This could potentially be applied both to dating different layers within the same midden, when the temporal resolution is such that it is possible to resolve the internal stratigraphy, and also to correlate the age of different deposits on a regional scale e. Amino acid racemization AAR dating of shell middens has a long history. Masters and Bada , compared the extent of isoleucine epimerization in radiocarbon dated shells and found distinct divergences.
They attributed these to both inaccuracies in radiocarbon dating of carbonate and the isoleucine method on shell.
Amino acid racemization dating of marine shells: A mound of possibilities
Wehmiller showed that the shell radiocarbon and racemization data were consistent with shallow ground thermal effects. A third factor which was considered was the likelihood of sample mixing within the midden. Moreover, the possibility of burning and human-induced heating of edible molluscs is a general concern for AAR dating: If unidentified, this could significantly affect the reliability of the technique in archaeological contexts e.
Masters and Bada, The main advance is in the isolation of a fraction of amino acids intracrystalline from the shell which behave as a closed system during diagenesis. The extent of protein degradation within this system can be used as a secure indicator of the age of a molluscan sample.
The analysis of the intracrystalline fraction therefore represents an important step forward for the reliability of AAR dating of mollusc shells e. This paper shows how the recent advances in the AAR dating method can be effectively applied to shell midden deposits.
The examples presented come from a range of samples from Holocene sites in Scotland Latitude: Detailed temporal and stratigraphical information was not available for all sites, hindering the possibility of considering shallow temperature burial effects. These can be particularly important for middens where the samples have not been submerged during burial and where the length of time at high shallow ground temperatures can be large in proportion to the age of the sample Wehmiller, Within this study it was not possible to investigate the effect of different within-site thermal environments during burial: The recent methodological advances in AAR dating are briefly summarized and a series of tests recommended to check for reliable AAR dating using the new closed system approach is proposed.
Finally, the reliability of the technique is tested on archaeological material associated with independent chronological information, and conclusions drawn on the utility of AAR dating for the dating of shell midden deposits. A mollusc shell contains both a mineral and a protein fraction; the biochemical functions of proteins in the process of biomineralization have been widely investigated, but many aspects still remain unclear e.
After the death of the organism, the proteins undergo diagenesis: Within this intracrystalline fraction, the extent of protein diagenesis is solely dependent on the thermal age of the fossil shells, i. Conversely, the majority of the other proteins intercrystalline are not trapped in the crystals and therefore behave as an open system. The level of protein diagenesis is generally estimated by measuring the extent of amino acid racemization AAR. Most of the amino acids can arrange their atoms in space in different configurations called enantiomers while maintaining their chemical properties: When an amino acid differs from its mirror image, it is defined as a chiral amino acid.
The majority of the natural amino acids possess at least one asymmetric carbon atom a chiral centre , as a result of the four different substituents bonded to the alpha carbon.
David A. Plaisted
By analysing the extent of protein breakdown within the intracrystalline fraction, secure relative aminostratigraphies can be established for a series of molluscan samples: This assumption is limited to: Here, the main steps used for sample preparation and the chromatographic analysis of multiple amino acids, performed with a modified method of Reverse Phase High Pressure Liquid Chromatography RP-HPLC of Kaufman and Manley , are briefly reported. Each shell was sub-sampled for amino acid analysis, by snapping off a fragment of a few square millimeters; for Patella , the edge of the shell was specifically targeted, to provide a consistent calcitic structural layer for analysis Demarchi, Each shell fragment was first sonicated and rinsed at least five times in ultrapure water After the bleaching agent was removed by washing in ultrapure water 5 cycles and methanol 1 cycle , the dry powders were further split into two subsamples.
For analysis, a modified analytical method of Kaufman and Manley for an automated system of RP-HPLC, described in Penkman , was adopted, allowing the routine analysis of l and d isomers of 10 amino acids. This can be crucial for the analysis of archaeological material, which is often scarce or too precious to analyse destructively using large samples. Secondly, the high automation of the system allows for maximum efficiency of the analysis, increasing the number of samples which can be processed, minimising the analytical variability and therefore improving the statistical significance of a given dataset.
