East Pacific Rise Part 2

So we knew there’d be a plume. Why come here? 
We know what ³He does in the plume – pretty much nothing! It gets diluted as seawater mixes. But, we don’t know what other elements coming out of the mantle are doing. 20 years ago it was believed that most elements from the lava, such as iron (Fe), precipitated out of the water near the vent. Now, models have shown there must be a significant source of dissolved Fe to the ocean from hydrothermal activity. In the last 10 years, Chris has begun looking at how these small amounts of dissolved Fe from the lava can allow for localized growth of marine organisms. Our research expedition is prepared to look at large chunk of the elements on the periodic table, helping answer the question of what really happens to these elements in plumes. 

Chris excitedly looking at the CTD trace and noting our plume.

Chris explains, “Starting at this station and heading west, we’re going to fly right down the core of a big jet of ³He that we know spreads west from here, so we’ve been collecting samples here to know what the [elements of] the periodic table are doing right at the ridge axis. Then we’ll be able to go and see how the elements behave over the next month. We know that the only way the ³He concentrations can change is through dilution; by comparing all the different elements to ³He, if everything else dilutes the same we’ll end up with the same ratio of each element to ³He. And if any of the ratios go down it’s because the elements must have done something else, like going away into the sediments underneath.”

A better view of the CTD trace. The plume is best shown by the disruption of light transmission, because there are often particles suspended in plume waters. [Red = temperature, orange = salinity, yellow = light scattering sensor, green = redox chemistry sensor, blue = oxygen, purple = light transmission sensor.]

What does the seafloor look like?

Envision a freshly surfaced highway – very smooth and shiny and hard. There are vents that look like little chimneys sticking out of the surface, which are ~10 cm across and not very tall. The hydrothermal plume originates from these vents, where fluid trapped by the now-cooled lava is wriggling its way out, carrying along with it high concentrations of elements. Chris estimates a field of vents that is 100 m across (size of a football field) can generate a plume that is a mile or two in extent.

Chris hopes to return to this site in a few years along with the Alvin 2 to actually see what is going on at the ocean floor. In the meantime, I got him to speculate on what we’d see today. He tells me the dominant animals here are giant tube worms (8 feet long). Their outsides are made of chitin (same thing as your fingernails) and their insides have red filaments colored by hemoglobin (same thing that gives our blood its color). The worms’ hemoglobin has specially evolved the ability to transport oxygen and hydrogen sulfide separately into their “guts” where bacteria mix the 2 gases in the presence of carbon dioxide. As a result, the carbon dioxide is converted into organic carbon (similar to sugar) – the process acts as a source of energy to the bacteria and the “sugar” acts as source of food for the worms. In this sense, the worms don’t eat like terrestrial worms do; they are chemotrophs, literally meaning they eat chemicals. All of this is probably happening under our ship right now – which is pretty awesome!!! 

Can we actually pull up any of the seafloor?

While we have a coring device – something that captures the surface ~30 cm of ocean floor for us – we couldn’t use it here. The seafloor is literally rock hard! 

Instead, we utilized a technique that is over 100 years old. Oceanographers before our time would attach a hard ball to the end of a line, covered in tallow. The Boson (head sailor) would carefully let the line out all the way to the seafloor, knowing he had hit bottom because of the reduced tension in the rope. Then, the fathoms were carefully counted as the ball was pulled back on board – with a bit of the surface seafloor encrusted in the tallow. We didn’t have to raid the galley for lard, as the engineers were happy to provide us with some white waxy stuff which we coated on a lead weight, held by rope to the bottom of Rosie on her deep cast. 

We were delighted when Rob Palomares (Scripps Institute of Oceanography / Ocean Data Facility) pulled the “wrecking ball” back up and found small shards of shiny black lava rock (obsidian) smooshed into the wax!