The Amino Acid Racemization Dating Method
As a matter of fact, the ages obtained from racemization dating must rely on other techniques such as Carbon 14, and if the dating of Carbon 14 is not accurate, racemization dating can never be certain. It is, perhaps, best considered to be a calibrated relative dating technique which puts it somewhere between relative and chronometric methods.
See additional references on the relative nature of amino acid racemization dating in the addendum section Link. Clearly, all of the above described variables for amino acid racemization rates create great difficulty for AAR as a dating technique. In fact, the difficulties are so great that this technique cannot be and is not used as any sort of "absolute" dating technique. So, how is it thought to be at all helpful?
Well, it is thought to be helpful as a "relative" dating technique. To overcome the various uncertainties inherent to amino acid dating, the method must be "calibrated" based on other more reliable techniques such as radiocarbon dating carbon 14 dating. These values are used to solve for a constant or "k" in the formula used to estimate ages based on the calibration sample. Of course, the " major assumption required with this approach is that the average temperature experienced by the 'calibration' sample is representative of the average temperature experienced by other samples from the deposit.
At first "cubic transformations"' and then later "power function transformations" were used that seemed to show a "strong correlation with time, but did not explain the observed kinetics.
What this basically means is that amino acid dating is not based on any sort of understanding about how racemization takes place, but is strictly a function of correlation with other dating techniques, such as the radiocarbon technique.
So, if there is any problem with the basis of the correlation i. In this light, it is interesting to consider what happened in when some of the major proponents of amino acid dating Bada et al decided to analyze the Paleo-Indian skeletal material from Del Mar, California. Their estimated age of 48, years before present BP "stunned" the archaeological community who generally believed these bones to be less than 10, years old.
Bada went on to date other skeletal specimens between the 35, and 48, year range with one specimen from Sunnyvale being dated at an astonishing 70, years BP. Then, in the s, something very interesting happened.
Conventional plus accelerator mass spectrometry AMS radiocarbon dating Taylor et al. The Oxford dates were all between 4, and 8, years BP and the Arizona dates were between 3, and 6, years BP.
They did note that there appeared to be a direct relationship between the extent of racemization and the level of preservation of collagen in the bones. Those samples with the most racemization had the lowest amino acid content and this poor preservation of protein would contribute to anomalous AAR results. Later, based on AMS radiocarbon dates, Bada calculated a new value for k asp for the Californian samples.
He used the Laguna skull and the Los Angeles Man skeleton as 'calibration' samples for this. They all fell within the Holocene but had much larger error estimates than those of the AMS values. Pollard and Heron also point out that there is poor concordance between the conventional and the AMS radiocarbon dates and there is no concordance between the uranium series dates and any of the other dates either.
At best three of the four methods put the bones in the Holocene. Because of these problems AAR dating of bone and teeth teeth in different locations in the same mouth have been shown to have very different AAR ages is considered to be an extremely unreliable practice even by mainstream scientists.
That is because the porosity of bones makes them more "open" to surrounding environmental influences and leaching. Specimens that are more "closed" to such problems are thought to include mollusk shells and especially ratite bird eggshells from the emu and ostrich. Of course, even if these rather thin specimens were actually "closed" systems more so than even teeth enamel they would still be quite subject to local temperature variations as well as the other above-mentioned potential problems.
For example, even today " very little is known about the protein structure in ratite eggshell and differences in primary sequence can alter the rate of Asu formation by two orders of magnitude [fold] Collins, Waite, and van Duin Goodfriend and Hare show that Asx racemization in ostrich eggshell heated at 80 o C has complex kinetics, similar to that seen in land snails Goodfriend The extrapolation of high temperature rates to low temperatures is known to be problematic Collins, Waite, and van Duin A pilot study would be necess ary and a reliable relationship between racemate ratio and time could remain elusive.
The Changing Correlation Constant k. Also, there is a poten tial problem with radiocarbon correlations that is quite interesting. Note what happens to the correlation constant k with assumed age of the specimen in the following figures. Interestingly enough, the racemization constant or "k" values for the amino acid dating of various specimens decreases dramatically with the assumed age of the specimens see figures.
Note that these rate differences include shell specimens, which are supposed to be more reliable than other more "open system" specimens, such as wood and bone. Is this a reasonable assumption? Well, this simply must be true if radiocarbon dating is accurate beyond a few thousand years.
