GLACIER MELT OR PRECIPITATION?

One of the main objectives of my research is to simply identify the water source of each high-elevation pond. I want to know if frogs are perhaps relying on glacier-fed ponds more than ponds created by seasonal precipitation. Ponds connected to glacier melt should stay wet longer compared to ponds that are cut off from meltwater, because the contribution from precipitation during the dry season is almost zero. Since the daily freezing temperatures at this elevation limit the speed at which tadpoles can develop into frogs, glacier-fed ponds that persist longer may make better reproductive habitat than more ephemeral ponds.

Stick with the blog to learn how I’ll figure out how tadpole development times are affected by ponds drying up and water temperature. For now, I’d like to tell you about using water chemistry to distinguish glacier-fed ponds from precipitation-fed ponds.

A primer on stable isotopes. Everybody knows that water molecules are made up of two atoms of hydrogen and one oxygen. The lesser-known fact is that there are different stable versions (a.k.a. stable isotopes) of both Hydrogen and Oxygen out there in the world; they’re in your drinking water, and in your body. Note that many other elements also have stable isotopes, and by “stable” I just mean that they don’t exhibit radioactive decay. An example of an unstable isotope is 14C, whose radioactive decay occurs at a known rate and is used to date organic material less than ~50,000 years old.

So, hydrogen has two stable versions: the light isotope 1H (1 proton and 0 neutrons in the nucleus) and the heavier isotope 2H (1 proton and 1 neutron in the nucleus). We call 1H the light stable isotope because it has a smaller mass than 2H. Oxygen has three stable isotopes (16O, 17O, and 18O). In nature, the heavier stable isotopes  of O and H are more rare than the light stable isotopes.

Through extensive global sampling of surface ocean water and precipitation, we know the global ratio of light stable isotopes to heavy stable isotopes for hydrogen and oxygen. For example, 1H accounts for 99.98% of all hydrogen in the oceans and 2H is the other 0.02%. This is key because these data provide a point of comparison for all other waters. Measurements of the proportion of light to heavy stable isotopes in other waters (like in the Amazon River, or in my frog ponds) vary from this global average, and that variation can be used to group water from similar sources together.

Why does the ratio of light to heavy stable isotopes ever change? I mean, they’re stable, right? Yes, but because the isotopes have different masses, they have slightly different chemical properties. For example, the heavier isotopes require more energy (=higher temperatures) to evaporate. This means that water made with 1H will evaporate at a lower temperature than heavy water made with 2H. Therefore over time in evaporating surface ponds 2H will accumulate; the ratio of 2H will increase (i.e., become enriched) relative to 1H.  To illustrate the point:

MWL is the Meteoric Water Line, the line drawn by plotting the ratio of heavy and light stable isotopes of oxygen and hydrogen from global precipitation. Samples to the right of the line are enriched by evaporation. As an example of how to interpret this figure, the Darling River in arid Australia is characterized by more evaporation than the mighty Amazon. The Amazon River stable isotope signature is very similar to rainwater.

Surface ponds that don’t have glacier meltwater as a source should become enriched in heavy stable isotopes of both hydrogen and oxygen because of evaporation. Glacier-fed ponds should remain relatively depleted in stable isotopes, much like the glacier ice itself. So, I expect my samples to fall right about where it says “MWL” on the figure above because isotope signatures from colder climates are at the bottom of the global line. The surface ponds not connected to glacier melt should be farther to the right than glacier-fed ponds.

Only extensive sampling will tell if this hypothesis is correct. I am collecting water samples from each pond and each water source (example: the glacier edge, precipitation events) every month to track the contribution of glacier melt over the course of one full dry and wet season.

I hope that soon I’ll be able to post real data from my field site! As soon as the isotope analyzer begins to cooperate…

I welcome your comments and, even better, your questions!

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