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.

T879-young_lava.jpg (78097 bytes) 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.

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A stalked crinoid has already colonized the 1986 lava surface (laser dots are 30 cm apart for scale).
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?

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.

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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.

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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.
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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.

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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.
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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! 

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