Chapple, R. M. 2012 'Bubbling over: archaeological lipid analysis and the Irish Neolithic. Review' Blogspot post
Bubbling over: archaeological lipid analysis and the Irish Neolithic: Review Originally posted online on 6 March 2012 at rmchapple.blogspot.com (http://rmchapple.blogspot.co.uk/2012/03/bubbling-over-archaeological-lipid.html)I recently attended one of the PCC Lunchtime Seminar Series talks (Booms and Busts inEurope’s Earliest Farming Societies given by Prof. Stephen Shennan) at the School ofGeography, Archaeology and Palaeoecology at QUB. It was a fascinating talk that Ienjoyed very much and I decided that I would make every effort to attend the next one.Yesterday (6th March 2012) we had Dr. Jessica Smyth, currently of the OrganicGeochemistry Unit, in the School of Chemistry, at the University of Bristol, speaking onthe topic of lipid analysis and their application to the study of the Irish Neolithic.She began by introducing the topic of lipid analysis, explaining how it involves theinvestigation of surface and embedded fats to reconstruct past diets. This field ofresearch was largely pioneered by Prof. Richard Evershed and Smyth explained how sheis working closely with him on this post-doctoral research project. She explained thatone of the thrusts of her PhD research (Neolithic settlement in Ireland: new theoriesand approaches, completed at UCD) was to provide a counterpoint to the traditionalnarrative of the Irish Neolithic. In the past much of the debate had centred on themegalithic tombs and the relatively small number of excavated houses. However, by theearly to mid 2000s, much new and challenging evidence was being uncovered as part ofthe large infrastructural projects and the topic was ripe for re-evaluation (See my takeon the benefits of the ‘Celtic Tiger’ years here). One of the bounties of the ‘Celtic Tiger’period was the vast increase in recovered ceramics, many of which may have beensuitable for lipid analysis. Following on from her PhD, Smyth applied to the Marie CurieActions foundation and was awarded a Marie Curie Intra-European Fellowship forCareer Development for lipid analysis for the ‘SCHERD’ project. ‘SCHERD’ stands for“Study of Cuisine and animal Husbandry among Early farmers via Residue analysis and
radiocarbon Dating” and, as these things go, is a pretty snappy title for the project andgets all the main points across.The first question that Smyth posed was ‘why is it important?’ She emphasised that theNeolithic is a significant stage in human development with the introduction ofmegalithic architecture, farming, and generally increased social complexity. Lying at theedge of Europe, Ireland is also chronologically and geographically at the end of thisprocess, which makes an even more interesting research proposition. She explained howher research interests partially overlapped with those of the LeCHE group, aconfederation of researchers examining the origin of dairying in Neolithic Europethrough the use of lipid analysis, DNA etc. However, their research focuses on centraland south-eastern Europe, leaving Ireland previously unstudied.The aims of the SCHERD project are to firstly identify the contents of the pots via lipidanalysis. This is particularly important in the case of Ireland where generally poorpreservation of animal bone has constricted our ability to understand some of thedevelopments and mechanisms of Neolithic agriculture. In this way the analysis of lipidsmay be developed for use as a proxy for the economy of the past. It is also hoped todistinguish patterns of regional variation that may shed light on differing diets,economy and traditions. It is also hoped that, similar to the research carried out byMukherjee et al. (2008) for England, that diachronic changes may be discerned, such asa change in emphasis on cattle rearing to pig production. Another aim of the project is toprovide more secure date markers for the Irish Neolithic by directly dating surface andabsorbed residues from selected vessels in key assemblies. In this way, it is hoped to addto or, assist in the revision of, Bayesian chronologies for the Neolithic, a process begunwith the volume Gathering Time: Dating the Early Neolithic Enclosures of southernBritain and Ireland. Smyth also highlighted the problem that much of our chronology ofthe Irish Neolithic is relatively poor as it is based on typological assessments, chiefly ofchanging pottery styles. For all of these reasons, the chronology requires urgent revisionand the SCHERD project hopes to play a prominent role in that process.
