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Biogeochemistry

Volcanoes of the Caribbean: Science Swashbuckling with a Laser Spectrometer

Article Michel, A. P. M., Wankel, S. D., Kapit, J., Sandwith, Z., & Girguis, P. R. (2018). In situ carbon isotopic exploration of an active submarine volcano. Deep-Sea Research Part II: Topical Studies in Oceanography150 (2017), 57–66. https://doi.org/10.1016/j.dsr2.2017.10.004

Launching a remotely operated underwater vehicle off the side of a research vessel. It is carrying a suite of underwater sensors to explore an underwater volcano. Source: nautiluslive.org

An actively venting volcano in the deep ocean probably sounds like something you’d try to stay far away from.  To ocean scientists, however, these dynamic environments offer a fascinating opportunity to probe the mysteries of the deep.  One research team  recently succeeded in exploring the most active volcano in the Caribbean – with a little help from a laser-wielding underwater robot.

 

A Volatile Situation

Carbon dioxide and methane are greenhouse gases – gases that trap heat in the atmosphere, thickening the “Earth’s blanket” and warming it up.  Normally, greenhouse gas emissions are balanced out by the natural “sinks” that remove the gases from the atmosphere.  Since the Industrial Revolution, however, human activities have added excessive amounts of greenhouse gases to the atmosphere through the burning of fossil fuels and agricultural practices.  This has thrown a wrench in the natural carbon cycle, and the ability of the Earth system to keep up with this increase is flagging.

One perhaps surprising area where vast amounts of carbon are recycled is the deep ocean.  In particular, subduction zones and underwater volcanoes are key sites where carbon is buried and released.

Subduction zones mark the boundary where two of Earth’s tectonic plates have collided.  When these massive plates crash into each other, one slides under the other and is pushed into the hot interior of the planet.  As it is forced into the inferno, the sinking plate drags its carbon-containing sediments with it.  This is not the carbon’s final resting place, though – eventually, it gets released by underwater volcanoes.

Given the current global warming trend, which is widely believed to be due to increased greenhouse gas concentrations, scientists are greatly interested in obtaining a better understanding of the role of the deep sea in carbon cycling.  How much excess carbon from fossil fuel emissions can the ocean really store?  How much carbon dioxide and methane is leaked out of undersea volcanoes?  And does the carbon dioxide produced from these volcanoes contribute to ocean acidification?  Answering these questions will require cutting-edge tools that can operate in the hostile deep sea.

 

Illuminating the Deep

One way to measure gases is to use light.  Anna Michel, a scientist at Woods Hole Oceanographic Institution (WHOI), is pioneering new devices that use laser spectroscopy.  In this technique, a laser beam is shot through the sample (either air or water), and a receiver measures the amount of light that makes it through.  Different chemical compounds absorb different wavelengths of light, and the greater the amount of gas present in the sample, the less light that makes it through.

In 2014, Michel and other scientists from WHOI and Harvard University tested a new laser spectroscopy-based device at a volcano named Kick ‘Em Jenny in the Caribbean Sea.  The laser spectrometer measures carbon isotopes (see Box 1), shedding light on the underwater sources of carbon dioxide and methane.  It was mounted on a remotely operated vehicle (ROV; an underwater robot that is controlled by a pilot aboard a ship) and then sent 180 m (590 ft) below the water to Kick ‘Em Jenny’s crater.

Kick ’em Jenny, the most active volcano in the Caribbean, is located off the coast of Grenada.

 

During a 21-hour mission, the team investigated venting within Kick ‘Em Jenny’s inner crater, targeting both focused vents and diffuse vents.  These two types of vents have different “plumbing” systems –  focused vents resemble small, well-sealed pipes that transport hot fluids from underground to the seafloor, while diffuse vents are more like leaky pipes that releases fluids over a larger area.

The laser spectrometer made measurements of both volcanic fluids and gas bubbles.  In fluid-sampling mode, the ROV positioned a sampling wand directly into target fluids, from which gases were extracted by passing the collected fluid past a membrane.  In gas-sampling mode, the ROV maneuvered a funnel over bubble streams to capture the rising bubbles.  The laser spectrometer then analyzed the gas samples on site.

The remotely operated vehicle placing a funnel over bubbles emerging from a vent. The gas content of these bubbles will be analyzed by the laser spectrometer.

 

Passing Gas: Whodunit?

By measuring the carbon isotope composition of the fluids emitted from a vent system, one can track where the carbon came from.  For example, it can arise from magma, the molten material within the Earth’s crust (“magmatic”).  Or, it can be emitted as buried carbon materials are broken down in the hot crust (“thermogenic”).  A third source is release from sunken carbon reservoirs (“outgassing”).  Using the carbon isotope values measured by the laser spectrometer, the scientists determined that Kick ‘Em Jenny’s methane has a mainly thermogenic origin, whereas its carbon dioxide is mainly magmatic.

 

An Underwater Lab

As you can probably imagine, creating technology that can operate underwater and withstand the high pressure of the deep ocean is no easy feat of engineering.  Why bother going through all the hassle?

As it turns out, bringing the lab to the ocean can result in immense payoffs.  The underwater laser spectrometer is non-invasive, meaning that it doesn’t disturb the environment it is measuring – which ensures greater accuracy of measurements.  Furthermore, since the instrument measures chemicals on-site, there is no need to bring samples back to the lab for analysis, circumventing the painstaking process of preserving the integrity of the samples.  Direct measurements also mean that a greater volume of data can be collected, allowing for more systematic ocean exploration.  For example, the traditional method of collecting individual gastight samples results in 2-4 samples per dive; with the newly-developed laser spectrometer, 31 measurements were made.

 

Self-Driving Laser Spectrometer?

All told, the laser spectrometer device represents a significant advance in deep ocean gas analysis technology.  Besides exploring underwater volcanoes, it can be applied to a variety of other deep sea environments – for example, detecting the methane emitted by deposits beneath the ocean floor.  Warming of the ocean could destabilize these deposits, releasing large amounts of methane to the atmosphere.

In addition to taking laser spectrometry underwater, the research team pushed the boundaries of another technology: telepresence.  During the mission, only one team member was actually aboard the ship, while the rest were sitting at their laptops at the University of Rhode Island and live-streaming the action.  Telepresence via a satellite connection enabled a two-way audio communication between the ship and scientists on land, as well as the sharing of real-time HD video feed from the ROV.  This successful demonstration is a step toward making the laser spectrometer more autonomous – which means that in the future, the device could be pre-programmed and sent on data collection missions on its own.  Just as self-driving cars may soon hit the road, autonomous laser spectrometers could roam the ocean in the near future.  Such deep-sea sensors will be key to monitoring our changing oceans.

Shipboard ROV operators discuss the day’s dive. Telepresence enabled scientists on land to follow the action occurring on the ship and in the water.

 

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