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Moreover, it is possible to detect the enantiomers of multiple amino acids, thereby increasing the level of resolution available. The conventional method of AAR dating generally focused on the racemization epimerization of a single amino acid, isoleucine e. It follows that unexpected differences in the dl ratios of some amino acids can be used to detect compromised samples see Section 4. The technique proposed is therefore cost-effective and efficient, and yields accurate quantification of multiple amino acids in the sub-picomole range.
It is expected that the concentration of amino acids in the intracrystalline fraction would be lower than in the whole shell. Therefore this method is highly appropriate for testing the ability of the bleaching treatment to isolate the amino acids from the intracrystalline fraction. Not all molluscs are suitable for closed system AAR dating and tests must be performed to investigate the behaviour of protein degradation in each taxon. This paper presents results on six different taxa: Extensive bleaching and heating experiments on modern specimens were performed for Patella and a large database collected Demarchi, Bleaching tests demonstrated that Patella retains a fraction of intracrystalline proteins which can be isolated by a 48 h bleaching step.
The effectiveness of 48 h bleaching in isolating the intracrystalline amino acids from Patella is in agreement with the data from other shell taxa analysed in the NEaar laboratory. However, the behaviour of this fraction during long term diagenesis could be such that it is inappropriate to use it for dating purposes. High-temperature kinetic experiments have traditionally been performed to monitor the behaviour of protein diagenesis, particularly amino acid racemization AAR , within laboratory timescales e.
Hare and Mitterer, Heating experiments can be performed to test the closed system behaviour of the intracrystalline proteins from any molluscan species, and check whether leaching of the amino acids from the biomineral into the external environment occurs. A number of tests were performed to investigate the reliability of Patella for protein geochronology Demarchi, For the bleached shells the concentration of amino acids in the water was similar to background levels.
In contrast, the concentration of amino acids in the water for unbleached shells was three orders of magnitude higher nanomoles Fig. This demonstrates that leaching is particularly marked for whole-shell unbleached Patella , which therefore does not represent a closed system. On the contrary, the intracrystalline amino acids in Patella approximate a closed system with regard to diagenesis, thus providing a robust substrate for reliable AAR dating Demarchi, On the contrary, the loss of amino acids from bleached shell powder is negligible.
Bleaching and heating experiments on modern samples are crucial for assessing the reliability of molluscan taxa for the closed system approach of AAR geochronology. However, such rigorous testing of each species is time consuming.
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This is a disadvantage when pilot data are required to assess the suitability or otherwise of different shell taxa which have never been previously investigated for closed system AAR. This information is useful during archaeological excavations in order to optimise the sampling strategies on-site and to develop research plans which include a detailed AAR investigation of the deposits. In order to test the suitability of the species available from the Red Sea middens, a simple initial experimental test for closed system behaviour was devised, which can be performed directly on archaeological shells from a site.
It is recommended that these tests should be performed routinely on new species of shells in order to provide an initial assessment of the potential of protein geochronology for each different taxon and site. Five species of shells, from different midden sites and among the most abundant in the archaeological record in this area, were targeted and the results obtained were used for informing further field sampling see Section 4.
These shells were collected from a group of middens located in an area north of the Harid bay, and are likely to be broadly contemporaneous based on their inland location and linear distribution Williams, in preparation. Chicoreus shells were collected from a basal layer in the shell midden. The bleached shells were heated at high temperature in sealed glass tubes under hydrous conditions to test for closed system behaviour.
This was performed for each of the five taxa. Three replicates were prepared for each time-point. The concentrations were comparable with background levels and, in most cases, fell below the limit of detection Fig. Note that only Strombus and Chicoreus intracrystalline values can be considered significantly higher than the limit of detection. The Limit Of Detection LOD was calculated on the basis of the amino acid concentration detected in procedural blanks used in this study: The low protein contents detected may be due to the sampling strategy, as each shell was sampled only in a single location.
