Exploring the Davidson Seamount
The Davidson Seamount is an impressive geologic feature that has intrigued people since it was first mapped as a sea mountain in 1933. It is located 120 kilometers southwest of Monterey, just outside the sanctuary boundary, due west of Point Piedras Blancas. The seamount is an inactive volcano that last erupted about 10 million years ago; its summit is far below the ocean surface at a depth of 1,300 meters. Rising above the relatively flat abyssal plane, the seamount dominates the ocean floor like Mount Shasta dominates northern California and is as tall as much of the Sierra Nevada Mountains (2,300 meters). Its as long and as wide as Monterey Bay.
|The ROV Tiburon is able to travel far below the oceans surface and gather images and organism samples from around the Davidson Seamount. photo 2000 NOAA/MBARI
Following decades of curiosity about the seamount, the new NOAA Office of Exploration funded a sanctuary-led, multi- institution expedition to characterize the distribution and abundance of creatures living there.
Though Davidson Seamount is not far away geographically, it is only recently that technology has been available to navigate, gather high quality images, and collect delicate organisms from these ocean depths. The sanctuary contracted with Monterey Bay Aquarium Research Institute (MBARI), one of the few organizations worldwide that has a ship and robot able to access these areas effectively on a regular basis. Imagine sending a robot down on 3,600 meters of cable into the pitch black, following specific routes up the side of the seamount and along particular ridges, and documenting areas never or rarely seen before.
After months of planning, we set off in May on a one-week cruise, with the main objective being to characterize invertebrates and fishes of the seamount. We also wanted to involve the public in the exploration through the Internet, evaluate the seamounts relevance to the sanctuary, gather geologic samples, make opportunistic bird and mammal observations, and collect organisms for study and display at the Monterey Bay Aquarium.
|The blob sculpin (Psychrolutes phrictus) and sponges on the Davidson Seamount at 1,317 meters (4,321 feet). Blob sculpins are opportunistic feeders, most commonly eating sea pens, snails, and crabs. photo 2002 NOAA/MBARI
After the cruise, we learned that creatures living on the Davidson Seamount could be grouped in depth zones from the base to the summit not unlike terrestrial plant life on mountains. Like tourists driving up a mountain at night and noting all the wildlife in their headlights, we cruised up the sides of the seamount and noted what was found in the lights of MBARIs deep-sea robot, the Tiburon. However, what we saw was much more colorful and abundant than tourists are likely to have seen from their cars. At the base of the seamount are common bottom fishes, like the rattails exhibited at the Monterey Bay Aquarium. Near the base, we found large, crab-like creatures called sea spiders. These can also be found on rocky shores, but never as big as the Davidson Seamounts foot-long spiders; in fact, many of the organisms we encountered were much larger than their relatives in the much warmer and shallower areas of the California coast.
Mid-way up, we saw species of fishes never seen live before, with names such as toadfish and witch eel. Some of the most impressive lava flows and geologic features were evident in the mid-range, as sediments down low and animals above covered the seamounts rock surface.
Invariably, it was the ridges at the top of Davidson Seamount that had the most spectacular life forms: twelve-foot-tall corals, large vase- and horn-shaped sponges, and a variety of sea stars. In a habitat where sunlight never reaches, we were surprised by bright yellow, pink, red, and purple organisms. As noted above, the size of most organisms surprised us all. We found sponges as large as a phone booth and others as wide as a soccer goal. We carefully photographed giant red corals that reached fifteen feet tall along several ridge lines. One cone of the volcano was completely blanketed with one sponge or a colony of sponges a density of cover that surprised us all. Currently, we are carefully quantifying the distribution and abundance of species, but our initial observations have already influenced the public and resource managers alike.
Above the seamount, we made observations of birds and mammals during the daylight hours. We encountered a total of nine different species of mammal, including killer whales, as well as fifteen species of bird, of which the Black-Footed Albatross was the most common. We followed a pod of sperm whales with a small boat launched from the ship but were not able to obtain skin samples requested by NOAA Fisheries for genetic analysis.
A key component of our expedition team was the educators, who were team members along with the scientists and resource managers. Our expedition was shared with students and the public on a web page that consisted of daily updates and video clips along with an ask the explorer e-mail option to link us with the rest of the world. Whether it was the unique creatures, the geology, or the technology, we piqued the interest of the public with up to 140,000 visitors per day to our web site and a story on the CBS Evening News.
|Pink gorgonian coral growing on hard substrate at 1,573 meters (5,161 feet). Shrimp, brittle stars, and crabs were often found associated with this gorgonian. photo 2002 NOAA/MBARI
Resource managers came to the conclusion that the Davidson Seamount is a unique habitat, based on the number of new and rare species, large and long-lived species, and the potential fragility of this habitat. Currently, there are no seamount habitats under protection in any of the thirteen national marine sanctuaries around the United States. As part of the current sanctuary management plan revision process, a diverse working group of interested parties is assessing the necessity of including the Davidson Seamount within the sanctuary boundary.
