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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