Category Archives: Stable Isotopes

On the Necessity of Specialist Meetings – #ASITA2013

I am writing this from a classroom at the University of Calgary, where I’m attending a conference called ASITA (Advances in Stable Isotope Techniques and Applications). I’ve been tweeting about it here (Apparently, though, I’m the only one tweeting. Maybe others will join next year.)

Continue reading “On the Necessity of Specialist Meetings – #ASITA2013”

The Mystery of the Delta Value – Why ‰ isn’t Parts Per Thousand

I’m about to gripe. But it’s a science gripe. It’s a technical gripe. It’s about stable isotopes. If you aren’t interested in stable isotopes, I suggest you tune out now.

There’s this thing in stable isotopes, especially the so-called ‘light’ isotopes like hydrogen, oxygen, carbon, and nitrogen. It’s called the ‘delta’ value. When we say what the stable isotopic measurement of something is, we say ‘the delta value is blah-blah-blah.’ The value is always in the units of permil (‰).

“δ13C of warm-season grasses is -14‰.”Continue reading “The Mystery of the Delta Value – Why ‰ isn’t Parts Per Thousand”

Random Science Poems Written on the Spot

Tonight, I was at a loss for what to blog about. The prompts I have available are not as inspiring as usual. So I asked the hive mind for suggestions:

Continue reading “Random Science Poems Written on the Spot”

What Can A Clam Teach Us About Climate Change?

One of the many projects I work on involves the study of climate change in the fossil record. I’ve put a bit of it on-line here. What I’ve published thus far deals mostly with interpreting general climatic and environmental factors using bulk geochemistry (all isotopes) from rocks and the fossil contained therein. That is to say, I take a big rock or fossil and grind it (or part of it) down into a single sample. I analyze that and call that a ‘average’ for that entire rock layer.

It turns out that clams (and mollusks in general) do a good job of recording environmental signals not just in bulk, but on a fine scale, such that we can see yearly, monthly, even daily records of weather.Continue reading “What Can A Clam Teach Us About Climate Change?”

Lake Effect Snow – An Isotopic Study: Some Results

A while ago, I proposed an experiment in which I collected snow at regular intervals during a Lake Effect Snow event. I made some predictions and collected the snow, and have now finally succeeded in analyzing the waters. The results weren’t quite what I expected.

I had predicted that isotopic values in the snow would not change over the course of the event. This was because all the snow would be forming directly off the lake, which is only a few miles away from the collection point. (This is in contrast to other synoptic storms, where we have precipitation coming from a single vapor mass, which will evolve isotopically over time. Read more about that here.) The temperature of the lake water, and its isotopic value would not change consequentially over the course of such a short event.

What I saw instead was an increase isotopic values overnight, and then a decrease the next day.

Results from January 22-23 Lake Effect Snow Event
Results from January 22-23 Lake Effect Snow Event. Click to enlarge.

I compared the isotopic values with measures of air temperature during that period of time. I selected first the air temperature measured at the Rochester International Airport (ROC). Isotopic values do track temperature changes, thus I realized that what is most likely happening is the fractionation of isotopes (the selective evaporation of the heavier versus the lighter water) in both the formation of the water vapor off the lake and more importantly the freezing of that vapor into snow which is changing over time due to temperature.

I realized that ROC is actually sufficiently removed from the lake, that its measured temperatures are likely to be different than those directly adjacent to the lake. Shoreline temperatures are moderated by the warmth of the lake water itself. Temperatures between ROC and the lake shore are known to differ by as much at 20 or 30 degrees. I retrieved data from a WeatherBug weather station right on the lake shore (Forest Lawn Beach, FLB on the plot. Thanks to Parker Zack and Kevin Williams for helping me find this.) as it happens, for this snow event, temperatures at ROC and at FLB track each other quite closely for much of the event, until the event peters out. In either case, isotopic values track air temperatures.

The snow gets isotopically ‘heavier’ during the colder overnight hours. Does this make sense?

Under warmer conditions, more of the heavier isotope will be incorporated into water vapor. In isotopic terms, this means that δ18O and δ2H of the vapor will be more positive when air temperatures are warmer. For freezing (or crystallizing snow), one might expect that more of the heavy isotope would remain in the vapor when the air temperatures are warmer. Or, since we’re measuring snow, warmer air temperatures means isotopically ‘lighter’ snow. If it’s colder, more of the heavy isotopes go into the snow, causing the δ18O and δ2H values of the snow to become more positive.

