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California Margin’s Ancient Volcanic History—MBARI Cruise to California Seamounts
An interdisciplinary team of scientists recently undertook a seamount cruise along the California margin as a followup to a cruise completed in October 2003. Both cruises were conducted by the Monterey Bay Aquarium Research Institute (MBARI) and led by MBARI volcanologist Dave Clague. Other participants in the 2004 cruise included Alicé Davis, Jenny Paduan, Lonny Lundsten, and Joe Jones of MBARI; Jim Hein and Brandie McIntyre of the U.S. Geological Survey (USGS); Kathie Marsaglia of California State University, Northridge; and Tessa Hill of the University of California, Santa Barbara. The 2003 cruise explored Pioneer and Rodriguez Seamounts off central California; the 2004 cruise expanded the study to seamounts south of Point Conception, plus one dive on the Patton Escarpment, a steep scarp that delineates the ancient subduction margin of North America. The two-week cruise began on April 27, 2004, when we departed at 0600 from Moss Landing, CA, only to return 3 hours later to replace a powerboard on the remotely operated vehicle (ROV) that was damaged during a power spike the day before. Leaving again at 1730 necessitated modification of our next day's dive plan and left the team wondering if this cruise would be similar to its October predecessor, which suffered many delays as the result of high winds. Despite this less than auspicious beginning, we experienced spectacular weather for the rest of the cruise, and the full complement of 10 dives were completed successfully: two dives on Rodriguez Seamount, three on San Juan Seamount, two on Northeast Bank, one on the Patton Escarpment, one on Little Joe Seamount in the Ponderosa seamount group, and one on San Marcos Seamount. Like the seamounts studied during the 2003 cruise, these extinct volcanoes are examples of a previously unrecognized type of oceanic volcanism, reflecting changes in the Pacific/North American Plate boundary as it shifted from a convergent to a transform margin. The team used the specialized ROV Tiburon housed on the research vessel Western Flyer to explore and collect samples from the seamounts. We collected samples from depths ranging from 500 to 3,600 m, including 332 rock samples, 26 sediment push cores and sediment scoop samples, and 175 animals, in addition to the multitude of critters clinging to the collected rocks. Our main objectives were
Highlights of the cruise included two additional dives on Rodriguez Seamount, bringing the total to six dives for the 2003 and 2004 cruises. These dives provide a thorough view of the seamount's biology and geology and show profound differences in the characteristics of the many cones that make up the seamount. We had an exciting find on San Marcos Seamount, where we collected an extremely large (7 cm diameter at the base), nearly dead bamboo coral. The coral was so large that the ROV's manipulator couldn’t break it, and so, to collect it, we had to drive the vehicle into the coral to snap it from its base. The annual growth rings of this specimen and several other large bamboo corals that we collected will help Tessa Hill decipher past climates and ocean circulation during her postdoctoral fellowship at the University of California, Davis. These corals may record 100 to 200 years of ocean history with annual resolution. Another interesting find was a fossil-bearing layer underlying volcaniclastic deposits on Northeast Bank. The sea-floor surface was littered with partly dissolved fossil solitary corals that had been eroded from the outcrop. The corals were of a type not observed on the seamount and may be as much as 5 to 10 million years old. On the second dive at Northeast Bank, evidence was found for the shoreline of an ancient island, confirming that the seamount was subaerial in the past. A beach rock composed of rounded pebbles was collected at about 550-m water depth at the break in slope that delineated the ancient shoreline. The single exploratory dive on the Patton Escarpment recovered a range of rock types and sediment samples that Kathie Marsaglia will compare with clastic sand samples recovered at several nearby Deep Sea Drilling Sites. The entire 3.5-km traverse showed outcrops of compositionally varied breccia and talus, completely encrusted by Fe-Mn crusts that are far thicker and more extensive than anticipated. We had expected deformed sedimentary rocks related to subduction at the former North American margin. Marine biologist Joe Jones collected translucent-shelled clams to provide a more thorough picture of the evolutionary processes that influence the distribution and genetic composition of these unique animals, and to analyze their reproductive cycle on the seamounts. He was rewarded with samples from Rodriguez Seamount, San Juan Seamount, the Patton Escarpment, and Northeast Bank. He also has a new, larger species to study that was recovered from relatively shallow water. Jim Hein and I will study the formation of Fe-Mn crusts observed on all of the seamounts, primarily focusing on the many paleoceanographic conditions that can be deciphered from a careful study of sequential crust layers. We will also determine the growth rates and compositions of these unusual accumulations of metals. The seamounts appear to young southward, and so we anticipated only thin (a few millimeters thick) Fe-Mn oxide crusts; but on several of the seamounts, such as Northeast Bank and Little Joe Seamount, crusts were abundant, covering most of the sea floor and measuring as much as 40 mm thick. This cruise was especially fruitful, adding at least 200 samples to our collection. One of the real surprises was the amount of hydrothermal Mn oxide collected during the cruise, considering that these seamounts are mostly Miocene and volcanism ceased millions of years ago. These hydrothermal deposits are relicts from the early history of the seamounts, when hydrothermal activity was an important process along the California margin. Now, if we can only find the time to analyze all of it! Each dive was a great success, providing new observations about submarine eruptive styles, benthic-animal distribution, and even some great midwater squid and jellyfish observations. The biology and geology of the California Continental Borderland is so varied and fascinating! We observed a vast array of sponges, including glass, lacy, and barrel; many types of corals, including golden and paragorgia; a mysterious and beautiful purple hydroid; brittlestars, seastars, and slime stars galore; dinner-plate-size sea urchins; stalked and swimming crinoids; crabs; a type of sea cucumber affectionately known as “Pinky the slime bag”; different species of squid and octopi; jellyfish; and even a 6- to 8-ft-long sleeper shark! The ROV traveled over volcaniclastic debris flows; onto blocky, jointed basalt ledges; and up to pelagic sediment pools at the tops of seamount cones. Overall, the cruise was a huge success that will contribute much to our knowledge of the California margin, and it reinforces how important and useful interdisciplinary studies are for science. For additional information about the cruise, visit MBARI's California Seamounts Web page.
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in this issue:
Cruise to California Seamounts
Scientists Interviewed for Documentary The National Map Comes to Florida
Geologist Joins USGS in St. Petersburg |
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U. S. Department of the Interior | U.S. Geological Survey
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