Let's Talk with Susan Humphris about the Chemistry of Deep Sea Vents

AMNH: Why should kids know about deep sea vents?

Susan: Apart from them being a whole new ecosystem on Earth, our home, I also think they are a marvelous example of the interplay between geology, physics, chemistry, and biology. That can help kids understand how any study of Earth's dynamic forces requires knowledge in many different fields of science.

AMNH: How can students everywhere be good stewards of our oceans?

Susan: Learn everything you can. The discovery of the vents shows us that there is still much to learn about our own planet. Kids often get turned off by doing the same old predictable experiments in the lab; these give the impression that we know everything. But the deep sea vents are a very visual demonstration that there is still a lot to be discovered. The thrill of discovery is great—and kids need to know that exploration is still very much part of science.

AMNH: What's so important about your field of study?

Susan: Deep sea vents are important for a number of reasons. They remove heat from the Earth's interior and put it in the ocean. They may also play a role in regulating the chemical composition of seawater; that's what I study. Deep sea vents form deposits composed of minerals rich in iron, copper, and zinc. Over millions of years, some of these deposits get thrust up on to land and become valuable ore deposits. By studying deep sea vents, we can study the way in which mineral deposits are formed. And then there's the biological importance of the vents; although I am not a biologist, the oases of life around deep sea vents intrigue me. Just think—until 1977, we did not know that these communities of organisms even existed!

AMNH: So, you study the chemical composition of seawater at the deep sea vents?

Susan: I study how the vents may be affecting the chemistry of the oceans. You see, the mid-ocean ridges represent the longest chain of volcanoes on our planet, stretching for more than 40,000 miles. The water down there is very cold; and cold seawater percolates down into the rocks that make up the volcanoes, just as rainwater percolates down into the ground. Beneath the volcano, there are molten rocks—in other words, VERY hot rocks. As the cold seawater descends into the volcanic rock, the molten rocks heat it up. The seawater starts reacting chemically with the rocks; its chemical composition actually changes. Once the water gets hot—and I mean possibly up to about 350-450°C (660-750°F), it rises, just as a hot air balloon rises into the air. The water discharges at the seafloor, creating the deep sea vents. During this process of hydrothermal circulation, seawater passes through the volcanic rocks of oceanic crust and reacts chemically with them.

AMNH: And you study those chemical reactions?

Susan: Well, this process changes the composition of the seawater flowing through the vents; that changed seawater is then returned to the ocean. I am interested in how that process affects the composition of the whole ocean. To answer that question, I need to know two things—first, I need to know how the water-rock chemical reactions change seawater. Then I need to know how many hydrothermal vents there are on the seafloor, where they are, and where they are in relation to each other. In my work, I study rocks from the oceanic crust to see how they have changed during chemical reactions with seawater; and I also try to find new hydrothermal vents and understand why they occur where they do.

AMNH: Okay, let's say a group of kids read this interview and then go to the beach after school. Is the water in which they swim being affected by deep sea vents X miles away, and X meters below the surface of the sea?

Susan: Yes, for some elements, vents are regulating the chemistry of the seawater that those kids swim in. You see, the relative proportions of the elements in seawater have stayed constant over a long period of time. That means that if an element is added to the ocean, an equal amount of that element is taken out. Hard to picture? Imagine you have four kids in a classroom and a few outside in the hall. Tell them that they can run in and out of the classroom, but the number in the classroom must always equal four.

AMNH: In that case, the four kids in the room would be like the amount of elements in the water. It always has to be four, but some stay in and others go out.

Susan: Exactly. Take magnesium. Before we knew about vents, we couldn't figure out how enough magnesium was removed from seawater to keep the concentration the same. Where was it going? Then we discovered that volcanic rocks take up all the magnesium from seawater when they react; the water coming out of the vents has virtually no magnesium in it. So the riddle as to how to remove magnesium was solved.

AMNH: Wow. So the vents are yet another example of how the dynamic processes on Earth are all interconnected.

Susan: Yes. The source of heat beneath the Earth's surface allows seawater to circulate through the crust by convection, much like the way soup cooks on a stove. The hot fluid rises up from the bottom, cools, and then sinks again. That circulation allows seawater to react chemically with the rocks, changing the composition of seawater and providing the chemicals that support life around the vents.

