note is that our results suggest that these biomarkers may be present in a residue without a statistically significant FRO.
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. This has been particularly true in northern Europe but has also been identified as a potential problem in other regions [
Our results indicate that there is a very high positive correlation between the percentage of C from fish in residues and FROs. However, there is no such direct relationship between the fraction of raw fish cooked in a pot and the fraction of fish C in the residue. Statistically significant FROs may occur when fish constitute <1% of the raw resource mix, but may also not occur until fish represent over 90% of raw resources depending on the resource(s) with which it was cooked and the size of the 14C age error. Our results also indicate that the complete sets of biomarkers, in this case the presence of ω-(o-alkylphenyl)alkanoic acids with 18 and 20 carbon atoms and phytanic acid, may be detected when fish C contributes little to residue formation. Thus, it is possible for aquatic biomarkers to be identified in a residue in the absence of a statistically significant FRO when a freshwater reservoir effect was present. This in turn emphasizes the need to assess the potential for ancient carbon reservoirs for specific periods of time in question prior to considering charred, encrusted residues for radiocarbon dating. Contemporary water chemistry and aquatic organisms are not adequate analogues for prehistoric reservoirs because modern land practices have significant effects on freshwater reservoirs [
Moreover, our results demonstrate compound-specific14C analysis (CSRA) could also be applied to issues surrounding FROs from charred cooking residues. To date, applications of CSRA to pottery residues have targeted C16:0 and C18:0 fatty acids, which typically are the most abundant fatty acids preserved in potsherds [
We thank Rick Morse, Bryan Weatherwax, and Jeremy Wright (New York State Museum) for capturing the fish used in this study; Brian Bird (New York State Museum) for obtaining the maize; Robert Feranec (New York State Museum) for providing help in the lab with sample preparation; Alexandre Lucquin (University of York) for assisting with the lipid analysis; Susan Winchell-Sweeney (New York State Museum) for