The past and present of trenches - why should deep-sea research focus on trenches?

Editor’s Note:

On November 25, 2022, the "Struggler" full-sea-depth manned submersible successfully completed the first leg of the first China-New Zealand joint deep-sea diving scientific expedition and returned to the Port of Auckland, New Zealand. In the first leg, the "Struggler" full-sea-depth manned submersible dived 16 times, 14 of which were at a depth of more than 6,000 meters, including 2 at the 10,000-meter level, and collected a wealth of deep-sea macroorganisms, rocks and sediment samples, providing important support for a deep understanding of the evolution and adaptation mechanism of life in the hadal, the evolution of the hadal sedimentary environment, and the subduction of plates and material exchange flux.

The first China-New Zealand joint deep-sea diving expedition completed the first leg of the Kermadec Trench expedition

In a person's life, he goes through the embryonic period → infancy → childhood → adolescence → youth → middle age → old age, and finally to death. Birth, aging, illness and death are the laws of nature. Did you know? The ocean also goes through a similar process, embryonic period → juvenile period → young adult period → decline period → residual period → extinction period. This process is called the Wilson cycle. The hadal trench, as a product of this process, is an excellent place to study major scientific issues such as the evolution and adaptability of hadal life, the evolution of the hadal environment and the effects of resources and environment, the hadal tectonic activities and geological evolution.

Appearance is determined by the mind - the theory of plate tectonics

We often say that a person's appearance is determined by his heart, and his appearance is determined by his inner self. This phrase can also be used to describe the basic landforms on the earth's surface.

The Earth is divided into the core, mantle and crust from the inside out. The crust is divided into the continental crust and the oceanic crust in the horizontal direction, and the mantle is divided into the upper mantle and the lower mantle in the vertical direction. The top of the upper mantle and the crust constitute the lithosphere. The lithosphere is divided into the continental lithosphere and the oceanic lithosphere in the horizontal direction. The continental lithosphere is generally thicker and less dense than the oceanic lithosphere.

At the top of the upper mantle and the bottom of the lithosphere, there is a critical layer of material called the asthenosphere , which is highly plastic and basically distributed globally. It is the source of magma. The average density of the asthenosphere is smaller than that of the overlying oceanic lithosphere, but larger than that of the continental lithosphere. The top surface is undulating, which causes the overlying lithosphere to be seriously unstable and laterally divided into plates of different sizes. These plates float on the asthenosphere and make large-scale lateral movements, colliding, squeezing or separating, and then forming the basic landforms on the earth's surface.

When the continental plate collides with the oceanic plate, the oceanic plate will subduct under the continental plate due to its high density, often forming a convergent plate edge, that is, a subduction zone, where 90% of the world's earthquakes and volcanic activities are concentrated. The boundary where the plates converge has the lowest terrain, which is called a trench . When two continental plates collide, huge mountain ranges are often formed, such as the Himalayas. In areas where plates are split and separated, rift valleys and oceans are often formed. The Great Rift Valley of East Africa and the Atlantic Ocean were formed in this way. The edge of the plate becomes the most intense zone of geological activity (magma, earthquakes, metamorphism, deformation, sedimentation, etc.).

Tips: The world is mainly divided into seven plates, namely the Pacific Plate, the Eurasian Plate, the North American Plate, the South American Plate, the African Plate, the Indo-Australian Plate and the Antarctic Plate. The Pacific Plate is almost entirely in the ocean, while the other six plates include large pieces of land and large areas of ocean. The large plates can also be divided into several smaller plates.

The Life of the Sea - Wilson Cycle

Oceanic crust basins (which can be simply understood as oceans) do not exist forever. They generally undergo a development process of cracking, expansion, contraction, and closure . Oceanic crust basins are formed between the split continental lithosphere. The entire process of the horizontal separation and splicing of the continental lithosphere is the Wilson Cycle , which is also the process of the ocean from birth to extinction. The beginning and end can be expressed as embryonic period → juvenile period → young adult period → decline period → residual period → extinction period . The corresponding example is the East African Rift Valley → Red Sea Gulf of Aden → Atlantic Ocean → Pacific Ocean → Mediterranean Sea → Himalayas .

(1) Embryonic period

Under the action of regional stress, the continental lithosphere first undergoes cracking in the weak zone of the upper continental crust, forming a continental rift. The fault zone at the rift is a strong active zone of modern volcanoes and earthquakes, but no marine environment has been formed, such as the Great Rift Valley of East Africa;

(2) Childhood

The continental crust continues to crack, and the molten mantle material rises on a large scale. Oceanic crust has appeared in some areas, and the original rift valley has become a narrow ocean basin, such as the Red Sea and the Gulf of Aden.