Amino acid dating - Infogalactic: the planetary knowledge core
But, what if radiocarbon gets significantly worse as one moves very far back in time beyond just a few thousand years? In other words, what would it mean for one to assume that the k-values remained fairly constant over time as would seem intuitive? Well, with the k-values plotted out horizontally on the graph, the calculated ages of the specimens would be roughly affected as follows: Current Fossil Age Assignment. Adjusted Fossil Age Assignment with horizontal k-values.
Clearly this is a dramatic adjustment that seems to suggest that amino acid racemization may be more a reflection of the activities of local environmental differences than any sort of differences in relative ages. This seems especially likely when one considers that each type of specimen and each different location have different k-values meaning that the radiocarbon-derived constant in one region or with one type of specimen cannot be used to calculate the age of any other specimen or even the same type of specimen in a different location.
Add to this the fact that radiocarbon dating is also dependent upon the state of preservation of the specimen. They dated a number of fractions ranging from insoluble collagen to individual amino acids from each of a selection of differentially preserved mammoth and human bone.
Age estimates from the fractions within a bone were consistent if it was well preserved. Thus the final irony is that the poorly preserved Californian Paleo-Indian bones would return Holocene 14C dates even if they were actually Pleistocene. The state of preservation of the bone appears to be as important an issue for radiocarbon dating as it is for AAR dating.
So, what do we have? In short, it seems like the claims of some scientists that amino acid racemization dating has been well established as reliable appears to be wishful thinking at best.
The huge number of confounding factors and a complete inability to explain the calibrating k-values in terms of amino acid kinetics leaves those with even a tiny pessimistic bone in their bodies just a bit underwhelmed. For many decades the observation that petroleum shows optical activity, usually favoring L-enantiomers, was used to prove the biogenic origin of petroleum.
However, more recently there have been scientists who have argued for the non-biogenic origin of petroleum, citing situations where optical activity can be produced in non-organic materials, to include hydrocarbons.
A particularly impressive proof of this hypothesis was the discovery of L-enantiomers in proteins within meteorites Link. Subsequent analysis by Bada et. However, in research showed that individual amino-acid enantiomers from Murchison were enriched in the nitrogen isotope 15 N relative to their terrestrial counterparts, which seemed to suggest an extraterrestrial source for an L-enantiomer excess in the Solar System Link.
Then in a paper by Pizzarello and Cooper again seemed to confirm the contamination argument for the origin of optical activity for amino acids within meteorites. There is currently still some debate, but the consensus seems to currently favor the contamination theory Link. This is all very interesting because optical activity decays over time toward a racemic state.
Optically active petroleum is usually found with temperatures of 66 degrees Celsius. At such temperatures, optical activity should not be maintained for more than 10 or 20 million years at most.
Yet, optical activity within petroleum, usually of the L-enantiomeric type, seems to be maintained in significant degrees despite ages assumed to be over million years old?
How is this explained? Brown, Amino Acid Dating, Origins 12 1: Regarding the relative nature of amino acid racemization dating - i. In the case of the California paleoindian skeletons, the original racemization ages were derived using the Laguna skeleton dated by conventional radiocarbon method Beta-counting. Amino-acid ratios can be used for either relative or absolute dating. Absolute dating requires calibration with radiometric techniques, such as radiocarbon dates, and knowledge of the temperature history of the fossil.
Once such information is established for a region, amino-acid dating may be used with confidence. However, amino-acid time calibration cannot be extended beyond the area of study due to regional differences in temperature history. In those articles I show that after 'calibrating' the amino acid racemization reactions using a radiocarbon dated bone, it is then possible to date other bones from the same site, which are either too old or too small for radiocarbon dating.
The only assumption in this approach is that the average temperature experienced by the calibration sample is representative of the average temperature experienced by the other sample.
Ages thus deduced are in good agreement with radiocarbon ages determined on the same samples. Carroll uses amino acids with C dating to come up with a calibration curve.
To overcome the problem of inherent uncertainty in the temperature history of sub-fossil bone Bada and colleagues Bada and Protsch ; Bada et al. A bone from a site was chosen as a.
These values were substituted into equation 2. The result was an in situ kasp value for the site. After substituting in this k asp value equation 2. The major assumption required with this approach is that the average temperature experienced by the.
More recently researchers have developed calibration curves using a number of age estimations by independent dating methods e. Such transformations allow a strong correlation with time but do not explain the observed kinetics. The apparent implication is that either the geochronologic age is incorrect or the samples were contaminated by recent organic material.
The four magnitude 10 4 range of data for both rate constant and age require a logarithmic scale on each axis. These data include mollusk shells, foraminifera shells, foraminifera ooze, coral, bones, wood, and parchment Dead Sea Scrolls.
