World Oceans Day | Our bond with the ocean is a two-way journey

June 8, 2024 is the 16th World Oceans Day and the 17th National Oceans Publicity Day. This year's theme is "Protecting the Marine Ecosystem and Harmonious Coexistence between Man and Nature". Humans were born in the ocean and benefited from the ocean, but in the process of human development, they also had "friction" with the ocean. Our "bond" with the ocean is actually a two-way journey.

01 People come from the sea

3.8 billion years ago, the earth's land was still dead. The excess carbon dioxide in the atmosphere and the harsh climatic conditions made it difficult for living things to survive.

The Earth in the Archean Era (copyrighted image from the gallery; reprinting may lead to copyright disputes)

At this time, in the ancient oceans of the Earth, early life on Earth had already begun to develop. The oceans are rich in water and nutrients, especially in shallow sea areas, where there is sufficient sunlight and a suitable temperature. A large amount of nutrients from land are carried into the sea by rivers, which is very suitable for the survival of organisms. Early life thrived in such a comfortable shallow sea environment.

Among them, cyanobacteria capable of photosynthesis set off a great oxidation revolution 1.3 billion years later. From 2.5 billion years ago to 1.8 billion years ago , countless cyanobacteria absorbed a large amount of carbon dioxide and released a large amount of oxygen through photosynthesis , oxidizing the iron-containing substances on the seabed into hematite , leaving rich iron ore resources for modern humans.

However, for many creatures at that time, this oxygen was "poison", causing a rapid decrease in anaerobic organisms and a rapid increase in aerobic organisms . It was not until 550 million years ago that a magnificent explosion of life finally appeared in the ocean, represented by the Chengjiang Biota in Yunnan. The ancestors of humans and even all vertebrates were born in this grand event of life.

About 470 million years ago , some green algae seemed to have the ability to survive on the coastline. The frequent rise and fall of tides made it possible for them to gradually adapt to the terrestrial environment and paved the way for other plants to land. Terrestrial plants have higher photosynthesis efficiency and can produce more oxygen, which prepared oxygen and food for the vertebrates that landed later. One branch of these early vertebrates eventually evolved into humans.

02 Living by the sea

From the landing of early vertebrates to the evolution of humans, it seems that humans who have always lived on land have nothing to do with the ocean. In fact, from the land beneath our feet to the white clouds above our heads, from the rivers that gave birth to civilization to the creatures that share weal and woe with humans, they are all closely related to the ocean, and we can see that humans have always made a living from the sea.

Many of the mountains and lakes under human feet are formed by the collision and compression of plates , turning the vast sea into mulberry fields. Today, the east and west coasts of the Pacific Ocean are getting closer to each other, and perhaps in the future they will become an intercontinental sea like the Mediterranean Sea, which is surrounded by land. As the plate movement continues, the ocean disappears, and the two continents collide to form a huge mountain range similar to the modern Himalayas.

Arid climate is not conducive to human survival. From the current population distribution map, we can see that densely populated areas are mostly distributed in coastal humid areas . Water vapor evaporated from the sea surface can be transported to the land through atmospheric movement to form precipitation. The closer to the sea means more abundant precipitation . At the same time, the specific heat capacity of seawater is greater than that of land, which can also play a role in regulating temperature .

Water on land flows from high to low. If there is no large amount of water vapor transported from the sea surface to supplement it, the big rivers will dry up sooner or later . The Yellow River, Yangtze River, Indus River, Nile River and other rivers that gave birth to several important civilizations in the world, their main water supply sources are atmospheric precipitation brought by the water cycle between sea and land .

Water cycle between land and sea (copyright image from the library, reprinting may cause copyright disputes)

03 People go to the sea

Humans living on land have never really left the ocean. On the contrary, humans have always lived by the sea. The ocean is a mysterious and huge treasure house, providing abundant biological, mineral, energy, seawater compounds, space, tourism and other resources for human production and life. From ancient times to the present, from near sea to offshore, from shallow sea to deep sea, human exploration has never stopped.

Initially, human use of the ocean was mainly concentrated on the coast and offshore areas, which were manifested in the benefits of fishing and salt production and the convenience of navigation . The coastal mudflats can be converted into salt fields for drying salt to meet human consumption and salt chemical needs. After draining the excessive salt content in the coastal soil, trees, grass and crops can be planted .

Raoping fishing boats go out to sea when the fishing season begins in 2022 ( Source: Xinhua News Agency )

There is also abundant energy between the tides, which can be converted into electricity through tidal power stations . Abundant biological remains are buried on the seabed, forming rich oil and natural gas resources on the shallow sea continental shelf, which can be mined by building drilling platforms. There is also methane-rich combustible ice, which is an important clean energy.

Coastal placer mines rich in titanium, iron and chromium, seabed phosphate mines rich in phosphorus, manganese nodules rich in manganese, and seabed hydrothermal polymetallic deposits rich in metal sulfides are all important mineral resources that can be used by humans.

In addition, in some areas with shallow waters, it is possible to reclaim land, build artificial islands and cross-sea bridges and tunnels, and use the seabed space to store materials .

04 Heading to the Deep Sea

Nowadays, people have deployed more satellites and survey aircraft over the ocean, there are various types of ships on the sea surface, there are many submarines and probes under the sea surface, and even many underwater tunnels have been built on the seabed, but human understanding of the ocean is still very limited.

There are many risks in human exploration of the ocean. The ocean weather conditions and seawater movement are changeable, seawater is highly corrosive, and sea ice is highly destructive , requiring a lot of technology and capital investment. Especially in the deep sea, there are complex and special environments such as darkness, high pressure, low temperature, and lack of oxygen , which hinder the pace of human exploration. Humans are also accumulating experience step by step and learning how to get along with the sea better.

Every time humans further understand the ocean, it can bring huge social impact. During World War II, in order to allow submarines to better detect the ocean, humans invented more accurate sonar instruments and discovered that there is a mid-ocean ridge in the middle of the Atlantic Ocean that runs through the North and South Poles, also known as the mid-ocean ridge. It turns out that the deep sea is not as flat as people imagined.

Later, through paleomagnetic and radioactive isotope dating studies on the rocks on both sides of these ridges, earth scientists found that the rocks closer to the ridges are newer, and the rocks farther away from the ridges are older, and they are symmetrically distributed along the ridges. From this, the famous seafloor spreading theory was proposed, believing that a new ocean was being born here. Based on this, the later popular plate tectonic theory was established.

The ocean drilling program started in the 1960s, through drilling deep-sea sediments and rock samples, restored the climate change process since the Cenozoic era, allowing people to understand the structure and material composition of the oceanic crust. There is also the Mariana Trench, which is 11,000 meters deep. Now there are deep-sea submersibles such as the "Struggler" landing on the bottom, and more and more mysteries are being revealed.

Human beings came from the sea, lived on the sea, and now they are going to the sea, heading towards the deep sea.

Author: Yang Shuaibin, PhD in Quaternary Geology, Chinese Academy of Geological Sciences, Geography Teacher at Beijing No. 11 School

Reviewer: Dong Hanwen, Associate Researcher, Institute of Geology, Chinese Academy of Geological Sciences

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.