(3) Young and middle-aged

The narrow ocean basin continues to expand and develops into an open ocean. The middle of the ocean is the ocean ridge, and the upwelling magma forms new oceanic crust, which continues to grow on both sides. The developed oceanic crust accumulates thick marine sediments, a typical example being the Atlantic Ocean.

(4) Decline

Although the mid-ocean ridge continues to expand and grow, the oceanic crust continues to lose heat, cool, and become heavier during the expansion process, while the stable continental margin is covered by extremely thick sediments, and the heat in the ground is not easy to dissipate, so it continues to expand and become lighter due to heat. Fractures occur in areas with large differences in density and rock mechanical properties. The heavier oceanic crust subducts under the lighter oceanic crust or continental crust, enters the mantle and melts. This location is called a subduction zone. **In the subduction zone, a trench forms between the two plates, which becomes the deepest unit in the ocean. **The overlying plates in the subduction zone form island arcs, and volcanoes and earthquakes occur frequently. The ocean area gradually shrinks. A modern example is the Pacific Ocean;

(5) Residual period

As the oceanic plate further subducts, the oceanic crust area gradually shrinks, and the continental crust blocks on both sides approach each other, with only small oceanic crust basins remaining in between, such as the Mediterranean Sea.

(6) Extinction period

The collision of continents and the disappearance of the ocean caused the original sediments on the edge of the continent to be strongly deformed and uplifted into mountains, such as the Himalayas and the Alps.

There is no law of law - everything has its own law

The movement and interaction of plates on the asthenosphere, the opening and closing of ocean basins, have created the life of each ocean. The life span of the ocean ranges from 20 million to hundreds of millions of years, and there is no certain regularity. This is related to many factors such as deep tectonic processes, the diffusion rate of continental crust, the subduction rate of plates at the extinction boundary, and the characteristics of passive continental margins. Although this process is called the "Wilson Cycle", the ruptured continent cannot close again, and even if it closes again, it cannot split in the original place. In this sense, it is not cyclical. Since the total land area is relatively stable, when the ocean enters a contraction phase, it also means the expansion or rebirth of other oceans. Organisms are somewhat similar. Individual organisms will age and die, and through reproduction, there will be new and vibrant life, which will multiply from generation to generation.

Similar to humans, the ocean also has a life cycle, the length of which is determined by many factors, and the end of one life is followed by a new one. Similarly, celestial bodies such as the sun and the earth are also in constant motion and have a lifespan, so what about the universe? What about outside the universe?

The trench, as the deepest point in the world, may become the highest point in the world when the plates on both sides of the ocean collide and squeeze upward.

One of the benefits of studying geology is that geologists can look at problems from a more macro and long-term scale. The changes in the world are not only a little bit romantic and emotional, but also contain profound philosophy and wisdom, which is worth our lifelong appreciation and pursuit.

Touching the Deepest Point on Earth - The Abyss Trench

Distribution map of major hadal trenches around the world

(The trench marked with a red five-pointed star is the diving area for the first leg of the first China-New Zealand joint deep-sea diving expedition)

During the decline and residual period of the Wilson Cycle, the magma chamber at the bottom of the mid-ocean ridge develops and continuously upwells to form new oceanic crust, which pushes away from the sides, gradually cools, and becomes denser. The denser oceanic crust subducts under the lighter oceanic crust or continental crust, and the two plates meet on the seabed to form a trench.

Hadal trenches usually refer to seabed troughs with a depth of more than 6,000 meters , and are an important part of the earth's surface geomorphic units. There are 37 trenches in the world with a depth of more than 6,000 meters , 5 of which are distributed in the Atlantic Ocean and 4 in the Indian Ocean. The western Pacific Ocean, which is in a period of decline, has the most, with 28, including 5 10,000-meter-class hadals , from north to south: Chiba-Kamchatka Trench, Mariana Trench, Philippine Trench, Tonga Trench and Kermadec Trench . The Mariana Trench, known as the "fourth level of the earth", has a depth of 10,909 meters, making it the deepest area in the world . The Kermadec Trench, where the first leg of the first China-New Zealand joint hadal deep-diving scientific expedition was conducted, is located east of the Kermadec Islands in the southern Pacific Ocean and is connected to the Tonga Trench. It is about 1,200 kilometers long and has a maximum depth of 10,047 meters. It was formed when the Pacific Plate sank into the Indo-Australian Plate.

