Interpreting the Data: Isotope terminology
Let's quickly review
some basic isotope terminology, so you know how to compare your isotope
values to foods and other visitors on the graphs. When we analyze
a fingernail sample for isotope ratios, we are comparing the relative
amounts of a "heavier" isotope and a "lighter" isotope
in your fingernail to trace what you ate. These isotopes are
"stable", meaning that they are not radioactive. For carbon, the
heavy isotope is ^{13}C and the light isotope is ^{12}C.
For nitrogen, the heavy isotope is ^{15}N and the light is ^{14}N.
Did You Know?

The international reference
standard for nitrogen isotopes is nitrogen gas found in common air! The nitrogen
isotope composition of air changes very little from place to place and
over time.
The international
reference standard for carbon isotopes is VPDB, which is shorthand
for "Vienna Pee Dee Belemnite". The original PDB sample was a
sample of fossilized shells of an extinct organism called a belemnite
(something like a shelled squid) collected decades ago from the banks
of the Pee Dee River in South Carolina. The original sample was
used up long ago, but other reference standards were calibrated to that
original sample. We still report carbon isotope values relative to PDB
but now use the term “VPDB” to indicate that the data are
normalized to the values of that standard.
After learning
that nitrogen in air is used as the nitrogen isotope reference
standard, you might ask why we couldn't simply use the carbon isotope
composition of air (CO_{2} gas) as the standard for carbon
isotopes, instead of using fossil shells. Good question! The short
answer is that a reference standard needs to have a constant value,
because all analyzed samples are compared to it to calculate the d^{13}C value. Because the d^{13}C value of atmospheric CO_{2}
changes significantly over time and space, different laboratories (and
analyses at different times) would give very different d^{13}C values for your fingernail
if that were the standard! The early isotope pioneers understood this
and wisely selected VPDB as the standard.


When isotope
biogeochemists (like us USGS folks) analyze samples for isotope
composition, we report the results in a special notation, called
"delta notation", that has a similiar format for carbon
and nitrogen
isotopes. The carbon isotope ratio values, (written as d^{13}C and pronounced "delta
see thirteen") and nitrogen isotope ratio values (written as d^{15}N and pronounced "delta en
fifteen"), that we determined by analyzing your fingernail sample, are
simply a way of comparing the isotope composition of a sample with that of
an international reference material (called a "standard").
Specifically, we
compare the ratio of the heavier to lighter isotope in the sample with the
same ratio in the standard. Your body parts and most living things have a
smaller ratio of ^{13}C to ^{12}C isotopes than the
carbon standard (i.e., relatively less ^{13}C), so the d^{13}C values are negative. For
example, plants like wheat have d^{13}C
values that average about 27‰ (pronounced "minus twenty seven
per mil", and meaning 27 partsperthousand). In contrast, animals
typically have a higher ratio of ^{15}N to ^{14}N isotopes
than the nitrogen standard (i.e., relatively more ^{15}N), so the d^{15}N values are positive.
Finding your 2006 isotope value on the graphs
To find where your
fingernail isotope values for 2006 plot relative to foods and other Open
House visitors on the graph, first go to
the data table and find the number you
were assigned when you gave your fingernail sample. Write down the d^{13}C and d^{15}N values for your number from
the table. Recall that these values have units of "per mil"
(symbolized by "‰"), which is parts per thousand difference
from the isotope ratio of the reference standard. You can locate your value
on either the plot of all the 2006 data, or
on separate plots for Saturday visitors and
Sunday visitors.
Next, locate your d^{13}C value on the d^{13}C scale (bottom axis) of the
chosen graph and draw a vertical line up from that point, across the graph.
It may help to print out the graph first. Then locate your d^{15}N value on the d^{15}N scale (left axis) of the
graph and draw a horizontal line across the graph at that point. The place
where your lines cross is where your fingernail plots.
Finding your isotope values
Let's say I was
visitor number 118 during the Open House in 2003 (see
example). I find 118 in the left column of the data table and write
down my isotope values listed to the right: d^{13}C = 18.8 and d^{15}N
= 9.3. Next, I go to the graph and locate 18.8 on the d^{13}C scale (bottom axis). It may
seem odd at first to deal with negative numbers like this. But the
important thing to remember is this: the larger the number after the
negative sign, the farther left you'll move to find it. Therefore, I know
that 18.8 is between 19 and 18, but closer to 19.
Starting at the 18
mark, I imagine ten equallyspaced divisions between 18 and 19 on the
graph scale and move left, counting off 8 (for the 0.8, or 8 tenths of a
unit on the scale). The mark halfway between 19 and 18 (i.e., 18.5)
should provide additional help with locating values. As an analogy to help
with the graph, you might think of the isotope values as money: so find
$18.80 between the $18.00 and $19.00 marks on the graph, using the $18.50
mark to help. When I locate 18.8, I draw a line straight up across the
graph.
Next, I find my d^{15}N value of 9.3 (or $9.30) on
the d^{15}N scale (left axis) in the same
way. This one is a bit easier, because the numbers aren't negative.When I
locate 9.3, I draw a horizontal line to the right across the graph. My
fingernail plots where this line and the vertical line drawn for the d^{13}C value cross.
What does it mean?
The carbon and
nitrogen isotope compositions of various
types of diets are known and can be compared to the compositions of
your fingernails. Hence, the graphs of your
fingernail d^{15}N and d^{13}C values can help to identify
your diet. In general, if you eat more fruits, vegetables, grains, and meat
fed with these foods (like beef), your fingernail will plot farther to the
left. In contrast, eating more corn, sugar, and cornfed meat will result
in fingernails that plot farther to the right. People who eat more animal
protein, and especially marine fish, will plot higher up on the graph, whereas
those who eat less meat will plot lower down. Keep in mind that, on
average, your isotope values are 1 unit higher in C isotopes and 3 units
higher in N isotopes than your average diet (see food
web diagram).

