03-074 Oceanusmktivey.Indd

http://oceanusmag.whoi.edu/v42n2/mktivey.html

The Remarkable Diversity of Seafloor Vents Explorations reveal an increasing variety of hydrothermal vents

By Margaret Kingston Tivey, Associate Scientist Juan de Fuca Ridge off the Pacific North- thrive in the absence of sunlight. It was Marine Chemistry & Geochemistry Department west coast. Expecting to find common also becoming clear that the relatively con- Woods Hole Oceanographic Institution seafloor rocks called basalts, scientists stant chemistry of the ocean was in part n late summer of 1984, I anxiously were surprised to pull up fragments and sustained by hydrothermal activity. Iawaited my first trip to the seafloor in boulders of massive sulfide covered with What my colleagues and I were only the submersible Alvin. There was a delay small tubeworms. They had discovered beginning to realize then was that hydro- in launching the sub, but I resisted the the fourth and, at the time, newest site of thermal vent systems are like snowflakes— urge to have a drink, anticipating one fi- hydrothermal venting on the seafloor, a no two are ever exactly alike. nal trip to the bathroom before crawling place now known as the Main Endeavour into Alvin’s three-person, 6-foot sphere for Field. These rocks helped launch my sci- Long-standing mysteries eight hours. I was excited not only about entific career. When scientists in Alvin discovered my first chance to dive, but about visiting In the years leading up to my 1984 the first active hydrothermal system in the home of the seafloor rocks I had long dive, I had learned that hydrothermal vent 1977 at the Galápagos Rift, they found been studying for my master’s thesis. systems played a significant role in trans- warm fluids, later determined to be a Since 1982, I had spent most of my ferring heat and mass from the solid Earth blend of cold seawater and hot vent flu- waking hours examining pieces of seafloor to the ocean, and that the vent sites host ids, seeping from seafloor crust. Never- vent deposits that had been recovered dur- unusual biological communities, including before-seen organisms were present at ing a routine dredging operation along the tubeworms, bivalves, crabs, and fish that the vents, including large clams and tall, IMAX film by William Reeve and Stephen Low, Stephen Low Productions Low Stephen Low, Reeve and Stephen William film by IMAX When hot hydrothermal fluid jets from the seafloor and mixes with cold seawater, fine particles of dark metal sulfides precipitate out of solution, creating the appearance of black “smoke.”

1 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu A Seafloor Observatory

In 2000-2001, scientists collaborated for the most comprehensive study of a hydrothermal vent system to date, making complementary and continuous observations at the Main Endeavour Field off the Pacific Northwest coast. This schematic depiction shows the various instruments deployed and vehicles used (though the vehicles were not used simultaneously). They measured currents, pressure, vent fluid temperatures and flow rates, chemical properties, and seafloor magnetic properties. The project, funded by the National Science Foundation, is a model for future long-term seafloor observatories.

The Autonomous Benthic Explorer (ABE) collects data on seafloor depths, water properties above the vent field, and magnetic properties of the seafloor. The remotely operated vehicle Jason makes acoustic images of vent structures and venting fluids.

The human-occupied submersible Alvin “eavesdrops” on data being collected by downloading information without removing the instrument from a vent site. E. Paul Oberlander E. Paul

