May
31

Tending the patch …

Filed Under (Uncategorized) by Robert on 31-05-2009

During SUCCES, two tracers (Fluorescein and SF6) are being used to delineate water from the bottom boundary layer (BBL), and allow us to determine the advection (transport) and dispersion (spreading out) of the body of water (“the patch”).  By marking the water from the BBL, scientists on the cruise can measure biogeochemical changes taking place in the water to ultimately determine controls on, and the fate of, carbon from the BBL.

Why use two tracers?  Each tracer has its advantages and disadvantages.  For instance, Fluorescein, which is a fluorescent dye, can be measured easily with an in situ fluorometer at high resolution.  However, it is relatively expensive, so only a limited quantify of dye could be used. SF6 is less expensive and has a greater dynamic range than fluorescent dyes, which means it can be accurately measured over a broader range of concentrations (i.e., we can still measure the SF6 after the dye has become too dilute to measure).  Also, because SF6 is a gas, we can use it to examine the dynamics of gases dissolved in ocean water, and the transfer of gases between the atmosphere and the ocean.

One focus of this experiment is the methane produced in seafloor sediments that diffuses into the water column, and whether upwelling events can transport this methane to surface waters and ultimately to the atmosphere.  Methane is a potent greenhouse gas, so understanding fluxes of methane from oceans to the atmosphere is necessary to quantify the contribution of methane to climate change.  Because it is a gas, SF6 allows us to quantify transfer of gases between the sea and the atmosphere, and can thus be used to determine how much methane has been lost to the atmosphere.

For the past 2 weeks, a 20 ft container on the R/V Wecoma has been home to a group of us—David Ho and Sara Ferrón-Smith (University of Hawaii), and Matthew Reid (Princeton University)—who are measuring SF6 on the SUCCES cruise.

Matt on deck preparing a tank of SF6-saturated water to pump into the Bottom Boundary Layer

Matt on deck preparing a tank of SF6-saturated water to pump into the Bottom Boundary Layer

Sara, hard at work changing a sample loop on the automated Gas Chromatograph, used to measure SF6 concentrations in the water

Sara, hard at work changing a sample loop on the automated Gas Chromatograph, used to measure SF6 concentrations in the water

We have employed the tracer technique in rivers, estuaries, wetlands, and oceans all over the world.  However, this is the first time we have used it to mark a patch of water from the bottom boundary layer. Our instrument for measuring SF6 is fully automated and has worked very well during this cruise.  This leaves us with plenty of time to get (re)acquainted, explore the Internet, and discuss science, politics, history, culture, and whatever other topics comes to mind.

The figure below shows one day’s worth of preliminary data of SF6 concentrations at the seafloor.  The plot is a “map” (note latitude and longitude axis) and the color represents the concentration of the tracer SF6 in the bottom boundary layer.  The SF6 “patch” is colored in red and non-labeled water in blue.

This "map" shows the SF6 tracer concentration (in parts per trillion) in the bottom boundary layer of our study site.  This tracer is mixed away to undetectable concentrations in a few days.

This "map" shows the SF6 tracer concentration (in parts per trillion) in the bottom boundary layer of our study site. This tracer is mixed away to undetectable concentrations in a few days.

If you are interested in more information on tracking a tracer patch at sea (or in the dynamics of gas transfer across the air-sea interface), the Southern Ocean Gas Exchange Experiment Blog has some stories from a cruise last spring in which SF6 was used as a tracer in an experiment focused on air-sea gas exchange.

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