Oh thank goodness! It does make sense! That is if the changes in isotopic value of the snow is directed by air temperatures during the crystallization of the snow and we assume that air temperatures have minimal effect on the fractionation during evaporation.

Can we make the latter assumption?

I think we can. The temperature of the water is close to freezing (approximately 4 degrees C, data found here). Evaporation stops if the water freezes. The difference in fractionation of evaporating water at 4° C and 0° C is negligible (see article here). We can assume it is essentially the same. Thus any isotopic change we see must be due to changing air temperatures during the freezing of snow.

Other observations

Snow was collected at two sites in Wayne County affected by this Lake Effect event. One site in the Town of Williamson, and one about seven miles further west in the Town of Ontario. Isotopic values of snow from these two sites are essentially the same and follow the same pattern. Thus we can say there is likely to be little lateral isotopic variation in snow isotopic values. That makes sense given that the snow is all coming from evaporation off the same lake.

Further work

If the isotopic value of the original lake water is known, along with air and water temperatures, it is possible to look at the extent of fractionation both during evaporation of the lake water and crystallization of the snow. We were unable to collect a lake water sample at the onset of this event, but we do have one collected from November of 2011, as well as snow measurements also from 2011. Alas, for the November 2011 event, we lack temperature data. But we can make some assumptions and try to look at fractionation. I’m working on those calculations now. And they make my head hurt.

For the next lake effect event, I’m hopeful we can get a sample of Lake Ontario water for a starting point. We will also collect snow from the weather station at FLB to see if there is a gradient in the snow isotopes from nearer the source to farther outboard (like Williamson). Sublimation may be occurring in the clouds, which might cause the snow to be isotopically heavier than ordinary fractionation would predict, in which case we would predict that shoreline snow would have more positive δ18O and δ2H values than snow collected further inland.

An Isotopic Study of Lake Effect Snow

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.

Predictions:

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.

Our water analyzer, Norm, analyzing waters from hurricane Sandy.
Our water analyzer, Norm, analyzing waters from hurricane Sandy.

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!

UPDATE 1-21-13

After waiting for 24 hours, there has not yet been any snow. But I’m assured it’s on the way!

UPDATE 1-22-13
Snow is falling. Finally.

 

Friday Headlines: 1-4-12

Friday Headlines, January 4, 2013

THE LATEST IN THE GEOSCIENCES

 

FIRST METEOR SHOWER OF 2013 PEAKS THIS WEEK

Quadrantid. Photo by Brian Emfinger in Ozark Arkansas, January 2, 2012

The Quadrantids are a meteor shower that happens in January. They seem to come from an area in the sky between the handle of the Big Dipper and the head of the constellation Draco.

(source: EarthSky Communications, Inc.)

Alas, by the time this is published, the peak will be just past, having been Wednesday night into Thursday morning. Plus, the waning moon (and all the snow where I live) make it difficult to actually observe this meteor shower.

PLANET’S OLDEST FOSSILS FOUND IN PILBARA, EXPERTS SAY

 

In the Pilbara region of Australia are some of the planet’s oldest rocks, dating back to about 3.4 billion years ago. In these rocks are various evidences for ancient life, including textures (like minute strands connecting to each other in a network similar to that of modern bacteria) and geochemical tracers. Yes, folks, there be isotopes there!

Metabolic processes in bacteria result in an isotopic signature wherein there is more ‘light’ carbon (carbon-12) than ‘heavy’ carbon (carbon-13) than would be expected for a limestone that formed without bacteria present.

Strelley Pool in the Pilbara, where 3.4 billion-year-old fossils have been found. Photo: David Wacey

What’s important is that finding these bacteria in such ancient rocks might suggest that the Earth’s atmosphere had oxygen in it a billion years before we previously thought. Oxygen in the atmosphere has had a profound effect on both the evolution of life on Earth and as well as it’s geologic history.

A GUIDE TO SNOWFLAKES

Snowflake classes

This is just cool. Who knew snowflakes were so complex? In light of all the snow we’ve received of late, this gives me something to look for in the next snowfall.