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2025-01-15 Update From: SLTechnology News&Howtos shulou NAV: SLTechnology News&Howtos > IT Information >
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This article comes from the official account of Wechat: Earth knowledge Bureau (ID:diqiuzhishiju), author: editorial Department of Ball Bureau
Author: true fruit juvenile cake
Revised manuscript: gu Hanying / Editor: tuna
Bohai Sea, which is located in the east of China, is a part of the western Pacific Ocean. However, compared with the vast Pacific Ocean with an average depth of more than 4000 meters, the average depth of the Bohai Sea is only 18 meters, and the deepest depth is only 70 meters, which is even shallower than inland lakes such as Sailimu Lake and Qinghai Lake.
Compared with the vast and deep Pacific Ocean
The Bohai Sea is really a very small and shallow sea area ▼.
As a member of the ocean family, why is the Bohai Sea so shallow and how will it change in the future?
If you look carefully, it is actually a sea ▼ made up of several bays.
Where does the Bohai Sea come from? it is believed that the first water on the earth came from the degassing of magma during volcanic eruptions. The water contained in the magma erupted from the depths of the earth was released into the atmosphere and landed on the surface as volcanoes erupted, becoming the original "source of life" on the blue planet. what's more, it laid the foundation for the formation of an ocean covering more than 2/3 of the earth.
The Pacific Ocean is the only ocean that can cover almost half a sphere.
Although the words "sea" and "ocean" are often used together, they are actually very different. The main body of the sea is generally above the continental shelf, and below it is the continental crust, which usually surrounds the continent. The ocean is different. Apart from having a vast oceanic crust, it is incomparable to the sea in depth and area.
The Bohai Sea, which lies entirely within the baseline of the territorial sea
Not even the marginal sea, which belongs to China's inland sea ▼.
For example, the Pacific Ocean has a total area of 181.3 million square kilometers, with an average depth of more than 4000 meters, of which the deepest Mariana trench even reaches 11034 meters, which can be called "the closest place on earth to hell." Compared with the large and deep ocean, the Bohai Sea, which is distributed on the edge of the Pacific Ocean, is simply "a little sesame meets a big watermelon."
If you compare the ocean with the Bohai Sea in terms of sea area and average depth
The Bohai Sea is really only a spoon of the ocean (found in the picture) ▼
The Bohai Sea, which is located between Liaodong Peninsula and Jiaodong Peninsula, belongs to the shallow sea of continental shelf in China, which is about 346km wide from east to west and 550km long from north to south. It has a total area of only more than 78000 square kilometers and has the basic geomorphological features of "three bays, one gorge and one basin".
Three bays: Liaodong Bay, Bohai Bay and Laizhou Bay
Yixia: Bohai Strait
A basin: Bozhong depression ▼
Although the area of the Bohai Sea is not large and the sea water is not deep, it has made a qualitative leap for the Bohai Sea, which has grown from scratch and from a lake to a sea.
In the Mesozoic era, about 250 million to 6500 years ago, the earth was undergoing great geological changes, the United continent was disintegrating, the Paleo-Tethys Ocean gradually shrank under the Indosinian movement, and the Yanshan movement subducted the Pacific plate to the west. gradually change the geomorphology of northern China. During this period, the Bohai Sea area showed a Bohai Sea subsidence basin, and continental sediments developed gradually.
From the end of the Mesozoic era to the Tertiary period of the Cenozoic era, the process of shallow compression and deep movement continued, and the "key role" of the ancient Bohai Lake also appeared in this period. The ancient Bohai Lake was invaded by sea water more than 1 million years ago and became a shallow sea about 300,000 years ago.
The time came to the most recent Quaternary period, during which many large-scale transgression and retrogression took place in the Bohai Sea. In the early Pleistocene period of Quaternary, the climate was mainly dry and cold, and the Bohai Sea was still a dry plain connecting Jilu Plain and Lower Liaohe Plain.
In the Middle Pleistocene, the climate was relatively warm, and several sea-level rises led to transgression, which made the Bohai Sea take shape. The two large-scale transgressions in the late Pleistocene made the story the most exciting part of the story, and the Bohai Sea basically formed its present scale.
Before the large-scale transgression, it was basically a big lake ▼.
