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Results | Food Web and its Function | Interesting Isotopic Studies

Fingernail Results

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 13C and the light isotope is 12C. For nitrogen, the heavy isotope is 15N and the light is 14N.

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 (CO2 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 d13C value. Because the d13C value of atmospheric CO2 changes significantly over time and space, different laboratories (and analyses at different times) would give very different d13C 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 d13C and pronounced "delta see thirteen") and nitrogen isotope ratio values (written as d15N 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  13C to 12C isotopes than the carbon standard (i.e., relatively less 13C), so the d13C values are negative. For example, plants like wheat have d13C values that average about -27‰ (pronounced "minus twenty seven per mil", and meaning -27 parts-per-thousand). In contrast, animals typically have a higher ratio of 15N to 14N isotopes than the nitrogen standard (i.e., relatively more 15N), so the d15N 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 d13C and d15N 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 d13C value on the d13C 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 d15N value on the d15N 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: d13C = -18.8 and d15N = 9.3. Next, I go to the graph and locate -18.8 on the d13C 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 equally-spaced 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 d15N value of 9.3 (or $9.30) on the d15N 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 d13C 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 d15N and d13C 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 corn-fed 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).

Fingernail results

Interpreting your data

Comparing 2003 with 2000



Comparing 2006 with 2003


Where you fall on the food chain


Results | Food Web and its Function | Interesting Isotopic Studies

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This page was last changed in June 2006.
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