31 March 2004
Wednesday, 31 March 2004
It’s déjà vu time
As Mike Harvey mentioned in the previous weblog, we had decided to re-infuse what remains of our patch with more iron and tracers. This started last night.
Complicated procedures often seem to go more smoothly the second time around, and the second iron/tracers infusion was no exception. Unlike the first time, where it took many days (and a detour) to have a working GC/TCD (gas chromatograph/thermo conductivity detector), Andrew Marriner and Peter Hill were able to get the GC/TCD up and running after less than one hour. The instrument is used to detect the high concentration of 3He and SF6 in the tanks.
On board the ship, we have two identical large (ca. 4000 L) tanks for tracer saturation. Tank #1 was used for the initial infusion. Tank #2 had been saturated with SF6 along with tank #1, so all that needed to be done was inject the 3He. Since 3He cost more per unit weight than saffron, we had to be careful in handling it and make sure that we did not loose it to the atmosphere. Andrew and I were in charge of this, and it went off without a hitch.
We created a headspace in the tank with 3He, and turned on a pump to circulate the tracer through a gas diffuser hose in the water in order to dissolve the tracer. With just the right amount of water, tracer, internal pressure, and pumping rate, we were able to create the perfect symphony of bubbles right below the intake of the circulation pump. Andrew described it as "poetry in motion". Our original intention was to bubble for 20 minutes. However, with 20 minutes approaching, things were going so well that we did not want to turn the pump off. I suppose it is akin to turning off free cable television, with the fear that it will not come on again. But alas, Andrew and I both realize that all good things must come to an end so one of us flipped the switch.
The re-infusion of iron and tracers soon commenced, and lasted for 8 hours and consisted of laying down two streaks over the previous patch. After the infusion, it was business as usual. In the morning, after a little SF6 mapping, we had the usual biological and gas casts for water samples. The calm conditions allowed various investigators to take two separate trips on the rigid hull inflatable boat to deploy instruments: Brian Ward and Craig Stevens went and deployed their autonomous profilers, while Cliff Law deployed his flux chamber. Then came more SF6 mapping, before we left for the "out" station. Tonight, it will be more SF6 mapping, punctuated by the nocturnal tracer cast for 3He and SF6.
My main role on the voyage is the sampling of 3He. Combined with SF6, these two tracers will allow us to calculate how rapidly gases transfer between the atmosphere and ocean. Knowledge of this is crucial to predictions of climate change, since the ocean acts as a large store for greenhouse gases such as carbon dioxide. Samples for 3He are taken in copper tubes, closed off by stainless steel pinch-off clamps and stored for measurement in the laboratory by mass spectrometry. See www.ldeo.columbia.edu/etg for more information about the 3He extraction and measurement.
David Ho (Lamont Doherty Earth Observatory)