First published January 19, 2013 at paleopix.com/blog by Penny Higgins:
I’ve written a few blog posts about what can be done with isotopes from precipitation, and how that might assist us in understanding how to interpret isotopic data collected from ancient rocks and fossils. (Look here and here.) As I live here in western New York state, close to Lake Ontario, I frequently have opportunities to further study how the isotopes from precipitation (in this case Lake Effect snow) are related to the isotopes of the water that originally evaporated to make the clouds that do all the snowing.
Right now, we’re looking at a Lake Effect snow event that’s due to start sometime tomorrow, so I’m throwing together is quick and fun isotopic study that I’ll share with you when the data come in. I’ll describe it here.
As review, let’s think about isotopes in water. First, what do I mean by isotopes? The term worries people, because they immediately think of radioactive isotopes and OMG, we’re gonna die! No, it’s not like that. The word isotope just refers to the fact that some atoms of the same element are heavier or lighter than the others.
Water is composed of hydrogen and oxygen (H2O). Hydrogen comes in two types. Most of it has a mass (think of it as weight) of 1. Some of it has a mass of 2. (The hydrogen with a mass of 2 is called deuterium. It’s one of the few isotopes that has its own name.) So water is mostly made with hydrogen atoms of mass 1, but some water has hydrogen of mass 2. The water with the mass 2 hydrogen is heavier than the water with the mass 1 hydrogen.
Similarly, oxygen comes in two important isotopes. The most common form of oxygen has a mass of 16. A more rare (but not radioactive) form of oxygen has a mass of 18. Either type of oxygen can be in a water molecule, but the water with the mass 18 oxygen is heavier.
With mass spectrometry, we can measure water to see how much of it has the heavier hydrogen and the heavier oxygen. This is what I do for a living.
To get any kind of precipitation (rain or snow), water must first evaporate to make a vapor mass in the atmosphere. You can think of this as just making a cloud or a storm. In the case of Lake Effect precipitation, the water that’s evaporating is the lake itself. When the water evaporates, the lighter water evaporates more than the heavier water because, well, it’s lighter. So the cloud that you get from evaporation is isotopically lighter than the lake it evaporated from.
We measure ‘lighter’ or ‘heavier’ with isotopes using what we call ‘delta notation.’ The numbers we get are given in ‘permil’ (‰) even though they’re not a concentration. What’s important is that more positive delta values means that there’s more of the ‘heavy’ element. More negative values means there’s more of the ‘light’ element. So, if the lake has a delta value of -1‰, then the cloud should have a more negative value, like -3‰. When a cloud rains or snows, the heavier elements fall out first, because they’re heavier. If the cloud has an isotopic value of -3‰, the snow should have a more positive value, like -2‰.
The change between lake and cloud, or between cloud and snow, is called fractionation, and is controlled in part by temperature. (This means that the numbers I just gave you are completely made up.) The fractionation is also different for hydrogen and oxygen, and we measure these separately. (Hydrogen and oxygen isotopes in water do tend to vary together, but it can get pretty complex.)
As a cloud rains, it loses its heavy isotopes. If we take a cloud or storm (or say a hurricane) and take it from its water source (a lake or the ocean) and move it over land, this fractionation will go on. If no more water vapor is added, then the cloud gradually gets isotopically lighter. This means that the precipitation will also get lighter (but will always be heavier than the cloud). This process is called ‘Rayleigh Distillation,’ and is an important assumption in isotope geochemistry. Luckily, it has been shown to be a good model.
All right, let’s get back to Lake Effect snow. We’re looking at a Lake Effect event that is expected to start sometime tomorrow. We can get snow bands off of the lake that make great stripes of snow across the landscape.
What Lake Effect snow from Lake Ontario teach us?
We know that the snow will be forming from water evaporated off of Lake Ontario, so it will be useful to know the isotopic values of that water as a baseline. We have no way of measuring it isotopic values of the water vapor (the cloud) but we can find out the air temperature close to the lake surface and calculate the the isotopic value should be.
Then, we can measure the isotopic value of the snow that falls. We can collect snow that falls right at the lake (that which first forms from the freshly evaporated water) and we can look at snow that falls some distance away. We can make predictions about what patterns we might see.
1) Snow collected near the lake will be isotopically heavier than snow collected further away. Even though it’s only a few miles, Rayleigh Distillation should have some effect.
2) Over time, the snow collected at one location should not change in isotopic value, unless air temperature at the lake varies significantly. Because the cloud will be continuously replenished from Lake Ontario, I don’t expect to see any variability over time. The isotopic value of the lake water should not change consequentially. What can change is the air temperature, which will alter the fractionation of the isotopes (when it’s colder, less of the heavy water will evaporate). Also, colder temperatures could result in freezing of the lake surface, effectively moving the shoreline further into the lake.
It’s a pretty simple thing to test these predictions. I just need to collect some snow samples (and recruit other people to do the same). Specifically, I’ll be collecting every six hours, since I’ll be measuring snow depth at that time interval anyway. Collecting every twelve hours would probably be sufficient. I run a laboratory that has a liquid water isotope analyzer, so analysis will be easy. Once I’ve got the results, then it’ll be a quick write-up that everyone can benefit from here. It’ll be interesting to see how well my predictions hold.
If you live nearby and think you might be interested in helping out with this little project, let me know in the comments below. The more the merrier!
After waiting for 24 hours, there has not yet been any snow. But I’m assured it’s on the way!
@paleololigo It’s coming; 1 week from tonight I think you’ll have 1-2′ on ground.Tues PM you will get a foot, Wed, Thu, Fri will see more!
— PARKER ZACK (@PARKERzack) January 21, 2013
Snow is falling. Finally.
A snow sample for isotopic analysis. (Ziplock bags aren’t the best, but they’ll work!) twitter.com/paleololigo/st…
— Penny Higgins (@paleololigo) January 23, 2013