by Ken Kostel
Much of what is surprising about the deep ocean results from the extraordinary conditions found there—frigid temperatures, crushing pressure, unusual chemical and biological processes, and the complete absence of sunlight. For those who study the deep sea, those extreme conditions mean that the very act of bringing many samples to the surface often changes or destroys them, leaving little or no trace of the things that scientists want to examine most.
Unable to study deep-sea phenomena in their labs, Sheri White and Chip Breier from the Woods Hole Oceanographic Institution (WHOI) are developing instruments that will allow them to bring their laboratory down to the seafloor.
Take gas hydrates, for instance, curious seafloor substances that offer both promise and peril for people around the world. Gas hydrates are molecules of methane and other natural gases that—under the high pressure and low temperature of the deep sea—become encased in a cage of ice. When brought toward the surface, they dissipate into gas and water, so scientists have had difficulty studying them.
Scientists know surprisingly little about them, but they certainly would like to know more. Packed into their icy crystalline structure, the energy potential of gas hydrates worldwide is estimated to exceed the total known reserves of natural gas, leading many people to search for ways to exploit this largely untapped resource. Meanwhile, others fear warming oceans will melt hydrate deposits, causing large amounts of methane—a potent greenhouse gas—to leak back to the atmosphere and exacerbate human-caused warming. Sudden conversions of gas hydrates into gas could also trigger catastrophic submarine avalanches and tsunamis.
A laser to enlighten
To study deep-sea phenomena like gas hydrates up close, White and Breier are working on ways to convert a laboratory device known as a laser Raman spectrometer for work in the difficult conditions of the deep sea. Raman spectrometers can tell scientists what almost any substance—solid, liquid, or gas—is made of. Geologists use them to study the mineral composition of rocks. Anesthesiologists use them to monitor gas mixtures going in and out of patients during surgery. Forensic specialists—in real life and on the television show “CSI”—use them to help solve crimes. They have even been compared to the tri-corders that the starship Enterprise crew used to identify unknown materials instantly.
Raman spectrometers work by focusing a laser on a target object. Most of the beam scatters off the object at the same wavelength as the laser. However, a small part of the beam (one in 100,000,000 photons) interacts with the chemical bonds of molecules inside the target area and scatters at different wavelengths. Which wavelengths come out depends on the chemical composition and the molecular structure of the material the laser hits. This is called Raman scattering, after C.V. Raman, who won the 1930 Nobel Prize in Physics for its discovery.
By reading the wavelengths of the Raman-scattered light, scientists not only can determine that an object is made of carbon, for example, but they can also distinguish whether the carbon atoms are arranged in a regular, crystalline pattern, as they are in diamonds, or the jumbled, irregular configuration of carbon in graphite. This versatility has made it a tool much favored by field geologists and geochemists—not to mention a natural choice to study the oceans.
“If you (had) to ask, ‘What could I take down to the bottom of the ocean that would tell me the most?’ and keep it in one package, Raman’s a darn good answer,” said Jill Pasteris, a geologist at Washington University in St. Louis and an expert in Raman spectroscopy who assisted a team at the Monterey Bay Aquarium Research Institute (MBARI) aiming to modify a laboratory Raman spectrometer for use on a deep-sea submersible.
ALISS and DORISS
White joined the MBARI group in 2001, but was no stranger to developing deep-sea instruments. As a doctoral student in the MIT/WHOI Joint Program, White helped build and test a device called ALISS (Ambient Light Imaging and Spectral System) to measure “vent glow,” the ghostly light emanating from hydrothermal vents on the seafloor. Heated by molten rock beneath the seafloor, hot, mineral-rich fluids spew out of these vents into the ocean, feeding surprisingly lush communities of life and bringing minerals such as iron and manganese from Earth’s crust up to the ocean. With ALISS, White was able to determine that most of the energy in vent glow stemmed from thermal radiation produced by the extremely high-temperature (sometimes in excess of 660°F, or 350°C) vent fluids.