red-tipped tubeworms. posed by geophysicists: How is heat trans- The chemistry of the ocean In 1979, a second active hydrothermal ferred from Earth’s interior to the oceans? The discovery of vents allowed sci- system was discovered along the East Pa- Earlier studies had shown that, contrary to entists to begin to answer another major cific Rise. At that site, much hotter fluids model predictions, not as much heat was question: How does the ocean maintain its (350°C) jetted from tall rock formations being transferred by conduction (particle- relatively constant chemical composition? composed of calcium sulfate (anhydrite) to-particle transfer) near the ridge crests. Over time, rivers drain materials into and metal sulfides. When the clear hot Scientists hypothesized that heat was the oceans, and winds blow in particles, fluid jetting from these chimney-like also transferred by convection, as fluid some of which dissolve to add chemical structures mixed with cold seawater, fine circulated within the crust near mid-ocean elements to the oceans. Some of these ele- particles of dark metal sulfides precipitat- ridges. Sure enough, cold seawater is en- ments, in turn, exit ocean waters, settling ed out of solution, creating the appearance tering cracks and conduits within seafloor in ocean sediments, for example. of black “smoke” (hence the name “black crust. It is being heated by underlying Many components of seawater—in- smoker chimneys.”) rocks and rising and venting at the sea- cluding lithium, potassium, rubidium, The discovery of these vent systems floor, carrying significant amounts of heat cesium, manganese, iron, zinc, and cop- immediately answered a question long from Earth to ocean. per—enter the oceans via vents. They are

2 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu leached from seafloor crust by subterra- The Birth of a Black Smoker nean chemical reactions with hot hydro- The key to the creation of black smoker chimneys is an unusual chemical property of the thermal fluids. Other hydrothermal vent reactions draw elements out of seawater mineral anhydrite, or calcium sulfate (CaSO4). Unlike most minerals, anhydrite dissolves at low temperatures and precipitates at high temperatures greater than ~150˚C (302˚F). and place them back into the earth. For example, magnesium eroded from land is carried to the ocean by rivers. Yet P A

R magnesium concentrations have not in-

T I

C creased in the oceans. Scientists puzzled L E S for decades over where all the magnesium could be going. Scrutinizing hydrothermal vents, researchers found that seawater entering seafloor crust is rich in magnesium, but fluids exiting the vent are free of it. Oceanogra- phers surmised that magnesium is left be- 10°C hind in the crust, deposited in clay minerals as seawater reacts chemically with hot rock. 200 °C

350°C Dive to Main Endeavour Field 2°C 2°C seawater seawater In those early years when observations

E were few and samples fewer, my thesis L IC T rocks provoked considerable interest. In R

A P some ways, the rocks were similar to those recovered from the East Pacific Rise, but in other ways they were quite different. 350°C The Main Endeavour Field samples Anhydrite were rich with amorphous silica, which 2°C seawater Chalcopyrite should only precipitate if the hydrother-

350°C mal fluid had cooled without mixing with seawater. And they did not contain anhy-

0.5 – 5 inches drite, a common vent chimney mineral diameter that dissolves in seawater at temperatures 350°C hydrothermal fluid Hydrothermal fluid Different minerals less than about 150°C (302°F). (containing dissolved and seawater mix precipitate at different We theorized that the dredged pieces 2+ Ca ions) exits the through the nascent temperatures. must have come from the low-lying mounds seafloor and mixes anhydrite wall around Chalcopyrite is that lay beneath and around black smoker with cold seawater the vent opening. Sulfide deposited against chimneys. Our dives to the Endeavour Field (containing dissolved and sulfate minerals the inner wall, which in 1984 would tell us if we were correct. 2+ 2- Ca and SO4 ions). precipitate within pore is adjacent to 350°C The trip to the seafloor took 90 minutes. A ring of anhydrite spaces of the wall, which hydrothermal fluid. As we approached the seafloor, the pilot precipitates around a gradually becomes Anhydrite, iron, copper- asked me to look out my viewport and let vent opening and the jet less permeable. The iron, and zinc-sulfide him know when I saw bottom. Alvin’s lights of hot fluid. anhydrite wall also minerals (such as pyrite, were turned on. It was like the curtain go- provides a substrate, or marcasite, bornite, ing up in a dark theater and the stage lights foothold, on which other sphalerite, wurtzite) going on. We were hovering over the same minerals can precipitate. precipitate at lower basaltic rocks that I had spent countless temperatures within hours studying in photographs. pore spaces of the Then to my right, I could see a rock chimney wall. wall rising from the seafloor. It was obvi-

Jayne Doucette Jayne ous from the hedges of pencil-diameter

3 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu Hydrothermal Circulation

Seawater percolates through permeable seafl oor crust, beginning a complex circulation process. The seawater undergoes a series of chemical reactions with subsurface rocks at various temperatures and eventually changes into hydrothermal fl uids that vent at the seafl oor.