It is therefore possible that this specific sampling area was enriched in mineral, but very poor in proteins, leading to the low amino acid concentrations observed. Further investigation of possible sampling strategies may help clarify this issue. The data recovered from Anadara , Trochus and Tibia were therefore not meaningful for the interpretation of intracrystalline protein diagenesis patterns. The results concerning the intracrystalline fraction within Anadara are particularly interesting, as this genus has been used in the past for traditional whole-shell AAR geochronology e.
On the contrary, both Strombus and Chicoreus samples showed high concentrations of amino acids in the intracrystalline fraction and so were tested further Fig.
Amino Acid Racemization Dating
No significant amounts of amino acids were leached out of the intracrystalline fraction, which therefore appears to behave as a closed system. Error bars represent one standard deviation around the mean. A sample falling outside the expected trajectory is likely to have been compromised e. Preece and Penkman, This is the case for the amino acids which yielded the best chromatographic resolution and were therefore targeted for this study: Therefore Ser was not included in the spider diagram in Fig. The racemization values for the species analysed were very high, as expected for an area of low latitude such as the Farasan Islands, where the higher temperatures would accelerate the reaction.
However, the high temperature experienced by Red Sea samples during their burial history limits the use of Asx as an age indicator at these latitudes. However, for Strombus the extent of racemization was lower Fig. Two more protein breakdown indicators can also be considered when testing the behaviour of a molluscan species with regard to protein diagenesis: Both species analysed showed increasing percentages of free amino acids with increasing heating time, with Chicoreus displaying more extensive degradation Supplementary information. This is likely to be due to the decrease in analytical precision at the low concentrations of Ser similar to background levels, see Supplementary information at these higher levels of protein decomposition, due to the quasi-exponential nature of diagenesis.
This represents the decomposition of Ser into Ala with the progression of diagenesis. Bleaching and heating tests can be used as a quick, routine protocol to detect the most suitable shell substrate to be targeted for geochronological investigations of a new geographical and climatic area. Three out of the five taxa targeted for the Red Sea pilot study show very low protein content in the intracrystalline fraction: As the amino acid concentration for these species is barely distinguishable from the background noise, the collection of these shells from archaeological sites for intracrystalline AAR studies is not recommended at this preliminary stage.
However, further research may be able to clarify if this is a bias introduced by sampling in areas of the shell characterised by low proteic content. They are therefore suitable for targeting for intracrystalline AAR dating. This initial assessment protocol thus provides useful information to aid the sampling strategies of excavations Section 4.
The bleaching and heating tests demonstrated that Patella , Strombus and Chicoreus retain an intracrystalline fraction of amino acids which behave as a closed system during diagenesis. These taxa were therefore used for the AAR investigation of shell-bearing archaeological deposits from two geographic areas: The extent of protein degradation was compared with available independent age information. Because of the persistence of a species effect within the closed system as well as the difference in thermal regimes between Scotland and the Red Sea, the extent of protein degradation cannot be directly compared.
This section describes the results obtained on fossil Patella shells from different archaeological sites in Scotland.
Patella specimens were analysed from two shell midden sites Table 1: Independent chronometric information was available, enabling the testing of the ability of closed system AAR to distinguish samples of different ages. Error terms represent one standard deviation around the mean for the site. Sand is a rock shelter associated with a well-preserved shell midden which shows evidence for discontinuous human occupation from as early as the late 7th millennium BC to the Neolithic.
Samples analysed for AAR dating courtesy of P. Coire Sgamhadail 1 has a small assemblage of shells and cultural material. Two radiocarbon determinations have been reported Hardy and Wickham-Jones, For the purpose of this study, it was therefore assumed that the site age ranged between and cal BC. Two sites of historic age were also considered.