Smyth outlined her sampling regime of 15 key sites, spread across the entirety of theIrish Neolithic. In the Early Neolithic her sites include two enclosures (Donegore Hill,Co. Antrim, and Magheraboy, Co. Sligo) and five houses (including Ballygalley, Co.Antrim, and Monanny, Co. Monaghan). The sites from the Middle Neolithic includedone enclosure (Tullahedy, Co. Tipperary), two pits (including Goodland, Co. Antrim),and one pit/spread. The late Neolithic sites examined by Smyth include one enclosure(Ballynahatty, Co. Down), one spread (Longstone, Co. Kildare), one house(Ballynacarriga, Co. Cork), and one pit (Lowpark, Co. Mayo). To ensure that astatistically viable sample was investigated, where possible, a minimum of 30 sherdswere examined from each site. As Smyth points out, the lipid analysis process isdestructive. However, she argues that considering the potential rewards in terms of newdata and insights, the removal of 2-3g of pottery per analysed sherd is a pretty minimalprice to pay.The process of preparing the sherd is as follows: the surface is cleaned with a modellingdrill to remove all forms of surface contamination. To release the lipids from the claymatrix of the pottery, the sample is ground into a fine powder. The resulting dust isplaced in furnaced glass phials and solvents are added to release the lipids. The solventsused are most usually of the form of chloroform/methanol compounds. After some timethe resulting complex mixture is purified and separated. The resulting Total LipidExtract (TLE) amounts to approximately 2ml. This is kept in refrigerated storage and allanalyses are conducted using small portions of this core sample. The TLE is furthertreated to make it suitable for use in various tests, including use in a mass spectrometeror a gas chromatograph. Only once all of these steps have been completed can analysisactually begin.Smyth explained that one of the research aims pursued by Prof. Evershed was thatextensive experimental work was necessary to provide a library of lipid signatures thatwould allow researchers to confidentially infer the former presence of fats. She showed anumber of chromatograms showing Triacylglycerol signatures (TAGs). She alsoexplained how, over time, these degrade into Diacylglycerols (DAGs) and theninto Monoacylglycerols. Finally, these will degrade into simpler, free fatty acids. Lipidsrecovered from archaeological sherds are most usually of this latter type. It is estimatedthat of the original volume of deposited lipids, only 1% is likely to survive. Some of thesites investigated show excellent rates of survival (such as Tullahedy, Co. Tipperary),though survival rates are thought to be related to variables such as climate andtemperature etc. As an example, Smyth cited research work carried out in dry countries,such as Greece. Here, less than 10% of sherds retained lipid traces. Experimentation hasshown that there is vast and rapid degradation of the lipid remains on sherds stored inaerobic conditions over a 40 day period. It is for this reason that the sherds selected forthis form of analysis should come from sealed, anaerobic contexts. Smyth also notedthat the storage of sherds in plastic bags (extremely common these days) may also leavea lipid signature on the pottery. Her concern is that while it is not an insurmountableproblem, it does need to be noted and it may also mask genuine archaeological signals.In examining what makes a high quality sample, Smyth noted that upper body sherdsand rim sherds have shown the best level of survival. At face value, this seems counter
intuitive as this is the region in least contact with any boiling liquids in the potteryvessel. As noted above, sherds from sealed, anaerobic contexts are also preferred overfinds from the plough soil etc. It is also important that finds are well archived and areretained with their meta data, indicating their contact with plastics and other forms ofpost-excavation processing. While, from a chemical point of view, the need for sherdsfrom well stratified deposits is not essential, it is of the highest archaeologicalimportance so that the fullest data may be relayed back into our site models. Finally, it isalways good to have a complementary faunal record or other proxy data against which tocompare results.Addressing the question of the feasibility of this project, Smyth noted that earlier workon the Tullahedy, Co. Tipperary, assemblage reported a rate of five out of six analysedsherds retaining lipids. At another Middle Neolithic site on Rathlin Island, Co. Antrim,50 sherds were analysed. This showed a recovery rate of 28 of the 50 analysed sherdscontaining lipid residues. Smyth was also keen to stress that, with 14 