��

�� E. Paul Oberlander E. Paul

4 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu Cross Section of a Black Smoker Chimney As hydrothermal fl uid vents from the seafl oor, different minerals in it precipitate at different temperatures, creating a multi-layered black smoker chimney. Iron sulfi des on the outer wall are exposed to oxygen Outer layers contain in seawater and are oxidized to form iron oxides. a mixture of minerals that precipitate at lower temperatures, including anhydrite

(CaSO4 which precipitates at ~150°C and contains sulfate from seawater), pyrite

(FeS2), and other ﬁ ne-grained metal sulﬁ des.

A ﬁ nal inner coating of sphalerite (ZnS), which precipitates at ~300°C, can form as hydrothermal ﬂ uids cool.

The initial chimney structure restricts mixing with cold seawater.

Chalcopyrite (CuFeS2) which precipitates at >300°C, is deposited on the inner walls. Tom Kleindinst Tom

tubeworms sticking out of the tops and so much silica precipitating at this site? as much as we could. Hypotheses were sides of the cliffs that I was looking at large The fluid chemistry provided an- advanced, only to be proven wrong by yet hydrothermal vent structures.The view swers: This vent site’s fluids were rich in another discovery. was nothing like what had been described ammonia (NH3). As the ammonia-rich More visits to these seafloor hot springs + at the East Pacific Rise. Instead of low-ly- fluids cool, NH3 takes up excess H ions made it clear that all vent fluids are not the ing mounds of sulfide debris topped by to form NH4, which raises the pH of the same. Rather, the chemical composition active smokers, we saw steep-sided struc- fluids. The higher pH likely allows silica to changes from ridge to ridge—and from tures standing 15 meters (50 feet) high. precipitate within Main Endeavour Field time to time. Why was this site so very different? structures; at sites with no NH3, low pH Researchers returning to some vents How could these chimneys stand there like probably inhibits the formation of amor- found that the chemistry of vent flu- multi-story buildings without collapsing? phous silica. ids was not constant, changing on scales As with almost every visit to a new ranging from days to years. These vent A ﬂ uid environment vent site, our survey of the Main Endea- sites were all associated with sites of re- Answers to these questions came from vour Field raised as many questions as it cent magmatic activity, with recorded studying new samples. We learned that the answered. The differences among known earthquakes and evidence that dikes had tall chimneys structures were essentially hydrothermal systems and the revelations been intruded into the ocean crust and, “cemented”—silica filled pores in the vent that accompanied each new discovery at some sites, that lava had been extruded structure walls as the emerging fluids cooled, provoked oceanographers to hunt for new onto the seafloor. The vent fluid com- making the structures sturdy. But why was sites. We were explorers trying to learn positions were changing as these dikes