The trench system mainly refers to the seafloor boundary layer interface where the trench and its surrounding hydrosphere and lithosphere interact. Of course, studying the trench is not the same as simply studying this interface. There is extensive material and energy exchange between the hadal trench and the upper ocean and under the seabed, involving material and energy cycles in multiple spheres. The trench is dark and lightless, has extremely high hydrostatic pressure, slow water exchange, scarce food supply, peculiar life mechanisms, complex seafloor topography, and active tectonic geology . These characteristics have won the attention of scientists in the fields of tectonics, geology, biology, pharmacy, physical oceanography, ecology, environmental science, chemistry, climatology, and even aerospace.

Subduction factories - subduction zones

Trenches indicate the presence of subduction zones. The total length of active subduction zones worldwide is about 43,500 kilometers, which is roughly the same as the total length of volcanic island arcs.

The subduction zone is figuratively likened to a subduction factory. The raw materials of the factory are the subducting plates, which include ocean floor sediments, igneous oceanic crust and part of the lithospheric mantle. These materials release water and volatiles as pressure and temperature increase during the subduction process. The fluids and sediments are "processed" to form magma, which returns to the continent through volcanic island arcs or back-arc areas, accompanied by fluid/gas release, earthquakes and other phenomena. These fluids, gases, magma and other materials are the products of the factory. After being processed by the subduction factory, the residues continue to subduct deeper into the mantle, undergo dehydration and decarbonization, participate in the deep water cycle, carbon cycle, and rock cycle, and then feed back to the atmosphere through phenomena such as volcanic eruptions.

Subduction factories are focal areas for the exchange of matter and energy between the atmosphere, hydrosphere, biosphere, lithosphere, and mantle of the Earth. They are related to almost all the Earth's spheres and are a key link in the operation and evolution of the Earth system. The operation of subduction factories has triggered most of the world's earthquakes, tsunamis, and volcanic eruptions, bringing serious disasters to mankind. Therefore, it is particularly important to study the material and energy cycle of the subduction system and the triggering mechanism of earthquakes.

The Izu-Ogasawara-Mariana arc system is located in the western Pacific Ocean. It was formed by the subduction of the Pacific Plate under the Philippine Plate. It has a complete trench-arc-basin system and a clear evolutionary history. It is an ideal research object for studying the processes related to the subduction factory system. The Mariana Trench in this system is an important window for studying the subduction factory as the shallow manifestation of the subduction factory and the deepest part of the world.

Subverting cognition: reconsidering the origin of life

"Everything grows with the sun" has always been regarded as a golden saying about the source of energy for life on Earth, including marine life. People have always believed that the energy source of shallow-sea creatures is the algae, plants, and marine bacteria on the surface of the ocean that convert and store solar energy through photosynthesis, which is then eaten by marine herbivores, which are then eaten by carnivores, forming a food chain in sequence; while the energy source of deep-sea creatures depends on the feces, corpses, and organic debris of shallow-sea animals. In short, the beginning of energy is solar energy. So, did the earliest life on Earth also survive on solar energy?

In 1979, the Alvin manned submersible discovered an unusually thriving life scene in the black chimney area on the seabed at a depth of 2,500 meters, reshaping people's understanding of the origin of life.

Black chimneys, anemones, shrimps, hydrothermal area, southwest Indian Ocean, depth 3312 meters

Photography/"Deep Sea Warrior" manned submersible

As the deepest part of the world, the hadal trench is characterized by high pressure, no light, low temperature, and oligotrophy. It was once considered a forbidden zone for life. It is hard for us to imagine that there is life at the bottom of the Mariana Trench at 1,100 atmospheres of pressure. However, exploring the unknown and transcending the boundaries of cognition is exactly what scientists do. In recent years, with the support of hadal research programs in various countries, scientists have used deep towing, grabs, sediment column samplers, biological traps, landers, buoys, manned or unmanned submersibles and other technical means to carry out scientific research work focusing on the 10,000-meter trench in the western Pacific Ocean, and have achieved a series of results that refresh cognition. The deep sea abyss is not only full of life, but also a lot of it!