During the Holocene that followed, the story of the Bohai Sea began to come to an end. During this period, with the rapid expansion of the ocean, the separation of Jilu Plain and Lower Liaohe Plain, coupled with the influx of river system and the role of human factors, the Bohai Sea was finally formed. The legend of ups and downs is written in the on-off and on-off continental tectonic evolution and the cyclic geological evolution.
In ancient times, the vicissitudes of life is actually the area at the junction of the Yellow Sea and the Bohai Sea.
It can be seen that the area of the Bohai Sea has changed greatly.
(today's Bohai Sea is actually very young) ▼
Today, it is as far north as Laotieshan in Liaoning and 32 small islands in Penglai, Yantai, Shandong Province, the Miaodao Islands. Like a pearl necklace, it is generally connected above the sea to form the natural dividing line between the Yellow Sea and the Bohai Sea. The oil and gas resources contained in the basement faults of the Bohai Bay Basin have also made important contributions to the social and economic development of our country.
Bohai Oilfield, Dagang Oilfield, Shengli Oilfield, Huabei Oilfield
All ▼ in this Mesozoic petroliferous basin.
From the development of geological history, we can see that the sea and land changes in the Bohai Sea are closely related to climate change and geological structure. Climate change leads to the growth and decline of solid water and liquid water, while the opening and closing of plates leads to changes in the volume of sedimentary basins.
Different from the changes in the geological history, today's Bohai Sea is affected not only by the above two restrictive factors, but also by river systems and man-made factors.
Both sea reclamation and land reclamation have a great impact on the area of the Bohai Sea.
The smaller and smaller Bohai Sea river is a kind of depression flow formed by the convergence of atmospheric rainfall into low-lying grooves, which is an important part of continental strata. Among the 40 rivers that flow into the Bohai Sea, such as the Yellow River, Haihe River, Luanhe River and Liaohe River, the Yellow River, our mother river, is most closely related to the development of the Bohai Sea coastline.
If we compare the coastline of the warring States period
You will find the Yellow River in the southwest and the Liao River in the northeast.
Created a lot of land in 2000.
But in terms of scale, the scale of land reclamation of the Yellow River is much larger.
(in fact, you can tell from the color of the coast) ▼
The Yellow River originates from the Yogu Zonglie Basin at the northern foot of the Bayan Kara Mountains on the Qinghai-Tibet Plateau and is the second longest river in China. It flows through nine provinces (autonomous regions) of Qinghai, Sichuan, Gansu, Ningxia, Inner Mongolia, Shaanxi, Shanxi, Henan and Shandong, and finally merges into the Bohai Sea in Dongying, Shandong Province. The Yellow River, which has rich experience, is drifting all the way, from the pure plateau upstream to the downstream, which is no longer as clear as it was at first.
The name of the Yellow River comes from its huge sediment content.
Its serious flooding is on the one hand.
On the other hand, it also brings fertile soil to the downstream.
Due to the jacking action of sea water, the flow velocity of the Yellow River slows down, and the terrigenous clastic materials it carries form a very thick loose accumulation near the estuary of the Bohai Sea.
According to statistics, the average annual sediment transport of the Yellow River reaches 1.6 billion tons, accounting for more than 90% of the total sediment input into the Bohai Sea every year, shaping various estuarine landforms such as alluvial fans, impact plains and the Yellow River Delta.
Among them, the Yellow River Delta, known as the "three major deltas of China" with the Yangtze River Delta and the Pearl River Delta, occupies a certain "right to speak" in the change of the area of the Bohai Sea.
At the mouth of the Yellow River, the river carries a huge amount of sediment to form a delta.
In recent years, the Yellow River Delta shows a trend of continuous expansion. Data show that the average coastline of the Bohai Sea advances 390 meters every year, and the newly built continent covers an area of more than 30 square kilometers, constantly squeezing the Bohai Sea, which is not deep. However, the Yellow River Delta does not always make land, and there will be "land swallowing".
A very spectacular close-up of the Yellow River estuary
Still marching into the Bohai Sea (photo: Changguang satellite) ▼
Since the Yellow River seized the Daqing River into the Bohai Sea in 1855, under the combined action of natural and man-made factors, breaches and diversion have occurred in the delta for more than 50 times, resulting in continuous changes in siltation and erosion in the coastal section. From the early 1940s to the 10s of this century, the area of the Bohai Sea has shrunk by nearly 5700 square kilometers.