months left to run,she is still at a relatively early stage in her project and the results she has thus farobtained are preliminary in the extreme. With regard to the site at Donegore Hill, Co.Antrim, the recently published monograph and the Gathering Time volumes haveprovided a secure dating range. Given the enormous quantity of recovered ceramics (c.45,000 sherds), only the rim sherds were formally analysed. These were estimated torepresent approximately 1,500 individual vessels. Unfortunately these sherds wereunavailable for lipid analysis. As none of the body sherds were identified to individualvessels, a selection of fabric types was selected, spread across the entirety of the site. Atthe Neolithic house site at Ballygalley, Co. Antrim, the upper body and rim sherds wereavailable for analysis, though the results of these analyses are yet to be completed. Theenclosed Middle Neolithic settlement at Tullahedy, Co. Tipperary, produced a very richcereal assemblage – in fact it is the most abundant assemblage of its type throughout theentire Irish Neolithic. However, the site had remarkably poor bone preservation. Earlierinvestigations of some sherds from Tullahedy confirmed the presence of free fatty acidson a high proportion of the examined sherds. While early results are promising, she wasquick to stress that further analysis, including isotope ratio analysis, would be necessaryto confirm them.Looking towards the future of the SCHERD project, Smyth hopes to document thechanging nature of Irish agriculture over time. Evidence from such sites as the CéideFields, Co. Mayo, and Kilshane, Co. Dublin - the latter with its exceptionally large LateNeolithic assemblage of animal bone – have been taken to suggest that there was a moveaway from a cereal-based economy to a stock rearing economy over the course of theNeolithic. Smyth believes that with enough good quality samples, and sufficient time onthe various machines, the SCHERD project can make significant contributions to thestate of our knowledge.In the question and answer session at the end of the presentation, Smyth was asked if itwas possible to date lipids directly, using radiocarbon dating. She replied that it didappear to be possible, though they had not attempted it so far. For example, some of thesherds, especially those from Donegore Hill, had produced over 5mg of lipid residue,more than enough for an AMS date, so the potential is definitely there. Smyth was also
asked if it was possible to distinguish between sheep and cattle in the lipid residues. Sheexplained that it was, though the analysis of subtle differences in the signatures ofTriacylglycerol (TAGs) and Diacylglycerols (DAGs). One other contribution related tothe description by Peacock, during an 18th century tour of the Hebrides. He hadwitnessed the native islanders creating coil-built pottery vessels and firing them in openhearths – essentially the same manner as in the prehistoric past. However, Peacocknoted that before they were placed in the hearths, they were filled with milk, which wasallowed to evaporate during the firing process. The question then was that could thisform of activity be what we are witnessing, rather than the continued use of the vessel tohold a variety of different cooked meals. Smyth admitted that it could complicatematters, but that it was most likely that the firing process would not just evaporate themilk, but destroy any lipid residues in the process. One further intriguing problem wasraised during this discussion. This was the discovery of certain keytones in somesamples that indicate that the contents were fired to a temperature over 300°. While it isthought that the average open heath would struggle to achieve this temperature, thereality is that little food would survive for long even if it could be achieved.As Smyth stressed throughout this presentation, she is currently only able to presentpreliminary results. Nonetheless, she has already produced some interesting, andpotentially controversial, findings. I, for one, eagerly await her return trip to Belfastwhen she has been able to fully verify her findings and process more samples.Notes:I hope that I have done justice to Jessica Smyth’s lecture and managed to convey the gistof her ideas and results. Nonetheless, I do sincerely apologise if I have misrepresentedor misquoted the speaker. If so, please feel free to contact me, and I will endeavour toset the record straight.I have not been able to give the full references to any of the literature referenced in thelecture – sorry.Some photographs of the 1984 season of excavations at Donegore Hill are available forpublic viewing, at The William Dunlop Archaeological Photographic Archive.