Atlantis Diary X: Reaction Zone

Reaction Zone

The bizarre hydrothermal vent field discovered a little more than two years ago surprised scientists not only with vents that are the tallest ever seen –the one that’s 18 stories dwarfs most vents at other sites by at least 100 feet — but also because the fluids forming these vents are heated by seawater reacting with million-year-old mantle rocks, not by young volcanism. The field is unlike any seen before, according to chief scientist Deborah Kelley, a University of Washington associate professor of oceanography, and co-chief scientist Jeff Karson, a Duke University professor of earth and ocean sciences. Both have visited fields of black-smoker hydrothermal vents that scientists have been studying since the 1970s.

Now the two scientists who were the first to travel in a submersible to the field after its serendipitous discovery Dec. 4, 2000, are leading a National Science Foundation-funded expedition to map and further investigate the field. The ‘Atlantis Diaries’ chronicles the expedition returning with 24 scientists onboard an exploration vessel, the Atlantis, during their 32-day expedition that spans April 21 to May 22.

 


Monday, May 19, 2003
David Butterfield

The objective for fluid chemists on this expedition was to characterize the full range of fluids that are venting at the Lost City field. The fluid composition may help us understand the seawater-rock reactions, the reactions that occur during formation of the carbonate chimneys, and the sources of chemical energy for microbes living within and on the vent formations.

Our main activity during the expedition was to find and sample fluids wherever we could. We took samples with the titanium major and gas-tight bottles, then followed up by using the Hydrothermal Fluid and Particle Sampler, commonly known as the Beast. We took 37 titanium major samples, and 40 fluid samples with the Beast.

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The Beast sampling fluid from a carbonate chimney growing out of a near-vertical wall. Credit: lostcity.washington.edu

The tectonic environment near the Atlantis fracture zone makes Lost City unlike any known volcanic hydrothermal system. One of the most exciting features of Lost City is that we can access the ocean crustal section directly beneath the active vent field. Tectonic forces have literally stretched and ripped the crust apart at Lost City, and by diving and exploring the near-vertical wall on the edge of the field, we can look at the underlying plumbing that feeds the vent field. For those of us who have been looking only at the upper surface of hydrothermal systems, this is a fantastic prospect.

Throughout the proposal process and planning stage for this expedition, we have been thinking about sampling fluids coming directly out of the host peridotite rock, potentially giving us access to "reaction zone" fluids. These fluids have not been affected by transport through the massive, 50-foot-thick carbonate structures.

However, when this fluid meets seawater, the result is carbonate formation. This results in fractures filling with carbonate, and massive deposits form where fluids leak out on the seafloor. Virtually all cracks in the host rock are filled with carbonate.

Lost City is the first and only known example of a vent field driven by serpentinization, the chemical process that occurs when mantle rocks are exposed to seawater. Wherever there is active flow of serpentinization fluids, there are large carbonate structures. Even on the near-vertical wall, 20 to 40 meter tall carbonate chimneys grow. So, even though we have a fantastic exposure to the plumbing system beneath the vent field, we do not have direct access to unadulterated fluids created by the reaction of seawater with peridotite.

While this may be a disappointment to fluid chemists who had hoped that Nature would supply a fountain of pure source fluids, it is certainly not a cause for despair. The fluids may not be as immediately revealing as we imagined — the full picture will only become clear after detailed laboratory analysis on shore — but we still have a remarkably full picture of the system.

Our shipboard work on the fluids tells us that there is not a great deal of variation in the major element chemistry (e.g. the calcium concentration). All fluids have pH higher than seawater, but there is a relatively large variation in the reactive compounds that are commonly involved in biological productivity (e.g. hydrogen sulfide, hydrogen, and methane). Detailed trace element and isotopic analysis of fluids and rocks should give us a much clearer picture of how this system operates, and we are eager to get the samples back to the lab and start working on them.

Tuesday, May 20, 2003
Betsy Williams

Now that we’re on our way to Bermuda, you might think we’re lounging around watching movies, sunning on the deck, and playing lots of Ping-Pong. Larry and Linda did provide a delicious barbecue for us last night, but mostly, we’ve spent the past couple of days preparing preliminary results for the "cruise report."

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Ken Rand of the R/V Atlantis heads out to pick up the Alvin divers. Credit: lostcity.washington.edu

The computer lab has been crowded (and a bit crazy) recently. People jockey for the CD player and their favorite computers as we all try to pull together our reports by the deadline. We just had a science meeting where each group tried to summarize in ten minutes everything they’ve learned in one month. It’s impressive, and it reminded me how lucky I am to be working with so many smart, capable people on such an exciting project.

