Shell middens are one of the most important and widespread indicators for human exploitation of marine resources and occupation of coastal environments. Establishing an accurate and reliable chronology for these deposits has fundamental implications for understanding the patterns of human evolution and dispersal. This paper explores the potential application of a new methodology of amino acid racemization AAR dating of shell middens and describes a simple protocol to test the suitability of different molluscan species.
This protocol provides a preliminary test for the presence of an intracrystalline fraction of proteins by bleaching experiments and subsequent heating at high temperature , checking the closed system behaviour of this fraction during diagenesis. Only species which pass both tests can be considered suitable for further studies to obtain reliable age information. This amino acid geochronological technique is also applied to midden deposits at two latitudinal extremes: Northern Scotland and the Southern Red Sea. Results obtained in this study indicate that the application of this new method of AAR dating of shells has the potential to aid the geochronological investigation of shell mounds in different areas of the world.
Shell midden sites, found throughout the world, provide a range of important archaeological information, including the use of coastal resources, consumption practices and human impact on the environment.
These deposits are especially found after the establishment of modern sea level in the mid-Holocene, and have been recorded in their hundreds of thousands around the coastlines of the world, often forming large mounds containing many millions of shells. Earlier deposits are much less frequent, most probably due to Holocene sea level rise, resulting in the submergence of palaeoshorelines and the associated archaeological evidence Bailey and Flemming, However, earlier midden deposits are present in some areas of the world, often in deep cave sequences but also in some open-air locations.
Middens from the Last Interglacial Eemian and before have been reported in Africa e. The study of midden deposits, including their dating, must incorporate an accurate evaluation of the processes which have operated through time to produce the assemblages as they are observed today. Unfortunately, dating of these deposits can be problematic e.
Stein and Deo, Radiocarbon dating is usually applied to develop chronological frameworks for shell midden deposits. It is a relatively costly procedure, so in most cases only a limited number of samples can be selected for dating; ideally these have to have an established provenance within a clearly identified stratigraphic sequence in order to produce reliable dating results for further interpretation.
Shell middens, however, often do not meet these requirements.
Amino acid racemization dating of marine shells: A mound of possibilities
Often they accumulate relatively rapidly within the margins of error of radiocarbon dating, or are subject to high levels of disturbance, mixing and inversion of materials, requiring large sample sizes of dates to resolve issues of intra-site chronology e. In this context, a quick, cost-effective and above all reliable dating technique could be useful, allowing the processing of a large number of samples and an assessment of intra-site chronology and formation processes within shell midden deposits.
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. 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.
Amino Acid Dating. Is it reliable?
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.
- Amino acid racemisation!
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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. 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. 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.
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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.
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- Amino acid racemization dating of marine shells: A mound of possibilities!
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. 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.
Amino Acid Racemisation
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. 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.