Spec Tech: The Lost City of the Deep

In 2000, scientists crammed into a tiny deep sea submersible vessel caught sight of 60 meter tall spires of white, crystalline calcite rising from the black depths of the sea floor. It was a sight that captured the imagination and earned these spires the name: The Lost City.

Many people immediately think of Atlantis when they hear the phrase “the Lost City.” Unlike Atlantis, however, this Lost City has been explored with submersibles and extensively documented scientifically. This Lost City is real – though it is neither lost, nor a city.

The Lost City is a hydrothermal vent field that was original discovered by the MARVEL cruise AT3-60 in the year 2000. The cruise’s original purpose was to explore and document the structure and geology of Atlantis Massif, an undersea mountain similar in size and topography to Mt. Ranier in the United States. That expedition found something more fantastic than they had hoped – the Lost City – and cruises were sent to that location in 2003 and 2005 specifically to investigate the site.

There are several different types of deep sea hydrothermal vents. You may be familiar with one type – the “black smoker,” made famous in the later 70s and early 80s by nature programs that followed the exploration of the ALVIN deep sea submersibles. Black smokers (and their somewhat gentler cousins, the white smokers) vent superheated water into the ocean and create an unusually acidic local environment, normally with a pH of 5 or 6. Black smokers are often remembered for the strange life that is found around them – particularly giant tube worms. Black smokers are geologically and chemically fascinating – but we’ll talk about them more some other time.

The Lost City was (and still is) a departure from everything that was known about deep sea hydrothermal systems. The water coming from these vents is markedly hotter than the surrounding sea water, of course, but is still less than 100 degrees Celsius. And while we’re used to hydrothermal environments being highly acidic due to their unique chemistry, the Lost City turned that norm on its ear as well.

The waters of the Lost City hydrothermal vents get up to a pH of 11. For comparison, the average pH of seawater is 8.2, though humans are helpfully lowering that a bit at a time by constantly pumping carbon into the atmosphere. And instead of pouring forth waters that are enriched with metals into the ocean, the low heat water of the Lost City is enriched in calcium and hydrogen as H₂. The high amounts of calcium in the waters are what causes the precipitation of the calcite and aragonite (CaCO₃) that make up the bulk of the white towers. (Calcite and aragonite are also the two minerals most sea animals make their shells from.)

Most deep ocean hydrothermal systems at mid-oceanic ridges get their heat and chemistry by interacting with the freshly made basaltic oceanic crust that the ridges are constantly manufacturing. The Lost City is a special case that where waters interact with rocks from the Earth’s mantle, called peridotite. Basalt has a higher magnesium and iron content than the igneous rocks we normally see on continents; the term ‘mafic’ is shorthand for this. Peridotite is ‘ultramafic’ – it contains far more magnesium and iron than basalt.

It’s very unusual to have ultramafic rocks so close (relatively speaking) to the surface. Ultramafic volcanic rocks haven’t existed since the Archean, when the Earth was a much hotter and less pleasant place. Usually the peridotite of the mantle stays put and doesn’t directly interact with the surface of the Earth. However, Atlantis Massif – the site of the Lost City – was not formed by volcanic activity. It was actually formed by the sea floor being put under extreme tension that caused faulting and uplift. This faulting (and the major degree of uplift) brought mantle rocks close up to the surface in a highly deformed shear zone, thus giving ocean waters a way to contact hot rocks of very different chemistry.

The ultramafic rocks are the source of the high amounts of H₂ seen at the Lost City. Peridotite is composed primarily of olivine, a mineral that forms at extremely high temperatures and pressures; that’s where it’s happiest. At the (relatively) low pressures near the sea floor, olivine is very unstable and interacts enthusiastically (exothermically) with water to form a secondary mineral called serpentine, magnetite, and a hydrogen gas.

The chemistry of the water coming through the Lost City vent system is highly reducing. Normal sea conditions are oxidizing – in large part due to the normally plentiful amounts of dissolved oxygen. At the Lost City, the water conditions are highly reducing. What we think of as normal respiration for animals is impossible. Primitive bacteria and archea are the organisms best suited to that sort of environment, and they thrive there, surviving using chemosynthetic metabollic processes. There is abundant H₂S and CH₄ in the waters thanks to the highly reducing conditions. Both of these can be oxidized by bacteria to release energy.

There are some larger organisms that survive in the Lost City by building a symbiotic relationship with these chemosynthesizers – gastropods, crabs, and several types of corals call the area home. Life isn’t as plentiful at the carbonate chimneys as it is at the acidic and much hotter black smokers, but there is still a community of organisms that thrive.

The deep ocean is still a largely unexplored space on our planet, where life thrives at conditions completely alien to those of us at the surface. Much speculation has surrounded hydrothermal systems were archea and primitive bacteria thrive, since these extreme, reducing oceanic conditions might give us insight into the conditions under which life first evolved. What new, beautiful landscapes we might find in the depths of our oceans is at the moment a question for the imagination – as is what wonders could exist in alien oceans.

For more information about the Lost City, start at the University of Washington research website.

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