An interdisciplinary-research cruise conducted by the Monterey Bay Aquarium Research Institute (MBARI) under the direction of MBARI volcanologist David Clague recently made several discoveries about Rodriguez Seamount off Point Conception, CA, including new information about how the seamount evolved and what types of organisms it hosts.
The origin and geologic evolution of seamounts off California are poorly understood. In addition, seamounts are among the least understood biological habitats in the ocean basinsmore poorly understood even than the abyssal plains, which were recently the subject of study as part of the Census of Marine Life (CoML) program.
To learn more about the geologic and biologic makeup of seamounts, a week-long cruise was designed to use the remotely operated vehicle (ROV) Tiburon on MBARI's research vessel Western Flyer to study seamount environments. Participants included volcanologists Dave Clague and Alicé Davis of MBARI and Brian Cousens of Carleton University, Ottawa, Canada; paleoceanographer and geochemist Jim Hein of the U.S. Geological Survey (USGS); biologists Veronica Franklin (Monterey Bay Aquarium), Charles Messing (Nova Southeastern University, Florida), and Lonny Lundsten (MBARI); geographic-information-system (GIS) experts Jenny Paduan and Nadine Golden (MBARI); and computer guru John Graybeal (MBARI).
The cruise embarked from Moss Landing, CA, on October 11, with the goal of studying the Taney Seamount chain off San Francisco. Plans changed immediately upon departure, however, because the largest low-pressure zone in the North Pacific was parked over the Taney Seamounts, producing gale-force winds and high seas. So, we headed for Pioneer Seamount, at the base of the slope just north of San Francisco; but after the ROV had been on the bottom for 4 hours, the weather caught up with us, with seas approaching 15 ft and projected to be 18 ft, and winds to 30 knots. We had to abandon Pioneer Seamount, and because the forecast predicted worsening conditions for the next several days, we headed south for Rodriguez Seamount, on the lower slope off Point Conception. There we found refuge from the storm and several days of excellent conditions for ROV dives. Seamounts are common in the Exclusive Economic Zone (EEZ, a zone stretching from the shore to 200 nautical miles offshore) off the Western United States, and a wide selection is available for study.
We discovered that Rodriguez Seamount comprises numerous cones composed predominantly of bedded volcaniclastic rocks and fewer cones composed of basalt flows. One cone that we explored has a volume of 2.6 km3, making it one of the largest known cones on any seamount. The volcanologists were elated to find pristine volcanic glass in collected samples. Analysis of that glass will help determine the environment of seamount formation, such as depth and temperature of eruptions and whether magma was stored for long periods in underground chambers before erupting.
At about 700-m water depth, we discovered ancient beach deposits and associated basalts that had flowed across the ancient shoreline into the sea. These findings indicate that the seamount was an island, subaerially exposed probably some 10 million years ago, which subsided as it approached the subduction zone that was in the region at that time. Jagged piles of lava about 10 m thick may have originated as subaerial aa (lava flows with a rough, rubbly surface) inland from the beach.
Debris chutes and landslide scars characterize the flanks of the seamount. Rocks on the seamount summit and flanks are coated by iron-manganese-oxide crusts that increase in thickness (to as much as 5 cm thick) with increasing water depth. These samples will be used by Jim Hein (USGS) to study the paleoceanography of the California margin and, with accompanying data such as oxygen content of the water column collected by the ROV, to determine mechanisms for the acquisition of metals. This is the first research cruise to allow for the correlation of seawater composition with iron-manganese-oxide crust compositions at precisely known water depths and in well-understood geomorphologic environments.
The flanks of Rodriguez Seamount are generally rugged; however, extensive flat areas covered with crust pavements were mapped by the ROV. The seamount has very little sediment cover, even though primary productivity is high in surface waters. Limestone samples collected on the seamount were once foraminiferal ooze before undergoing diagenesis and cementation, indicating that the seamount's sediment cover may have been greater in the past.
Organisms were collected from water depths of 700 to 2,100 m. Amazingly, biological communities varied significantly even in similar habitats at similar water depths on different parts of Rodriguez Seamount. The seamount's megafauna consist of a vast array of sponges, including large, brilliant-yellow barrel sponges; many types of coral, including large gorgonians and huge golden coral seafans; abundant brittlestars, crinoids, clams, seastars, polychaete worms, crabs, tunicates, sea urchins, sea cucumbers, octopi, and many more. The giant sponges housed communities of other organisms. New species were found, and some species were rediscovered after not being observed for decades.
The summit of the seamount occurs within the oxygen-minimum zone, where oxygen contents fall as low as 0.2 milliliters per liter, which reduces population size, but biota were still remarkably abundant. Populations were also smaller along debris chutes and in the headwall and debris apron of a landslide. Abundant dead and dying sponges were found in some places.
The little research that has been done on seamounts shows that they provide important hard-rock environments for marine organisms within the U.S. west coast EEZ. They provide important clues to the geologic evolution of the continental margin, and host deposits of paleoceanographic and potential economic interest. Our interdisciplinary effort to understand these important features should provide exciting results. We collected about equal numbers of rocks and organisms, and now the real work begins, as samples are analyzed by a wide variety of techniques.
in this issue: Seamount Environments off California