Direct dating of pottery from its organicresidues: new precision usingcompound-specific carbon isotopesR. Berstan1, A.W. Stott1,2, S. Minnitt3, C. Bronk Ramsey4,R.E.M. Hedges4 & R.P. Evershed1∗

Techniques for identifying organic residues in pottery have been refined over the years by ProfessorEvershed and his colleagues. Here they address the problem of radiocarbon dating these residuesby accelerator mass spectrometry (AMS) which in turn dates the use of the pot. Fatty acids fromcarcass and dairy products cooked in the pot were isolated from early Neolithic carinated bowlsfound at the Sweet Track, Somerset Levels, England, and then dated by AMS. The results werevery consistent and gave an excellent match to the dendrochronological date of the trackway. Themethod has wide potential for the precise dating of pottery use on sites.

Keywords: England, Somerset Levels, Neolithic, pottery, organic residues, fatty acids,radiocarbon, dendrochronology

IntroductionArchaeological ceramics have traditionally played a vital role in the development ofchronological sequences, with relative dating techniques such as typology, stratigraphy andseriation all used extensively. Direct radiocarbon dating of pottery is relatively uncommondue to the presence of carbon sources with differing ages, for example geological carbonremaining in the clay after firing, added organic temper, carbon from the fuel of the kilnand exogenous contaminants absorbed from the burial environment – although most ofthese substances have been dated (De Atley 1980; Gabasio et al. 1986; Evin et al. 1989;Hedges et al. 1992; Kolic, 1995; Gomes & Vega 1999; Nakamura et al. 2001; Mihara et al.2004; Yoshida et al. 2004). Surface residues in particular provide a valuable resource fordating programmes (Bayliss & Bronk Ramsey 2004) although their ill-defined chemicalcompositions can be sources of uncertainty.

A more promising source is provided by lipid residues absorbed into the pot wall, sincethese should relate directly to periods of use (Heron & Evershed 1993). An increasingly wide

1 Organic Geochemistry Unit, Bristol Biogeochemistry Research Centre, School of Chemistry, University of Bristol,Cantock’s Close, Bristol, BS8 1TS, UK (Email: r.p.evershed@bristol.ac.uk)

2 Present address: CEH–Lancaster, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP,UK

3 Somerset County Museum, Taunton Castle, Castle Green, Taunton, TA1 1AA, UK4 Oxford Radiocarbon Accelerator Unit, Research Laboratory for Archaeology and the History of Art, Oxford

University, Dyson Perrins Building, South Parks Road, Oxford, OX1 3QY, UK∗ Author for correspondence

Received: 17 August 2007; Revised: 4 March 2008; Accepted: 3 April 2008

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range of organic commodities has been identified from lipidic components of archaeologicalpottery, including those derived from beeswax (Charters et al. 1995; Heron et al. 1994;Evershed et al. 1997b; 2003), birch bark tar (Charters et al. 1993a), leafy vegetables (Evershedet al. 1991; 1992; 1994; Charters et al. 1997), plant oils (Condamin et al. 1976; Copley et al.2001a; 2001b; 2005a) and marine oils (Copley et al. 2004; Hansel et al. 2004; Craig et al.2007). However, degraded animal fats are the most common class of lipid encountered(Needham & Evans 1987; Rottlander 1990; Evershed et al. 1992; 1997a; 2002; Dudd &Evershed 1998; Copley et al. 2003). These are readily identified by the dominance of C16:0

and C18:0 fatty acids, either as free components or as fatty acyl moieties of mono-, di- ortriacylglycerols (Evershed et al. 2002).

The origins of these degraded animal fat residues can be classified further throughcompound-specific stable carbon isotope determinations (δ13C values) of the C16:0 and C18:0

fatty acids; this technique, exploiting fundamental differences in the diets, metabolisms andphysiologies of different species of animals, has previously been employed to distinguishunambiguously between degraded residues derived from ruminant (e.g. cattle and sheep) andnon-ruminant (e.g. pigs) adipose/carcass fats (Evershed et al. 1997a; Mottram et al. 1999;Mukherjee et al. 2007; 2008). Furthermore, by exploiting differences in the biochemicalsources of their C18:0 fatty acids, degraded ruminant dairy fats can be distinguished fromruminant adipose/carcass fats (Dudd & Evershed 1998; Copley et al. 2003; 2005b; 2005c;2005d; 2005e; Berstan et al. 2004).

