May 1, 2004, Day 5
Monitors in the control room on the ship display views from cameras and sensors mounted on the ROV. The video feeds are distributed throughout the ship on closed circuit TV, so everyone can participate in the dive. Dave Clague (left, in the shadows) is at the controls of the science camera.
An animal we've never seen before, one of a group of four. It stands on a tapering stem about 18 inches tall. Short spicules, just like the fibers in fiberglass, are aligned to make up the stem, so we now know that it must be a hexactinellid (glass sponge).
Today we returned to San Juan Seamount where we had done our second dive. The
dive sequence has been a bit strange and challenging in that several of our
dives sites are within military target practice ranges and there are areas and
days when we are not allowed to work. San Juan Seamount happens to be inside one
of these target ranges and so we could not simply continue our dive sequence on
San Juan on Friday. Hence our excursion to the south and Northeast Bank
yesterday. The San Juan area will be off-limits Monday through Thursday, so
Sunday is our only shot to finish the third dive we had planned there.
Unfortunately, we had also planned another dive within the target range and that
will we will have to relocate to an alternate site. On the plus side, it was
planned to be on the Patton Escarpment, which continues along the western margin
of the California Borderland for many miles, so we have many alternate sites
among which we can choose.
Now, back to San Juan Seamount and the dive we completed today. We worked on the northeastern fissure zone and worked our way upslope from volcanic cone to volcanic cone, collecting lavas and animals along the way. The deeper parts of the dive encountered almost entirely pillow lava flows, many that draped steep to nearly vertical slopes. When lava flows down such steep slopes the pillow lavas that form are narrow and elongate tubes instead of bulbous pillows. [Caption for image to the left: Elongate pillow lava tubes streaming down a steep slope, accentuated by light-colored sediment that has filled in between.] In addition, many of the pillows drain as the pillow advances more rapidly than the lava can replace it from upslope, so the pillow stretches and is disrupted. Many of these flows end up as hollow drained pillows or tube-shaped flows. [Caption for image to the right: Drained pillow (center). Just after this pillow's outer rind began to chill, the pillow broke open and lava continued to flow. The roof of the small cavern here is the remains of the thin rind of the original pillow, and the "tongue" is the lava that broke out from the pillow. To the left of the drained pillow is an elongate pillow, and to the left of that, a bulbous pillow.] These types of pillow lavas are quite rare on most submarine volcanoes, yet we saw numerous examples today. Other slopes were covered in pillows that had detached from the flow that fed them and rolled down the slope, making a deposit of talus that consisted of large nearly spherical pillow lavas, but none in place. Other flows are loose or Mn-oxide-cemented rubble with rounded to angular blocks and clasts. These appear to be the submarine equivalent of subaerial a'a flows. Finally, there are volcaniclastic deposits that can be coarse breccia or fine-bedded sandstone consisting of volcanic fragments. The lavas that make these different types of flows and deposits have a range of compositions and eruption temperatures. The hotter lavas (probably >1150 degrees centigrade) tend to make pillow lavas with small and elongate pillows whereas the cooler lavas (perhaps as low temperature as 1100 degrees centigrade) make thick pasty flows and tend to erupt explosively because they also contain more gas. We will be trying to quantify these relations after we determine the compositions of the lavas when we return to our laboratories.
On top of all this good geology, we also found several new (at least to all of us) animals including a weird sponge?, coral?, sea pen? More clams (the ones Joe described yesterday) were collected as were several bamboo corals that Tessa will tell you all about next. [Caption for image at left: Bamboo coral presiding over multitudes of anemones.]
- David Clague
Throughout the Seamounts cruise, we have been observing, photographing, and collecting deep-sea corals for study back in the lab. [Caption for image at right: Bamboo coral being placed in the ROV's sample drawer.] In addition to trying to understand the ecology of deep-sea corals, we are collecting one type of coral to study the temperature history of the deep sea. Bamboo coral grows both upward (in a branching morphology) and at the same time adds concentric layers to its skeleton. Because of this growth pattern, we can examine the base of the coral skeleton, the thickest part, and see all of the layers that represent the whole lifetime of the coral - much like looking at the rings of a tree! These layers of skeleton are very valuable to ocean geochemists because as the corals precipitate calcium carbonate for their skeleton, they record the ocean chemistry at the time the skeleton was growing. Bamboo corals can live for up to centuries, so they are valuable recorders of how the deep ocean has changed over the past 10's to 100's of years. These timescales are important, because there is preliminary evidence that short-term climatic oscillations like El Nino and Pacific Decadal Oscillation can cause changes in the temperature and geochemistry (i.e. pH, nutrients, salinity) of waters at these depths. We have collected a few living bamboo corals and have also picked up specimens that were already dead and resting on the sea floor - these corals will provide us with a unique record of the history of the deep ocean.
- Tessa Hill
Bamboo coral skeleton, cleaned of tissue (scale-bar is in centimeters). The segments of light-colored calcium carbonate and dark organic matrix are characteristic of bamboo corals. Concentric rings within the segments will be analyzed for stable and radioactive isotopes, trace metals, and other indicators of environmental conditions through the coral's life.
Tessa draining water from a Niskin bottle attached to the ROV. The water was collected at the same location as one of the living corals and will be analyzed for many of the same elements as the coral skeleton.