Sampling for helium: A beginner's guide

As promised in an earlier post, we now take a closer look at what we’re doing with helium!

Once the CTD is safely out of the ocean and back in the hanger, samples are taken from each of the niskin bottles, which have been closed at different depths to collect seawater from the entire water column. Some of these samples are analysed on the ship for oxygen content and salinity, while others will be stored and shipped back to the UK, where they will be sent for helium analysis at Woods Hole Oceanographic Institution. Since many materials are permeable for helium and we need the samples to remain in the same condition as when they are taken, ordinary storage methods are not sufficient. We must use a material with low permeability, which is why we collect our samples in copper pipes.

Copper pipes containing samples of seawater from a CTD cast.

To trap water samples in the pipes, we use a process called cold welding. The copper is cut to the right length to create two samples, as it is good practice to have backups in case anything goes wrong. The middle sections of each sample length are flattened at this point, to slightly reduce the volume. The reason for this will become clear later.

When the time comes to collect samples from the niskin bottles, the water is passed through the copper pipe via plastic tubes attached to either end. The scientist taking the sample must hit the copper pipe repeatedly, watching the outflow tube for air bubbles which are dislodged. Once there are no bubbles remaining, the plastic tubes are clamped to trap water inside the pipe.

Miguel watches for air bubbles while taking a water sample.

The copper is then taken to the lab, where it is cold welded to produce two closed pipes containing water samples. The cold welder consists of hydraulic jaws powered by a compressed air pump. The jaws close over the pipe to seal it.

The cold welder in action.

Once this is done at both ends of a sample, we use a re-rounder to take out the flattened section in the middle and increase the volume of the pipe. This serves to reduce the pressure exerted by water inside the copper, and avoid the sealed ends breaking open. The small vacuum created by this process will cause the pipe to make a clicking sound when shaken, which is how we check that the sample has been stored successfully.

Re-rounding the copper to reduce the internal pressure.

Now you know how the water samples are securely stored, the real question is why are we so interested in helium anyway?

Well, what the analysis will tell us is the ratio of isotopes helium-3 (³He) and helium-4 (⁴He). By far the more common of the two isotopes, on this planet, is ⁴He. The rarer ³He is mostly contained within the Earth’s mantle, below the sea bed. When water is drawn down into the crust, it picks up this isotope, and when it is then ejected from a hydrothermal vent the comparatively ³He-rich water is introduced back into the ocean.

The ratio of helium isotopes will allow us to find out where our water samples have been. In the atmosphere there are a million atoms of ⁴He for every one of ³He, so if a sample of seawater contains a similar ratio it probably originated at the surface. If we find higher amounts of ³He, this will tell us that some of the water has come from a hydrothermal system. So the ³He isotope is a way of tracing water which has passed through a hydrothermal vent. Thus we will be able to get a better idea of the circulation in the Panama basin, and to what extent water from hydrothermal systems is mixed into the abyssal ocean.