Faults, Vents, and Seeps Logbook
Leg 6, Day 3: Rounding up bubbles/Tubeworm city
April 10, 2012
Overnight, we traveled to the Guaymas Basin to make two dives on the northeast side of the transform fault zone. First, we dove at Pinkie's Site, a well-studied area with extensive methane gas vents. Peter Brewer's team visited this site on an earlier leg of this expedition and MBARI researchers originally sampled the gas in this area during the 2003 Gulf of California Expedition. Nine years later, we returned to measure the gas composition of the methane bubbles and determine if there has been any change over the years.
Rounding Up Bubbles
Today’s dive site was rich in gas emanations from the seafloor. Most likely this gas is methane produced by bacteria. However, at the pressure and temperature conditions of this site—1,500 meters (5,000 feet) deep with a low temperature of two to three degrees Celsius (35 to 40 degrees Fahrenheit)—high concentrations of methane do not stay in the gaseous phase or dissolve in water. Instead, methane combines with molecules of water to form a solid combination of methane-water called gas hydrate. This solid accumulates in the subsurface sediment, but due to the volume expansion in the conversion to a solid phase, the surrounding sediment deforms and cracks creating fissures through which methane that is not trapped in solid form can escape before combining with the surrounding water.
The seafloor at this site had multiple locations where gas bubbles could be seen escaping from the seafloor and floating upwards into the overlying seawater forming bubble plumes. These bubbles, though small in size, contain large quantities of gas. In fact, the gas pressure in these bubbles has to at least match the seawater pressure, which at this location was 2,200 pounds per square inch. We deployed a gas sampler developed at MBARI that collects the bubbles inside an inverted funnel and diverts them into an empty cylinder.
Methane bubbles escaping from the sediment combine with seawater and acquire a coating of gas hydrate. In the picture below, bubbles coated with hydrate have accumulated on the edge of the camera lens and frame; the gas sampler is in the background. Gas measurements conducted onboard later in the day confirmed that this was nearly pure methane with traces of ethane and propane.
We relocated to a site 1.6 kilometers (one mile) north of Pinkie's Vent to do another dive in the afternoon. This site was chosen because the autonomous underwater vehicle (AUV) bathymetric surveys collected last month from the R/V Zephyr showed an approximately 15-meter-high by 100-meter-wide (50-foot by 330-foot) mound which has a distinctive rough surface texture characteristic of seafloor gas venting areas. It proved to be a fascinating site with large blocks of beautiful carbonate rock cut by meters-deep fissures, large patches of white bacterial mats, numerous thickets of tubeworms, beds of clams, and lots of other animals, which were sampled.
When one push core was taken, gas was observed to gush out of the bottom and pieces of gas hydrate were seen floating away. Although this push core was put back in the core holder in the rack of the remotely operated vehicle (ROV), it floated away before the ROV was recovered, an indication it contained considerable amounts of gas and/or gas hydrate.
The exposed carbonates we saw were formed within the sediments at shallow sub-bottom depths and are associated with methane-rich seafloor sites. Methane and sulfate do not both occur in most marine sediments. However, in methane-rich seafloor environments, methane moving upwards encounters sulfate diffusing downward from the overlying seawater, and microbes utilize the sulfate to oxidize the methane to obtain energy. As a byproduct they exhaust bicarbonate and hydrogen sulfide, both of which have profound effects on the environment. The increased bicarbonate in the pore waters stimulates the precipitation of carbonate into rocks. The sulfide can diffuse to the seafloor, where it is used as an energy source by the chemosynthetic organisms like the worms and clams we collected. This dive was a great illustration of the combined impact of geological, chemical, and biological processes on the seafloor. All the scientists were excited!
Today we hit the tubeworm jackpot! Our goal was to find patches of vestimentiferan tubeworms that aggregate in bush-like clumps. Two species of tubeworms live together in these clumps. One species, Escarpia spicata, makes long, straight tubes and has a spike about 12 millimeters (half-inch) long at its front end. The second species, Lamellibrachia barhami, lacks the spike and makes a curlier tube with periodic "flutes" that look like the end of a clarinet. The tubes are made of chitin, the same material that makes up a crab's shell. The worms have no mouths or guts; instead, they have a structure called the trophosome, which houses a very large colony of bacteria. The bacteria are symbionts, providing food for their worm hosts by a process known as chemosynthesis. It is similar to photosynthesis, but instead of using sunlight as a source of power, chemosynthesis uses chemicals released from the gas seeps found in the Gulf of California. The worms absorb hydrogen sulfide gas from the seeping fluids and deliver it through their blood to the trophosome and bacteria. They also use their blood to deliver carbon dioxide absorbed from ocean water to their bacteria. The bacteria then use the two gases to convert the carbon dioxide into sugar, their source of nutrition. In turn, the worms benefit from nutrients that leak from the bacteria. Together these symbiotic partners thrive in deep-sea environments without sunlight.
