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Thursday, August 4, 2011

RV Celtic Explorer Venture Survey: Day 5 +

The VENTuRE Cruise: Discoveries from the Deep

On arriving at the target site there was growing excitement on board the RV Celtic Explorer for the scientists and crew.  More than a year before, signs of hydrothermal vents had been detected by UK scientists – tiny signals of cloudiness in the water at depth indicating the presence of vents producing sulphide rich black smoke.  Now the job of the combined Irish and UK team was to try and locate this small target up to 3000m below the RV Celtic Explorer.  More than two days away from the Azores the site was about as far from land in any direction as is possible in the vast expanse of the Atlantic.

Luckily the weather was now on our side, as we began to hunt for the plumes of smoky water expelled from hydrothermal vents. For this we used sensors on a CTD rig designed to measure depth, temperature, cloudiness of the water and Eh (which indicates the presence of sulphide rich water). Work began at half an hour after midnight on the 21st of July and went on day and night for the next three days.  The CTD was let out to more than 2000 metres depth and regularly raised and lowered through the water column while being towed at a quarter of a knot (a procedure called Tow-Yo CTDing).  In that time two ROV dives down to the bottom to check out promising signals revealed nothing. 

By day three we had many plume signals but still no positive sighting of the vents themselves – this was truly becoming a frustrating search for a needle in a haystack. Normally, searching new hydrothermal vents takes several weeks, sometimes years, but we had only a few days.  Finally, a strong Eh signal was followed closely by strong temperature signal, indicating a 'hot spot' close to the vent site. The fluid emitted directly from the vents is hot, roughly 350cC, instantly diffusing to 5cC a few meters from the vent as it mixes with cold seawater. The background temperature in the deep sea is about 3cC. After spending forty hours searching, we had plume signals boxed in, in record time! But the site appeared to be close to a 200m high cliff. The search would require skilled and careful ROV work.

The ROV Holland I was deployed with a full suite of cameras recording continuously in four directions. Tense with excitement, the team followed the progress of the ROV over lava flows covering the sea floor to the edge of the cliff at 2800 metres – and over and down.  The day watch team on the 24th of July first spotted traces of smoke in the water at 2900 metres. Powerful lights and lasers on the ROV caught glittering sulphide particles and black soot in the water column.  Soon billowing clouds of black smoke engulfed the ROV. The vent was close!  News traveled throughout the ship like wildfire and the science lab was filled with scientists and crew.  The ROV crept deeper and deeper 2950, 2980, 2990 metres.  Suddenly, just before midnight, after more tense navigation through swirling black smoky water, crusty columnar chimneys belching black smoke emerged into view.  A new vent site had been found! All hell broke loose in the science lab……
  

First time ever seen by human eyes, the Moytirra vent field. Picture shows chimneys of metal sulphides (black and rust coloured) at 3030m below sea level formed. They are precipitated from hot fluid erupting from the volcanic mid-Atlantic Ridge. The white mineral is anhydrite.
Another view of the main chimney in the Moytirra vent field. This time the mid-section. This chimney by global standards is huge with an impressive girth and stands over 10m tall. Laser dots are 10cm apart.


Another view from the Moytirra vent field. "Black smoke" (volcanically heated hot vent fluid issuing from the seabed with high concentrations of metal sulphides) can be see leaking from half way up the chimney. Shrimp make a home on the vent. 

 The summit of the chimney with a large concentrated plume of metal sulphide enrich vent fluid issuing from the top.

 
A cooler less active (but still venting) chimney complex with less life.

For more on the VENTuRE cruise, please check out the Science blog spot:

http://scientistsatsea.blogspot.com

and UCC's student website: http://blogs.ucc.ie/wordpress/bees

And the GSI blog: http://www.infomar.ie/blog/index.php

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