With a few exceptions among ages in the less than years range, the age assignments are based on radiosotope data, either carbon or disequilibrium of uranium daughter products.
The specimens represented in figures 3 and 4 were obtained from a wide range of arctic, desert, temperate, and ocean floor environments. Some of them may have inconsistent age assignments. The most impressive immediate impact of these plots is that for a particular amino acid there is no characteristic racemization rate constant that can be used to estimate the age of every fossil containing that amino acid. The demonstrated clustering about a line which slopes downward indicates that the apparent racemization rate constant is actually not a constant, but is related to fossil age, diminishing as age increases.
This observation has been made frequently in the literature e. Given the sensitivity of racemization rate to temperature, could the general trend in figures 3 and 4 indicate that the older a fossil the lower the average storage temperature it has experienced? These simplified estimates fully establish that the pattern of figures 3 and 4 cannot be explained on the basis of lower temperature on land and on the ocean floor in the past Miller and Hare , p.
Other factors that may account for the pattern of figures 3 and 4 will be considered subsequently. It has been asserted that amino acid age dating corroborates radiocarbon ages and age assignments based on disequilibrium of uranium decay products e. With this simplified introduction to a highly complex topic one can appreciate the following comments from a paper presented at the conference on Advances in Biogeochemistry of Amino Acids Miller and Hare , pp.
Amino Acid Dating. Is it reliable?
From Figures 3 and 4 it is apparent that for any specific amino acid there is not one characteristic racemization rate constant that is appropriate for all ordinary circumstances at all times, as is the case for radioisotope dating. The racemization rate of an amino acid has been determined to be dependent on the following factors Smith and Evans , Kriausakul and Mitterer a, b. This complication can be avoided by restricting analysis to amino acids or amino acid components which are unlikely to have been affected by bacteria, or to samples which do not contain excess amounts of the amino acids that are characteristic of bacterial activity.
In consideration of the ten factors listed above, it is evident that equations 3 and 4 yield an effective average k that represents the combination of a large number of specific rate constants that have been descriptive of various individual amino acid molecules at different times throughout the history of a specimen.
Historical Geology/Amino acid dating - Wikibooks, open books for an open world
On the basis of this insight one can expect that during the early history of a fossil D-amino acids will accumulate relatively rapidly from L-molecules that may be described by a relatively high value of k, and that as time progresses accumulation will be increasingly limited to L-molecules associated with lower values of k. Such a model is the favored explanation for the pattern displayed in figures 3 and 4. Since the observed range of variation in k at any particular fossil age is much less, and over the total range of fossil age is much greater, than the range allowed by the available data on racemization rate as a function of protein molecule size, explanation of the pattern displayed in figures 3 and 4 in terms of a progressive decline in the effective average value of k is at least questionable.
As an alternative to the model based on D-amino acid accumulation becoming increasingly limited to L-molecules initially associated with lower k, one could propose that as a fossil ages the breakdown of proteins continually renews the supply of L-molecules in locations with the higher values of k.
It is significant that temperature, water concentration, and alkalinity to which the racemization rate in a fossil is particularly sensitive are also factors which are particularly conducive to the breakdown of larger protein molecules to smaller components.
Before conclusions may be drawn with confidence concerning change of the average racemization rate with time we should have studies such as that represented in Table 1 for each of several samples with well-determined fossil age assignments ranging from years to 1,, years. This possibility is clearly indicated if the average probability for conversion of L-amino acids to the D-form in a sample actually remains roughly the same from century to century.
Many difficulties would be resolved, and much anomalous data would be reconciled if fossil ages were adjusted to make the data points in figures 3 and 4 scatter about a horizontal line, rather than a line sloping downwards.
While the data discussed in this paper provide an adequate scientific basis for such an adjustment, the extent to which it "would undermine other conclusions" is a price very few members of the scientific community would be willing to pay.
The course of preference is to assign the apparent inverse relationship between effective average racemization rate constant and fossil age to some time-dependent factor that is not yet fully understood. A more sophisticated data analysis would be required before such claims should be made. Due to the strong dependency of racemization rates on temperature, water concentration, and alkalinity, uncertainties regarding conditions of preservation can leave age relationships among even similar fossils open to question.
The present status of amino acid dating can be summarized by the conclusion from the 19th International Symposium on Archeometry and Archaeological Prospection that "the time when [amino acid racemization] can provide a problem-free dating service is still some way off" Hedges In addition to the wide range of fossil age associated with a given value of a racemization rate constant and the wide range of racemization rate constant associated with a given fossil age, there is a dominant trend for the effective racemization rate constant to decrease with putative fossil age.
