World Builders™
World Builders™
Session Five  --  Seaweeds
Session Five  --  Seaweeds



Black Smokers
Black Smokers

Deep under the sea, water carrying sulfide minerals cools, depositing rock chimneys.

 
     We know that tectonic plates move away from mid-ocean ridges, places where the sea bed is stretched open by currents rising in the mantle of the earth.  The stretched part opens up to form a trench and lava comes out, pushing the seabed away from the opening and  creating undersea volcanoes.

    Volcanism is often accompanied by geothermal features such as hot springs, geysers, and hot pools.  We can see these features on land in Yellowstone National Park, where the geyser Old Faithful erupts several times a day for astonished visitors.

     Hot water also accompanies the volcanism under the sea.  The sea floor is made of cooled volcanic rocks, and is cracked in many places.  Some of the sea water seeps down through the cracks in the rocks.  If the cracks are shallow nothing much happens, but water that goes down into the really deep cracks is warmed by the heat of the magma  further down in the earth.  This hot water loses its dissolved oxygen and becomes acidic.  Chemicals, including such metals such as gold, iron, copper, and zinc, dissolve in this hot acidic water.  Another chemical that is picked up is hydrogen sulfide, which is important for life forms.

    What happens next?  The hot water rises.  As it comes out of the chimney it builds up the chimney pipe a little. In seconds the hot water and chemicals are released into the cold waters of the ocean, and the chemicals join with the oxygen in the water and come out of solution as tiny black particles.  The jet of water is dark and goes upward like smoke, which is why these structures are called black smokers.  The little particles fall down and are deposited on the sulfide mounds and hot, hollow, chimney-like columns.

     Is this exciting?  You'd better believe it!   The water is rising  by convection because it is so hot, and hot water is less dense than cold water.  As the hot water rises, cold water is drawn down into the deep cracks to refill the deep reservoirs that keep this process going.  Shallow cracks also fill with water, but the water remains cool because it is not near the hot magma.

     So how hot is this hot water?  That seems to vary by location, some vents spewing water at around 63o Fahrenheit (17o Celsius), others as high as around 700o Fahrenheit (between 350o Celsius to 400o Celsius).  The deep ocean water is barely above freezing, so there is quite a gradation in the temperatures of the waters around the vents.  

     The vents are located at a variety of depths, from perhaps about 100 feet (30 meters) to 11,972 feet (3650 meters).  Different web sites give somewhat different numbers, which is not surprising, as the field is new and under intensive investigation.

    Scientists were astonished when they found the black smokers in 1977.  The geology is amazing, but the most amazing thing of all is that there are life forms down there in the darkness.  Until the discovery of these life forms it was believed that all life on earth depended on photosynthetic organisms.  The life forms that crowded around the black smokers did not photosynthesize, because the deep sea is in total darkness.

     Conditions around the black smokers seem hostile to life.  The temperatures could cook life forms, and the water, filled with metallic sulfides. is toxic.  Yet communities of life forms live around the vents, choosing their locations where the temperatures suit them and relying on their own autotrophs (food makers) as the base of their food chain.

     And what are these autotrophs?  Thermophilic archaea!  Bacteria!  These unicellular organisms  are sulfur eating, heat loving bacteria and archaea that use oxygen to break down the hydrogen sulfide and create carbon compounds, carbohydrates, that can be used by the other life forms in this biome.  These bacteria and archaea are the first life forms to colonize new vents of hot water break that break through the sea floor.  As the micro-organisms crowd together and reproduce in the scalding water, they are so numerous that they look like a snowstorm.  Soon they are also settling on the rocks like white and colored mats, becoming grazing grounds for eyeless shrimp and other tiny animals.  Other larger animals include giant (3 feet long) tube worms, giant clams a foot across, long necked barnacles, snails, mussels, and white crabs.

     One of the problems for these undersea communities is that the vents eventually stop working. New vents ones open up, of course, but they may be far from the original ones.  As the water between the vent systems is barely above freezing, the organisms have to find new, hospitable environments when established vents cool. 

     Adults of vent-dwelling species, such as the tube worms, are rooted to the rocks.  The tiny larvae of these species drift in the cold waters, and only a very few find new places where they can survive.  As in much of nature, the journey of life is perilous, and only the strong and the lucky have a chance of

     Although the ecosystems around the vents work, and must have been working for many, many millions of years, it is difficult to see how these life forms could evolve into land-dwelling communities of autotrophs and heterotrophs. Could they?  Who knows?  Life is full of surprises. If you choose to use the deep sea vents as one of your biomes, think ahead carefully.   


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Header and photos  from NOAA Ocean Explorer
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at eviau@earthlink.net .