About the Deep Sea

Hidden below kilometres of water, the sea floor is the most dynamic part of the solid Earth. Along a 60 000 km long ridge and mountain chain, the oceanic crust is constantly being created by volcanic activity and the process of seafloor spreading. Here 90% of Earth's volcanic eruptions occur. Over the last period of about 200 million years – a short time in Earth’s 4.6 billion year history – millions of volcanic eruptions have formed the seafloor we know today.

The ocean spreading ridges are a locus for the most vigorous and extensive interactions between water and rock on Earth. This hydrothermal activity supports chemosynthetic ecosystems – ecosystems that rely on chemical energy rather than solar energy and photosynthesis.

Until the 1970s people thought the deepest parts of the ocean were devoid of life, but when the first hydrothermal vents were discovered on the sea floor (in the eastern Pacific Ocean) scientists were surprised to find them home to a variety of unique organisms. We now know that the deep sea harbours the greatest amount of biodiversity on Earth. Researchers have discovered that the process of chemosynthesis supports a deep biosphere that extends kilometres into the subsurface ocean sediments. Here we find organisms living under conditions that are at the limits for life. Temperatures approach 130°C (or higher!) and the chemical conditions are poisonous to most known life forms. These "thermophilic" or heat-loving organisms are changing our understanding of the roots of the evolutionary tree of life. It may be that the deep sea is the nearest modern analogue for the beginnings of life on Earth.

Sediments accumulate on the seafloor slowly, at rates of a few millimetres per thousand years. Gradually, over time, the rough volcanic landscape becomes covered by hundreds of metres of sediments. When extracted in a sediment core, these layers of sediments are like pages in a history book and can take us back in time – up to 200 million years ago. Geologists can read the story the sediments tell us about changing oceans, volcanic eruptions, hydrothermal activity, glaciations and catastrophic events – when life on Earth almost went extinct.

The sedimented seafloor covers a major part of the globe. It is a complex and vital interface where geology and biology interact. This interaction extends deep into the subsurface. One of the challenges today is to decipher the information in the sediments to better understand this dynamic relationship.

In July 2008, a scientific expedition led by Rolf-Birger Pedersen from the University of Bergen discovered the Loki’s Castle vent field in the Arctic Ocean between Iceland and Svalbard. This was the first hydrothermal vent system discovered on an ultra-slow spreading ridge. A remotely operated vehicle (ROV) was instrumental in this discovery, as the five hydrothermal vents that make up Loki's Castle are located 2,300 metres below the surface of the ocean. The shape of the vents reminded the scientists of a castle's turrets and the name Loki was chosen as reference to the Norse god of mischief, Loki.

Location of Loki's Castle in the north Atlantic Ocean

Photo:

CDeepSea/UiB

Hydrothermal vents form along Earth's tectonic plate boundaries. When these plates pull apart, linear ridges called mid-ocean ridges form and host volcanoes and volcanic activity along them. The tectonic activity creates abundant faults and cracks in the crust at these ridges, allowing seawater to enter the ocean floor, where it can come into contact with the magma heat sources beneath the surface around these volcanic areas.

This heat drives convection of the seawater that enters the crust, much like what you see in a boiling pot of water. As the seawater is heated, it immediately starts to dramatically react with the rocks and minerals beneath the seafloor, becoming very corrosive, gassy and rich in dissolved metals. It also becomes extremely buoyant and rises rapidly to vent at the sea floor.

Where these waters emerge — sometimes at temperatures as high as 400°C — hydrothermal vents form, allowing the hot water to mix back into cold seawater. These sites become oases for life in the deep sea because of the rich chemical composition of the fluids. Microbes that consume the gases and metals contained in the hot spring water thrive, forming the basis of a deep-sea food chain. The metals in the fluids also precipitate out, forming metal sulfide chimney structures and mounds.

Here, we find ecosystems with unique organisms that live in darkness under extreme environmental conditions. Light cannot reach these depths so microorganisms use gas as an energy source instead of light. The microorganisms perform chemosynthesis instead of photosynthesis. Perhaps this was the energy source for the first life on Earth? The microorganisms lay the foundation for more advanced animal life in and around the hydrothermal vents.

Compared to animal communities at hydrothermal vents in other ocean areas, Loki's Castle stands out in several ways, and many of the species the scientists from the Centre for Deep Sea Research have found here have never been observed before.

Their adaptation to extreme environmental conditions makes the microorganisms interesting in their own right, but also in terms of potential benefits for humans. Particularly useful is that they have enzymes that can withstand extremely high temperatures.  

Life in extreme environments has extreme properties and already there is research showing that such life forms contain biomolecules that may be valuable for industrial processes and medical use. Such untapped biological and genetic resources may result in an important bioeconomy.

No one knows how “blue” the future will be, but we know that new knowledge is needed to fully understand the potential and the significance of the deep sea.

Nations are scrambling to claim areas of the seafloor for mineral and resource exploration, with deep-sea mining already commencing in some regions in the coming years. However, although there are vast seafloor and subseafloor metal resources, it is unclear if the resources that are accessible are substantial enough to justify activities such as large-scale deep-sea mining. The environmental impacts of such industrial activity are also unknown. Much research remains to be done.