Sixty Degrees South

by srosengard

Today, we touched the southernmost point of our trajectory through the Southern Ocean, the closest we’d get to Antarctica–16:00 marked 60° South, and when that hour arrived the science crew celebrated by donning giant orange body suits (i.e., mustang suits) on the stern. It’s something that might be a Southern Ocean tradition of sorts, but I haven’t yet confirmed the origin of the idea.

Hours later, we were on our way back North. Two and a half weeks have now passed, and all the research groups on board just had to send a progress report on our cruise to the National Science Foundation. With all the hours outside collecting seawater, wet and cold, and inside doing chemistry, still wet but warmer, my group has managed to gather some initial data.

60 South!

On the stern of Revelle, suited up for the southernmost point of our cruise. I'm not sure why everyone wore mustang suits, but some things are better left unexplained. (Photo courtesy of Matthew Durham).

My research on sinking ocean carbon requires two types of data collection: (1) short-term, which I collect during the cruise, and (2) long-term, which will I collect months from now. The reason for the timing is the element thorium-234 (234Th), my principal tool for understanding sinking carbon. Like other radioactive elements, 234Th turns into another element, protactinium-234, which shortly turns into uranium-234, all at fixed pace. It is the nature of radioactive elements; they must change identity in order to become more stable. Each time one 234Th atom changes (the proper term is decays), it releases a unit of non-hazardous radiation: a beta particle. Thus, by counting beta particles in a seawater sample, I can infer the amount of 234Th that is supplying them.

I analyze Thorium once at sea and again after 6 months. The reason is that with every seawater measurement, there is a natural presence of beta particles that do not come from 234Th. If I count these extra particles, I will overestimate the amount of 234Th in my seawater. So in order to rule out these particles from my samples, I cannot just use what I have been measuring on Revelle. Instead, I must wait until all of the seawater 234Th has decayed, which takes just about 6 months, and then re-measure the samples. The count at the end of 6 months measures the extra beta particles, allowing me to remove them from my initial count.

We take great time and care to measure thorium properly because it can tell us something about the amount of organic carbon sinking to ocean depths. As an element, 234Th sticks to organic carbon particles as they sink. Measuring the radioactivity of 234Th, therefore, gives us clues to the existence of the organic carbon it is stuck to in seawater.

To some, it might seem indirect to use Thorium to get to carbon, and there are indeed other ways. But each creative path provides a nuanced understanding of the bigger picture, and the convergence of these paths, if possible, is the true reward. In fact, there is one other person who is investigating carbon here: Helen Smith, a graduate student at the University of Southampton in England. Her proper role in the scientific mosaic of Revelle is the snowcatcher, a device that she uses to collect all the sinking particles at a specific depth range underwater. What her catcher finds reveals not only the amount of carbon that might be sinking through the ocean depths, but also its shape and form. We are both curious to see how our two methodological paths might ultimately merge in future analysis.