Using techniques first applied to sedimentary lipids (Eglinton et al. 1996; 1997), werecently employed preparative capillary gas chromatography (PCGC) to isolate individualfatty acids from absorbed lipid residues extracted from archaeological pottery in sufficientamounts for radiocarbon dating by high precision accelerator mass spectrometry (AMS)analyses (Stott et al. 2001; 2003). We reasoned that lipids absorbed within pottery areexcellent candidates for routine 14C dating as they are widespread at most archaeologicalsites and often occur in high abundance. Lipids have fast metabolic turnover rates whichensure 14C ages close to the date of death of the organism. Moreover, their hydrophobicnature gives relative chemical stability and immobility within the burial environment. Inaddition, through chromatographic, mass spectrometric and stable isotope analysis, theirindividual structures, distributions, sources (e.g. animal fat, plant wax, etc.) and potentialcontaminants (e.g. phthalates) can all readily be determined. The possibility of eliminatingcontamination serves to highlight the elegance of the compound-specific radiocarbon datingapproach compared to ‘bulk’ dating of whole residues, as the targeted compounds can beunequivocally linked to the commodity originally processed within the vessel.

In this investigation we isolated and dated individual fatty acids from Early Neolithicpottery associated with the Sweet Track, a preserved timber trackway excavated in theSomerset Levels in south-west Britain. The trackway itself is well dated by dendrochronology,offering a direct test of the precision of the method.

Background to the Sweet TrackThe Sweet Track is one of the earliest elevated wooden trackways in Europe and originallyspanned 1.8km of the Somerset Levels, an area of low-lying wetland, between the Polden

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Hills and Westhay Island in south-west England (Coles & Coles 1986). Most of the trackwaystill remains today, submerged and protected under a peat layer. Using dendrochronology,the date of construction of this Early Neolithic structure was determined as between thewinter of 3807/6 BC and the spring of 3806 BC, with evidence of repair observed between3804 and 3800 BC (Hillam et al. 1990). It has been suggested that the track may only havebeen used for a period of up to 10 years, before it became unusable as a result of flooding inthe winter and summer reed growth (Coles & Coles 1986; Coles 1999). These precise datesfor the construction and use of the Sweet Track enable equally accurate ages to be assigned tothe wide variety of Early Neolithic artefacts (e.g. pottery, flints and tools) recovered adjacentto the track. The pottery assemblage consisted entirely of fine wares of the Early NeolithicBowl tradition (Smith 1976; Kinnes 1979); this being the earliest pottery style in Britaincoinciding with the advent of British agriculture (Gibson 1986).

Dating of Sweet Track pottery usage by lipid residue analysisThirteen Early Neolithic potsherds (SW1-13) discovered adjacent to the Sweet Track weresupplied by the Somerset County Museum, Taunton. Most of these potsherds derived fromseparate vessels recovered from near the southern end of the trackway (Smith 1976; Coles &Orme 1984). The sherds were reportedly consolidated after excavation with a poly(vinylalcohol) known as Mowiol, but none was detected in any of the analyses performed. Solventextraction of lipids from the Sweet Track potsherds and subsequent gas chromatography(GC) and GC/mass spectrometry analysis (see technical summary, below) revealed 10 ofthem to contain absorbed lipid residues in appreciable concentrations (>5μg g−1) rangingfrom 56 to 13 806μg g−1. Table 1 lists the lipid compositions for each potsherd. SW1,SW2 (Figure 1) and SW8 all yielded lipid residues dominated by free fatty acids (mainlyC16:0 and C18:0) and acylglycerols (mono-, di- and tri-) in distributions consistent withdegraded animal fat. This was corroborated by identification of trace amounts of cholesterolin residue SW2. A substantial plant lipid contribution was identified in a number of residues,in particular those from SW10 and SW11. These consisted of long chain fatty acids (upto C30:0), long-chain alcohols (up to C28:0), plant sterols (β-sitosterol and stigmastanol)and ω-hydroxy fatty acids (up to C30). Long chain fatty acids with carbon chain lengths>C22 were also identified in SW9, SW12 and SW13, providing further evidence for plantcontributions to these residues. Mid-chain ketones with carbon chain lengths ranging fromC29 to C35 were identified in six of the lipid residues. These compounds have previouslybeen shown to form through the thermal condensation of fatty acids at temperatures inexcess of 300◦C (Evershed et al. 1995; Raven et al. 1997), and thus provide evidence thatthe vessels were exposed to high temperatures either during or after use.

