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.

The Earth's Energy

by Rob Harris from Oregon State University.

I am interested in understanding the energy budget associated with geologic processes.  I think this understanding leads to better insights into how the Earth works.  For example, plate tectonics – the creation, motion, and destruction of plates – reflects Earth cooling.  About 70% of the Earth’s heat loss is through the ocean floor and is reflected by the cooling and subsidence of oceanic plates as they move away from spreading centers.  The upper layer of these plates, the oceanic crust, is cooled efficiently by hydrothermal circulation.  It turns out that the entire volume of the global ocean circulates through the oceanic crust every few hundred thousand years.  This hydrothermal circulation is important because it leads to significant exchanges in energy, mass, and solutes between the ocean and crust.  These exchanges modify the chemistry of the ocean, the chemical and physical properties oceanic crust, and supports a globally significant biosphere.

 

Rob with his heat probe. Note the thin thermistor string supported by the thick lance.

I am excited about participating on the OSCAR project because it touches on many aspects of these processes, the cooling and evolution of oceanic plates, hydrothermal circulation, and the impact of heat exchange between the ocean and crust.  Specifically, I am using heat flow measurements to better understand how and where fluids are moving in the oceanic crust and how this fluid flow changes as the plate ages.  One mystery is how and why the fluid flow wanes as the plate ages.  Clearly part of the reason is that the plate cools so there is less energy to drive the system, but other processes are involved as well.  This data will also be used to better understand the nature of energy transfer between the crust and oceans.  One idea is that warms fluids emanating from the crust may stimulate the flow of bottom water.

 

The heat probe being deployed, showing the weight stand at the top, and the long lance and thermistor string pointing downwards

The picture shows the heat flow probe I use.  The weight stand contains the data logger, power, and acoustic telemetry so we can monitor its performance from the ship.  These heat flow measurements are made by plunging the probe, under the force of gravity, into sediments.  The lance supports the thermistor string keeping it straight.  We house the thermistor string in a small tube so that the thermal response time is relatively fast, decreasing the time each measurement is made.  This style of probe with a sensitive thermistor string supported by a larger mass is called a violin-bow probe. 

Heat flow is the product of the thermal gradient and thermal conductivity.  Once the probe is in the sediment, we measure the thermal gradient by measuring the temperature at each of the 11 thermistors in the thermistor string and knowing the distance between them.  A heater wire also extends along the thermistor string that generates a short heat pulse.  The way the heat decays lets us determine the thermal conductivity.  These two measurements yield the heat flow.  Low heat flow measurements can indicate areas where cold bottom water enters the oceanic crust.  High heat flow measurements can indicate area where the now warm water exits the oceanic crust.

Moorings, landers, and squid

It’s only been a week and a half since we left Panama behind, though we’ve accomplished a lot of science since then. As well as numerous CTD casts mentioned in the previous post, we’ve done almost as many VMP deployments (vertical microstructure profiler) which measure the turbulence in the water column, indicating the extent of water mixing. Additionally, we have deployed 5 semi-permanent moorings, which will measure the hydrography of the ocean for the next few months until collection on a different cruise, 12 MT (magnetotelluric) landers, and started our heat flow measurements. This goes to show that although this is a long cruise (at 6 weeks), time is absolutely of the essence. It’s very lucky to get so many days of sea time to do this research, and we must make the most of it.

Three of the moorings we deployed consist of a set of several instruments, connected in a long vertical array with a weight to keep them anchored to the seafloor. They are supported by buoyancy along the length of the chain, resulting in a floating set of instruments from the seabed to approximately 1500 m above it. These instruments include: 

- micro-CATs, which stands for Conductivity And Temperature (and doesn’t refer to small felines). These are like a smaller version of CTDs and give measurements of the salinity and temperature of the water. 

- Current meters, which measure the strength and direction of the currents throughout the water column. 

- And a bottom pressure recorder, which sits on the seabed at the base of the mooring and measures the pressure every minute, which can tell us about the changes in water depth over time, such as that caused by the tides.

Paul and John attaching an instrument to the mooring rope

The moorings are specially designed for their position to best measure changes in the hydrography of the water column, depending on what is already known about the location. Each length of rope or chain is measured and cut to the specific size long before the ship sails, and then the mooring is put together on deck as it is being deployed, piece by piece. Our first mooring was assembled in an incredibly heavy rain storm, with everyone assisting soaked through within 10 minutes, and staying that way until the last piece went over the side an hour and a half later. Naturally the rain stopped almost as soon as we had finished.

The bottom-pressure recorder and weight going over the side

The last two moorings were ADCP moorings, which stands for Acoustic Doppler Current Profiler. These float 50 m above their anchor on the seafloor and measure the currents in the water column using the Doppler Effect (from movement of particles in the water) to about 500 m above their position. With these, we hope to measure the flow of water into and out of the Panama Basin.

A quick wildlife update

For a few days, we had a blue-footed booby accompanying the Cook on its voyage. Christened Glinda, the bird settled on the aft-deck to oversee the mooring deployments, and obviously pleased with the crew's efforts, flew to the forecastle deck afterwards to sun herself. Glinda has sadly left us now, perhaps to oversee mooring deployments elsewhere.

We were also visited by a squad of squid during one of our CTD casts. They were hunting little whitebait-sized fish that were leaping out of the water, and you could see their tentacles reaching above the surface sometimes to try and grab them. 

Glinda, the blue-footed booby

Sampling from the deep

After having left the coast of Panama, we have been steaming into the Panama Basin for the past couple of days. 

Since then, we have performed four CTD casts! CTD stands for Condutivity-Temperature-Depth, and it is an instrument that measures these properties of the water column (with conductivity giving salinity, and depth coming from pressure measurements). As well as this, a carousel of ‘niskin bottles’ is attached to the instrument, which collects seawater samples from different depths. Other measuring instruments can also be added, including an SVP (sound velocity probe) which measures the speed of sound in the water, and devices which measure the oxygen content and turbidity among other things. All in all it’s a pretty fantastic instrument!

The CTD being lowered over the side. The measuring instruments are attached to the metal frame at the bottom and the side, with the carousel of 24 bottles above (credit: Emma)

The CTD is lowered over the side of the ship using the winch system, right down to within meters of the bottom of the ocean. Once we passed the edge of the continental shelf, this was down to ~3000 m deep, so a lot of cable is needed. The continental shelf is where the ocean-covered continental crust changes into proper oceanic crust, which has a lower elevation due to its higher density. This leads to a relatively steep increase in depth over the shelf. Often, there is increased upwelling of nutrient-rich water at continental shelves, which generally means they’ll be more wildlife around. And sure enough, we did see a few pods of dolphins leaping around the ship, though sadly too far away and quick for good photos.

CTD emerging at the surface (credit: Jowan)

CTD being brought back on deck (credit: Jowan)

Once the CTD is back to the surface, we take water samples from the niskin bottles for analysis of oxygen and salinity content, and sometimes helium content too. More about each of these will come in future posts. It was strange thinking that as we were taking the samples from the first bottles, we were being splashed with cold water (~6 °C) from over 2000 m deep in the ocean, a place none of us will ever go (at least for now!). This contrasted to taking the samples from the last bottle, which contained water from close to the surface of the ocean and was at ~27 °C, warmer than the current air temperature and made us want to go for a swim.

Taking water samples from the niskin bottles (credit: Jowan)