We sampled two discrete clumps of tubeworms, taken about 1.6 kilometers (one mile) apart at seeps along different walls of the Guaymas Transform Fault. This fault system eventually ducks under California and emerges as part of the San Andreas Fault system. The reason we are seeking discrete clumps of these worms is that we want to know whether the bacterial symbionts differ among different clumps that grow in slightly different environmental patches. The geologists and chemists onboard are simultaneously getting information on the chemical environment associated with these worm patches. For the answer, we will have to wait to conduct genetic and chemical analyses when we get back to our laboratories at MBARI and elsewhere. For now, we just have to wonder why some patches have many large worms and others have very small worms.
R/V Western Flyer
The R/V Western Flyer is a small water-plane area twin hull (SWATH) oceanographic research vessel measuring 35.6 meters long and 16.2 meters wide. It was designed and constructed for MBARI to serve as the support vessel for ROV operations. Her missions include the Monterey Bay as well as extended cruises to Hawaii, Gulf of California and the Pacific Northwest.
ROV Doc Ricketts is MBARI's next generation ROV. The system breaks new ground in providing an integrated unmanned submersible research platform, with many powerful features providing efficient, reliable and precise sampling and data collection in a wide range of missions.
MBARI's heat-flow probe is mounted on the side of the ROV Doc Ricketts inside the vertical stainless steel box. This both protects the delicate probe and provide the track so that the probe can be inserted into the sediment along a totally straight path. The probe contains five high precision platinum sensors which are used to measure the vertical temperature gradient in the sediments. This gradient along with some knowledge of the heat capacity of the sediment allows scientists to calculate the rate of heat loss from the sediments into the ocean.In situ gas sampler
These are devices that are used to collect and sample gaseous gases bubbling out of seafloor vents. The way they work is by having small pressure vials (like tiny scuba tanks) from which the air is pumped out with a vacuum pump on the surface and sealed with the valve. On the bottom gases are captured underneath an overturned funnel so that a large gas headspace is developed. Then the value on the pressure vial is opened, gas is sucked into the vial, and the vial's value is re-closed. This way a sample of the gas at the high seafloor pressures is recovered.In situ ultraviolet spectrophotometer (ISUS)
The ISUS is a sensor used to measure concentrations of dissolved chemicals directly from their Ultraviolet Absorption Spectrum. A variety of chemicals absorb light in the UV and each of these chemicals has a unique absorption spectrum. We can determine the concentration of these chemicals directly, with no chemical manipulation, by measuring the absorption spectrum of seawater in the UV and then deconvolving the spectra to yield the concentration of each component. ISUS has been used to determine nitrate concentrations while deployed on CTD/Rosette profilers, undulating towed vehicles such as a SeaSoar or SeaSciences Acrobat, and on deep-sea moorings. It has also been used to measure sulfide flux from cold seeps in Monterey Bay while deployed on the ROV Ventana.
A push-core looks like a clear plastic tube with a rubber handle on one end. Just as its name implies, the push core is pushed down into loose sediment using the ROV's manipulator arm. As the sediment fills up the core, water exits out the top through one-way valves. When the core is pulled up again, these valves close, which (most of the time) keeps the sediment from sliding out of the core tube. When we bring these cores back to the surface, we typically look for living animals and organic material in the sediments.
Vibracoring is a common technique used to obtain samples from water-saturated sediment. These corers work by attaching a motor that induces high frequency vibrations in the core liner that in turn liquefies the sediment directly around the core cutter, enabling it to pass through the sediment with little resistance.Gravity corer
Device lowered off the ship to the seafloor on a wire which consists of a long tube that extends below a moderately heavy weight. When the device encounters the bottom, the weight forces the tube into the sediments. When it is pulled out of the bottom the tube will contain a sediment sample (i.e., core) of the upper layers of the ocean floor.
R/V Western Flyer
ROV Doc Ricketts
Leg 6 Research Team
Charlie Paull has been a marine geologist and geochemical stratigrapher at MBARI since January 1999. The central theme of Charlie's work involves investigating the fluxes of fluids and gases through continental margins. Over the past decade his primary focus has been gas hydrate research on the Blake Ridge gas hydrate field on the continental rise off of southeastern North America. Assessing the global distribution of gas hydrate and interstitial gas is a continuing interest as well as the development of new techniques to detect the presence of gas hydrate in marine sediments. Charlie's other ongoing work is focused on the geology associated with seafloor seepage sites, including investigating the deposits associated with chemosynthetic communities, determining the processes that occur at the methane-sulfate boundary, and understanding the origin of pockmarks and other potential seafloor fluid venting sites.