In all but two of the sherds yielding lipid residues, the C16:0 or the C18:0 fatty acids werethe most dominant lipid components. However, SW7, SW9 (both from the same vessel) andSW13 all exhibited unusual distributions, with ratios of C18:0 to C16:0 of 9.2, 6.8 and 14.4,respectively. Typically, degraded animal fats exhibit similar abundances of C16:0 and C18:0

fatty acids, while plant remains display a dominance of C16:0 (Evershed et al. 2002). Thereduced abundances of C16:0 suggest some post-burial loss through leaching or microbialaction.

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Table 1. Potsherd information and organic residue concentration and composition. Key: FFA – free fatty acids; MAG – monoacylglycerols;DAG – diacylglycerols; TAG – triacylglycerols; ALC – n-alcohols; HFA – ω-hydroxy fatty acids; K – mid-chain ketones; CHL – cholesterol;PST – plant sterols; tr – trace; nd – not detected; * from vessel SLP SWR E3; ** from vessel SLP SWR P80.

Laboratory Lipid concentrationnumber Sample code (μg g−1) Major lipid components Lipid residue origin(s)

SW1 130/1986/2453 (SWD 1299) 13806 FFA, MAG, DAG, K Ruminant adipose/carcass fatSW2 130/1986/2452 (SWC 124) 4900 FFA, MAG, DAG, TAG, K, CHL Ruminant dairy fatSW3* 130/1986/2432 (SWR E3) tr FFA UnknownSW4 130/1986/2465 (SWR P1) nd nd ndSW5** 130/1986/2466 (SWR P80) tr tr UnknownSW6 130/1986/2433 ([SW] G1) 62 FFA (tr), K Animal fat?SW7* 130/1986/2432 (SWR E3) 348 FFA, K UnknownSW8 130/1986/2464 ([SW] E3 VII/VIII) 665 FFA, MAG, DAG, TAG Ruminant dairy fatSW9* 130/1986/2432 (SWR E3) 280 FFA, K Unknown + minor plant contributionSW10** 130/1986/2466 (SWR P80) 1070 FFA, PST, ALC, HFA Plant contributionSW11 130/1986/2435 (SW H3) 332 FFA, PST, ALC, HFA Plant contributionSW12* 130/1986/2432 (SWR E3) 423 FFA, K Unknown + minor plant contributionSW13 130/1986/2434 (SW E9) 327 FFA Unknown + minor plant contribution

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Figure 1. Partial gas chromatogram of trimethylsilylated lipids extracted from SW2. Chromatographic peak identities are:C10:0FA to C20:0FA, saturated straight chain fatty acids with 10 to 20 carbons atoms; C18:1FA, monounsaturated fattyacid with 18 carbon atoms; C15:0br and C17 :0br, branched chain fatty acids with 15 and 17 carbon atoms; M14 to M18,monoacylglycerols with 14 to 18 acyl carbon atoms; D30 to D36, diacylglycerols with 30 to 36 acyl carbon atoms; T42 toT54, triacylglycerols with 42 to 54 acyl carbon atoms; K31 to K35, mid-chain ketone containing 31 to 35 carbon atoms;CHL, cholesterol; IS, internal standard, n-tetratriacontane (n-C34 ).

The δ13C values for the fatty acids from SW1, SW2 and SW8 were plotted and overlaidwith confidence ellipses (1σ ) representing the values recorded in modern reference fats(Figure 2). The residue from SW1 was found to be consistent with a ruminant adipose/carcassfat origin, while those from SW2 and SW8 corresponded to ruminant dairy fat, as all plottedwithin or on the edge of the corresponding reference ellipses. This demonstrates that sherdSW1 derived from a vessel that processed ruminant adipose/carcass meat, while SW2 andSW8 were from vessels used to process dairy products, probably butter (Copley et al. 2005d).