AMNH: Which chemicals support life at the vents?

Susan: Mostly dissolved gases, such as hydrogen sulfide.

AMNH: How do they support life?

Susan: Any biological community has an organism at the base of its food chain. At the base of the vent community food chain are microbes that use chemicals as a source of energy. The chemicals are created by that heat source below the Earth's surface. If the heat source were to be turned down, like the stove under the soup being turned off, the seawater wouldn't get heated and wouldn't circulate; and the chemicals needed by the microbes wouldn't be available—and the whole biological community would collapse. That's interconnection! We call those interconnections the consequences of the feedbacks between physics, chemistry, and biology; and they're all dramatically displayed at deep sea vents.

AMNH: When you're out in the field, you're studying so many different sciences at once!

Susan: Well, I'm always very busy! But I love going to sea. I go on one cruise every year or so, for two to eight weeks at a time.

AMNH: While you're at sea, are you mostly on board the research vessel or down at the vents?

Susan: I work a lot with the submersibleAlvin and the remotely operated vehicleJason down at the seafloor. If I'm working onAlvin, I actually go down to the vents. But if I'm on a team that is working with the remotely operated vehicle,Jason, we're up on board.Jason can stay at the bottom continuously because it receives power through a cable; we stand watch in the vehicle control van, taking notes on the dataJason sends and making decisions about whatJason should sample and where it should go next.

AMNH: Do you take samples from the vents to study back at the lab?

Susan: I collect rock samples while I am at sea. We don't analyze rocks at sea, but we can cut them open so that we can catalog and describe them. Every rock has its own catalog number so that when we get home, it can be put in our storage facility. Back at the lab, I crush, powder, and analyze the samples that I bring back from cruises in order to analyze their chemical composition. If you have ever looked at a rock, you will know that it is not homogeneous in composition—it is made up of different minerals. When we crush the whole rock up, we create a powder that represents all of the minerals in the rock. We can then dissolve the powder or subject it to X-rays to determine the minerals present and the chemical composition of the whole rock.

AMNH: That's how you examine its chemistry.

Susan: Yes. By looking at the rock and analyzing how it has changed in composition during the reaction with seawater, we can tell which elements have been lost or gained from the rock. That way, we know that the seawater must have gained the elements that are missing from the rock, and lost the elements that are in greater abundance in the altered rock compared with the fresh rock. All the rock samples we collect, by the way, eventually become available for anyone in the scientific community who wants to study them.

AMNH: How do you know which rocks to collect?

Susan: We try to take some samples that are representative of what we are seeing, and some that are unusual as well. Sometimes it is very tempting to take only unusual-looking rocks, but we are very careful to take representative samples, too, no matter how dull they look in comparison to others.

AMNH: Obviously, you can't just stick your arm outsideAlvinto pick up the rocks; how do you do it?

Susan: The pilot holds the rock up with a robotic arm so that we can see it and describe it. The pilot then tells us in which part of the basket on the front ofAlvin orJason he put it. That way, when the vehicle comes up, we can pick each sample out and recognize it.

AMNH: You end up using bothAlvinandJasonto do your work. Do you prefer either one?

Susan: Both have their value, and I enjoy using both.Alvin is great because sometimes it is really important to go down and get the perspective of seeing the vent in all of its surroundings. ButAlvin trips are short.Jason is great because it stays down a long time, and everyone on the ship can see what is going on. Of course, you see only the view that a TV screen gives you—you often wonder what is off to the side, just out of view! There are advantages and disadvantages to each submersible, but they are both wonderful because they provide the most critical element of deep sea vent research—getting there!

AMNH: What was it like the first time you "got there"?

Susan: I'd been working on rocks dredged from the seafloor for over 10 years before I finally got to go down inAlvin and see a vent. We were at the first vent site discovered in the Atlantic Ocean, and I was diving with a colleague of mine from MIT. We came upon some huge chimneys over 40 feet tall at the top of a huge mound of mineral deposits. The surfaces of the chimneys were completely covered with billions of swarming shrimp that seemed to be fighting to get onto the surface of the chimney. It was a spectacular sight—black smoker fluid pouring out of the chimney and its surface covered in white shrimp. I'll never forget my colleague's description of the spectacle: "They look like maggots on a piece of rotting meat!"

