MBARI Ridges 2005 Expedition
Juan de Fuca Leg: August 7–18, 2005
Gorda Leg: August 22–September 2, 2005
August 13, 2005
Tiburon dive 879, North Cleft Flows, Juan de Fuca Ridge.
The seas have picked up. There is a hurricane that originated in the Gulf of California and headed west, and is now somewhere south of us in the Central Pacific. Swells generated by it are traveling at us from the south against the usual swells from the north. The seas are confused, making a rougher ride for us. The conditions are still well within the operating range for the Tiburon and are not expected to get worse, so we anticipate good diving for the next several days at least.
In contrast to the surrounding older lavas, the 1986 lava
flow is darker and has much less sediment cover. A close up view shows that it still has its
glassy outer surface, making it shiny, although not as shiny as right
after the eruption, when it was observed from camera tows and an Alvin dive.
Bill Chadwick writes:
Today we made a dive at the northern end of the Cleft segment at a site
that we know erupted in the mid-1980’s. In fact, this was the first
site in the world where we were able to document a historical eruption
on the mid-ocean ridge system. Before then lots of young-looking lavas
had been found, but we had no way of telling how young they were. The
age of the lava flows at north Cleft could be determined because they
are so thick (pillow lava ridge up to 45 m high!) that they show up as
areas of significant depth change between bathymetric surveys before and
after the eruption. Camera tows showed that young glassy lavas were
located exactly where the depth changes were found. In addition, a
giant plume of warm water, called a “megaplume”, was found by chance
over this site in 1986, and was probably produced by the eruption.
Since north Cleft is the oldest eruption site of known age in the
northeast Pacific, it is a critical test of the idea that the rate of
colonization of historical lava flows by sessile fauna (animals that are
attached to the bottom and cannot move) can help us estimate the ages of
other flows of unknown age. It is totally unknown how long it takes
many deepsea species to colonize and grow, but at the historical
eruption sites we have a key piece of information: when the seafloor was
last “repaved”. We found no sessile animals growing on the 1998 lava
flow at Axial that we visited a few days ago, so apparently 7 years is
not enough time for colonization. Is 19 years (the age of the north
Cleft flow) enough time?
Image on right: A stalked crinoid has already colonized the 1986 lava surface (laser dots are 30 cm apart for scale).
Our dive today showed that the answer is YES! Colonization of the north Cleft lava flow by sessile species is already happening. We found sparse stalked crinoids up to 30 cm long and small spherical sponges growing on the 1986 flow. In a few days, we will visit another historical lava flow that erupted in 1993 on the CoAxial segment, and this may help us to further refine the pattern and rate of biological colonization on young lava flows.
Montage of animals seen today (see Linda's writeup).
Linda Kuhnz writes:
While traversing both older flows and newer lavas
that covered the seafloor in the mid-1980’s, we saw a variety of
animals living on the bottom. We find long-legged crabs on nearly
every dive. There were also some large red shrimp bouncing along the
lava pillows and beautiful frilly worms swimming from rock to rock.
A new sight was a long worm, nearly translucent with a large lobed
hood near its head. These animals feed on the nutrients in the
sediment that settles in between the big pillow lavas over time.
Fish are rare; there are two types swimming above the bottom
(rattails and ophidids), and today we found a bottom-dwelling form
called a zoarcid.
There is not much color variation as we fly along the seafloor here, so the shade of an occasional pink or orange anemone looks very bright. We find sessile (attached) white sponges and soft corals as well. All of these organisms feed by filtering or catching small particles from the water. Sticking up, out, or growing a long stalk helps the animal reach into the water column where small bits of food move by.
Left image:
The older lavas surrounding the 1986 lava flow at north
Cleft have sediment accumulated between the pillows and large sessile animals
growing on them, such as this Gorgonian coral. Right image:
The crust of this pillow cracked open while lava still flowed in the
tube, and it drained to form another pillow downslope. A feather star (crinoid) is clinging to the rim. It is capable of swimming. Mobile
species like this can't reliably be used to help determine the age of a
flow.
Left image:
Laura is sieving sediments collected with a push-core. We sampled the
sediments on and off the young lavas for volcanic glass fragments that
will tell us if pillow eruptions show evidence for the mild explosive
activity that we have found associated with more fluid sheet flow
eruptions elsewhere. Right image: Glass fragments sieved from push-core samples are
drying under heat lamps in the lab. A board from one of the
high-temperature probes, which flooded when we broke it free from the black-smoker chimney at South
Cleft, is also drying. We are confident that we'll be able to get the data off it!