A Different Way of Living——Abyssal Life

The semi-enclosed and funnel effect of the "V-shaped" landform of the hadal trench makes it a natural gathering area for marine sediments. The sediments are rich in refractory organic matter and volcanic materials, and are active areas of microbial activity, breeding rich and unique life forms. The complex topography of the trench and its surroundings makes the ocean currents at different depths complex and changeable, and induces the diversity of the seabed ecological environment. The trench is not unobstructed on the ocean floor, but is divided into several discontinuous sections by some ocean floor plateaus or ridges. The extension direction of each section has a significant turn, and the depth varies. The organisms in different sections of the trench have the same or similar species, as well as a large number of different species. With the increase of the depth of the sea water, the number of large organisms and higher animals will generally decrease. Whether in the horizontal or vertical direction, the hadal biological community shows unique regional specificity.

The hadal trenches not only have a wide variety of microorganisms, such as bacteria, archaea, fungi, and viruses, but also a large number of macroorganisms, such as fish, echinoderms, cnidarians, sponges, arthropods, and mollusks. In 2021, the "Struggler" manned submersible discovered during its scientific expedition to the Mariana Trench that a large number of gammarids, sea cucumbers, anemones, polychaetes, and other organisms live in the 10,000-meter abyss .

Anglerfish, Kermadec Trench, 5736m

Photography/"Struggler" manned submersible

Rare deep-sea starfish, Kermadec Trench, 6700 meters deep

Photography/"Struggler" manned submersible

Brillouin eel, Kermadec Trench, 6500m

Photography/"Struggler" manned submersible

A piece of hard rock becomes a common home for starfish, anemones, small corals, sponges and sea squirts, Kermadec Trench, depth 6124 meters Photo/"Struggler" manned submersible

Starfish, Mariana Trench, depth 9580 meters

Sea anemone, Mariana Trench, depth 9254 meters

Polychaete, Mariana Trench, 7120 m

Sponge, West Philippine Basin, 7464 meters

Sea cucumber, Mariana Trench, depth 10812 meters

Sea anemone, Mariana Trench, 9000 meters deep

A row of anemones (under rocks), Mariana Trench, 9347 meters deep

Photography/"Struggler" manned submersible

Eight Immortals Crossing the Sea, Each Showing Their Magical Powers - Abyssal Creatures with Special Skills

Abyssal life mainly relies on organic matter that settles from the upper seawater to maintain life, and some also rely on substances from the deep earth to obtain energy through chemosynthesis. In order to adapt to the extreme environment, life in the abyss has a life mechanism and special metabolic pathways that are different from those of surface and shallow sea creatures.

The abyss lionfish is considered to be the deepest-dwelling vertebrate on Earth and has been found at a depth of 8,145 meters. Its skin, muscles and bones all show adaptation to the high pressure of the deep sea.

Lionfish School, West Philippine Sea Basin, 7731 meters

Amphipoda gammarus - one of the protagonists of the abyss, its food is obviously controlled by the productivity of the upper seawater. Any biological debris that falls into the abyss will be located and eaten by it. On the other hand, gammarus is also a source of food for other large predators. For example, after the lionfish caught by the Chinese Academy of Sciences' Hadal Expedition Team at 7,400 meters in the Mariana Trench was dissected, its stomach was filled with a large number of relatively complete gammarus. In addition, in addition to obtaining energy from food, gammarus and lionfish also receive necessary nutrients and immune defense from the symbiotic microorganisms in their intestines.

Gammarus, Mariana Trench, 10,900 meters

Photography/"Struggler" manned submersible

In order to adapt to the abyss of ten thousand meters, sea cucumbers even abandon the bone needles that support their torsos and turn themselves into small transparent creatures.

In addition to the above-mentioned macro-organisms, abyssal microorganisms also have their own unique skills. Some microorganisms play an important role in the nitrogen cycle, sulfur cycle and energy conversion in the abyssal environment of the Mariana Trench. This year, an article in Nature Communications analyzed sediments from 13 stations in the Mariana Trench and found that the dominant microorganisms in the sediments can convert organic nitrogen accumulated in the abyss into nitrogen gas and release it into the upper water body, which may contribute to the global nitrogen balance.

In the sediments of the Mariana Trench, scientists have also detected a wide variety of viruses, which regulate and influence the composition and material and energy cycle of the marine ecosystem by changing the material and energy metabolism of microorganisms.

Another study found that there are a large number of "oil-eating" microorganisms - hydrocarbon-degrading bacteria - in the 10,000-meter seawater of the Mariana Trench. They can effectively degrade alkanes, which suggests that alkanes may be an important "fuel" for microorganisms in the 10,000-meter water body.

Not only that, scientists have also discovered some "amphibious" microorganisms in the sediments of the Mariana Trench, which have both aerobic and anaerobic respiration genes, that is, they can survive tenaciously in both aerobic and anaerobic environments .

