Spending most of my childhood summers near the mangrove swamp, I find the smell of sulfide in the water sample coming up from the abyss on each Alvin dive strangely familiar. On this cruise, I perform a number of on-board chemical analyses on the vent fluid samples (including concentrations for ammonia and sulfide) and collect samples to bring to the lab back in Woods Hole.
On most days, the routine is more or less the same. Alvin surfaces in the late afternoon and we retrieve the Major samplers (which we use for collecting vent fluids) after the submersible is on deck. My Hydro Lab teammates and I carefully sample from these samplers one by one. We keep good notes on the white board for the parameters we measure like pH and salinity. These two measurements, despite sounding seemingly basic, can tell us so much about the water we sample. In most low-temperature samples (like those of less than 30°C in situ temperatures), they have very similar salinity to that of the bottom seawater (35-37 parts per thousand: 35-37 gram of salt in 1kg of seawater solution) simply because they have a lot of seawater mixed in it. Their pH values ranges between 5 and 6 (like those of black coffee or milk) On the other hand, the high-temperature vent fluids from the black smokers which have in situ temperature as high as 360°C tend to have lower salinity and also a lower pH of 3-4 (like those of tomato juice or vinegar).
After the pH and salinity measurements are made we proceed with taking samples for other analyses including sulfide and ammonia. Hydrogen sulfide is the chemical that gives the vent fluid a distinctive rotten egg smell, which permeates the entire Hydro Lab. Disgusting to most of us, this smelly sulfide however is a good source of chemical energy for many microbes inhibiting around the hydrothermal vents. They can harvest the energy stored within sulfide for their growth and metabolisms. Similarly, some microbes can also utilize ammonia in the similar manner. As a PhD student, I am particularly interested in the production and fate of ammonia in the hydrothermal vent fluids by looking at the nitrogen isotope ratios in different nitrogen-bearing ions and compounds.
Prior to participating on this cruise, I had worked with the water samples collected by other scientists. So I had very limited knowledge of the processes and hard work that go into the sample collection. Of course, getting to work in the Major Sampler team and getting a dive in the famous Alvin changed all that. I was a starboard observer on Dive#4901 on May 4, 2017. I first learned about Alvin in my undergrad Biological Oceanography class many years back, so it was unimaginable that I would get a chance to ride in it as a PhD student. All that I learned about light in the ocean was experienced first hand as we made a descent down. Looking out of the porthole, I observe the water turned from the light shades of blue to the darker hues as we exit the photic (i.e., well-lit) zone of the water. Jellies are everywhere until we hit the layer called oxygen minimum zone, which at least in this part of the ocean is completely devoid of oxygen. Once we made it out of those depths, the dark water was decorated with the sparkles of bioluminescence (an absolute sight to see).
We had a long list of what we needed to accomplish on this dive including releasing our large volume pump back to the surface, deploying our Vent-SID, taking water samples with Major Samplers, and investigating the crab traps and microbial colonizers; all of which were masterfully executed with ease by our seasoned Alvin pilot, Pat Hickey. We spent quite a bit of time around black smokers at the sites called Bio9 and Biovent. I had seen the photos of these stunning structures numerous times before, but nothing remotely compared to what I observed with my own eyes from Alvin; truly a spectacle they are. The sheer number of animals around the vents is also amazing. It was such a stark difference seeing merely a couple of them here and there off axis compared to vast expanse of mussel beds and giant tubeworm patches.
As a bonus to our dive since we managed to accomplish all tasks on the list relatively early, we decided to travel further north to a site called Hobbit Hole. Yes, the site has a structure similar to those you have probably seen from the Lord of the Ring movie: a big hole in the basaltic rock. We scouted around for a little bit for some warm spots of diffuse flows before we made our way back to the surface.
Before I sign off, I would like to thank the Alvin Team, the crew of R/V Atlantis and of course our Chief Scientist, Stefan Sievert, for such an amazing experience to be on this cruise and of course to be an observer in an Alvin Dive. My oceanographer’s dream had come true.