Easter Microplate Expedition
April 2, 2005 Day 22
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Alvin dive# 4097,
Since we felt we had sufficiently explored our first target area at 23°S during dives #4095 and #4096, we opted to move on to a secondary site farther south along the same ridge. One benefit to this move is that the new site is 20 nautical miles (nm) closer to our final target area at 26°S, over 200 nm away. While 20 nm doesn't seem like a lot, it would make a 1.6 hour earlier arrival time, which could make the difference in whether or not we'd be able to get in all 14 planned dives: if we arrive tomorrow after 12 noon, it will be too late and we won't be able to launch a dive until the following day.
Today's dive (#4097) is an unusual one for several reasons. First, this is our only dive at this site. Previous surveys by a towed TV camera suggested the presence of dead mussels shells and extinct sulfides. However, this data was collected 15 years ago. Hydrothermal vents usually don't last much longer than several decades since the conduits for the super-heated water become clogged. As a consequence, the hydrothermal vent community will slowly disappear from the area unless new areas of venting arise nearby. Second, this dive is a PIT dive (Pilot in Training) which means that our pilot today is Mark Spears and the starboard observer is
Alvin chief pilot, Bruce Strickrott. As a consequence, I will be the only scientist (to date) who will see the ocean floor at this particular site. While exploring vents is an exciting adventure, it is also a lot of pressure to balance the priorities of the dive with the limited amount of time and
Alvin battery power.
The dive itself started off with a 1.5 hour descent in the darkness (no bioluminescence this time, in fact, sunlight was visible to 400 m!) About 100 m from the bottom, Mark turned on the sub's lights. To my surprise, we saw white flocculent material in the water- a tell-tale sign that there is a smoker nearby. Since the pilot is busy looking out of the front view port for the bottom and smokers, each observer must be paying careful attention to anything that may resemble a black smoker or structure that could damage the view ports or Alvin. On the Juan de Fuca ridge system in the NE Pacific, there is a 330oC black smoker that is over 45 m tall (appropriately named Godzilla)! As you approach the bottom of the seafloor in Alvin, you become acutely aware how dangerous it is to be in a 6 foot diameter sphere at 260 atmospheres of pressure (approximately 3850 pounds, or two elephants, per square inch) approaching a bottom that may or may not have such a tall black smoker positioned directly below you.
Iron weights (the two stacks of rust-colored slabs, just left of center) are attached to Alvin before the dive. The extra weight makes the sub descend to the bottom. One or two weights are dropped when they reach the bottom. This makes the sub closer to neutrally buoyant and more responsive to fine adjustments in the variable ballast system, which pumps water in and out of tanks. The rest of the weights are dropped at the end of the dive, so the sub becomes positively buoyant and ascends to the surface.
We reached the bottom and sat for a few minutes while we got good fixes on our navigation and adjusted our variable ballast (VB - like a scuba diver's BC) to compensate for the 200 lbs of iron weights we had just dropped. We landed a bit west of our target site so we could work uphill (minimizes the amount of dust we have to navigate through). We arrived at the target site and to my amazement a huge field of sulfide sediment stretched out before us. We went a little bit farther and saw an extinct sulfide chimney that was 2 meters in diameter and about 3 meters tall. Surrounding this chimney was an immense number of dead mussel and clam shells.
As we moved to the north, we saw an increasing number of Bythagraid crabs. As Bob says, follow the animal trail. So, we continued north looking for more of these vent endemic crabs. After a while, we reached the end of the Bythagraid crabs and started seeing lots of stalked crinoids and anemones. We cut back to follow a fissure to see if we could find the mysterious origin of the white flocculent. The fissure dropped off about 15 meters and we followed it for a while until Bruce noticed some cracks in the sheet flow where there was diffuse venting. Mark flew over to the area and then suddenly out of my view port, I saw a clump of Tevnia tubeworms no larger than 5 square feet. We sat down amongst all of the diffuse flow coming out of the sheet flow surrounding us and surveyed the situation. Hidden amongst these rocks and Tevnia tubeworms were several individuals of the large Riftia tubeworm! We have only seen a few other Riftia tubeworms at 38°S and one at 32°S. Only one Riftia was within the reach of the Alvin's manipulator, so we decided to try and collect it. About this time, the top lab called down on the underwater phone and said the wind was starting to pick up and we may have to start our ascent soon. The pressure was on us, but Mark collected the Riftia tubeworm and put it in the drawer. We collected a few water samples for microbiology since we were sitting right on top of a diffuse flow (only 4.2°C). We continued our survey course to find some more active vents but in the end only returned to our original target site with the large extinct sulfide. We slurped some fissures in the lava for glass samples and small invertebrates right before we dropped the last of the iron weights and began our ascent.
Left:Branchipolynoe polychaete worm: dorsal and ventral views and closeups of organs and eggs, from dive# 4088. This worm lives within the mantle cavity of the Bathymodiolus vent mussel. Right: Clump of Oasisia (vestimetiferan worms) and attached Paralvinellida (polychaete worms), from dive# 4092. The Oasisia tubes are stained by the mineral-rich vent water. Watercolors by Karen Jacobsen, In Situ Scientific Illustration.
People asked me if this dive was better than my previous dive. It's hard to compare. This site had an amazing abundance of crinoids and fish whereas the other site that I dove on (38°S) had "Sebastian's Steamer". I'd have to say that both sites were equally amazing to me.
Cookout Number Two! Since we pulled the dive a little early to get to the next site, we had time for a cookout, our second. Once again, we sat on the fantail and stuffed ourselves with great food. This time, the wind was a little more gusty-- keeping food on our paper plates and our plates on the deck was difficult. Most of us had to get up at least once to chase a rolling can or a flying napkin before it went overboard and sailed off to sea.
I've been diligently working on my undergraduate honors thesis. It is a small comfort to know I am not the only one on the ship who has approaching deadlines and increasing levels of writing-induced stress. My favorite way to unwind is to stand outside on the deck and enjoy the gorgeous blue ocean and refreshing sea breeze.
Left: Ana and Megan working on their theses in the lab. Right: Robbie is scheduled to dive tomorrow. It is critical that the oxygen masks fit snugly without leaks to reduce fire danger in case there's an emergency in the sub, so men must shave their beards
This is a stalked yellow crinoid and a small isopod found clinging to the crinoid, from dive# 4089. It is a deep-sea animal that clings to hard rock, and is not necessarily associated with vents. Watercolor by Karen Jacobsen, In Situ Scientific Illustration.
All underwater photos were taken with the submersible Alvin, and are courtesy of Woods Hole Oceanographic Institution.