Interestingly, the triacylglycerols surviving in SW2 and SW8 also suggest a ruminant dairyfat origin due to the detection of low molecular weight components (Figure 2, inset), whichare negligible components of adipose fat (Davies et al. 1983). Commonly during degradationthe short chain acyl moieties are readily hydrolysed from the glycerol backbone (Dudd &Evershed 1998), resulting in triacylglycerol distributions resembling that of adipose fat.In this case the heavier of these low molecular weight triacylglycerols, with acyl carbonnumbers of C40 and C42, have survived and can be attributed to a butter fat rather thanan adipose origin, thereby supporting the assignments based on fatty acid δ13C values (seeFigure 2; Dudd & Evershed 1998; Copley et al. 2003; Berstan et al. 2004). The lipid analysesshowed no evidence of significant post-excavation contamination and thereby provided animportant screening step in the compound-specific radiocarbon analysis protocol. They also

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Figure 2. A plot of the δ13C values for the C16 :0 and C18:0 fatty acids extracted from Sweet Track potsherds SW1, SW2 andSW8. The modern reference fats are represented by confidence ellipses (1σ ). Theoretical curves from the porcine adipose fatsto the ruminant adipose and dairy fats are plotted to illustrate the δ13C values expected through their mixing. All referenceδ13C values include the addition of 1.2Ğ, to adjust for fossil fuel burning (Friedli et al. 1986). The insets display histogramsrepresenting acyl carbon number distributions of triacylglycerols identified in: (a) SW2; and (b) SW8.

confirmed the identities of commodities processed in the original vessels, and indicatedthose sherds containing optimal concentrations of lipid for radiocarbon analysis.

Being the most abundant compounds present in the lipid extracts, the C16:0 and C18:0

fatty acids were targeted for the 14C dating. These were isolated as methyl ester derivatives,using the PCGC, with trapping sequences consisting of c . 120 GC runs. All the AMSdeterminations on the SW1 and SW2 fatty acids revealed consistent results ranging in agefrom 4950 to 4790 BP (Table 2). The agreement between the calibrated 14C dates andthe dendrochronology date of the track was precise, with three of the six measurementsoverlapping, two within 20 years and one within 130 years of the 10 year date range for

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Table 2. Radiocarbon dates of individual fatty acids from potsherds SW1 and SW2. The associatedtimbers used to make the Sweet Track were dated between the winter of 3807/6 BC and the spring of3806 BC by dendrochronology.

Calibrated calendardates (BC)

Sample/ Combusted Radiocarbonfatty acid 14C lab. number yield (μg C) % Modern14C age (BP) 1σ 2σ

SW1 (C16:0) OxA-V-1045-17 883 54.0 +/− 0.35 4950 +/− 50 3780-3660 3940-3640SW1 (C18:0) OxA-V-1045-18 996 54.5 +/− 0.35 4870 +/− 50 3710-3540 3780-3520SW2 (C16:0) OxA-X-850-13 ∼400 55.1 +/− 0.4 4790 +/− 60 3650-3510 3670-3370SW2 (C16:0) OxA-V-1045-19 1026 54.3 +/− 0.35 4910 +/− 50 3760-3640 3800-3530SW2 (C18:0) OxA-X-850-14 1140 54.6 +/− 0.4 4860 +/− 60 3710-3530 3780-3510SW2 (C18:0) OxA-V-1046-5 614 54.3 +/− 0.5 4900 +/− 80 3780-3630 3940-3520

the Sweet Track’s use (2σ precision; Figure 3). All of the measurements are statisticallyindistinguishable and pass a chi-squared (χ2) test at 5% (T = 4.7, 5% threshold 11.1;Shennan 1988).

DiscussionThe robustness of our analytical protocol (Stott et al. 2001; 2003) was again confirmed.Duplicate determinations on each of the C16:0 and C18:0 fatty acids from potsherd SW2(C16:0, OxA-X-850-13 and OxA-V-1045-19; C18:0, OxA-X-850-14 and OxA-V-1046-5)showed little variation, with the C16:0 and C18:0 fatty acids differing by 0.8 and 0.3%,respectively in their amounts of modern 14C (Figure 4; Table 2). In earlier reports (Stott et al.2001; 2003) we observed that the C16:0 fatty acids often gave slightly younger 14C ages thantheir C18:0 counterparts, which we tentatively attributed to the enhanced migration/leachingof the shorter chain fatty acid. No such trend was observed herein, as demonstrated throughchi-squared (χ2) tests, which revealed a level of association between the fatty acids fromeach potsherd to be >25% and >45% for SW1 (n = 2) and SW2 (n = 4), respectively.As the degree of consistency was sufficiently high (i.e. χ2 test >5%), the fatty acid agesfrom each individual potsherd were combined to achieve greater precision. This resultedin constraining the calibrated calendar date ranges (2σ ) to 3770-3640 BC and 3710-3540 BC for SW1 and SW2, respectively (Figure 3). These date ranges correlate well withthe dendrochronological dates for the Sweet Track timbers of 3807-3797 BC, the upperlimits of the calibrated date ranges of residues indicating that the pottery was in use within100 years of the cutting of the timbers.