Once the land is created, it will have an owner.
It will be very difficult to return it to the Bohai Sea.
And even without the Yellow River, people would take the initiative to build land.
(photo: google map)
In recent years, with the national control of soil and water loss in the upper reaches of the Yellow River and the protection of the environment of the estuary, the development rate of the Yellow River Delta has slowed down.
Related studies show that the coastline of the Bohai Sea continues to silt to the sea at the current estuary and retreats by erosion in the northern abandoned estuary, while the coastline of the southern Laizhou Bay area does not change much.
It can be seen that the coastal changes in the Bohai Sea vary, and it can be said that there are advances and retreats.
The Yellow River estuary and Laizhou Bay are important coastal wetlands.
A large number of migratory birds come here every year.
What needs to be paid attention to is not only the area, but also the ecological condition ▼.
No longer enclosing the Bohai Sea, in addition to the expansion of the estuarine delta caused by shoreline sediment deposition, the construction of artificial reclamation projects will also affect the area of the Bohai Sea. The area around the Bohai Sea is one of the areas with the highest intensity of reclamation in China, accounting for 46% of the total reclamation area in the country.
Before 2000, the reclaimed land was mainly the slow and disorderly expansion of aquaculture land, salt field and agricultural land. After 2000, the land type of the main reclamation growth around the Bohai Sea has been transformed into building land, which also makes the scale of reclamation in the Bohai Sea gradually towards a high-intensity orderly expansion, with an average annual shrinkage rate of 141 square kilometers. The length of the natural coastline has also shrunk year after year.
Comparison of coastal satellite images between 1979 and 2000 in Laizhou Bay
In the past 20 years, the coastal salt flats have been filled with salt farms.
(red is cultivated land, picture: wiki) ▼
According to the statistics of the researchers, in the 38 years from 1980 to 2017, the reclaimed area around the Bohai Sea increased by 1988.5 square kilometers, accounting for nearly 3% of the total area of the Bohai Sea. If it continues at this rate, the Bohai Sea may be filled in more than a thousand years.
At that time, the sea will change into mulberry fields again.
For decades, with the rapid development of coastal economy, the phenomenon of reclamation is becoming more and more common in many areas around the Bohai Sea. Although the reclamation activities of "important land to the sea" can provide land and considerable income for industry in a short period of time, it will also damage the coastal environment. For example, the deterioration of water quality, the depletion of fishery resources and the destruction of wetland habitat.
A large part of the sea enclosure is used for aquaculture.
Aquaculture is actually a highly polluting industry.
In view of this situation, in November 2018, the National Development and Reform Commission put forward the "Action Plan for Comprehensive Management of the Bohai Sea", which puts forward reasonable suggestions for the future sustainable development of the Bohai Sea from the aspects of sea current pollution, port pollution control, ship pollution control and so on.
The Bohai Sea is shallow and small, and the exchange rate with the Yellow Sea is slow.
But the surrounding area contains too much industry, agriculture and cities.
The huge discharge of sewage can not purify itself at all.
In the same year, the State Council issued the Circular on strengthening the Protection of Coastal Wetlands and strict Control of Reclamation, which proposed for the first time that the development of any reclamation projects in the Bohai Sea should be prohibited. It is also required to speed up the treatment of the remaining problems such as water pollution and reduction of coastal wetlands caused by reclamation in this area.
For example, Nandagang, located in the Bohai New area of Cangzhou, Hebei Province, used to be an industrial park dominated by petrochemical industry. With the implementation of the national policy, some areas of Nandagang have become wetland "post stations" for migratory birds to and from, and have been set up as provincial nature reserves.
In recent years, in order to practice the concept that "Green Water and Green Mountain is Jinshan and Silver Mountain", the state has paid more attention to the ecological environment of Bohai Sea and issued a series of ecological assessment and ecological protection and restoration programs. Through the demolition of unreasonable reclamation areas, the renovation of coastal wetlands and other measures, strictly abide by the red line of eco-environmental protection around the Bohai Sea.
Only when the environment is good will seagulls come to the wharf and shore.
The Bohai Sea may be small or shallow, and it may take decades of protection to continue to repair, but we have no other choice. after all, we do not have a second Bohai Sea.
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