Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Chasing the Tracer

Posted by sogasex on March 15, 2008

By Matt Reid, LDEO

I read a lot of comic books when I was younger, so it made sense that Roy Lichtenstein was the first artist I liked. I remember traveling to New York when I was in middle school and seeing a huge Lichtenstein mural in the lobby of a midtown skyscraper, and marveling at how, close up, the painting dissolved into a mosaic of individual brightly-colored points. So it’s fitting that on this cruise I am tasked with following the tracer patch, which is displayed as a collection of individual colored data points that coalesce into an image of the tracer patch and how it is evolving over space and time.

I’m with a team of collaborators from LDEO and NOAA/AOML conducting the dual tracer release of SF6 and 3He (see David’s post “What’s the patch?”), gases whose natural concentrations are so low that we know the relatively high concentrations we detect in the water are there because of our release. For the next two and a half weeks, we will track the patch as it is transported through the Southern Ocean and monitor the changing concentrations of SF6 and 3He. Once the sample analysis is completed, we will use the ratios between the concentrations of 3He and SF6 over time to determine the velocity at which gases are transferred across the air-sea interface, a process crucial to understanding global budgets of carbon dioxide and other environmentally-relevant gases. Dual tracer releases have been performed many times, but there is a scarcity of data at high wind speeds, which brings us to the “roaring fifities” of the Southern Ocean.

I work with the Underway SF6 system, which was built by David Ho and Paul Schmieder (LDEO). It uses a membrane to extract a gas stream from seawater pumped continuously onto the ship. We use Gas Chromatography to separate SF6 from the other components of the gas stream, and an Electron Capture Detector measures the concentration of SF6 in the gas stream. With this instrument, we are able to measure 1 sample of seawater per minute, providing a high-resolution map of SF6 concentrations as the Ronald H. Brown plies the high seas. Each data point is displayed as a dot on the latitude-longitude grid, with the color of the dot corresponding to the concentration of SF6 in the sea at that location. The yellows and reds are high concentrations, while the greens and blues indicate lower concentrations.

Sample collection and analysis are fully-automated, so my role is to determine where the tracer patch is moving, and to communicate with the bridge to devise a course which allows us to map the boundaries of the patch and measure the range of SF6 concentrations within the patch. I anticipated this would be fairly straightforward, but the variables of unexpected engine trouble, twice-daily CTD and optical casts at the patch’s center, daily “dumps” of wastewater outside the tracer patch, and a rapdily-evolving patch make for a dynamic process in which the time-series of tracer concentrations, current data from the ship’s ADCP, and a little intuition are used to predict the patch’s advection across the Southern Ocean.

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Underway SF6 System in the Hydro Lab

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A portion of the tracer patch, represented by colored data points which show the SF6 Concentration at different locations

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