under the seas

What secrets are guarded by the darkness of the ocean depths? If novelists, storytellers, and dreamers of all ages have largely devoted themselves to filling these unknown lands with sirens, krakens, and other mythical creatures, science has, too late, undertaken to unravel their mysteries. In 1840, the British naturalist Edward Forbes went so far as to suppose that no life is possible beyond 600 meters and into the deep sea. A few decades later, the Challenger expedition, the first expedition to focus on the study of the deep environment, disavowed the azo theory. Since then, explorations have multiplied, but the deep environment remains one of the least studied systems in the ocean. During an interview, Christian Tamborini, director of research at the Mediterranean Institute of Oceanography, invites us to dive into these dark waters.

under the ocean

The beautiful blue color of the ocean can only be admired at the surface, in euphotic zones. Between 0 and 200 m, sunlight penetrates the water, allowing in particular the formation of phytoplankton. These microscopic plants suspended in the water absorb part of the carbon dioxide in the atmosphere and release oxygen, which is an essential part of the mechanism of gas exchange between the ocean and the atmosphere. If not consumed by larger organisms during their lives in surface waters, they eventually fall into the water column when they die and sink into the ocean depths.

Take a deep breath, here we are now in the middle seas region. Between 200 and 1000 metres, the light gradually decreases to complete darkness. Phytoplankton, fecal pellets and the remains of surface organisms fall like ghostly snow in the middle of this chiaroscuro photo, delighting the few bacteria and fish that cross its path.

Do you have some oxygen left? A few kilometers below, between 1,000 and 4,000 meters deep, extends the deep-sea zone. The organic carbon flakes that residents of the upper floors did not use as a snack continue their slow descent towards the sea floor. Their journey will end when they reach the abyssal zone, where they are stored among the sediments. Only 1% of the sea snow that passes through the water column reaches the ocean floor, sequestering carbon that is pumped to the surface.

Thus, if we automatically prompt our collective imagination to associate the abyss with the deep ocean, that term actually covers a much larger area! In practice, we talk about ocean depths once we cross the 1,000-meter mark, or even the 200-meter mark. It is above all a matter of distinguishing the area of ​​the ocean where the maximum productivity – the surface area – is from the residual density – the ocean depths.

Light on the twilight area

This vast and deep environment is one of the least studied systems in the ocean. If surface waters are indeed a site of great activity, especially thanks to solar energy allowing primary photosynthesis, then the ocean depths are also the scene of many phenomena, as we saw during our underwater journey. Mineralization processes that convert surface-produced organic carbon into inorganic carbon occur on long time scales that are difficult to measure. But its importance within the ocean system is just as important!

Good knowledge of these mechanisms is absolutely essential because climate change will have consequences that are currently difficult to measure. Excess carbon dioxide emissions from human activity continue to be partially absorbed by the ocean biosphere. Today, we have not yet been able to determine the consequences of this massive uptake of carbon by the oceans. This will require a very accurate knowledge of the process of carbon mineralization throughout the water column. However, the researchers are unable to correctly identify the fluxes measured in the mesopelagic region. The person with the name Twilight Zone, Referring to the legendary American fantasy series The fourth dimension, still contains many mysteries.

Among the current research, scientists question what effect bioluminescence — the ability to produce and emit light — might have on this process. The lantern fish, a creative creature in the abyss, is far from the only living being to possess this amazing ability! Along the water column, 75% of living things will be able to produce light, attract prey and partners to themselves, or even defend themselves. Among them, it was discovered that some bioluminescent bacteria cling to sea snow flowing along the water column. Much easier to detect in the underwater core of darkness, this luminous snow would be consumed more than had been estimated up to then, which would have a significant impact on carbon fluxes measured in the Mesozoic region. The question remains unanswered: research must continue to better understand the impact of biodiversity on measured fluxes.

Data hunting is on!

The interest of science in the deep environment is very recent: for only a few decades, researchers have studied with greater interest this enormity that has long been left on the margins of their questions. Research on these issues is in the process of being organized. How much remains to be discovered makes you dizzy: the sea floor is less known than the soil of the moon!

This lack of knowledge is partly due to the technical difficulty in studying the deep environment. When one descends along the water column, the temperature decreases, and the hydrostatic pressure increases … At a depth of 3800 meters, the pressure is 380 bar, that is, 380 times the surface pressure! It is essential to have tools that withstand such conditions. In addition, studying the deep environment requires more time and organization than studying the ocean surface: it takes less than an hour to take a sample at a depth of 200 meters, when a sample at a depth of 3000 meters requires three hours of time. Thus, campaigns in deep media make it possible to obtain less information, since researchers spend more time on the implementation of each sample. As such, a major challenge for research in this field is developing more efficient monitoring tools, deploying sensors across the oceans, and establishing deep observatories to measure hydrological and biogeochemical parameters. (including oxygen concentration and ocean pH), diversity and biological activities.

The stakes are high! If we want to protect the ocean, we must first observe and understand it. We can only consider it in relation to its surface! To understand what is happening in the surface area, it is necessary to learn more about the regions of the mesoameric, surface and abyssal seas, and vice versa. Under the influence of ocean currents, surface waters and deep waters mix, and minerals and organic matter mix in the vastness of the ocean. Climate change can modify these currents, making their study all the more necessary. Unfortunately, researchers currently have very little data from the ocean depths. The models they rely on to operate are fed primarily with surface data, allowing only a partial understanding of the oceanic system. Data hunting is on!