White’s experience with ALISS prepared her to help develop DORISS, the Deep-Ocean laser Raman In-Situ Spectrometer, as well as for the challenge of making a precision laboratory instrument work in a pressure-proof container dispatched to the seafloor. For Raman spectrometers, a major hurdle turned out to be the fact that they only “see” the precise point where their laser beam focuses—an area smaller than the head of a pin. To learn more about how the greenhouse gas carbon dioxide is stored in the deep sea, they could focus the beam anywhere on samples of seawater inside a glass container. But focusing the beam exactly on the surface of a solid object, such as a gas hydrate or rock, proved to be much more challenging because the laser had to be positioned to within one-tenth of a millimeter—and held absolutely still for minutes at a time.
“When you’re sitting on a ship, and you see something very small on the bottom that you want to sample, it’s very different than doing it in a lab,” White said. In the “hands” of a manipulator arm on a remotely operated vehicle hovering near the seafloor and controlled from a ship at the other end of a 4,000-meter tether, precision control proved to be impossible, despite the skill of the vehicle’s shipboard pilots.
So White built what amounted to a three-legged lab bench for the bottom of the ocean: the Precision Underwater Positioner. With PUP, White and her team could make the minute adjustments on DORISS needed to study solid samples of minerals and bacterial mats living off chemicals in vent fluids. They were also able to make measurements of methane hydrates and mineral deposits on the seafloor nearly two miles beneath the sea surface.
A super new device called SuPR
In 2005, White returned to WHOI, where she spends time in the lab—one on land—optimizing her Raman spectrometers and refining techniques to study the minerals and gases emanating from beneath the ocean crust in a rush of fluids spewing from vents. And she is getting help from someone who hopes to add what amounts to an entire new wing to her underwater lab.
Chip Breier came to WHOI last year after a five-year stint in the U.S. Navy, where he was among a small group of officers who oversaw designs and testing of nuclear reactors (his specialty was reactor primary coolant pumps). He followed his tour of duty with a Ph.D. from the University of Texas at Austin, where he used naturally occurring radioactive chemicals to trace the pathways of groundwater discharging into coastal ocean waters. As a graduate student, he volunteered for a cruise to map a hydrothermal vent field in the Pacific Ocean and found the subject for the next step in his career.
Now a postdoctoral fellow, Breier is developing something called the Suspended Particulate Rosette (SuPR)—a device shaped like an oversized hockey puck with a rotating carousel of sample containers packed inside. Extremely fine filters inside each one can collect large quantities of tiny particles from minerals or microbes in a short time during a single deployment. Where existing particle samplers collect a single 2,000-liter sample, the SuPR collects 24 separate 100-liter samples. This gives scientists the ability to study ocean chemistry in much finer detail; throughout hard-to-access environments such as rising hydrothermal plumes. They would be able to collect many samples, not just one or two—either over time periods much longer than a research vessel can stay on site, or quickly, using a remotely operated vehicle, for example.
In November 2007, Breier successfully tested his SuPR in a vent plume in the Pacific Ocean. He is also working to put a SuPR right in the hands of the manipulator arm of WHOI’s remotely operated vehicle Jason, which can place the SuPR right in rising hydrothermal vent plumes, where emerging vent fluids mix with seawater and a flurry of chemical reactions take place.
As mineral particles in vent fluids rise from the crust, they chemically combine with trace elements in seawater such as vanadium and chromium, forming new minerals that precipitate and fall to the ocean floor where they slowly accumulate—processes that influence both mineral deposits on the seafloor and the chemistry of ocean water. The chemicals and microbial life gushing from vents also influences the unique life that accumulates around them.
The SuPR also contains room for optical instruments such as one of White’s Raman spectrometers. White and Breier hope to integrate a Raman spectrometer into the SuPR sampler and take it for a trial run in a vent plume. That will allow the samples to be analyzed immediately—effectively catching them in the act, before unstable materials dissolve, precipitate, disintegrate, or otherwise transform. By taking many samples in succession, as SuPR would allow, they hope to create a series of “snapshots” of what’s happening to the particles and chemicals in vent fluids as they interact with seawater. If they succeed, ocean scientists will have a powerful new tool to shed light on the murky depths.
White's laser Raman spectroscopy research was funded by a WHOI Green Technology Award and the WHOI Deep Ocean Exploration Institute, which also funded Breier's work on SuPR, which was developed in collaboration with Mclane Research Laboratories. Breier's postodoctoral fellowship was funded by the National Science Foundation's RIDGE 2000 Program.