Where’s the tracer, man?
Posted by sogasex on April 2, 2008
By Paul Schmieder, LDEO
I am a graduate student at Lamont-Doherty Earth Observatory (LDEO) working in the Environmental Tracer Group, and at sea I am assisting the LDEO and NOAA/AOML team with the collection and analysis of SF6. SF6 is my tracer of choice, both out in the open ocean and in coastal waterways.
Nearly two weeks ago, in the early morning hours of March 21st, we successfully injected a second patch of tracer containing both 3He and SF6 gases infused in seawater. Deliberately, this tracer patch was smaller in area than the first patch in order to obtain higher SF6 (and 3He) concentrations in the water. Higher concentrations would allow us to conduct a longer survey. Our plan to increase the SF6 concentrations was successful! Surveys through the patch conducted on the day following injection yielded concentrations as high as 1024 fmol/L (fmol = femtomol = 10^-15 mol). The peak concentrations have now fallen to ~20 fmol/L, a difference of 2 orders of magnitude since the start of the survey.
Since the time of injection, the patch has displayed a pulsed migration to the east, with periods of fast advection and moments where the patch remained stationary. Overall the center of the patch advected 80 km to the east. Approximately 8 days ago, the MAPCO2 buoy, which was deployed at the same time as the tracer, began to migrate along a different path than the portion of the patch we were following (picture below). We retrieved the buoy two days ago, and to my surprise there was tracer present 50 km to our south. Currently, the patch has decided to migrate in a new direction to the southwest. Using ADCP current measurements as a guide, we should be able to keep up.
Matt Reid (LDEO) and myself have been holding down the fort, mapping out the tracer 24 hours a day for 13 consecutive days now, and it looks like we might have 2 days of survey remaining. The routine of the daily SF6 surveys is punctuated, though, with the excitement of both ‘Pump and Dump’ and the pending CTD casts. It is our duty to direct the ship to the CTD station, and it is always a bit of a struggle to predict where we might find the highest tracer concentrations, and there is the added pressure to arrive at station on-time. We don’t always get the highest concentration, nor do we always hit our waypoints on schedule, but in the end I think we have fulfilled our duties and successfully obtained the samples we need.
This is a composite map of the SF6 concentrations for the second tracer patch. The concentrations are plotted on a logarithmic scale with units of fmol/L. The black dots show the position of the MAPCO2 buoy in time, migrating from west to east.
When David asks “Where’s the tracer, man?” this is my response…