Life without Volcanic Heat
|The Mid-Atlantic Ridge is one of the earth’s largest undersea mountain ranges at a length of nearly 6,200 miles. The dots represent active hydrothermal sites.
An 18-story undersea vent off the Atlantic, near what has been called the ‘Lost City’, has recently revealed itself as ripe with exotic microbial life. From the University of Washington oceanography team, led by Deborah Kelley, recent reports in Nature magazine point to a new way to build such towering vents from what is nearly 100% limestone.
Previous deep-sea finds of hot vents have not reached beyond 8-stories (80 feet) and also proved rich in a mix of black minerals (mainly iron-sulfides). But the new vent found atop the seafloor mountain Atlantis Massif, is nearly 10 stories taller. Not piled up atop volcanic cracks, the 180-foot vent instead formed from a chemical reaction between seawater and the exposed mantle along deep fracture lines in the mid-Atlantic. Typically minerals dissolve above such deep-water vents, mainly as warmer water below carries those minerals upward, but then contact colder water where they redeposit in spectacular chimneys, mounds and vents.
What "makes this potentially such an important and exciting find," says Kelley, "is that it is a completely different type of hydrothermal system not requiring volcanic heat, and the implications it may have for examining early Earth questions and hydrothermal systems on other planets. This opens the possibility that a much larger portion of the seafloor may host hydrothermal vents (and microbial life)."
The Journey to the ‘Lost City’
|The carbonate structures at the Lost City Field include this chimney more than 30 feet in height. The white, sinuous spine is freshly deposited carbonate material. The top shows evidence of collapse and re-growth, as indicated by the small newly developed cone on its top.
Credit: University of Washington/Woods Hole Oceanographic Institution
Getting to the snow-white vents that are active is half the game. While surveying over the Atlantis mountain, the research team first looked for a tell-tale signature of venting. "By sampling the plume fluids during towing of instruments that," says Kelley, "measure conductivity, oxygen, salinity, depth, particulates and temperature, and by measuring the gases onboard, it is possible to detect springs venting from mantle rocks."
But even after finding a good candidate to dive deeper for, actually getting to the Lost City is a navigational feat. "Much of this discovery was serendipitous…The rugged terrain, combined with the fact that the Lost City chimneys are typically greater than 20 meters tall makes navigation very difficult," says Kelley. "Perhaps a good analogy may be to imagine flying a small plane by radar in the dark through a redwood forest with no map and no easy way to locate yourself. If you fly down near the tree trunks it is very difficult to see the rest of the grove, but if you fly high, the individual trees can be spotted by radar. In a very similar way, this is the method we used for exploring the Lost City."
The ‘Poseidon’ Discovery
Once the seafloor was combed, what Kelley’s team found, they called Poseidon. Poseidon consists of an array of exotic looking mounds, spires and chimneys, first formed from rising columns of water which desalt when reaching colder top layers of water. Sometimes well-above atmospheric boiling temperatures (100 C), the mineral-rich water (40 to 150 C) first rises buoyantly, eventually cooling as top layers drop the temperatures below the solubility or dissolution range. This process is called hydrothermal circulation. The result is a tower of 100 percent carbonate, the white, gray or creamy colors found in limestone caves, and the snow-white opposite of so-called black smokers, which are formed from iron-sulfide minerals.
But the seafloor for Poseidon is an old crust layer, 1.5 million-years old, formed from underlying mantle material. The simple view of the Earth’s interior involves crust, mantle, and core wrapped over each other like an onion. Most previous vents formed around much younger parts of the crust, where so-called spreading centers split open to get filled in later — usually by volcanic eruptions and the heat associated with black smokers. Tectonics or the moving continental shelves force two parts of the ocean floor to rip or join. A fast spreading center will realign every 5-10 years, while more rare realignments like at Poseidon happen only once every 5,000 to 20,000 years. Because the Atlantis Mountain area has such deep fractures, the crust doesn’t layer the mantle in parts, exposing raw minerals like olivine to sea water.
That’s where the chemical features of the Lost City differ from volcanic vents. The olivine reacts with seawater to make a mineral called serpentine, which generates more hot water columns and thus the highly columnar Poseidon vent. This ‘serpenization’ warms the deep ocean water to 40 to 75 C typically, but "may reach 150 C", according to researchers survey of thermodynamic tables for olivine changes. When that upwelling water cools as it rises, the carbonate drops out of solution, and crystallizes onto nearby rocks like Poseidon’s massive 18-story tower.
Life atop Atlantis Massif
|University of Washington oceanographer Deborah Kelley contrasts the white porous (almost wasp-nest-like) texture of a sample from the Lost City’s carbonate chimneys with a sample from the sulfide chimneys studied since the 1970s.
Credit: University of Washington
The chemical vents don’t have large animals that prey on exotic life like those near black smokers, but they do seem to foster enough microbes (100 million per gram of carbonate) to cover entirely some returned rock samples. "The Lost City carbonate vents host diverse and very dense microbial communities. The organisms form thick biofilms that cover the mineral surfaces. We believe that these systems will host extensive microbial communities that may include methane- and hydrogen-oxidizing bacteria."
"Unfortunately, instruments are not yet developed to characterize microbial populations," says Kelley, although culturing samples back in the lab has shown heat-loving microorganisms adapted to thrive in the thermophilic (50-70 C) and mesophilic (25 C) temperature ranges. "The long term goal is to develop seafloor observatory sites that will allow long term investigation of how volcanoes support life on the seafloor, and how submarine earthquakes affect the output of gases from the seafloor and microbial life. These are the types of questions that are of planetary scope (See Neptune surveys at: http://www.neptune.washington.edu/). Perhaps learning how to examine such questions will provide useful guides to exploration of other planets."
|A 5-foot-wide flange, or ledge, on the side of a chimney in the Lost City Field is topped with dendritic carbonate growths that form when mineral-rich vent fluids seep through the flange and come into contact with the cold seawater. Photo credit: University of Washington/Woods Hole Oceanographic Institution
Credit: University of Washington/Woods Hole Oceanographic Institution
Although chemically heated vents like the Lost City may prove common, pinpointing their exact location has been a challenge. Unlike volcanic vents, chemical ones may arise tens of miles away from the deep cracks where the ocean floor itself rips and rejoins. But better instruments are in the works, too, to help narrow where next to send submersible divers. "There is a significant amount of energy now going into developing instruments," says Kelley, "that can be placed in the vents for long periods of time to collect in-situ measurements."
Just within a mere 50-mile radius of the Atlantis Massif are three similar mountains subject to the same fracturing, the same intrusion of seawater and perhaps the same reactions with mantle material. And those four represent only a tiny fraction of the potential sites along the 6,200-mile Mid-Atlantic Ridge, (as well as the Indian ridges and the Arctic Ridge, also considered slow- and ultraslow-spreading centers). Kelley concludes: "How we most efficiently explore for such systems will be a learning process."
Collaborators include: Jeff Karson, Duke University, Co-PI and diver during the discovery; Matt Schrenk (an astrobiology graduate student at the UW School of Oceanography); P.J. Cimino (a NASA Space grant undergraduate); and John Baross, also a faculty member in astrobiology and oceanography.