MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 26 update - from Dave Clague (aka Dr. Limu)
Tiburon dive 887 - Sediment sampling in Escanaba Trough
Last night we steamed to the NESCA (Northern Escanaba) site, an active hydrothermal vent site about 300 kilometers west of Eureka, California. This site was studied extensively by the U.S. Geological Survey in the 1980’s and subsequently drilled by the Ocean Drilling Program on leg 169 in 1996. Our main interest in this area is a large (approximately 10-square-kilometer) lava flow that erupted from faults around an uplifted sediment hill, which currently supports active hydrothermal vents. We made some observations of this old lava flow during ROV Tiburon dives in 2000 and 2002, locating the eruptive vents and discovering that sediment on the uplifted hill contained significant amounts of limu o Pele and dense angular fragments of glass whose chemistry matched the adjacent lava flow.
This lava flow and eruption is relatively isolated from other volcanic sites. In addition, it sits in the midst of a plain of thick turbidite silt and mud that fills the axis of the Escanaba Trough (to a depth of 450 m). For this reason, it is an ideal site to investigate the dispersal of limu o Pele and other pyroclastic glass fragments that may have been released during the volcanic eruption. During the next five dives (over the next five days), we will attempt to measure the amount, size distribution, and morphology of the glass particles emitted during the eruption, and to find out how these characteristics vary depending on the distance and direction from the eruptive vents.
The thick sediment in this area means that we can use pushcores to sample and measure the abundance of eruption-related glass at various points on the seafloor. The glass may be moved vertically within the sediment by organisms that feed in the mud, but about 95% of the glass still resides in the upper 8 cm of the sediment, so that is how much of each core we will need to sieve to recover the glass particles. A few previous cores showed us that the maximum amount of glass occurs at a depth less than 1 cm. This information, coupled with known rates of sediment accumulation in the area, suggest that the lava flow erupted approximately 300 years ago.
We have selected 4 transects radiating out from the central eruption. Over the next few days, we will fly the ROV along each transect and collect numerous pushcores, starting about 5 kilometers away from the vent. At the farthest stations, we will stop and take cores every 250 meters, but as we approach the central vents, we will take cores more frequently. The hardest part about selecting these transects is that in some places, the seafloor is covered by lava flows and in other places the sediment has been disrupted by tectonic and magmatic processes. This means that in some cases it may be impossible to take push cores or difficult to be certain that the upper 8 centimeters of sediment has not been disturbed since the eruption occurred.
Today’s dive was the transect north of the eruptive vents and move south in the direction of the vents. We collected 30 pushcores, starting with 12 cores located 250 meters apart, then 5 that were 200 meters apart, then 4 that were 150 meters apart, then 5 that were 100 meters apart, and finally 4 that were only 50 meters apart. This sampling schedule was selected so that we could use the entire complement of 30 pushcores we can carry on ROV Tiburon to sample 5.3 kilometers of seafloor, with denser sampling closer to the source vent.
The coring went smoothly, although the dive was somewhat dull, since all we had to look at was mud, more mud, and the occasional animal residing in or on the muddy bottom. The end of the dive covered terrain with much more tectonic disruption of the sediment, so we had to spend more time searching for suitable locations to core. Along the way, we discovered two previously unknown massive sulfide outcrops and two lava flows exposed along small faults.
Near the end of the dive, we discovered three previously unknown hydrocarbon/sulfide seeps with large living vent clams. We took samples of these clams for our colleague Bob Vrijenhoek, who will analyze their DNA to determine what species they are and how closely they are related to other vent clams at cold seeps and hydrothermal vent sites in the Pacific. We also collected samples of a white bacterial mat at one of these vent sites. Rob Zierenberg selected several samples of the hydrothermal sulfide deposits to compare to others previously studied in this area. It was an exciting end to an otherwise rather dull dive. However, we are optimistic that, once we complete the lab work, the results will help us understand the style of undersea eruptions that produce the limu o Pele fragments.