Using a simplified model of 'racemization kinetics' of bone collagen over geological time Collins shows that almost any value of D-Asx can be obtained by varying the rates of collagen denaturation and leaching of the denatured product. The Interaction of bacteria and fungi with organic specimens may also be problematic. Such creatures have various enzymes that can digest various types of proteins, such as collagen, that are commonly used for dating.
Various studies, such as those dealing with 18 th and 19 th century burials, showed "unexpectedly high levels of aspartic acid racemization. So, special care is obviously needed in order to particularly avoid this sort of contamination. The rate of racemization can not be standardized by itself because it is too changeable. Thus, because of the rate problem, this dating technique must rely on other dating techniques to standardize its findings. 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.
Amino acid dating
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.
Amino acid racemization dating of marine shells: A mound of possibilities
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.
Amino acid dating - Wikipedia
The Changing Correlation Constant k. Also, there is a poten tial problem with radiocarbon correlations that is quite interesting.
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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. 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?
Related where has amino acid racemization dating been used
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