Diving to an Underwater Volcano – NSF Cruise AT26-17A, Description of Science and Engineering

The theme of this cruise, the common thread that binds all the research going on, is future instrumentation and sensor development. Researchers from Woods Hole Oceanographic Institution, Lamont–Doherty Earth Observatory, University of Idaho, University of Minnesota, Institute GNS Science (New Zealand), and of course, Arizona State University’s School of Earth and Space Exploration have come together on this cruise to push the boundaries of measurement in the most extreme place on earth, the hydrothermal vents at the bottom of the ocean.

The world of the deep ocean is way beyond intuitive experience. The first difference between surface water and deep water is the total lack of light. The last of the sun’s photons having been absorbed nearly 1.3 kilometers above you in the first 200 meters of ocean called the photic zone. It is absolutely pitch dark at the bottom, the only light we see is what we bring and what is produced by occasional marine animals. There are no plants here, only animals. The second difference is the pressure. Seawater is about three times as dense as a skyscraper – no steel beams or concrete but much less empty space. The Empire State Building has a density of about 342 kilograms per cubic meter and seawater has a density of about 1027 kilograms per cubic meter. That means 1.5 kilometers ocean depth is equivalent to having a 4.5 kilometer tall building on your head! Ironically, the only thing that saves you is that it also presses in on you in all directions. Alvin’s small titanium sphere (that three of us squeeze into), “the ball”, is spherical precisely because we exchange structural strength for material strength through the use of the sphere’s basic shape. The third difference is the water itself. In contrast to the approximately 2 to 4 °C ambient water temperature at these depths, water escapes from the vents at temperatures ranging from 60 to 460 °C. Around Axial these days we are hard pressed to find anything much above 350 °C. Due to high hydrostatic pressure, deep heated water can exist in both liquid form and as a supercritical fluid, possessing physical properties between those of a gas and those of a liquid. Realize that it’s not boiling even when it’s 460 °C! Besides being superheated, the water is also extremely acidic, often having a pH value as low as 2.8 – approximately that of vinegar.

Now let me explain in detail what we are placing on the seafloor and why anyone would want to do that:

Our project involves two different components. The first component relates to a novel, high-temperature glass material which is being developed for future use as a subsurface fluid flow tracer. The second component further develops a new approach to high-speed underwater sensing and wireless communications networks. Both project components are ambitious experiments that progress the mission of the National Science Foundation by providing foundational engineering research with the long-term potential to transform our approach to ocean science, education and policy.

The goal of the first component is to test the stability of a new type of non-toxic, chemically-inert fluorescent glass in hydrothermal vent fluid. If the material can withstand the complex chemical environment of high-temperature vent fluid for an extended duration, it could potentially be used as a tracer for mapping subsurface fluid flow in the future. Such tracer studies will help to address some of the most difficult but fundamental questions we have about the Earth, including: How deep within the Earth does life live? What limits the growth of life in these extreme environments? How large is the subseafloor biosphere, and what role does it play in the carbon cycle? We will test this inert non-toxic material by attaching it to temperature probes and placing the probes in direct contact with high-temperature hydrothermal fluid for 2-3 weeks. Probes will be placed into hydrothermal vents using the Alvin Submersible. We will examine the material before and after vent fluid exposure using fluorescence microscopy, and evaluate any changes in its physical and optical properties.

The goal of the second component is to characterize the range and stability of an optical multi-hop sensor network. Sensor networks employ a spatially distributed array of communicating nodes, in which each node collects and transmits data to its neighbors in a web-like fashion. Sensor networks allow scientists to monitor dynamic phenomena over an extended area simultaneously. Optical multi-hop networks will form an important part of the communication backbone for distributed, underwater sensor data collection to help monitor ocean phenomena over wide areas and volumes. Such networks can be joined by passing Remotely Operated Vehicles ROVs, Autonomous Underwater Vehicles AUVs, or other sensors (like those used to monitor the tracers described above) to relay data to each other or onto a cabled observatory or surface buoy for real-time reporting. On this cruise, we will test two optical modem modules deployed multiple times at varying distances apart along a cable. The data will be statistically combined in order to model and plan for future sensor network missions.

This was a long one,  not really suited for a blog perhaps. I hope you made it through and maybe learned a little too.

 

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