ConclusionsOrganic residue analyses demonstrated excellent preservation of lipids in the Sweet Trackpottery, with 10 out of 13 potsherds yielding appreciable residues. Based on the lipidcomponents present, it was revealed that degraded animal fats were present in three of thepotsherds and plant residues in a further two. Interpretation of the remaining five residueswas more complicated, possibly as a result of diagenetic alterations and/or mixing of different

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Figure 3. Radiocarbon calibration graphs for the combined 14 C ages of fatty acids obtained from potsherds SW1 (n = 2)and SW2 (n = 4). The calibrated calendar ranges are compared with the associated dendrochronological date (3807/6 BC).χ2-Test: df, degrees of freedom; T, calculated χ2 value; (5%, 3.8) and (5%, 7.8) are the values T must be less than for a5% association between measurements for SW1 and SW2, respectively. Calibration was performed using Oxcal v3.9 (BronkRamsey 2001) with the INTCAL 98 calibration curve (Stuiver et al. 1998).

commodities. Classification of the three degraded animal fat residues, via the δ13C valuesof their C16:0 and C18:0 fatty acids compared with modern reference fats, revealed thatone of the vessels was originally used to process ruminant adipose/carcass products and theother two dairy products; the interpretation of the latter two assignments were corroborated

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Figure 4. Plot of the calibrated date ranges obtained from the radiocarbon dating of individual fatty acids (C16 :0 andC18:0) extracted from potsherds SW1 and SW2. Calibration was performed using Oxcal v3.9 (Bronk Ramsey 2001) withthe INTCAL 98 calibration curve (Stuiver et al. 1998). The ‘associated date range’ refers to the date of the timber trackway,dated to 3807/6 BC by dendrochronology.

by their intact triacylglycerol distributions. These results demonstrate that domesticatedruminant animals, probably cattle or sheep, were present and exploited for their meat andmilk during the use of the Sweet Track, previously dated from 3807 BC to around 3797 BC.The finding of dairy fat residues in Early Neolithic pottery concurs with our other resultsfrom other southern British sites from this period (Copley et al. 2003; 2005d; 2005e).

The tightly constrained dendrochronological dating for the Sweet Track and the potteryassociated with it provided an excellent opportunity to test procedures for radiocarbon datingthe well-preserved residues in the vessels. The six resulting radiocarbon determinationsfor the targeted fatty acids (C16:0 and C18:0), including replicates, agreed very favourablywith each other and with the dendrochronological age of the Sweet Track. Thesefindings emphasise the power of combining compound-specific stable carbon isotope andradiocarbon determinations to the study of organic residues preserved in archaeologicalpottery vessels. The results raise confidence in using fatty acids isolated from residues todate pottery from sites not blessed with such robust associated dates as the Sweet Track.

Technical summaryThe analytical protocols and instrumentation employed during the analyses have beenpublished in detail previously (e.g. Charters et al. 1993b; Evershed et al. 1990; Dudd &Evershed 1998; Stott et al. 2001; 2003; Copley et al. 2003). Potsherds yielding relativelyhigh concentrations of lipid residue (c . >500μg g−1) and containing components thatcould be directly linked to commodities originally processed within vessels, were selectedfor compound-specific 14C analyses. To increase the total yield of lipid, larger pieces ofpowdered potsherd (c . >5g) were extracted using a Soxhlet apparatus; the lipid extract

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treated with base, methylated and screened via GC. Preparative capillary GC was thenused to isolate individual fatty acids as their methyl ester derivatives (FAMES) in amounts>200μg (amassed from c . 120 GC runs). Before submission to AMS, the purity andconcentration of the target FAMEs (C16:0 and C18:0) were established via GC analyses ofsmall aliquots with a co-injected standard. To ensure no isotopic fractionation occurredduring trapping sequences, δ13C values of individual FAMEs were determined before andafter PCGC analyses (Stott et al. 2003).

AcknowledgementsWe would like to thank the Natural Environment Research Council (NERC) for funding this research(GR3/10641) and for supporting the organic, stable isotope and AMS facilities at Bristol and Oxford. Wealso thank J. Carter, A. Gledhill and M. Humm for technical assistance.

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