AMNH: You've gone down so many times, but there's always something new.

Susan: Vents are spectacular environments that never cease to excite me. They're beautiful, too, and I never tire of looking at them or at videos of them.

AMNH: Do you study other deep sea environments besides the vents?

Susan: I've been on a large drilling ship run by the Ocean Drilling Program; this ship drilled holes into a deep sea vent to figure out what minerals were inside a big mound of mineral deposits created by the seafloor vent. We were also trying to find out how circulating seawater had reacted with the volcanic crustal rock.

I've also worked along parts of the volcanic mid-ocean ridge system where there are no vents, just black volcanic rocks and very little biology. The volcanic rocks have their own beauty; they form different types of rocks, depending on whether they flowed in a sheet or formed bulbous fragments as they solidified, and their surfaces show a variety of textures. Unlike the vents, where there is lots of action, the action at these sites is frozen in the shape and texture of the rock.

AMNH: What are you looking for in those "non-vent" environments?

Susan: We try to determine where the molten rock came from in the Earth, and what has happened to it since it was a liquid—or since it wasmagma, as molten rock is called—deep in the Earth along its path to the seafloor. We look at its mineralogy and chemical composition, just as we do for the rocks found at and below vents.

AMNH: Now we know a little about the science you conduct; what about life on board the research vessel?

Susan: Life on board develops into a routine, punctuated by watches and sleeping. When you are at sea, you work in shifts called "watches" so that the ship can be operated and science can be conducted 24 hours a day. The watch system means that at any time of the day, people are sleeping between their watches. Some people prefer to work at night so they have their days free, while others like to keep the schedule that they are used to on shore. When setting up watches, we try to give everyone the time slot they want. However, sometimes, to make sure that every shift has the right number and combination of people, certain individuals have to stand a watch that they would not normally choose.

AMNH: Who's "in charge"?

Susan: Usually, a senior scientist on the cruise will be put in charge of each shift. The chief scientist often does not stand a watch because he or she is on call 24 hours a day; often the chief scientist will have to stay up for very long periods of time, especially when things are not going as planned.

AMNH: When you do get to sleep, where do you sleep?

Susan: Most people share a cabin with one or three other people, with a bunk for each person in the room. There are also cupboards and usually a desk. Bathrooms, or "heads" as they are called on ships, are usually shared between two rooms. As I get older, the lack of privacy does bother me more! What we try to do is put roommates on different "watches" so that each person has a time when the room is his or her own.

AMNH: What about eating and relaxing? Where does that happen?

Susan: Meals are very important at sea, as they break up the day and they are something to look forward to. There is a crew of people, usually about three, who deal with the "hotel" services of the ship. This includes providing meals, which are usually very good, and making sure there is food available for people working at night. This crew also provides bed linens and worries about general cleanliness and hygiene. For leisure time, there is some gym equipment for working out; and on the cruises that I go on, I like to get a group together to do aerobics on the bow—although not in bad weather, of course! The ship also has a library, both of books and of videos, for entertainment.

AMNH: So that's life at sea. In addition to grinding up rock samples for analysis, what else do you do when you get home?

Susan: I do spend time writing papers and giving presentations at scientific meetings on my findings. I also make time for writing proposals to the National Science Foundation for new projects that I would like to do. I do a small amount of teaching. I am also currently director of a newly formed Earth-Ocean Exploration Institute at the Oceanographic Institution, so now I spend a lot of time giving presentations related to that, and providing funding to other researchers. This institute brings together scientists who study different subjects—including biology, chemistry, and geology—to study different Earth systems. As I said before, deep sea vents provide a wonderful example of the need for different kinds of scientists to work together in order to truly understand the system.

AMNH: You lead a very full life. What did you have to do to prepare for your career?

Susan: After concentrating on physics, chemistry, and mathematics in high school, I decided I was much more interested in applying science to the environment. I also wanted subjects that would let me spend time outside. So I studied environmental science in college. I completed my undergraduate work in England; then I came to the U.S. for my Ph.D. I returned to England for postdoctoral work and then again returned to the U.S. in 1978. I've lived here ever since.