Seamounts Cruise
May 4, 2004, Day 8
On each of our cruises we try to squeeze in one dive where we explore
an area about which little is known. Today we did that dive on the
southern Patton Escarpment. The dive site is an unexplored part of the
former North American margin and is thought to be a relict from
subduction that ceased when the San Andreas fault system formed. No
previous sampling or geophysical surveys have been done at this site.
Typically, exploratory dives like this develop into new areas of study.
We were expecting to find deformed sedimentary rocks. Instead we found
that the surface was encrusted by thick Fe-Mn coated and cemented talus
and breccia. The Fe-Mn crusts were far thicker and more extensive than
we expected to find. Their great thickness suggests that the surface has
been undisturbed for tens of millions of years, which is unusual for
subduction complexes. These deposits covered the entire 3.5 kilometers
we traversed during the dive. With such thick Fe-Mn crusts, we will 
have
to wait until we can carefully cut up and examine the samples to
determine what types of rocks are hidden within the coatings.
The biologists were a bit disappointed in that the fauna were not very
diverse nor very abundant. This was in stark contrast to all our
previous dives on submarine volcanoes. However, we did find the deep-sea
clams that previously had been seen only on the volcanoes. Our first
image of the bottom 
included several Poralia jellyfish, commonly found
near the bottom, but here were actually on the sediment. We encountered
them numerous times during the dive, but only saw one up in the water
column.
--Dave Clague
Today our dive was at the Patton Escarpment. We had intended to dive
much farther north along the escarpment, which was a subduction zone
before the San Andreas fault became the plate boundary, but due to the
Navy's bombing practice plans, this was the farthest north we were
allowed to go today.
There were a few outcrops, but much of the dive traversed slopes
littered with rounded talus broken from outcrops we never saw. Some of
what we sampled was poorly lithified sedimentary material that broke
into many pieces in the ROV's sample drawer. The thick manganese crusts
on many of the rocks made Jim very, very happy.
Predatory tunicates
(Megalodicopia) were
more abundant here than on our previous dives. Besides a few sponges,
sea cucumbers and urchins, there were few animals overall. We wonder if
the thicker manganese crusts, which contain arsenic and heavy metals,
might make these old exposures toxic to animals that are thriving on the
younger seamounts nearby.
--Jenny Paduan
One of the jobs in the ROV control room is to operate the video annotation system. Here Lonny records observations of animals, geology, and sample collecting events using specialized video annotation software. He also changes video tapes and takes framegrabs (still images from the video).
Another job in t
he ROV control room is to run the real-time mapping GIS
we call ArcNav. Sonar fixes on the vehicle are received every few
seconds, and shortly after, the new vehicle position appears on the map.
Here Brandie is marking the location of an important observation or
event. These marks and the comments provide a useful geographic record
of the dive.
After the dive, sorting out the rocks from the ROV sample drawer is a
major task. As the ROV is ascending, we run a script on the video
annotation file that makes a list of the samples along with framegrabs
taken when they were collected. Then we can use that list (shown here),
and hyperlinks to full-sized versions of the framegrabs, to confirm
which sample is 
which.
After each dive, a map like this is produced. It shows the locations of
all the samples, and has an inset showing where on the seamount the dive
occurred.
During this cruise we have recovered many types of sandy sediments and
sedimentary rocks. The sand-sized material can consist of various
combinations of biogenic (e.g., foram tests, coral debris, sponge spicules), volcanic and detrital mineral and rock debris. Biogenic
debris in a sand can provide information on the age and depositional
environment (e.g., depth of water) of the sediment. Volcanic components
can be produced during volcanic eruptions (e.g., pyroclasts) or by the
weathering and erosion of previously erupted volcanic rocks. The
composition of rock and mineral debris in a sand is determined by the
composition of the source rocks (e.g., igneous, metamorphic,
sedimentary), the climate in which the rocks were exposed, and the
transport history of the sediment. The sediment can then be modified by
compaction and cementation to form a sedimentary rock.
When we traverse a slope with Tiburon, our sampling is limited to a very
narrow path. The
surficial sediments recovered by taking short cores or grab bags can
provide us clues as to the upslope composition of outcrops or the nature
of submerged paleoshorelines. We can carefully extrude the core after the
dive to preserve the stratigraphy (the layers and history of sedimentation) In the case of purely pyroclastic sediments or rocks, the
glassy components tell us about the magma composition as well as
information on eruption processes. Older sedimentary outcrops provide a
history of sedimentation that we can link to larger scale processes such
as plate tectonics. Here in California Borderlands, where the tectonic
evolution involved a complex shift from plate subduction to transform
(strike-slip) motion 20 to 16 million years ago, highlands are now
submerged and do not easily yield their secrets. The sediments and
sedimentary rocks recovered from our dive on an unstudied area of the
Patton Escarpment will elucidate the complex tectonic evolution of this
fossil plate margin.
--Kathie Marsaglia
When we collected this sample, we all made wild guesses as to what it
was. Manganese crust? Whale bone? Trash? It turned out to be the
skeleton of a large sponge. We never saw any living sponges with this
morphology on the whole dive today.
One of the few outcrops we encountered today. This one may turn out to be sandstone. We will know for sure when we cut up the rock we collected here.
Odd potholes like these, rimmed with thick manganese crust, were frequently observed on this dive.