May 6, 2004, Day 10
We awoke today to the calmest conditions I have experienced offshore California in many years of going to sea. It was also warm and sunny, so we all had a hard time returning once again to the control room, which is kept dark so we can see the video more clearly. Very little previous work had been done on San Marcos Seamount, so our expectations were based only on our experience with other similar seamounts. At first landing we were all mesmerized by the abundance, great size, and variety of deep-sea corals we saw. After quickly collecting a lava sample where we landed (it is a geologic cruise!), we began a search for a large bamboo coral for Tessa's climate studies. We quickly found one that was not only very large (7 centimeters in diameter at the base), but also nearly dead, with only one branch at the top still alive. This seemed ideal since none of us were enthusiastic about collecting an animal that may have taken hundreds of years to grow to such a large size. The coral was so large that we could not break it off using the manipulator and had to drive the vehicle into it to snap it off. Then we collected the largest sections (photo has centimeter-sized marks). Tessa was elated to find such a large and presumably old coral.We continued the dive examining the volcanic terrain. Along the way, we collected small snips of coral branches from about a half dozen different corals we had not seen on previous dives in order to carefully identify them. One of these was a light pink Iridigorgia species that is up to several meters tall and shaped like a giant corkscrew. We had seen a white species of Iridigorgia during dives in Hawaii a few years ago, but had not observed any on the seamounts we have studied in the last 4 years. Unlike all other seamounts where corals have been abundant, large sponges were absent.
We continued the dive up and down a series of volcanic cones and ridges. Each was distinctive in character. The first cone was surrounded by loose talus and had pillow lavas at the summit, but the next cone was heavily Mn-Fe-encrusted and appeared to be blocky a'a like flows that were almost impossible to sample. Another cone was entirely composed of fine and coarse volcaniclastic deposits, and we repeatedly encountered large bulbous pillow lavas with thick Mn-Fe crusts that made them nearly impossible to collect. All the Mn-Fe crusts will keep Jim and Brandie busy for quite a while. Most of the cone of volcaniclastic deposits was coarse grained and appears to be spatter, despite its eruption at 2000 meters water depth where the pressure inhibits exsolution of volcanic gases that drive such eruptions. We think these lavas must have been gas-rich in order to produce such eruptions at such great depths. The lower slopes of this cone and the saddles between it and adjacent cones were sediment covered. However, the sediment had a surface layer of Mn-Fe encrusted pebbles and gravel on top of white pelagic sediment thick enough we filled the 30-centimeter push-core tube. We think the pebbly gravel may be the most distal spatter clasts from the cone, but confirmation will await a more thorough examination back in the laboratory. By the end of the dive we had collected 30 rock and 24 biologic samples. The rocks included a handful of erratics (rocks from elsewhere, carried out to the seamount in kelp holdfasts or as ballast stones swallowed by pinipeds) volcaniclastic rocks, lava samples, and Mn-Fe crusts.
The animal collection included the array of deep-sea corals,
several crinoids, a strange worm, a sea cucumber, cup corals, and a
variety of animals adhering to the rocks and on the corals. The worm,
while exotic, may be the same one or a closely related species to one we
have seen off the tip of Baja California last spring and on the
Ridge in 2002. It is large (about 10 centimeters long and covered with
3-centimeter long bronze-colored bristles. When we first saw one on
Gorda Ridge, we nicknamed it the "gold lame'" worm as it is pretty
Outcrop at 2028m that we believe could be welded spatter from a mildly explosive eruption.
Light-colored, probably pelagic, sediment topped with large pebbles and gravel that may be manganese-covered spatter.
This ocean sunfish (Mola mola) swam between the pontoons under the moon pool. It had several large parasites on it (the pink spot on its back is one, which we guess may be an isopod).
Two ocean sunfish swimming with a gull. They swam slowly, mostly on their sides, in a tight circle around and around the bird. We think they may have been offering for the bird to clean them of the parasites. The gull didn't get it. This gull has been hanging out with the ship for the last several days, sometimes resting on the deck. We are well beyond where gulls normally venture; the birds seen out here are usually albatross.
This expedition has given me an opportunity to work on an exciting and
largely unexplored marine habitat. I've been helping to direct the
collection of video data and specimens, which MBARI scientists will use
for species descriptions and molecular analysis, as well as
physiological, behavioral, and ecological studies. On this expedition we
have discovered several species, which are new to us and possibly new to
science, including bivalve mollusks, gastropod mollusks, polychaete
worms, and several corals. In particular, I've gained a better
understanding of the types of communities we find on seamounts and what
the dominant organisms are. What is becoming very clear is that the
distribution of organisms on these seamounts is patchy, to say the
least, with some portions of the seamounts being literally dominated by
one species, for example sponges, zooanthid anemones, or Paragorgia spp., while others are either completely absent of all life, such
manganese encrusted talus slopes. Yesterday's dives at San Marcos showed
higher species abundance and diversity of corals than previous dives on
this expedition, with upwards of nine species being found on a single
rock outcrop. Seeing this patchiness in abundance and diversity we ask
ourselves what factors might be influencing the presence or absence of
the species we find with reasonable responses being depth, pressure,
oxygen concentration, nutrient availability or proximity to nutrient
sources, average current speed and direction, substrate type, and larval