Saturday, August 3, 2019

How we Date an Eggshell - Amino Acid Racemization

Small eggshell fragments can be dated and studied for their isotope compositions

All living things use large molecules called proteins to do the work in a cell. These proteins are composed of building blocks known as amino acids. There are about 20 different amino acids that are found in proteins. Anywhere from 50 to several hundred amino acids are linked together to form a long chain-like protein. Almost every amino acid comes in two forms, levorotary (left-L) and dextrorotary (right-D). These forms of amino acids are mirror images of each other however cannot be overlaid onto each other, just like right and left hands. The words come from the fact that if you shine a light on left-handed molecules; they bend light to the left. The same is true for the right-handed variety. However, most living things only use the L amino acids. When a creature dies the L amino acids begin to convert to dextrorotary amino acids through amino acid racemization. Racemization is the process in which one mirror image molecule turns into another.
All organisms have 99.95% L-amino acids in their proteins. When a bird or mammal dies, the proteins in its bones and teeth start to degrade. As they degrade, proteins are decomposed by bacteria into the amino acid building blocks. When this happens, the L-amino acids are likely to flip over to their mirror image, the D-amino acids. This is called racemization. Because of this process, researchers can use the amino acids found in eggshell samples to approximately determine the time of death of a creature. Each amino acid has a different rate of racemization and this rate is affected by certain environmental factors. These factors include temperature, water concentration, and acidity in the environment. Typically, temperature is believed to have the greatest impact on the rate. In hotter temperatures, the rate of racemization is faster. Because temperature plays such a significant role, using amino acid racemization to date eggshells is not always completely accurate. However, because the more accurate radiocarbon dating is only effective for samples from 40,000 years ago and after, amino acid racemization dating must be used to age samples from before 40,000 years ago.
In the late 1960s it was discovered that there is a clear division in carbon-13 values among terrestrial plants. Some plants like trees, bushes, and some grasses that grow in colder climates have lower carbon-13 values than others such as corn, sugar, and dryland grasses. Plants with lower carbon-13 levels are now know as C3 plants while their higher counterparts are known as C4 plants. The isotopic division is created during photosynthesis. In most plants, during photosynthesis carbon dioxide is incorporated into either a 3-carbon compound (C3), and so we call it a C-3 plant. C3 plants are found in a much broader range of environments than C4 plants. All pine trees, most flowering plants, and most of the vegetable we eat are C-3 plants.

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