5 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu cooled, and as fluids penetrated deeper For example, a technique of “tow-yow- While these instruments were in place, into the crust. ing” has been developed, where a conduc- other researchers made acoustic images tivity-temperature-depth (CTD) sensor of vent structures and venting fluids. Still Searching for new vents is raised and lowered through the water others used the Autonomous Benthic Ex- In the early 1980s, after a number of column in a saw-tooth pattern above the plorer (ABE) to measure water column vent sites had been found in the Pacific, ridge to map the locations of plumes, and properties above the vent field, seafloor scientists began to wonder if hydrothermal then to home in and map the buoyant depth, and magnetic signatures. activity and active black smokers might portion of plumes coming directly from Later in the program, the team de- exist on the more slowly spreading Mid- active vent sites. This technique was used ployed a systematic array of current me- Atlantic Ridge. The hydrothermal vent successfully to find vent sites in the Pacific ters, thermistor strings, magnetometers, systems transfer large amounts of heat and Atlantic, and most recently in the In- and tilt meters. Scientists even tested from magma or newly solidified hot rock, dian Ocean. techniques to “eavesdrop” on the data be- but on slow-spreading ridges the spreading collected and download it without ing rate, and magma delivery rate, is much Seaﬂoor observatories removing the instrument from the vent. less (about 1/3) of that on the northern The need to explore the dynamics The result of this collective effort was the East Pacific Rise and Juan de Fuca Ridge. of hydrothermal systems over time has most comprehensive study of a hydrother- To our surprise, exploration from led to new technologies and the devel- mal system to date, and a model for future the mid-1980s through the early 1990s opment of seafloor observatories. New, seafloor observatories. gradually made it clear that hydrother- more precise and durable instruments al- mal systems may be spaced further apart low us to monitor temperature and fluid A continually unfolding story on the Mid-Atlantic Ridge, but they tend chemistry at vent sites for hours, days, or As we develop these new techniques and to generate much larger mineral depos- months—as opposed to observing those instruments, our ability to explore ongo- its. Expeditions to the Southwest Indian properties for brief moments and grab- ing seafloor processes will grow. More than Ridge in 2000 and the Gakkel Ridge (un- bing one-time samples. a quarter-century into our studies, we still der the Arctic Ocean) in 2001 revealed The future of hydrothermal studies was find ourselves constantly revising and refin- that hydrothermal venting occurs on even displayed in a recent series of coordinated ing our ideas about hydrothermal systems. the slowest-spreading portions of the mid- experiments. With support from the Na- At the same time, as we home in on the ocean ridge system. tional Science Foundation’s Ridge Inter- fine details of how these systems work, we disciplinary Global Experiments (RIDGE) continue to find new sites that completely New ways to ﬁnd vents program, a team of researchers built a sea- break the mold. As recently as December Two decades of study have taught us floor observatory on the Endeavour seg- 2000, researchers diving in the Mid-Atlan- that there is no single type of seafloor ment of the Juan de Fuca Ridge. tic discovered “Lost City,” a vent site located hydrothermal vent system. The plumb- During the summers of 2000 and 2001, far away from the ridge axis, on old rather ing systems beneath the seafloor are both scientists made complementary and con- than nascent seafloor crust, and with 15- diverse and incredibly complex. tinuous observations centered around story-high white minaret-like structures Ocean scientists today are posing ques- the Main Endeavour Field (the same site made of carbonate—a mineral that is not tions about the dimensions and evolution I first visited in 1984) and at vent sites to found at most other known vent sites. of the hydrologic systems beneath vent the north and south. The program goals So after 32 dives to the seafloor to sites. We puzzle over how hot these fluids included making more accurate measure- study vents, I am often still surprised, and get, how deep into the crust they descend, ments of the heat and mass flowing from I am always awed. Even when I return to a and how far they travel before venting at the system, and observing how the hydro- vent site that I’ve visited before, I still find the seafloor. And where does seawater en- thermal plumbing is influenced by tides it an unbelievably beautiful sight to watch ter these systems? and by high-temperature reactions that jets of hot fluid mixing with seawater, and To answer these questions, we will need separate elements into saltier liquids and unusual organisms that make their homes to continue exploring, not only over geo- more vapor-rich fluids (a process called near these vents. Like my colleagues, I graphic space, but also over time. In the “phase separation”). look for ways to make our studies more early years, most vents were discovered Instruments were deployed to continu- precise, more methodical, and more con- serendipitously, but as we’ve explored and ously monitor vent fluid temperatures, tinuous. But 20 years after my first dive, I learned more about these systems, we’ve flow rates, and chemical properties. Scien- still enjoy seeing it all live. It’s the differ- been able to develop systematic methods tists also used newly developed samplers ence between watching a movie of a water- for pinpointing sites. to collect fluids at regular time intervals. fall and standing next to one.

6 Oceanus Magazine • Vol. 42, No.2 • 2004 Woods Hole Oceanographic Institution • oceanusmag.whoi.edu