biology chemistry geology instruments
Lost City Expedition: Science

How Come Black Smokers do not Form at Lost City?

In pondering the novelties of Lost City, an important question to ask is “Why are the amazing towers at Lost City made up of calcium carbonate minerals (limestone) instead of iron and sulfide minerals that form black smokers?” It turns out that because the Lost City field sits on top of mantle rocks, the vent fluids have dramatically different chemistries than those of black smoker systems, and this governs what types of minerals precipitate or crystallize out from the vent fluids. Remember that 1) mantle rocks have much higher concentrations of elements such as magnesium, but much lower silica concentrations than basaltic rocks, and 2) that black smoker systems are generally underlain by basalt.

Chemical changes within the Atlantis massif: As seawater migrates deep into the Atlantis mountain, it begins to warm up and reacts with the mantle rocks that it touches. Some minerals are dissolved by the fluids, while other minerals precipitate from the fluids along small cracks and veins. This process of dissolving some minerals and crystallizing out others, dramatically changes the composition of the circulating seawater—by the time the fluids are warmed enough to rise buoyantly to the seafloor, their chemistries are completely different than seawater.

During alteration of the mantle rocks beneath Lost City, the fluids become extremely basic. This means that the fluids do not contain many hydrogen ions (H+)—much less than one hydrogen ion in 10 million molecules of water. We say that these fluids have a very high pH or are basic, which is another way of saying that they contain very small amounts of hydrogen ions. .

This high pH is very important because when the Lost City fluids are ejected from the chimneys and mix with seawater, this mixing causes a chemical reaction to occur that releases carbonate ions (CO32-) from the surrounding seawater. The carbonate ions bond with abundant calcium ions (Ca2+) that are also in seawater, and this causes calcium carbonate (CaCO3) to precipitate. As long as the Lost City fluids maintain a high pH, abundant limestone will continue to be deposited and the chimneys will continue to grow.

As an example of how different this system is compared to black smoker environments, take a look at the two figures below. Notice that the black smoker fluids are much hotter, more acidic (that is they contain very high concentrations of hydrogen ions), and they are enriched in metals such as iron, copper, sulfur, lead and zinc. This is because as seawater circulates through basaltic rocks at high temperatures, many hydrogen ions are released from the rocks, the fluids become very acidic, and they are then capable of leaching or dissolving out the metals that are finely distributed in the basaltic rocks. These fluids, which are heated by molten magma, or very hot rock, are emitted onto the seafloor at temperatures up to 400°C. As these superheated, metal rich fluids mix with cold ocean water, minerals such as pyrite (FeS2) that made out of metal crystallize out from the hydrothermal fluids.

One of the important questions we are exploring at Lost City, is how these differences in fluid chemistries impact the kinds and abundances of animals and microbes that grow within these oasis.

Carbonate Flange

  • May be common along slow-spreading systems
  • Volcanic Heat not required--exothermic mantle
  • Temperatures likely <150 C (300 F)
  • Fluids are enriched in CH, H2, low in metals & S, pH 9-10
  • Support diverse microbial communities, sparse macrofauna?

Black Smoker

  • Found on all spreading centers
  • Fueled by cooling volcanoes
  • Temperatures reach 407 C (765 F)
  • Pressures 200-400 X atmospheric
  • Fluids are enriched in CO 2, H2S, H2+metals, pH 2-5
  • Supports dense and diverse macro-faunal and microbial communities