D334: Monitoring ocean circulation in the Atlantic


Cruise diary

9 November 2008

Craig writes...

We’ve been working on and around the Mid-Atlantic Ridge now for the past three days (see map, right) and although the swell of the sea has picked up a little, the mooring operations are going great. So far, all have been recovered and re-deployed and all sensors, apart from two microCats have returned data.

Damaged sensors are dangerous, especially when the watertight chambers fail, as happened with the two microCats in this case.  This allows any air inside the unit to compress – to exactly the environmental pressure surrounding it.  Not a problem at the ocean surface, but when you’re 5km beneath the ocean that’s 500 times stronger than regular air pressure. Rapidly return the sensor to the surface and the trapped air inside can violently decompress at any stage.  Ends have been known to blow off microCats as a result of this and so damaged sensors are now carefully set aside to allow them to safely decompress.

One important physical property of water that sets it aside from air is its behaviour under immense pressure.  Imagine a closed can, or bottle, full of water that is rapidly sent to the ocean floor. When you brought it back to the surface, the pressure at the ocean floor would have crushed the can, right? Not so - even if the pressure of the seawater is hundreds of times that on the inside. This is because it is not possible to compress water in the same way that it is possible to compress air.

Right, top: Location of RRS Discovery on 9 November 2008
Right, bottom:Flooded microCat sensor from mooring ‘MAR0’ left to decompress on deck

To demonstrate this we conducted a small side experiment on today’s deep-ocean CTD (conductivity-temperature-depth) experiment which saw the ship’s CTD frame lowered to 5.5 km in order to collect bottom water samples (of Antarctic origin).  You can see from the photos that one unlucky can of Coke Zero was selected to participate. The internal pressure of this can is approximately one atmosphere – or just slightly over – hence the noise the can makes when you open it gas moves out of the can. Safely secured to the CTD frame it was then sent on an odyssey to experience pressures over 550 times that at the surface.  Five hours later and the can returned - intact and in great shape. In fact, the only signs of its voyage was a very slight deformation of the can where the gas content of the can (i.e. the carbon dioxide fizzy bubbles) had allowed a very small amount of compression. We’ll let you know if the taste has been affected after dinner!

The ship’s CTD frame returns to deck,
having visited a depth of 5.5km.

The can survives!

The unlucky Coke can gets fixed to the CTD frame!

This evening sees the ship about turn 180° and we head east to service the final moorings in the Eastern Boundary.  These lie in shallow waters off the Moroccan coast, although we have one or two deeper moorings to service on the way. It’s an eight day transit to get on station - some 1500 nautical miles. Welcome time for all to recover from the packed schedule on the Ridge and to prepare for the final leg of the cruise. 

More soon!

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