This relationship, together with the demonstrated survival of amino acids in fossils from the Paleozoic era, raises a question concerning the accuracy with which radioisotope age data have been used to represent the real-time history of fossils.
I am deeply indebted to reviewers of this paper for suggestions that have contributed to readability for the general reader and rigor for the specialist. To these reviewers, and also all readers, I must express my regret at not having the skill or patience to meet these goals more fully. Carnegie Institution of Washington Year Book 54 , pp.
Abelson , Philip H. Scientific American 1: Some aspects of paleobiochemistry. Annals of the New York Academy of Sciences Total amino acid content of fossil pectin shells. Bada , Jeffrey L. Racemization of amino acids in fossil bones and teeth from the Olduvai Gorge region Tanzania, East Africa. Earth and Planetary Science Letters Accuracy of dates beyond the 14 C dating limit using the aspartic acid racemization reaction. Racemization reaction of aspartic acid and its use in dating fossil bones.
Proceedings of the National Academy of Science Racemization of isoleucine in calcareous marine sediments: Kinetics and mechanisms of amino acid racemization in aqueous solution and bones. In Hare et al.
The racemization reaction of isoleucine used as a paleotemperature indicator. Accelerator mass spectrometry radiocarbon ages of amino acid extracts from California palaeoindian skeletons. Bender , Michael L. Reliability of amino acid racemization dating and paleotemperature analysis of bones. Bischoff , James L. Temperature calibration of amino acid racemization: Blake , Weston, Jr. Application of amino acid ratios to studies of Quaternary geology in the high Arctic.
Fossil shell "concholin" and other preserved biopolymers. Springer Verlag, New York and Amsterdam. Amino acid composition of some calcified proteins. Carnegie Institution of Washington Year Book 64 , pp. Racemization of amino acids in fossil shells. Carnegie Institution of Washington Year Book 66 , pp. Biogeochemistry of Amino Acids. Non-protein amino acids in fossil shells. Carnegie Institution of Washington Year Book 65 , pp. Laboratory simulation of amino acid diagenesis in fossils.
Carnegie Institution of Washington Year Book 67 , pp. Phylogenetic information derivable from fossil brachiopods. Necessity of reporting amino acid compositions of fossil bones where racemization analyses are used for geochronological applications: Species effects in the epimerization of L-isoleucine in fossil planktonic foraminifera.
Carnegie Institution of Washington Year Book 71 , pp. King , Kenneth, Jr. Effect of in situ leaching on amino acid racemisation rate in fossil bones. Isoleucine epimerization for dating of marine sediments: Kriausakul , Nivat and Richard M. Comparison of isoleucine epimerization in a model depeptide and fossil protein.
Geochimica et Cosmochimica Acta Some factors affecting the epimerization of isoleucine in peptides and proteins. Kvenvolden , Keith A. Advances in the geochemistry of amino acids. Interlaboratory comparison of amino acid racemization in a Pleistocene mollusk, Saxidomus giganteus.
Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology , archaeology , forensic science , and other fields. This technique relates changes in amino acid molecules to the time elapsed since they were formed.
All biological tissues contain amino acids. All amino acids except glycine possess an asymmetric carbon atom, which means that the amino acid can have two different configurations, "D" or "L". With a few important exceptions, living organisms keep all their amino acids in the "L" configuration. The rate at which racemization proceeds depends upon the type of amino acid, average temperature, humidity, acidity, pH, and characteristics of the enclosing matrix.
Temperature and humidity histories of microenvironments are being produced at ever increasing rates as technologies advance and technologists accumulate data. These are important to amino acid dating because racemization occurs much faster in warm, wet conditions compared to cold, dry conditions. Temperate to cold region studies are much more common than tropical studies, and the steady cold of the ocean floor or the dry interior of bones and shells have contributed most to the accumulation of racemization dating data.
Strong acidity and mild to strong alkalinity induce greatly increased racemization rates. Generally, they are not assumed to have a great impact in the natural environment, though tephrochronological data may shed new light on this variable. The enclosing matrix is probably the most difficult variable in amino acid dating. This includes racemization rate variation among species and organs, and is affected by the depth of decomposition, porosity, and catalytic effects of local metals and minerals.
Asparagine acidified to aspartic acid racemizes quickly and has frequently been used to date materials from the present back to around BP. Isoleucine racemizes much more slowly, and has been used to date materials from to 2 million years of age. Concentration thresholds and less comprehensive environmental histories produce much greater margins of error with older isoleucine measures. Other amino acids are less frequently used for dating, primarily because of difficulties in isolation.