Our cruise was exciting in terms of scientific discovery as well as educating the public and influencing resource management pro-cesses. Bringing educators and resource managers on what could have been a more standard science cruise was a successful experiment for us. Today, ocean exploration is clearly a wide-open field with many opportunities for public involvement and resource management. Were looking forward to finalizing our analyses of the collected video images and listing all the new patterns and questions that arise from our quantitative descriptions. Perhaps most importantly, we are eager to contribute to conservation efforts, if the public and formal decision makers decide that the Davidson Seamount deserves special protection.
The Davison Seamount expedition was a multidisciplinary effort with members from the following institutions: the sanctuary, Moss Landing Marine Laboratories, the Monterey Bay Aquarium, MBARI, the National Marine Fisheries Service, the Alliance for Coastal Technologies, and the Office of Exploration.
Monterey Bay National Marine Sanctuary
One Year on Pioneer Seamount
Fifty miles off the California coast, just over the edge of the continental shelf, an underwater mountain rises from the Pacific Ocean floor, cresting 900 meters below the ocean surface. This underwater aerie, twice as high as Mount Tamalpais, surveys the open ocean to the west, the Juan de Fuca Plate to the north, and, to the south, the teeming wildlife of the Monterey Bay National Marine Sanctuary. Pioneer Seamount is an ideal vantage point for observing everything happening in this part of the ocean.
Underwater observation is not done with light. In even the clearest of seawater, light is strongly absorbed: two-thirds of blue light is absorbed over a distance of fifty meters, and red light fares even worse. In the murkier water of the Pacific, a whale can barely see its own tail, much less its mate or a straying baby. In this situation, sound replaces light and ears become eyes.
In contrast to light, sound travels almost forever underwater. Frequencies of 50 hertz (Hz) and below, favored by some whales, travel long distances with little attenuation. In 1995 this led the scientists of the Acoustic Thermometry of Ocean Climate (ATOC) project to choose Pioneer Seamount as a site for transmission and reception of low-frequency signals.
At the end of the project, an initiative was undertaken to preserve the underwater cable to shore for use in non-invasive environmental monitoring, spearheaded by our group at San Francisco State University, with the support of David Evans, Director of NOAAs division of Oceanic and Atmospheric Research. Concerned environmental groups acceded to the logic of this proposal. A team of scientists led by Chris Fox of NOAAs Pacific Marine Environmental Lab (PMEL) and Jim Mercer of the University of Washingtons Applied Physics Lab then installed a small vertical linear array (VLA) of four hydrophones, covering the frequency range of 10 to 450 Hz. On September 1, 2001, the Pioneer Seamount Observatory came on line.
|Figure 1. Composite spectrogram showing four commonly observed acoustic signals
During the following year, the observatory suffered a variety of minor equipment problems and one failure that required bringing the wet electronics to the surface for repairs. This entailed a wait of four months for ship availability and suitable weather conditions. Even so, the observatorys live time averaged nearly 60 percent during a period of more than a year, and a large body of data is now available for analysis.
The accompanying figure (see p. 11) is a composite spectrogram of acoustic signals commonly observed at Pioneer Seamount. The spectrograms show frequency versus time, and most of the interesting phenomena can be located by scanning the spectrograms. Four signals of interest are shown.
Ship Propeller Sounds The most obvious and loudest feature is the pattern of nested parabolic lines covering most of the spectrogram. This is the signal of a ship passing over Pioneer Seamount. Sounds like this are the loudest noises observed at Pioneer Seamount. Because of the long distances these sounds travel, the sounds from distant ships also make a major contribution to the ambient noise level.
The complex pattern of this spectrogram is due to the interference among the four hydrophones of the VLA, whose signals are added coherently. Where the bright lines dip down to their lowest frequencies, the ship is at its point of closest approach, and the frequency at that point gives its distance. From the rate at which the interference lines diverge, the speed of the ship can be determined. The pattern shown corresponds to a ship passing about 350 meters from the arrays location, and traveling roughly in a straight line, at a constant speed of twelve knots.
