Atlantis Diary VI: Portal on the Past
Portal on the Past
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.
Friday, May 9, 2003
While the Atlantis sits on the surface all day, I often wonder how many people have sailed, steamed, or cruised over this very spot, or perhaps even stared into the deep without any notion of the remarkable ecosystem that lies beneath the water. Columbus? Conquistadors? Pirates? Marines, sailing for the shores of Tripoli? GIs headed to Africa and Normandy? We’re certainly not the first ones here: yesterday Alvin spotted a drink bottle on the seafloor beneath Lost City!
What would the ocean floor have looked like four billion years ago? Many geologists think it might have looked a lot like what we’re seeing now at Lost City (with one caveat: the four-billion-year-old bottle hypothesis remains controversial). Today ocean crust is made mostly of a rock type called basalt. But four billion years ago, it may have been mostly peridotite – just like at Lost City. And since peridotite reacts with seawater to create the hydrothermal system at Lost City, similar systems might have been common on the ancient ocean floor.
|Filaments attached to a chimney at Lost City. We don’t know whether these filaments are bacteria or simply material they have left behind. Credit: lostcity.washington.edu|
Therefore, Lost City might be a window into the past. Microbiologists like Billy and Mausmi will tell you that the most ancient forms of life that we know of are the bacteria and archaea that live at hydrothermal systems, and they are trying to determine what types of bacteria and archaea live at Lost City. Are the microbes living at Lost City similar to those that lived in hydrothermal systems four billion years ago? If so, what can we learn about the ancient systems? And what information about the microbes’ past and present can we gather from organic geochemistry techniques?
To answer these questions, I’m working with John Hayes and Roger Summons to study organic material preserved in the carbonates at Lost City. In particular, we’re interested in studying lipids, which are chemicals that organisms use to make their cellular membranes. Lipids are excellent chemicals to study – for one thing, they are often exceptionally well preserved: many old rocks still contain lipids long after the carbohydrates, DNA, and other organic material has been destroyed. The structures and isotopic compositions of lipids also contain a lot of information about the organisms that produced them.
If an ancient analogue to Lost City were found in the geological record, information about microbes would likely be preserved only as lipids. But here at Lost City, we can study the microbes, the environment, and the lipids simultaneously. On board, John and I use a technique called Thin Layer Chromatography (TLC) to determine how much lipid material is present in each carbonate rock retrieved by Alvin. A small amount of organic material is first removed from the rock, then spotted on a TLC plate that is placed in a jar with some solvent. The solvent moves upwards, along the TLC plate, and carries the lipid material with it. The TLC plate is made of a special silica gel that binds to some compounds more strongly than others, so our organic material gets separated.
When we get back to land we can analyze these lipids with more sophisticated techniques. For now, we continue to examine samples and think about this amazing system. If only the pirates knew what they were missing!
Saturday, May 10, 2003
I process the sonar collected by the Autonomous Benthic Explorer (ABE) on its nightly sojourns. The new SM2000 multi-beam sonar unit on ABE provides exceptionally detailed maps of the rough topography of the Atlantis Massif.
|A closeup of the the Lost City vent field. Bathymetry generated by ABE.Credit: lostcity.washington.edu|
The resolution of the maps has revealed many details about the Lost City site and surrounding terrain. The maps generated by ABE help us navigate the vent field, since they are at a much larger scale than what is visible out of Alvin’s viewports.
The large number of dive opportunities on this cruise has allowed us to try some new mapping techniques. The SM2000 can be swiveled to point up to 60 degrees to one side. This arraignment maps the steep cliff faces surrounding the Lost City site from the side and resolves their textures in detail.
Because ABE flies low, about 40 meters over the bottom, each sonar beam only covers a small portion of the terrain. This results in precise range-to-bottom data representing about a square meter of the ocean floor.
After ABE returns to the surface, I download the data onto my computer. Saving the raw signals on ABE allows for tremendous flexibility in processing, but it can be frustrating. There are a great many variables, some of which must be determined from the data itself. But when a map comes together and reveals features like sheer cliffs, tall spires, and a pancake flat summit, I find the work very satisfying.
Sunday, May 11, 2003
I have been doing chemical analyses of hydrothermal plumes and vent waters since 1984, but this is my first time in the Atlantic Ocean. If I could invite you to go up on the bow of this ship and look at the ocean surrounding us, you’d think it doesn’t look any different than the Pacific Ocean (where I usually go to sea). But from a chemist’s point of view, it is very different. To understand the chemistry of the hydrothermal fluids, we first must know something about seawater chemistry.
The deep waters of the Atlantic were surface waters more recently than the deep waters of the North Pacific. But how does the surface seawater become deep seawater? The answer is density. Cold seawater is denser than warm seawater and saltier seawater is denser than less salty seawater.
|A view of the sea from the starboard (right side) of the R/V Atlantis.Credit: lostcity.washington.edu|
Warm, dry climates cause more evaporation from the ocean, so the surface ocean becomes saltier (and thus denser). If currents transport this saltier water to a cold climate, the surface seawater becomes even more dense and it sinks under less dense seawater. This happens off the coast of Greenland and near Antarctica. The Mediterranean Sea also contributes to this, with outflow from the Mediterranean sinking to a depth of about 1,000 meters in the Atlantic.
Below 500 meters, there is eight times more silica in the North Pacific than there is here. This is due to deep-water circulation. Surface seawater is silica-poor because diatoms (a type of phytoplankton with a silica "shell") remove dissolved silica from seawater. When diatoms are eaten by zooplankton, their "shells" are excreted and fall to deep water, where the silica dissolves.
Deep seawater generally flows from the North Atlantic southward to the Pacific. By the time deep seawater reaches the North Pacific, there has been much more "rain" of diatoms and other organic matter over time, and therefore a higher dissolved silica concentration in the North Pacific.
My job here is to analyze hydrothermal fluid from Lost City for silica, ammonia, and dissolved sulfide. To find these chemicals, I react water samples with other chemicals to make colors. Then using an instrument called a spectrophotometer, I measure the amount of light absorbed by the colored compound I have made. The amount of light absorbed is proportional to the amount of silica, ammonia, or sulfide that was in the water sample.
The chemistry of the vents at Lost City is different from anything I saw at black smokers in the Pacific. These Atlantic hydrothermal fluids have some sulfide-like black smokers, but they have a basic rather than acidic pH. They also are low in silica – as much as 1,000 times less than a black smoker vent.
Not only is the concentration of silica very low here, but sulfide is present in concentrations high enough to cause problems, since sulfide interferes with both ammonia and silica analyses. To get rid of the sulfide I add acid to the sample, turning the sulfide into hydrogen sulfide gas. I then bubble nitrogen gas through the sample, and this removes the hydrogen sulfide. Our noses are very sensitive to low concentrations of hydrogen sulfide, which has an odor of rotten eggs. So the by-products of my work here are pretty colors but bad smells.
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.