Wednesday, 19 December 2012

8 - From seabed to sample pot

We’ve now had several successful dives with ROV Isis  and still the call of ‘she’s on deck’ brings eager and excited faces to get the rare fluid samples collected during the dive. The chemists on board can immediately tell whether their fluid samples are good quality, as they smell sickeningly awful if they are collected from inside the vent chimneys without mixing with too much seawater. The worse the smell of rotting eggs, the happier the chemists!

Scientists collecting the samples from ROV Isis  after she returns from a dive.
So what makes the fluids smell so terrible? The answer lies in how these vents form. Shallow bodies of molten rock (magma) pool in the Earth’s crust in areas close to tectonic activity, such as mid-ocean ridges where new ocean crust is created. Seawater trickles deep into the Earth’s crust through cracks and fissures on the seafloor in these tectonically-active areas, where they are heated to extremely high temperatures. At such high pressures and temperatures, the seawater reacts with the surrounding rock and transforms into a highly acidic, metallic and chemically-rich fluid which rises back up through the crust owing to its buoyancy. This process of circulation also strips oxygen from the fluids, causing sulphate (a major component of seawater) to reduce to the strongly odorous chemical, sulphide. As the fluids emerge into the cold water of the deep sea (about 0 °C), lots of the dissolved elements precipitate from solution as metal-sulphide minerals. These are black in colour and as a result, the vigorously-venting fluids look just like black smoke.

We are really interested in these fluids, as their chemistry can provide us with information about the deep-ocean crust, the processes of hydrothermal circulation and the origins of many important elements in seawater. As a result, we are keen to collect pure fluids from within the vents, before they mix with seawater, as well as water samples from inside the black smoke.

For collecting fluids directly from within the chimneys, we use large syringes made from titanium, which can withstand the extremely high temperatures. The manipulator arms of the ROV insert the syringes right into the chimney, and we wait for our temperature sensors to reach about 350 °C to indicate that we are in pure hydrothermal fluid. We then fire the syringes, sucking the precious fluids into our sample barrels as you can see in the pictures below.
Top: ROV manipulator arm guiding the titanium syringes towards the venting fluids
Bottom: inserting the probes into the chimney orifice.

Once the samples are on deck, we have to process them quickly as oxygen in the air reacts with the sulphide in the fluids resulting in the chemical composition of the fluids changing. We preserve the samples for the analysis of tracers of magmatic gases, pH, sulphide, carbon cycling, metal content and many other chemicals.

Throughout the world’s oceans there are hundreds of sites of hydrothermal activity, pumping fluids enriched in many different elements into seawater. Despite being a few miles down in the deep sea, these elements can be transported up through the water column in the rising hydrothermal plume, and swept away on ocean currents. Some of these elements are essential nutrients and stimulate primary production - the first rung of the food ladder - if they reach the surface waters.

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