As part of Thetis' recovery process, safety diver Vasilis Stasinos attaches large ropes to the Thetis submersible so it can be lifted out of the water and up onto the deck of the Aegaeo. Click image for larger view and image credit.
Detecting Disappearing Gases
June 27, 2006
36 N, 24 E
Dr. Richard Camilli
Assistant Scientist
Woods Hole Oceanographic Institution
Our mission during this morning’s second dive is to explore for hydrothermal vent sites off the coast of Milos. Over the previous days, our Greek colleagues from the Hellenic Center for Marine Research have used acoustic echo soundings from the ship to identify likely areas of venting. I wake up at 5:00 am following a few hours of sleep and begin mission preparations. Dawn breaks as I step out onto the fantail (the rear deck of the ship), and I can see the planet Venus shining over Milos against an orange-pink sky. A celestial Venus de Milo to greet me this morning.
Dive preparations leave us without time for breakfast, only a quick cup of Greek coffee, and by 7:00 am Thetis begins her descent into Aegean waters. Beginning at about 60 meters depth, we plunge into a flurry of tiny white and yellow particles outside of the submarine. These particles, often referred to simply as marine snow, limit our visibility to about two meters at best. This is our first clue that we are close to an active vent site. We arrive on the seafloor at a depth of 102 meters (334 feet) and transmit our position to the mother ship Aegaeo. One last systems check, then we initiate sensor startup sequence and start our search for the vents.
We first head east, then north. The onboard chemical sensors allow us to identify active venting areas within seconds of encountering it. We combine sensor data with acoustic navigation to provide the critical data needed to survey the site. We quickly gather water and biological samples to take back to the ship for analysis.
At one of the vent sites we notice what looks like a strange organism – cylindrical shaped like a sea sponge, but white with a shaggy surface. Kostas gently plucks this furry little wonder from the seafloor with Thetis’ robotic arm and brings it closer to the sub for us to see. To our surprise it is a plastic water bottle covered with a microbial mat! We place the bottle in the sub’s basket so we can analyze the microbes and also do a little trash collection. Plastic garbage on the seafloor is not unusual. Unlike most metals and organic matter, plastics tend not to decompose in seawater. As a result, a tremendous amount of plastic rubbish has accumulated on the world’s seafloors in the short time since humanity has developed and used these materials.
Geologists Andreas Sioulas and Christos Anagnostou prepare to handle rock samples collected during a Thetis dive while organic chemist Aleka Gogou smells the collected samples, confirming the presence of hydrogen sulfide. Click image for larger view and image credit.
Approximately an hour into our mission, the winds and seas increase to a point where it is critical that we surface immediately to be recovered by our mother ship, Aegaeo. This is slightly disappointing, but we must always operate with safety as the first priority for ourselves as well as Thetis’ recovery diver, Vasilis Stasinos. Vasilis is the strongest swimmer I have ever seen – an underwater superman. As a part of the recovery process he must dive into the water and swim to Thetis with large ropes. He attaches these to the submersible so that Thetis can be lifted out of the water and up onto the deck of Aegaeo.
One of the things we discover during this dive is the presence of hydrogen sulfide, an exciting find because hydrogen sulfide reacts rapidly with dissolved oxygen in the water and disappears. In fact, later Gas Chromatograph analyses of the samples we collect at the site do not show the hydrogen sulfide. It does reveal byproducts indicating hydrogen sulfide was present; but by the time the samples are returned to the ship’s lab for analysis, the gas itself is gone.
During a subsequent Thetis dive a geologist collects rock samples exhibiting a coating of sulfurous material. When brought back in to the ship’s lab our own noses confirm the presence of hydrogen sulfide, which smells like rotten eggs. We even have to open the doors and portholes to clear out the air! While human sense of smell may not be as scientifically accurate, having in situ chemical sensors on board Thetis allowed us to ‘sniff’ out and relay the presence of a gas that otherwise may not have been detected, providing more data about the vent chemistry of a particular site.
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