Bob Vrijenhoek leads MBARI's molecular ecology group, which focuses on using molecular tools to examine population structure and evolutionary relationships. His group is working on a number of projects studying gene flow and barriers to dispersal of deep-sea invertebrates associated with cold seeps in the Monterey Bay and hydrothermal vents at ridge sites throughout the world. The group also studies DNA sequence information from bacterial symbionts to examine their evolutionary relationships with their hosts and infer possible modes of transmission.
Krystle Anderson is a research technician working for Charlie Paull in the Continental Margins Lab. Krystle's background is primarily in the acquisition and processing of seafloor mapping data. She came from the California State University, Monterey Bay Seafloor Mapping Lab where she obtained her data processing and Geographic Information System (GIS) skills. Krystle spends a majority of her time processing and creating high-resolution maps of multibeam data collected from the mapping AUV. The high-resolution maps Krystle helps create will then be used to aid navigation for the ROV to explore particular areas of interest. On this expedition Krystle will assist with running the GIS system, and processing and cataloguing sediment samples and vibracores. This is Krystle's second research expedition with MBARI and she is very excited to be involved in this expedition.
Roberto is a geochemist by training. His interests lie at the intersection of marine geology and sediment and water chemistry. During cruises Roberto operates a custom-built, portable chemistry lab that includes a complete set of analytical platforms for measurements of fluids and gases. On this expedition, Roberto will be responsible for analytical measurements of pore water chemistry on samples taken from sediment cores. He will also be in charge of collecting gas samples emanating from fluid vents and performing hydrocarbon analyses on dissolved gases collected from pore waters, from gas vents and from seawater.
As a member of Ken Johnson's Chemical Sensor team, Josh spends much of his time analyzing chemical data collected from instruments developed by the group. One of the main goals of these measurements is to decipher how biology affects the cycling of the measured chemicals. In the Gulf of California Josh will be measuring the distribution of sulfide in waters overlying cold seep communities as well as helping to process biological samples for the molecular ecology group. Sulfide is one of the energy sources which fuels seep ecosystems. When not at work Josh spends much of his time on or near the ocean, fishing and hiking, or working in his garden.
Kris Walz works with the Midwater Ecology team at MBARI where she studies pelagic animals and their distributions using horizontal video transects collected from midwater time-series dives (1993 to present) in Monterey Bay. She joins the science teams on this leg of the Gulf of California expedition to assist with their research by recording and annotating video during the ROV dives, and processing biological samples collected from the ROV.
Brian specializes in sedimentary processes and stratigraphy, integrating insights gleaned from seafloor rock and sediment samples with information from remote-mapping products, such as close-up photographs of the seafloor, high-resolution bathymetric maps, and seismic-reflection profiles. His recent studies have focused on how sediment moves from the land to the deep sea, processes controlling submarine landslides, saltwater intrusion into coastal aquifer systems, marine pollution, seafloor habitats, and the Cenozoic history of the Arctic Ocean.
Juan Carlos Herguera
Juan Carlos is interested in the history of past oceans, how changes in climate and ocean circulation contribute to the ecology and biogeochemical cycling sustained by coastal environments in the California Current and the Gulf of California regions. During this cruise he will be involved in sampling benthic foraminifera to help characterize their genomic information, and, through their stable isotopic and metal compositions, to understand how these geochemical markers reflect their ambient conditions. He will further use planktonic foraminifera for dating the deep-sea cores with radiocarbon techniques, which hold important clues on the tectonic rupturing rhythm along the boundary between the North American and Pacific plates. He is fascinated by these new observation windows opened up by the ROV deployed from the Western Flyer, making possible the discovery of new vent environments along these fractured boundaries and the chemosynthetic oasis sustained by these leaky enclaves that connect the deep ocean with the lower crust and mantle dynamics.
Mary's interests focus on using microbiota (primarily foraminifera but also pollen) to investigate marine sediment transport, geohazards (faulting, landslides and paleotsunamis), climate change, and the pathways and impact of invasive species introductions using sediment records and molecular analysis techniques. She also uses foraminifera in biomonitoring marine pollution sites and carbon-14 chronostratigraphy—the study of the age of rock layers in relation to time.
Greg uses morphological and molecular data to assess relationships among animals. His morphological studies range across various adult and larval anatomies using transmission and scanning electron microscopy as well as confocal laser scanning microscopy. This is combined with molecular (DNA sequence) data to infer phylogenetic relationships and hence evolutionary patterns. His research interests include the biodiversity and distribution of hydrothermal vent animals from the eastern and western Pacific, as well as those from methane seeps in the eastern Pacific. This often involves the discovery and naming of new species of animals.
Sigrid is a postdoc at Scripps in Greg Rouse's lab. Sigrid received her Ph.D. in Austria working on Osedax from whale falls and has continued this work during her postdoc. She is interested in symbioses, vent organisms, and their relationships.