Data from the geochronological analysis of amino acid racemization has been building for thirty-five years. Stratigraphy , oceanography , paleogeography , and paleoclimatology have been particularly affected.
Their applications include dating correlation, relative dating, sedimentation rate analysis, sediment transport studies, sea level determinations, and thermal history reconstructions. Paleobiology and archaeology have also been strongly affected. Bone, shell, and sediment studies have contributed much to the paleontological record, including the hominoid.
Verification of radiocarbon and other dating techniques by amino acid racemization and vice versa has occurred. The 'filling in' of large probability ranges, such as with radiocarbon reservoir effects, has sometimes been possible. Paleopathology and dietary selection, paleozoogeography and indigineity, taxonomy and taphonomy , and DNA viability studies abound.
The differentiation of cooked from uncooked bone, shell, and residue is sometimes possible. Human cultural changes and their effects on local ecologies have been assessed using this technique. The slight reduction in this repair capability during aging is important to studies of longevity and old age tissue breakdown disorders, and allows the determination of age of living animals. Amino acid racemization also has a role in tissue and protein degradation studies, particularly useful to developing museum preservation methods.
These have produced models of protein adhesive and other biopolymer deteriorations and the concurrent pore system development. Forensic science can use this technique to estimate the age of a cadaver or an objet d'art to determine authenticity.
Amino acid racemization analysis consists of sample preparation, isolation of the amino acid wanted, and measure of its D: Sample preparation entails the identification, raw extraction, and separation of proteins into their constituent amino acids, typically by grinding followed by acid hydrolysis.
The amino acid hydrolysate can be combined with a chiral specific fluorescent, separated by chromatography or electrophoresis, and the particular amino acid D: L ratio determined by fluorescence.
Or, the particular amino acid can be separated by chromatography or electrophoresis, combined with a metal cation, and the D: L ratio determined by mass spectrometry.
Chromatographic and electrophoretic separation of proteins and amino acids is dependent upon molecular size, which generally corresponds to molecular weight, and to a lesser extent upon shape and charge. The Full Wiki Search: Many of our articles have direct quotes from sources you can cite, within the Wikipedia article!
This article doesn't yet, but we're working on it! See more info or our list of citable articles. From Wikipedia, the free encyclopedia Amino acid dating is a dating technique used to estimate the age of a specimen in paleobiology , archaeology , forensic science , and other fields. Articles lacking sources from March All articles lacking sources Wikipedia articles needing context from August Wikipedia introduction cleanup from August Articles needing cleanup from March All pages needing cleanup.
Got something to say? Your name Your email address Message. This content and its associated elements are made available under the same license where attribution must include acknowledgement of The Full Wiki as the source on the page same page with a link back to this page with no nofollow tag. Geologic time scale and techniques. Emerson Calibration of amino acid racemization in late Pleistocene mollusks: Wehmiller Comparison of amino acid racemization geochronometry with lithostratigraphy, biostratigraphy, uranium-series coral dating , and magneto-stratigraphy in the Atlantic coastal plain of the southeastern United States, Quaternary Research 18, Amino Acid Racemization or AAR is the process by which there is an interconversion of amino acid s from one chiral form the L - laevo amino acid s which are the building blocks of proteins to a mixture of L- and D- dextro forms following protein degradation.
In most biominerals but not all, bone is a notable exception amino acid s occur within the biomineral crystallites, where they modify the behavior of the biomaterial. Figure 2 illustrates this pattern for isoleucine, which together with aspartic acid has received the greatest attention in amino acid dating.
The present status of amino acid dating can be summarized by the conclusion from the 19th International Symposium on Archeometry and Archaeological Prospection that "the time when [ amino acid racemization] can provide a problem-free dating service is still some way off" Hedges Amino acid dating of Saxidomus giganteus at Willapa Bay, Washington by racemization of glutamic acid.
This project aimed to determine if oogonia are a viable source of sample material for amino acid geochronology. The racemization rate for each amino acid in a given taxon must be calculated in order to understand the age or thermal history of a sample.
However, they are the exception and just as amino acid s are found predominately in one configuration, so are the natural sugars, but this time in the D-form. The chirality of the amino acid s and sugars in turn cause proteins and DNA to be chiral handed , and thus are often the cause for highly specific biological function. If we symbolize the considered amino acid with AA, the amount of molecules of each isomer with m and n, and the constants that define the racemization speed with kL and kD, the reaction may be described as:.