When we get back home, my first task will be to create a detailed structural map of the Lost City field. ABE has provided us with an incredible bathymetric map of the area to use as a base, and the Alvin dives have provided great images, observations, and samples.

Using this data, I can map the faults, joints, rock types, deformation zones, and vents in this area, giving us a better idea of how the Lost City formed and how the fluids find pathways through the rock. I’ll put some of the images together into "photomosaics" to get a larger-scale view of the geology. In addition to the many hours of videotape, the Alvin dives generated about 60,000 images of the area, so I ought to have plenty of material to work with!

Even though the lunar eclipse was a few days ago, this is my first chance to write about it. People started appearing on deck around midnight, sprawling on beach towels, eating popcorn, chatting. The clouds were beautiful under the full moon – the puffy tops shining, isolated patches of the ocean sparkling. We saw the beginning of the eclipse, with the lower left corner of the moon looking bent out of shape. Then a big cloud covered the moon for about ten minutes, making us wonder if we would get cheated out of the show. But the clouds broke and we had a clear view as the moon went to full eclipse and glowed a gorgeous dusky orange. But the best part was the stars, clearly visible without city lights to drown them out. A few of us aren’t used to being able to see the Milky Way. There were even a few shooting stars to make us feel we got our money’s worth!

However, the scene was perhaps not as romantic as you might imagine. A ship is a very noisy place – the engines are loud and there are always lights on and unidentified chemical smells in the air. But still, this was an amazing place to watch the eclipse. And last night, we got another treat: green bioluminescent specks in the foam streaking past the side of the ship.

Tomorrow we’ll be packing everything from lab equipment to rock and water samples to office supplies. But we’re all excited to be headed for shore – only two more days to Bermuda. After a couple of days offloading in port, most of us will head for home — back to where floors are stationary, wallets and keys are necessary, and the days of the week have meaning.

Wednesday, May 21, 2003
Gretchen Früh-Green

Only one more day to go it seems like only yesterday that I was writing "only one more day until we get on site and start our science." I don’t have the opportunity to sail often, but when I do, the final days of a cruise always leave me with mixed emotions. On the one hand, I am excited about getting back to solid ground, my family, and my own bed. On the other hand, I feel melancholy that the expedition is over and it is time to leave the group and the intensive scientific interaction that we have had.

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Debbie and Jeff, the co-chief scientists, take a moment to watch the sun go down on another day, and reflect on the many accomplishments of the cruise. Credit: lostcity.washington.edu

We all feel that our expedition to the Lost City vent field has been an incredible success. One of the exceptional things about this project is the mix of diverse scientific disciplines and technological advancements. The ABE team has produced amazing maps that delineate the vent field and help us understand the tectonic and erosional processes that may control hydrothermal circulation and venting. Through their skill and patience, the Alvin pilots have helped collect the samples we need for our studies of the rocks, biology, geology, fluid chemistry, and organic geochemistry. We are also greatly indebted to the tremendous work of the ship’s crew, who made sure that we got where we needed to go, had plenty of good food, and a safe working environment.

This teamwork has led to a phenomenal amount of imagery, samples and data. One of my jobs was to keep a log of all the samples we collected during our expedition. In the end, after 19 Alvin dives, we collected 114 rock samples, 173 vent fluid samples, and 41 macrofaunal samples! 153 microbiological samples were taken from these. ABE was busy as well, covering a distance of 200 kilometers during 17 missions.

I was present when we first discovered the Lost City, and I don’t think I will ever forget our excitement that night. Many of us will be leaving the ship with a similar type of excitement. The sampling we were able to do, together with the fabulous ABE maps, will help us understand how seawater gets into the basement rocks, how it reacts along the way, and how the fluids provide nutrients for the microbial communities, who in turn provide food for the macrofaunal species. I am sure I’m not alone in feeling privileged to have seen the indescribably beautiful structures that Nature has created on the seafloor.

The success of our cruise – and the possibility to share this success with all of you on land – is due to the tremendous efforts of our chief scientist, Debbie Kelley. Deb, we would all like to thank you for the long days and sleepless nights you put into writing the proposal, putting together the scientific team, preparing the trip, and guiding us through this expedition.


The project includes scientists, engineers and students from the University of Washington, Duke University, Woods Hole Oceanographic Institution, U.S. National Oceanic and Atmospheric Administration, Switzerland’s Institute for Mineralogy and Petrology and Japan’s National Institute of Advanced Industrial Science and Technology. 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.