The spectacular underwater world of the ‘Lost City’ hydrothermal field is perched near the summit of a huge underwater mountain called the Atlantis Massif. Geologists were studying the Atlantis Massif long before Lost City was discovered because it is made up of rocks that originated from the Earth’s mantle, which have been carried to the seafloor along major faults and have a distinct chemistry compared to most rocks exposed on the seafloor. Mantle rocks contain large amounts of the mineral olivine (a Mg-Fe silicate), which reacts with seawater and forms hydrous Mg-silicate minerals called serpentine and magnetite, an iron oxide that is highly magnetic. This process – referred to as serpentinization – produces methane, hydrogen and heat, among other things. These rock reactions excite scientists because they represent possible fuels for life in the absence of sunlight, and they could be analogous to conditions early in Earth’s history or found on other planets.
The extreme conditions in the impressive white towers of the Lost City are determined by these chemical reactions occurring in the rocks of the Atlantis Massif. The towers, which can be 60 meters (200 feet) tall, are formed when warm alkaline waters from deep within the Atlantis Massif exit the seafloor and mix with seawater. Unlike almost all other hot springs on the seafloor, the chemistry and circulation of water venting from the Lost City chimneys are not driven by hot lava or by cooling of magma at depth. In other words, the Lost City is not volcanic. Instead, it is a geochemical reactor. The hydrothermal activity is driven by the chemical reactions between seawater and the mantle rocks of the Atlantis Massif, which are cooling down as they are exposed to seawater. These same reactions could be providing fuel and food for microbial life, making them intriguing subjects for studying the origins of life. Our studies of the water venting from the Lost City chimneys could provide clues about how the first biochemical pathways might have emerged from geochemical reactions in seafloor rocks on the ancient Earth.
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