What is even more surprising is that beneath the abyss, the fluids produced by the changes in rocks in the oceanic lithosphere in the subduction zone nurture special groups of life. These lives may survive 14 kilometers underground . This may be the deepest hidden life on Earth.

How many unknown lives are waiting for us to discover in the abyss? What are the similarities, differences and connections between abyssal life and life on land or in shallow seas? What is the relationship between different abyssal lives? What is the environmental adaptation mechanism of abyssal life? What role do microorganisms in the abyss play in global climate regulation? We have too many questions about abyssal life. Solving these questions will help build a resource library of abyssal characteristic species and a gene resource library, laying the foundation for the development of abyssal biological resources.

The Loss of the Last “Pure Land”——Pollution in the Abyss

The 10,000-meter-deep abyss, a place so far away from humans, should be a pure land. However, manned submersibles have found garbage in the abyss, such as plastic bags, cans, fishing nets, etc. Through sampling research in the Mariana Trench, it was found that microplastics, POPs, methylmercury and other human pollutants were found in the abyss water, sediments and even organisms. What impact will these pollution have on abyssal life and the global ecosystem? What chain reactions will there be? Scientists are also working hard to explore.

Garbage found in the Hadal Trench

Photography/"Struggler" manned submersible

The Hadal Trench: A Testing Ground for Studying Extraterrestrial Oceans

The hadal trench, as a typical representative of the deep sea, is a testing ground for exploring the extraterrestrial ocean world. In recent years, scientists studying deep space have found a new breakthrough - the deep sea. They found that some extreme environments in the deep sea are similar to the possible conditions of the subglacial oceans of Europa and Enceladus. Therefore, studying life in extreme deep sea environments will help to find life in extraterrestrial oceans . Today, scientists and engineers in the fields of deep space and deep sea in the United States, Europe, and China are working closely together to achieve future on-site detection of extraterrestrial oceans through the exploration of the extreme environment of the Earth's abyss.

A powerful tool for deep-sea research: manned submersibles

The extremely high hydrostatic pressure brings great challenges to the exploration of the abyss. Due to the constraints of deep-sea observation and sampling technology, the research on the life process in the abyss has been relatively slow. Manned submersibles can not only take scientists to the bottom of the abyss for personal inspection and take video data, but also collect various samples such as water, sediments, rocks, macroorganisms, and microorganisms. This is extremely important for studying major scientific issues such as the formation and evolution of deep-sea earth science systems, the origin of life and environmental adaptation, biodiversity and climate change , and has really solved a major problem for the study of abyssal science.

"Struggler" Business Card

Basic parameters:

Mothership: Discovery One

Maximum working depth:

11000m

Size: Length 10.26m, Width 3.17m, Height 4.4m

Weight: 36t

Payload:

200kg (in air)

10,000-meter deep sea diving time:

12h

Capacity: 3 people

Maximum underwater speed: 2.5 knots

Inner diameter of manned cabin: 1.8m

Number of observation windows: 3

Average ascent and descent rate:

60 m/min

The "Struggler" manned submersible is a deep-sea equipment developed under the support of the key project "Deep-sea Key Technologies and Equipment" of the National Key R&D Program. It is China's first important equipment with the ability to manned access to global waters, and it is also the first full-sea-depth manned submersible with independent intellectual property rights. In November 2020, the "Struggler" completed the 10,000-meter sea trial and set a Chinese manned deep-sea diving record of 10,909 meters . The Institute of Deep-sea Science and Engineering of the Chinese Academy of Sciences actively deployed and implemented the Global Trench Exploration and Dive Program (abbreviated as Global TREnD) . The successful completion of the first leg of the China-New Zealand joint deep-sea diving expedition in the Kermadec Trench represents a solid step forward for the Global TREnD program. In the future, the expedition team will continue to give full play to the depth and technical advantages of the "Struggler", expand China's deep-sea diving expedition to multiple typical trenches around the world, and carry out multinational joint, systematic, multidisciplinary and comprehensive deep-sea diving surveys of hadal geology, life and environmental sciences.

With such a powerful tool, I believe that in the near future, the mysteries of the abyss trench will be revealed one by one.

Information sources:

This article is reprinted from Kong Xiu. The past and present of the trench: Why should deep-sea research focus on the trench? [EB/OL]. "Ocean World Culture Unlimited" WeChat account, 2022-12-16.

Written by: Kong Xiu; Art editor: Shi Yuqing

Editor: Li Weishan Reviewer: Cai You