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.
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.
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.
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 the 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.
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.