Dispatch 15: More on Brice and Pat     Print

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Joey Wenig

October 5, 2014

Today, like the past three days, was spent mainly in transit. We arrived at station CB-10 for a quick post-supper CTD/rosette cast, then continued on to CB-11 (where we’ll have our first ice station) immediately afterwards. The going is slow now that we’re into heavy ice cover. Thick ridges formed by floes battering against each other are common, and the Louis has to make frequent use of its bubblers to create a fizzy layer of water between the hull and the ice and ease its passage. Sometimes forward progress is impossible, so the ship has to back up and take repeated runs at the pack to break through, or find an alternative route. Noise levels below decks are higher than ever. On multiple occasions I’ve seriously wondered, after particularly loud battering and ramming sequences, whether the hull had been breached, leaving me moments from an icy grave. In general, though, I’m happy to see the more substantial ice, knowing that we’ll be walking around on it sometime tomorrow.  

Given that today was another uneventful travel day, I’ll pick up where I left off talking about Brice Loose and Pat Kelly’s work aboard the Louis.

Recall that Brice and Pat are studying how sea ice either limits or increases (or has no effect on) the amount of gas exchanged between the ocean and the atmosphere. To do this they have been testing large samples of seawater drawn up with their submersible pump for two isotopes: radon-222 and radium-226. Radium-226 is the ‘parent’ of radon-222: when the former decays, it turns into the latter. Four relevant background facts on this parent-daughter pair: First, radium-226 has no sources within the bulk of the ocean. If a radium-226 atom brushes by your shoulder while you’re snorkeling in Florida, then it either escaped from sediments on the ocean floor, or was carried into the ocean through an estuary.  Second, radon-222, a gas, can escape to the atmosphere, but radium-226, a salt, cannot. Third, radon-222’s half-life (3.8 days) is vastly shorter than radium-226’s (around 1600 years). Fourth, radium-226 can be roughly considered (warning: oceanographic term) well mixed because it has a half-life that is comparable to the time it takes water to circulate throughout the world’s oceans. If what you just read seems confusing, then look at it this way: radium-226 sticks around long enough (before abruptly shooting off an alpha particle and turning into radon-222) to get mixed up enough that you would expect to find roughly the same amount of it in seawater whether you’re still happily snorkeling in Florida or, like me, freezing in the Arctic.  

Now, let’s imagine that you had god-like powers and strange plans for using them, and you somehow capped the ocean to prevent any gas from exchanging between it and the atmosphere for twenty straight days (about five half-lives of radon-222).  Trust me when I say that you would now expect to find directly proportional amounts of radium-226 and radon-222 in seawater. (Or rather, trust Brice and Pat. Or even better, Google secular equilibrium, bearing in mind fact three from above.) Finally, you take pity on the dolphins and flying fish and uncap the oceans again. If you were to measure the amount of radon-222 in surface waters now, you would likely find a deficit against whatever the capped value had been. Partly because of facts one and two above, the magnitude of this deficit would actually tell you about how much, and how fast, gas is going between the ocean and the atmosphere. This is what Brice and Pat are doing—using radon-222 as a ‘flux clock’ to answer questions like: Does sea ice act to prevent air-sea gas exchange? Or does it do the opposite, by stirring surface waters?

Problems with this approach arise in certain situations when the generalization of fact four—the bit about radium-226 being evenly distributed—isn’t completely true. Where we are in the Arctic, an example could be when a bunch of water spins off the continental shelf and moves quickly into the open ocean, so that it was recently in contact with sediments or river inflow. This actually happens—the fast-moving bit of water is called an eddy—and spells trouble for Brice and Pat, who, in order to use radon-222 as a flux clock, need to be able to assume that any radium-226 they find in the water has been around for a little while, and didn’t flow out of the Mackenzie River delta yesterday, or leach out of silt on the Chukchi Plateau after breakfast this morning. To catch any such situations, they also test for two other radium isotopes: radium-223 and -224. Both of these enter seawater the same way as their heavier cousin 226, but they have much shorter half-lives (11.1 and 3.6 days, respectively). Since they decay quickly, 223 and 224 are almost nonexistent in ‘blue’ water that hasn’t had recent contact with freshwater inputs or bottom sediments. If Brice and Pat find a lot of either 223 or 224 in a sample, they know they can’t trust it: the telltale signs of gas exchange will be misleading or obscured.