Scientists from the UK’s National Oceanography Center (NOC) have set their sights on unmasking the ocean’s ‘twilight zone’ - the area between 100 and 1000 meters deep where a small amount of the sun’s light can still penetrate.
This area has proved particularly troublesome for researchers to study, as scientific instruments are typically designed to either sink to the ocean floor or float on the surface. But this elusive region is teeming with ocean life that plays a key role in keeping atmospheric carbon-dioxide (CO2) levels 30 percent lower than it otherwise would be, according to the scientists from NOC.
“Just like the plants in your garden, phytoplankton - the plants in the ocean - absorb carbon dioxide from the atmosphere. The difference is when these guys sink they can go down into the deep ocean. And then the deep ocean of course isn’t in contact with the air anymore and so it’s captured and it’s stored down there for long periods of time,” ocean biochemist Dr. Stephanie Henson told Reuters, adding that understanding what processes are involved in carbon transport in the ocean could lead to better predictions of global environmental change.
Professor Richard Sanders is leading a project to develop a scientific instrument that can gather samples of these organisms. At the NOC headquarters in Southampton, Sanders and his team are testing a device they’ve built called PELAGRA, or the ‘pelagic lagrangian sediment trap’.
“This is specifically designed to attack and sample the key unknown part of the ocean; we call it the ‘twilight zone’, where there’s just a little bit of light. So what this does is it samples the bit between about 50 and 500 meters, that’s where a lot of the action is. We’ve got stuff sinking and there’s lots of organisms that live there eating it. And what we want to know is what they’re doing. What this device does is it captures the flux at different depths in that depth range, so we can work out what the organisms that live there are doing,” explained Sanders.
PELAGRA’s sediment traps will help scientists calculate the carbon entering the oceans by providing a sample of the volume of sediment that sinks in a given period of time. Known as ‘marine snow’ for its appearance as it falls through the ocean, this sediment consists of flakes of marine detritus - dead plant and animal plankton, and plankton faeces.
“Marine snow is composed of dead phytoplankton which sort of clump together to form flakes and then they’re heavy enough to sink down into the deep ocean. It can also be formed of little animals which eat the little plants and then they poop out that carbon, and then their fecal pellets are very heavy and they sink down to the bottom of the ocean, also carrying lots of carbon with them,” said Henson.
She added that scientists have known about marine snow as a mechanism for getting carbon out of the atmosphere and down into the ocean for about 50 years, but it’s only recently that technology has evolved to a degree that allows for accurate measurements.
But building a scientific instrument capable of effectively collecting samples of marine snow presented a multitude of challenges for Sanders’ team, who had to get the 100 kilogram PELAGRA device to drift in the ‘twilight zone’ without sinking or rising to the surface.
“If we can get the density of this instrument just right, it will sit neither sinking nor floating. Now that’s easy to say but it’s actually a considerable challenge to do... They weigh about 100 kilograms and we need to ballast them to approximately better than one gram. So that’s a considerable challenge. The way that we do that is by putting them in tanks of water... and adding weights until they’re just sinking; at which point we know they have the same density as the water that they’re sitting in,” said Sanders.
The team is preparing to embark on two missions to the south Atlantic Ocean in a bid to understand why marine snow is more prevalent in some areas than others and to collect enough samples to determine how important it is in removing CO2 from the air. The research, Henson said, will also lead to more advanced computer climate models that could more accurately predict future climate change.
“This new project that we’re working on is going to try and figure out what kind of things control when and where we get this marine snow occurring, how much carbon it takes down into the deep ocean and, ultimately, with the data we collect, we’re going to build new models which are going to help us to better predict the ocean storage of carbon and its impact on the climate.”