Seamounts Cruise
May 2, 2004, Day 6
We stayed at San Juan Seamount for our third and final dive here. This
dive explored the northern part of the summit region and progressed from
volcanic cone to volcanic cone, working our way generally upslope during
the day. In contrast to the dive yesterday where most of the cones
consisted of pillow lavas, today we did not see any pillow lava.
Instead, every cone was mainly made up of volcaniclastic deposits
ranging from bedded sandstone to coarse breccia. We also encountered
several blocky lava flows. Perhaps the most interesting observations
include a nearly vertical dike that stood several meters above the
present volcaniclastic surface. The surface must have eroded downwards
by at least the amount the height the dike stands above the surface. We
also found a ring of what appears to be spatter. Spatter deposits
commonly form elongate ridges along eruption fissures or surround
central vents. We collected a record (for the cruise) 37 rock samples,
with more than half lava flow samples or blocks from volcanic breccia,
and the rest finer-grained volcaniclastic rocks that are commonly
altered to a bright orange color. The geology was not nearly as scenic
as the pillow lavas we observed yesterday, but we ended up with samples
from numerous vents that will provide the basis for geochemical and
radiometric studies, once we return to our laboratories.
The biological observations and collections today were very different
from our two previous dives on San Juan Seamount. We collected no bamboo
corals today because the only ones we saw were very small and would
record only a few years of climatic record. We also saw no Paragorgia
(red bubble-gum coral), perhaps the dive was shallower than the
(unknown) depth range where it grows. We did find a number of new
deep-sea corals including a golden one that was extremely abundant on
several cones, but absent from others. We also found a bright purple
coral, a skinny red one, and several light pink ones. We also
encountered another bright purple animal that we think is a colonial
hydroid-it is very similar to a similar animal seen only a few times on
Davidson Seamount during previous dives. The biologic high points of the
dive, however, arrived through the water column. Two Humboldt squid
followed the vehicle for perhaps 15-20 minutes, lurking in the shadows.
One finally appeared right in front of our camera and grabbed a rat-tail
fish, but then let it go and darted away. The final highlight, only a
half hour before the dive was ended, was an encounter with a Vampyroteuthis squid at a depth of 620 meters. We followed it for
perhaps 15 minutes, collecting some great video and digital still
photos.
--Dave Clague
One of several erosion patterns we've observed in the volcaniclastic
rocks: deep grooves aligned with the slope of the volcano's flank.
Vampyroteuthis, a "living fossil", has features of squid and
octopus. It
has deep red skin over most of its body, charcoal-gray skin and
hook-like "cirri" under its webbed arms, and large, bright blue eyes. It
didn't invert into the perhaps defensive posture reminiscent of a
vampire dramatically wrapping its cape across its face, but swam by
flapping its small fins.
Rocks That Grow
An interesting aspect of all deep-sea rocks is that iron-manganese oxide
coatings (called Fe-Mn crusts) precipitate on the rock surface exposed
to seawater. These crusts precipitate out of cold ambient seawater at
the incredibly slow rates of 1 to 10 millimeters per million years.
Even with those slow rates of growth, crusts as thick as 25 centimeters
have been found and represent as much as 60 million years of growth.
Those thick and old crusts occur on seamounts in the central Pacific
where the seamounts are also very old. The Fe-Mn crusts on seamounts
off California that I am studying on this cruise are thinner because the
seamounts are relatively young, about 10-12 million years old. So,
crusts that we are finding usually have a maximum thickness of about 5
centimeters, and commonly are much thinner.
I am collecting and studying these Fe-Mn crusts for three important
reasons. First, because the crusts grow at such slow rates, they absorb
high concentrations of a large number of chemical elements from
seawater. The concentrations of some metals, especially cobalt,
nickel, and platinum, are so high that they have been considered as a
potential future mineral resource. For this reason, their temporal and spatial distributions and metal concentrations need to be determined.
Much of this work has been completed.
The second important aspect is that Fe-Mn crusts grow everywhere in the
deep oceans where hard rock occurs as a substrate for their growth.
Consequently, their huge global abundance, about 200 billions tons, and
the sorption of high concentrations of metals, together control the
concentrations and chemical form of some of those metals in seawater.
The best known examples of this are for tellurium and cerium, but may be
true for other metals as well. This is important to know because we need
to understand the sources and sinks of elements in the oceans before we
can understand how the influences of people on Earth alter that natural
system with our activities.
The third important aspect of crusts is that they record changes in the
chemistry, circulation patterns, and other oceanographic conditions that
they were subjected to throughout their history of growth, which for the
samples collected on this cruise is about the past 12 million years.
For example, different water masses in the oceans have different
isotopic signatures for some metals dissolved in seawater, lead and
neodynium being especially useful. Changes in oceanic circulation with
time can be traced by analyzing the isotopic composition of lead and
neodynium in the crusts that precipitated from the seawater. There are
many examples of paleoceanographic conditions that can be deciphered
from the careful study of sequential crust layers and this aspect of the
crusts is currently the primary focus of my research.
--Jim Hein
Rocks collected on yesterday's dive being examined and described by Kathie, Jim and Alice' in the lab.
Brandie carefully scrubbing a rock to take off encrusting biology without disrupting the manganese crust that she and Jim will later subsample and analyze.
Rock with a thick crust of iron-manganese-oxide (dark brown). The crust has grown in pattern reminiscent of lizard skin, and the high places have been buffed to a sheen by erosive particles carried in the bottom currents.
Humboldt squid (Dosidicus gigas), roughly 5 feet long, striking a rat-tail (Macrouridae, or on the dinner plate, a grenadier). It then released the fish.
Jenny at the controls of the science camera, giving Dave a break from the chief-scientist's station in the ROV control room.
Ridge of eroded volcaniclastic (fragmented volcanic) rock inhabited by a bright purple colonial animal.