Blue Whale Calls
On the lower left-hand side of the figure appears a series of five blue whale (Balaenoptera musculus) A-B calls. Each pair starts with an A call about twenty seconds long, with substantial power at 16 Hz (below the limit of human hearing) and at 90 Hz, near the fifth harmonic of the low-frequency fundamental. The B call follows about fifty seconds later and has its frequencies concentrated at 16 and 48 Hz, the first and third harmonics of the same fundamental. These sounds are generally played back at between four and ten times their true speed, moving their frequencies into the center of the range of human hearing. The A call sounds like a series of gurgles, and the B call that follows is a sad moan, dropping steadily in frequency during its fifteen-second duration.
The B call, the less complex of the two, is fairly easy to recognize with automated pattern recognition. An effective method described in the literature uses a matched filter consisting of a perfect sine wave at about 16 Hz, dropping slightly in frequency during the moan. This procedure identified about 5,000 B calls during the last year, most of them coming in the fall months of September through November. While data from a full year are not available, the difference between the busy fall and a silent spring is striking.
The large number of individual whale calls recorded may eventually provide a means of approaching the holy grail of marine mammal acoustics the identification of individuals from their calls. The most striking feature of the blue whale calls is their lack of variability, as if the whale were repeating the same word over and over. However, there is some variation in harmonic structure, length of calls, and spacing of calls. In the future, these and other details of the calls may provide a way to tag individuals, age groups, or sex groups.
RAFOS Timing Sources
The fine, nearly horizontal line on the spectrogram labeled RAFOS is the signature of a swept-frequency signal (a chirp) from one of the acoustic beacons that make up a sort of underwater GPS navigation array for the eastern Pacific Ocean. The signal shown is from a source moored 400 kilometers west of Portland, Oregon. The delay between the known broadcast time and the detection time can be translated into a distance from the source. The signals from multiple active sources permit the determination of the position of a drifting receiver. Plotting daily positions of each drifting instrument allows determination of the eastern Pacific subsurface ocean currents, something otherwise very difficult to measure. The Pioneer Seamount Observatory is used to monitor the timing accuracy of the sources.
Earthquakes and LFA
The signal from a small earthquake is indicated on the spectrogram. Such quakes are detected about once per day. These arrivals will eventually be integrated with seismometer data to study earthquakes in the Pacific floor, although at present there are no ocean-floor seismometers in this general area of the Pacific. At present, study of plate-tectonic motion along the California coast is hampered by the fact that most observations are made east of the plate boundary. The addition of a seismometer would be very valuable for earthquake geologists.
The recent announcement by the U.S. Navy of its intention to test the SURTASS LFA (surveillance towed array sensor system, low-frequency active) sonar system for submarine detection in the Pacific lends additional interest to underwater acoustic monitoring. The proposed source level of the LFA array is 240 dB re 1 µPa (water decibels, not air decibels; to convert from dB in water to dB in air, subtract 60 dB) at 1 meter. Operation 200 miles off the California coast would result in sound levels of 180 dB re 1 µPa (again water decibels, not air decibels) in the sanctuary, a sound level considered by some to be dangerous to marine mammals. Independent monitoring of these sounds at Pioneer Seamount during these tests would enable the sanctuary to quantify the noise levels produced and to look for the response of marine mammals to the noise.
The Future of the Pioneer Seamount Observatory Pioneer Seamount is the first, and only, publicly accessible underwater observatory. Its first year of operation revealed the variety and quality of information to be obtained from a cabled offshore acoustic observatory. Its data support basic research in physical oceanography, geophysics, ocean engineering, and marine mammal research as well as the sanctuary missions of tracking populations of marine animals and monitoring their acoustic environment. With the use of air guns for geophysical investigations (potentially including oil exploration) and the prospect of SURTASS operation nearby, an acoustic monitoring station takes on added importance.
Pioneer Seamount went off the air on September 24, 2002 at 12:07 universal time. The center conductor of the coaxial cable is apparently shorted out to sea water. This is only the second cable failure over the seven years that the cable has been in place. The first failure (and possibly the current damage) was caused when a bottom trawler snagged the cable, an unavoidable hazard of the marine environment. Once repaired, this unique window into the ocean will continue to help scientists and regulators protect the sanctuary environment.
Interested readers can listen to the sounds of the Pioneer Seamount at www.physics.sfsu.edu/~seamount/gallery.html.
Roger Bland (1), (2) and Newell Garfield (2), (3)
(1)Physics and Astronomy Department, San Francisco State University
(2)Romberg Tiburon Center for Environmental Studies, San Francisco State University
(3)Geosciences Department, San Francisco State University